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Abstract

The hydrogen isotope composition of plant leaf wax (δDwax) has been found to record the isotope composition of precipitation (δDp). Hence, δDwax is increasingly used for palaeohydrological reconstruction. δDwax is, however, also affected by secondary factors, such as vegetation type, evapotranspiration and environmental conditions, complicating its direct application as a quantitative palaeohydrological proxy. Here, we present δDwax data from soils along vegetation gradients and climatic transects in southern Africa to investigate the impact of different environmental factors on δDwax. We found that δDwax correlated significantly with annual δDp (obtained from the interpolated Online Isotopes in Precipitation Calculator data set) throughout the eastern and central South Africa, where the majority of the mean annual precipitation falls during the summer. We found evidence for the effect of evapotranspiration on δDwax, while vegetation change was of minor importance. In contrast, we found that δDwax did not correlate with annual δDp in western and southwestern South Africa, where most of the annual precipitation falls during winter. Wide microclimatic variability in this topographical variable region, including distinct vegetation communities and high vegetation diversity between biomes as well as a potential influence of summer rain in some locals, likely compromised identification of a clear relationship between δDwax and δDp in this region. Our findings have implications for palaeoenvironmental investigations using δDwax in southern Africa. In the summer rain dominated eastern and central region, δDwax should serve well as a qualitative palaeohydrological recorder. In contrast, the processes influencing δDwax in the winter rain dominated western and southwestern South Africa remain unclear and pending further analyses potentially constrain its use as palaeohydrological proxy in this region.

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... In general a deuterium enrichment in plant waxes is observed in areas with high evapotranspiration and lower amount of precipitation (e.g. Feakins and Sessions, 2010;Herrmann et al., 2017;Kahmen et al., 2013a;Krull et al., 2006). The enriched δD wax in the marine surface sediments observed in the central mudbelt ( Figure 5) may reflect a combination of these processes indicating input of terrestrial organic material from the dry Karoo Biome on the adjacent west coast. ...
... In the southernmost mudbeltdepleted δD wax , relative to the central mudbelt, may indicate terrestrial input from the Fynbos Biome experiencing higher MAP than the dry Karoo. This is consistent with earlier published δ 13 C wax and pollen data from the same sediments (Herrmann et al., 2016;Zhao et al., 2015), investigation of bulk sediments in the southern mudbelt (Birch, 1977) as well as δD wax in South African soils (Herrmann et al., 2017). The more depleted δD wax values in the northernmost mudbelt, in contrast, point to a source region with higher amount of precipitation and/or less evapotranspiration. ...
... We are aware that both samples are of different age (Herrmann et al., 2016) and thus represent different periods. However, considering that modern precipitation has more depleted δD values in the Drakensberg Mountains than in the lower altitudes of South Africa's SRZ (OIPC; Bowen and Revenaugh, 2003) and that δD wax in soils of the SRZ tracks well the δD of precipitation (Herrmann et al., 2017), it can be assumed that the Orange River flood deposit reflects a flood event with an origin in the Drakensberg Mountains. Differences in the δD wax composition of both flood deposits at location ORF29 might be due to different ages or contribution of n-alkanes by different sources as they also show slightly different n-alkane compositions (Herrmann et al., 2016). ...
Article
The summer rainfall zone (SRZ) in the South African interior experienced pronounced hydrological and vegetation changes during the Holocene inferred to be driven mainly by shifts in atmospheric and oceanic circulations systems. The exact mechanisms controlling these changes are still debated. To gain better insights into the Holocene environmental changes in the South African SRZ and their driving factors, we analysed compound-specific carbon and hydrogen isotopes of plant wax n-alkanes (δ13Cwax and δDwax) from a marine sediment core covering the last 9900 years. The core has been recovered offshore the mouth of the Orange River, predominantly draining the South African summer rainfall region. Our data indicate a dry early Holocene and a gradual increase of wetter conditions with a higher abundance of C4 vegetation towards the middle Holocene. Wettest conditions occurred around 3900 cal. yr BP. The last 3900 years were characterised by a gradual aridification overlain by variable wetter conditions. D
... So far, several studies have analysed the hydrogen isotopic composition of leaf waxes in sedimentary archives, e.g., lake, marine and peat sediments in this area (e.g., Hahn et al., 2016Hahn et al., , 2017Miller et al., 2019;Strobel et al., 2019). Moreover, regional calibration studies for δ 2 H n-alkanes exist (Hahn et al., , 2017Herrmann et al., 2017). However, as outlined above, the interpretation of δ 2 H alone is challenging, because it is difficult to disentangle the various influencing factors. ...
... Accordingly, different months were used for these calculations: June, July and August (JJA) for the WRZ; annual weighted mean for the southern YRZ; and December, January and February (DJF) for the SRZ and northern YRZ. We have to note that Herrmann et al. (2017) described the northern YRZ to be dominated by the isotopic signature of the summer months, which explains our comparison with the respective precipitation (DJF). ...
... It has to be noted that the range of the intra-zonal δ 2 H n-alkanes data is substantially wider than the range of δ 2 H p (Fig. 5). Overall, the positive correlation between δ 2 H n-alkanes and δ 2 H p of the vegetation period was also found in several studies in different ecosystems (Herrmann et al., 2017;. However, due to phenological adaptations, the definition of the vegetation period for each ecoregion in South Africa is not trivial, but is generally considered to be related to the main precipitation seasonality (Matimati et al., 2013;Wessels et al., 2011). ...
Article
The hydrogen isotopic composition of leaf wax-derived n-alkanes (δ2Hn-alkanes) is a widely applied proxy for (paleo)climatic changes. It has been suggested that the coupling with the oxygen isotopic composition of hemicellulose-derived sugars (δ18Osugar) - an approach dubbed 'paleohygrometer' - might allow more robust and quantitative (paleo)hydrological reconstructions. However, the paleohygrometer remains to be evaluated and tested regionally. In this study, topsoil samples from South Africa, covering extensive environmental gradients, are analysed. δ2Hn-alkanes correlates significantly with the isotopic composition of precipitation (δ2Hp), whereas no significant correlation exists between δ18Osugar and δ18Op. The apparent fractionation (εapp) is the difference between δ2Hn-alkanes and δ2Hp (εapp 2H) and δ18Osugar and δ18Op (εapp 18O), respectively, and integrates i) isotopic enrichment due to soil water evaporation, ii) leaf (and xylem) water transpiration and iii) biosynthetic fractionation. We find no correlation of εapp 18O nor for εapp 2H with temperature, and no correlation of εapp 2H with potential evapotranspiration and an aridity index. By contrast, εapp 18O correlates significantly with both potential evapotranspiration and the aridity index. This highlights the strong effect of evapotranspirative enrichment on δ18Osugar. In study areas without plant predominance using Crassulacean Acid Metabolism (CAM), coupling δ18Osugar and δ2Hn-alkanes enables to reconstruct δ2Hp and δ18Op with an offset of Δδ2H = 6 ± 27‰ and Δδ18O = 0.8 ± 3.7‰, respectively, as well as relative humidity (RH) with an offset of ΔRH = 6 ± 17%. The paleohygrometer does, however, not work well for our study areas where CAM plants prevail (reconstructed δ18Op, δ2Hp and RH are off by 3.1‰, 27.2‰ and 31.7%). This probably reflects plant-specific (phenological) adaptations and/or post-photosynthetic exchange reactions related to CAM metabolism. Overall, our findings corroborate that δ2Hn-alkanes and δ18Osugar are valuable proxies, and the paleohygrometer is a promising approach for paleoclimate reconstructions in southern Africa.
... The fractionation of hydrogen isotopes between the precipitation (δ 2 H p ) and leaf wax lipids in sediments is complex and depends on many variables like vegetation assemblage, evapotranspiration, and catchment size (Sachse et al., 2012). Herrmann et al. (2017) found a significant correlation between δ 2 H wax in soils and δ 2 H p of the same area of the southern African SRZ. However, in more arid parts of the savanna biome high evapotranspiration leads to 2 H enrichment, which excludes quantitative precipitation reconstructions (Herrmann et al., 2017). ...
... Herrmann et al. (2017) found a significant correlation between δ 2 H wax in soils and δ 2 H p of the same area of the southern African SRZ. However, in more arid parts of the savanna biome high evapotranspiration leads to 2 H enrichment, which excludes quantitative precipitation reconstructions (Herrmann et al., 2017). Therefore, as long as PET is stable, δ 2 H 31 values represent relative changes in δ 2 H p . ...
Article
Conventional continental geoarchives are rarely available in arid southern Africa. Therefore, palaeoclimate data in this area are still patchy and late Quaternary climate development is only poorly understood. In the western Kalahari, salt pans (playas, ephemeral lakes) are common and can feature quasi-continuous sedimentation. This study presents the first climate-related biomarker record using sediments from the Omongwa Pan, a Kalahari salt pan located in eastern Namibia. Our approach to reconstruct vegetation and hydrology focuses on biogeochemical bulk parameters and plant wax-derived lipid biomarkers (n-alkanes, n-alkanols, and fatty acids) and their compound-specific carbon and hydrogen isotopic compositions. The presented record reaches back to 27 ka. During the glacial, rather low δ²H values of n-alkanes and low sediment input exclude a strong influence of winter rainfall. n-Alkane and n-alkanol distributions and δ¹³C values of n-hentriacontane (n-C31) indicate a shift to a vegetation with a higher proportion of C4 plants at the end of the Last Glacial Maximum until the end of Heinrich Stadial I (ca. 18–14.8 ka), which we interpret to indicate an abrupt excursion to a short wetter period likely to be caused by a temporary southward shift of the Intertropical Convergence Zone. Shifts in δ²H values of n-C31 and plant wax parameters give evidence for changes to drier conditions during early Holocene. Comparison of this dataset with representative continental records from the region points to a major influence of summer rainfall at Omongwa Pan during the regarded time span and demonstrates the potential of southern African salt pans as archives for biomarker-based climate proxies.
... For the case-study application, we choose to simulate and compare two present-day climate cases: the first one for Europe where many GNIP station observations are available (IAEA/WMO, 2020) and the second one for Southern Africa. The second application case is chosen in order to further investigate the potential alteration of Southern African paleo records data by soil processes (e.g., Herrmann et al., 2017), and better understand past-and present-day climate variations in Southern Africa (e.g., Weldeab et al., 2013) with the help of water isotopologue modeling. Section 2 of the paper describes the implementation of WRF-Hydro-iso. ...
... These well-known altitude and continental effects are less clear in Figures 9d and 9e for Southern Africa. Indeed, Southern Africa is characterized by a high precipitation seasonality with a summer rainfall zone in the central and eastern regions and a winter rainfall zone near the west coast (e.g., Roffe et al., 2019), which also contributes to regional differences in isotopic concentrations (e.g., Herrmann et al., 2017). But the scarcity of the GNIP stations in Southern Africa does not allow to fully validate the modeled spatial distribution of isotopic concentrations in this case. ...
Article
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Water isotopologues, as natural tracers of the hydrological cycle on Earth, provide a unique way to assess the skill of climate models in representing realistic atmospheric-terrestrial water pathways. This study presents the newly developed WRF-Hydro-iso, which is a version of the coupled atmospheric-hydrological WRF-Hydro model enhanced with a joint soil-vegetation-atmospheric description of water isotopologues motions. It allows the consideration of isotopic fractionation processes during water phase changes in the atmosphere, the land surface and the subsurface. For validation, WRF-Hydro-iso is applied to two different climate zones, namely Europe and Southern Africa under present climate conditions. Each case is modeled with a domain employing a 5 km grid-spacing coupled with a terrestrial subgrid employing a 500 m grid-spacing in order to represent lateral terrestrial water flow. A 10-year slice is simulated for 2003–2012, using ERA5 reanalyses as driving data. The boundary condition of isotopic variables is prescribed with mean values from a 10-year simulation with the Community Earth System Model Version 1. WRF-Hydro-iso realistically reproduces the climatological variations of the isotopic concentrations urn:x-wiley:19422466:media:jame21444:jame21444-math-0001 and urn:x-wiley:19422466:media:jame21444:jame21444-math-0002 from the Global Network of Isotopes in Precipitation. In a sensitivity analysis, it is found that land surface evaporation fractionation increases the isotopic concentrations in the rootzone soil moisture and slightly decreases the isotopic concentrations in precipitation. Lateral terrestrial water flow minorly affects these isotopic concentrations through changes in evaporation – transpiration partitioning.
... While some studies suggest that ε 2H n-alkane/p in plants and topsoils remains relatively constant over latitudinal distances (Liu et al., 2016, Liu andAn, 2019;Vogts et al., 2016), many others show correlations of ε 2H n-alkane/p with relative humidity, mean annual precipitation (MAP) and the aridity index (AI) (Hou et al., 2008(Hou et al., , 2018Douglas et al., 2012;Tipple and Pagani, 2013;Berke et al., 2015;Herrmann et al., 2017;Li et al., 2019). This can be related to more evapotranspirative enrichment under more arid conditions, but plant physiological adaptations might also play a role, as well as changes in vegetation, as e.g., monocotyledons have more negative δ 2 H n-alkane values than dicotyledons (Sachse et al., 2012;Kahmen et al., 2013b;Hepp et al., 2020). ...
Article
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Compound-specific hydrogen and oxygen isotope analyzes on leaf wax-derived n-alkanes (δ2Hn alkane) and the hemicellulose-derived sugar arabinose (δ18Oara) are valuable, innovative tools for paleohydrological reconstructions. Previous calibration studies have revealed that δ2Hn alkane and δ18Oara reflect the isotopic composition of precipitation, but – depending on the region – may be strongly modulated by evapotranspirative enrichment. Since no calibration studies exist for semi-arid and arid Mongolia so far, we have analyzed δ2Hn alkane and δ18Oara in topsoils collected along a transect through Mongolia, and we compared these values with the isotopic composition of precipitation (δ2Hp-WM and δ18Op-WM, modelled data) and various climate parameters. δ2Hn alkane and δ18Oara are more positive in the arid south-eastern part of our transect, which reflects the fact that also the precipitation is more enriched in 2H and 18O along this part of the transect. The apparent fractionation Ɛapp, i.e., the isotopic difference between precipitation and the investigated compounds, shows no strong correlation with climate along the transect (Ɛ2H n C29/p = -129 ± 14‰ , Ɛ2H n C31/p = -146 ± 14‰, and Ɛ18O ara/p = +41 ± 2‰). Our results suggest that δ2Hn alkane and δ18Oara in topsoils from Mongolia reflect the isotopic composition of precipitation and are not strongly modulated by climate. Correlation with the isotopic composition of precipitation has root-mean-square errors of 13.4‰ for δ2Hn-C29, 12.6‰ for δ2Hn-C31, and 1.2‰ for δ18Oara, so our findings corroborate the great potential of compound-specific δ2Hn alkane and δ18Oara analyzes for paleohydrological research in Mongolia.
... Leaf wax derived long chain n-alkanes (>C 25 ) and their compound-specific stable hydrogen isotopes (dD wax ) provide insights into past hydrological conditions and are stable over geological timescales (Eglinton and Eglinton, 2008;Sachse et al., 2012;Schefub et al., 2005;Sessions, 2016). In the SRZ dD wax in surface sediments and topsoils has been shown to correlate with dD of local precipitation (dD p ) (Herrmann et al., 2017). However, dD wax can be influenced by various processes that may bias the precipitation isotope signal and complicate interpretations (Sessions, 2016). ...
Article
Due to the scarcity of natural archives for palaeoenvironmental studies, the climatic evolution of South Africa during the late Quaternary remains the subject of considerable debate. Peat deposits provide excellent archives to investigate past environmental and climate variability. Vankervelsvlei, a fen near the southern Cape coast, located 152 m above mean sea level within the year-round rainfall zone of South Africa, is ideally suited to investigate past environmental changes in this region. A 14.6 m long sediment sequence was retrieved from the fen, from which 8.85 m of sediment have been analysed using a multi-proxy approach. This includes elemental, macrofossil and micropalaeontological analyses. As a novelty in palaeoenvironmental reconstruction in this region, leaf wax n-alkanes and their compound-specific stable carbon and hydrogen isotopes were also investigated. The chronology of the sequence is based on 13 radiocarbon ages and reveals a basal age of 37,430 +1,570/-1,710 cal BP. The top of the investigated sequence has an age of 1,180 +340/-170 cal BP. Leaf wax n-alkane abundances and their compound-specific stable carbon and hydrogen isotopes, as well as various (in)organic (bio)geochemical parameters, indicate that the older section of the sequence (37,430 +1,570/-1,710 - 28,050 +510/-600 cal BP), which is composed of strongly degraded peat, represents a rather dry phase during MIS 3. This is followed by a hiatus of around 20,000 years (28,050 +510/-600 to 8,360 +730/-810 cal BP) reflecting the driest conditions during MIS 2. By comparing to supra-regional archives, this is hypothesised to have resulted from a larger extension of the Antarctic sea ice, which caused an equa-torward shift of the Westerlies, blocking the tropical easterlies and resulting in drier conditions along the south coast and the adjacent coastal platform. During the Early Holocene, the input of reworked soil into the depression and subsequently renewed peat formation from 6,820 +305/-365 cal BP to 1,180 +340/-170 cal BP in Vankervelsvlei point to moister climatic conditions. Contraction of Antarctic sea ice and a poleward shift of the Westerlies during the Holocene is consistent with this interpretation. Climatic driving forces are suggested to differ between centennial/millennial and orbital time scales. Evapotranspirative enrichment through stronger winds is assumed to be the main driver on centennial to millennial time scale within this hydrological system. However, a combination of evapotranspiration and precipitation amount seems to be the most prominent driver on the orbital time scales.
... Changes in stable hydrogen isotopic ratios measured in plant waxes are therefore mainly related to isotope shifts in precipitation, potentially amplified by isotopic enrichment in soil and leaf water under low humidity conditions (evapo-transpiration, e.g. Kahmen et al., 2013a,b;Herrmann et al., 2017). In regimes of highly seasonal rainfall as in monsoonal regions, however, this secondary enrichment was recently inferred to be of minor importance (Niedermeyer et al., 2016). ...
Article
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Terrestrial signals in marine sediment archives are often used for paleoclimatic reconstructions. It is therefore important to know the origin of the different terrestrial sedimentary components. The proximity to a river mouth is often the key location to determine the source. Especially in regions with strong ocean currents, such an assumption might, however, lead to considerable misinterpretations. To investigate the source of various terrigenous sediment fractions in southeastern Africa, a region with strong sediment redistribution, we have performed an extensive comparison between terrestrial material (pollen, plant lipids, detrital modes, and heavy minerals as well as bulk inorganic geochemical composition) from potential source regions and the same components in the adjacent coastal and continental shelf sediments. Onshore the proxy-indicators reflect small-scale diversity in sampling locations and associated environments (riverbank sediments, flood deposits, suspension loads, and soils). Nevertheless, the overall trends reflect significant environmental gradients along a SW to NE transect. We note a general comparability of the studied parameters between the continental and marine sediments regardless of their specific differences in transport and depositional characteristics. We propose that the influence of the Agulhas Current affects sediment deposition and distribution only seaward of the midshelf and that pockets of sediment remain preserved in the lee of coastal protrusions where they are protected from erosion. This study provides the essential prerequisite to allow the attribution of temporal variations of compositional changes in marine sediment cores to environmental changes in southeastern Africa.
... All rights reserved. (Sternberg, 1988;Pagani et al., 2006;Schefuß et al., 2011;Feakins et al., 2012;Bai et al., 2015;Vogts et al., 2016;Herrmann et al., 2017). These research efforts are largely motivated by the empirical relationship between dD n-alkane values from modern plants/sediments and precipitation dD observed at regional and global scales (Sachse et al., 2004Liu and Yang, 2008;Polissar and Freeman, 2010;Yang et al., 2011;Garcin et al., 2012;Collins et al., 2013;Tipple and Pagani, 2013;Duan et al., 2014;Feakins et al., 2016a;An, 2018, 2019). ...
Article
Sedimentary δDn-alkane values have been widely used as a valuable proxy for paleoenvironmental reconstruction. A number of studies have focused on δDn-alkane values that derived exclusively from leaves, while less attention has been paid to the root-derived n-alkanes and their impact on sedimentary δDn-alkane values. In this study, we sampled modern plant leaf and root materials from different growth contexts (slopes and seasons) on the Chinese Loess Plateau to compare leaf-derived n-alkanes with root-derived n-alkanes. Our results demonstrated that total n-alkane (C27–C33) concentrations varied substantially between leaf and root materials, with average values of 209 and 29.5 μg/g observed in leaves and roots respectively. The results suggest that ca. 12% of the n-alkane concentrations in sediments may be derived from plant roots. Furthermore, leaf-derived δDn-alkane values for Stipa bungeana (grass), Artemisisa vestita (shrub) and Bothriochloa ischaemum (grass) averaged −184‰, −152‰ and −198‰, compared with their root-derived δDn-alkane values of −199‰, −179‰ and −163‰, respectively. These statistically significant differences in concentrations and δD values between leaf-derived and root-derived n-alkanes suggest that the contribution of n-alkanes derived from plant roots is important for evaluating the resultant n-alkane compositions of sediments for paleoenvironmental reconstruction. Our findings indicates that the effects of root-derived n-alkanes on total sedimentary δDn-alkane values should be considered carefully in future paleoenvironmental reconstruction efforts.
... Here, dD wax signatures indicate increasing rainfall amounts towards the north, paralleling mean annual precipitation gradients. Soil samples taken along climatic transects in central and eastern South Africa also indicate that dD wax correlates significantly with annual dD precip and mean annual precipitation amount (Herrmann et al., 2017). Consequently, precipitation amount and ET are likely the main controls of the isotopic composition of soil and leaf waters, and subsequently of the leaf waxes within core GeoB20610-2. ...
... Long-chain n-alkanes are produced as leaf waxes in higher terrestrial plants and remain well preserved in soils and sediments over millennia due to their low water solubility and high resistance against degradation (Eglinton and Eglinton, 2008;Sachse et al., 2012;Sessions, 2016). They serve as valuable biomarkers because their hydrogen isotope composition (d 2 H n-alkane ) is driven by the isotopic composition of precipitation (d 2 H p ) (Herrmann et al., 2017;Strobel et al., 2020;Struck et al., 2020). However, various fractionation processes occur from the moisture source to its fixation in the leaf waxes, which may bias the precipitation isotope signal and complicate interpretations, with evapotranspirative enrichment being a prominent factor (Sachse et al., 2012;Sessions, 2016;Zech et al., 2015). ...
Article
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The Late Quaternary climate history of South Africa and, in particular, potential changes in atmospheric circulation have been subject to considerable debate. To some extent, this is due to a scarcity of natural archives, and on the other hand the available indirect hydrological proxies are not suited to distinguishing between precipitation originating from temperate Westerlies and tropical Easterlies. This study presents a paleoenvironmental record from Vankervelsvlei, a wetland located on the southern Cape coast in the year-round rainfall zone of South Africa. A 15 m long sediment record was retrieved from this site and analysed using a multi-proxy approach. This includes, for the first time in this region, analysis of both compound-specific δ²H and δ¹⁸O from leaf wax-derived n-alkanes and hemicellulose-derived sugars, respectively, to investigate hydrological changes during the Late Quaternary. Our data suggest the driest conditions of the past ∼250 ka likely occurred from MIS 6 to MIS 5e, which still caused sediment deposition at Vankervelsvlei, and MIS 2, during which time there was an absence of sedimentation. Moist conditions occurred from MIS 5e to 5a and during parts of MIS 3, while drier conditions prevailed between MIS 5a and early MIS 3 and at the transition from MIS 3 to MIS 2. Besides changes in the amount and proportional contribution of precipitation contributions from Westerlies during glacial and Easterlies during interglacial periods, relative sea-level change affected the continentality of Vankervelsvlei, with a distinct impact on the sites hydrological balance. High-resolution analyses of the Middle- and Late-Holocene parts of the record show moist conditions and increased Easterly/locally-derived summer precipitation contributions from 7230 ⁺¹⁶⁰/−210 to 4890 ⁺²⁸⁰/−180 cal BP and after 2840 ⁺³⁵⁰/−330 cal BP. Dry conditions, accompanied by the reduction of Easterly/locally-derived summer rainfall and increased seasonality occurred from 4890 ⁺²⁸⁰/−180 to 2840 ⁺³⁵⁰/−330 cal BP. Our findings highlight that source and seasonality of precipitation play a major role in the hydrological balance of the southern Cape coastal region. By comparing the Vankervelsvlei record to other regional studies, we infer a coherent trend in the overall moisture evolution along South Africa's southern Cape coast during the Late Quaternary.
... The hydrogen isotopic signature of plant nalkanes mainly depends on the δ 2 Η value of meteoric water, the main hydrogen source for terrestrial plants (Sachse et al., 2012), and, to a lesser extent, on isotope enrichment due to evapotranspiration and water-use efficiency of the respective plant (Kahmen et al., 2013a(Kahmen et al., , 2013bSachse et al., 2012;Tipple and Pagani, 2010). In particular, 2 Ηenrichment in waxes can result from (1) reduced rainfall, (2) increased evapotranspiration, (3) a high condensation temperature under warmer climate, (4) replacement of C 3 grasses by C 4 grasses, (5) downslope movements of vegetation belts, or (6) a change toward a 2 Η-enriched moisture source (Collins et al., 2013;Dansgaard, 1964;Herrmann et al., 2017;Hou et al., 2008;Jaeschke et al., 2018;Kahmen et al., 2013aKahmen et al., , 2013bRisi et al., 2008;Rozanski et al., 1993;Sachse et al., 2012;Tipple and Pagani, 2010). ...
Article
The youngest and one of the largest saline deposits of Earth history formed in the Mediterranean during the Messinian salinity crisis (MSC; 5.97–5.33 Ma, Late Miocene), when global climate was experiencing a trend of cooling and aridification. However, recent paleoclimate reconstructions indicate strong climate gradients across the Mediterranean Basin and the persistence of humid conditions in the northern Mediterranean during the earliest phase of the MSC. To characterize climatic and hydrologic variability across the onset of the MSC, we studied inorganic geochemical proxies, comprising Si, Ti, and Zr element distributions and organic geochemical proxies, such as sedimentary plant wax contents and their carbon (δ13Cn-alk) and hydrogen (δ2Ηn-alk) stable isotope compositions in deep-water (> 200 m), alternating shale and marl deposits of the Piedmont Basin (Govone section; NW Italy). Our results suggest that deposition was controlled by precession-driven climate fluctuations before and after the onset of the MSC, with shales deposited under moister conditions (lower element/ Al ratios and δ2Ηn-alk values) and marls when drier conditions prevailed (higher element/Al ratios and δ2Ηn-alk values). The observed δ2Ηn-alk pattern further suggests changes in the relative contributions of the sources of moisture for precipitation. At precession minima, shales were deposited and moisture was mainly sourced from the North Atlantic, as reflected by lower δ2Ηn-alk values. At precession maxima, marls were deposited and higher δ2Ηn-alk values point to a – probably western – Mediterranean source of moisture. The decrease of element/Al ratios as well as the coeval increase of plant wax abundances after the onset of the MSC indicate enhanced humidity and increased net precipitation over the northern Mediterranean. These changes coincided with a restriction of the Atlantic-Mediterranean gateways and resulted from an enhanced contribution of deuterium-enriched moisture sourced from seawater evaporation in the Mediterranean, evidenced by an increase of δ2Ηn-alk values.
... While the majority of the mud belt sediments is provided by the Orange River, it will not provide the coarse end member as the poleward undercurrent lacks sufficient energy to keep coarser grain sizes in suspension (Garzanti et al., 2014;Shannon and Nelson, 1996). Other studies have indicated the overprint of Orange River derived material in the mud belt by local-derived pollen (Zhao et al., 2016), plant organic material (Herrmann et al., 2016(Herrmann et al., , 2017, and inorganic sediments (Hahn et al., 2015). The latter showed that Nd and Sr isotope ratios at site GeoB8323-2 do not correspond to those offshore of the Orange River and instead are representative of the Olifants River catchment. ...
Article
Southwest Africa is an important region for paleo-climatic studies, being influenced by both tropical and temperate climate systems and thus reflecting the interplay of variable controls. The aim of this study was to unravel the interaction of sea-surface temperature (SST) changes in the southernmost Benguela upwelling system with precipitation changes in South Africa’s winter rainfall zone (WRZ) during the late Holocene. Therefore, a marine sediment core from the southernmost Benguela upwelling system was investigated to reconstruct climate changes in this region for the past ~2000 years. Grain size and geochemical analyses were conducted to reconstruct changes in fluvial sediment discharge and weathering intensity, while SST changes were estimated using alkenone paleo-thermometry. Results show that the southernmost Benguela behaves distinctly in comparison with the rest of the Benguela system reflecting amplified SST changes. Decreasing SSTs accompanied increasing river discharge during times of increased precipitation in the WRZ, reflecting northerly shifted westerly winds during austral winter. We infer a control of past SST changes by processes not analogous to modern processes driving seasonal SST changes by changes through upwelling intensity. The findings suggest that late-Holocene SST changes in the southernmost Benguela upwelling system and the precipitation in the WRZ were both driven by latitudinal shifts of the austral westerly wind belt and associated changes in advection of cold sub-Antarctic waters and/or changes in Agulhas leakage of warm Indian Ocean waters.
... Molecules of animal or bacterial origin Phospholipid fatty acids Microbial biomass Kramer and Phospholipid fatty acid: (PLFAs) Gleixner 2157 (1628) ( 2006), Kindler PLFA: 1525 (1140) et ), Ngosong et al. (2012), Malik et al. (2013 Compound-specific stable isotope signal of one or more of the above c δ 2 H (deuterium) Precipitation and Peterse et 2 H: 6 (4) paleo-elevation al. (2009), Bai Deuterium: 9 (7) et ), Luo et al. (2011), Sachse et al. (2012), Hermann et al. (2017 proxies in different plant species, plant specimens, and plant parts as a result of genetic or life stage variations and/or external factors such as climate, seasonality, or exposure to the sun (e.g., Nødskov Giese, 1975;Lockheart et al., 1998;Shepherd and Griffiths, 2006); ...
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The application of lipids in soils as molecular proxies, also often referred to as biomarkers, has dramatically increased in the last decades. Applications range from inferring changes in past vegetation composition, climate, and/or human presence to unraveling the input and turnover of soil organic matter (SOM). The molecules used are extractable and non-extractable lipids, including ester-bound lipids. In addition, the carbon or hydrogen isotopic composition of such molecules is used. While holding great promise, the application of soil lipids as molecular proxies comes with several constraining factors, the most important of which are (i) variability in the molecular composition of plant-derived organic matter both internally and between individual plants, (ii) variability in (the relative contribution of) input pathways into the soil, and (iii) the transformation and/or (selective) degradation of (some of) the molecules once present in the soil. Unfortunately, the information about such constraining factors and their impact on the applicability of molecular proxies is fragmented and scattered. The purpose of this study is to provide a critical review of the current state of knowledge with respect to the applicability of molecular proxies in soil science, specifically focusing on the factors constraining such applicability. Variability in genetic, ontogenetic, and environmental factors influences plant n-alkane patterns in such a way that no unique compounds or specific molecular proxies pointing to, for example, plant community differences or environmental influences, exist. Other components, such as n-alcohols, n-fatty acids, and cutin- and suberin-derived monomers, have received far less attention in this respect. Furthermore, there is a high diversity of input pathways offering both opportunities and limitations for the use of molecular proxies at the same time. New modeling approaches might offer a possibility to unravel such mixed input signals. Finally, the transformation and turnover of SOM offer opportunities when tracing such processes is the purpose of applying a molecular proxy while imposing limitations when they obliterate the molecular proxy signals linked to other phenomena. For n-alkanes several modeling approaches have recently been developed to compensate for (selective) degradation. Still, such techniques are in their infancy and information about their applicability to classes of components other than n-alkanes is lacking. All constraining factors considered can have a significant influence on the applicability of molecular proxies in soil science. The degree of influence strongly depends on the type of molecular proxy and the environmental context in which it is applied. However, the potential impact of the constraining factors should always explicitly be addressed whenever molecular proxies are applied in a soil scientific context. More importantly, there is still a serious lack of available information, in particular for compound classes other than the n-alkanes. Therefore, we urgently call for the consideration of more holistic approaches determining various factors during sampling and using as many compound classes as possible.
... Among them, leaf wax dD n-alkane values have been employed as an ideal proxy for paleoenvironmental reconstruction (Liu and Huang, 2005;Schefub et al., 2011;Feakins et al., 2012;Herrmann et al., 2017). Knowledge of the controls on leaf wax dD nalkane values is increasing because great effort has been taken to focus on the effects of environmental and biological factors (e.g., precipitation dD, evapotranspiration, and biosynthetic fractionation) on leaf wax dD n-alkane values in higher plants (Chikaraishi and Naraoka, 2003;Liu et al., 2006;Jia et al., 2008;Feakins and Sessions, 2010;Zhang and Liu, 2011;Tipple and Pagani, 2013;He et al., 2017) and in soil and lake sediments (Huang et al., 2004;Polissar and Freeman, 2010;Garcin et al., 2012;Pautler et al., 2014;Feakins et al., 2016). ...
Article
Leaf wax dDn-alkane values have been widely utilized in terrestrial paleoenvironmental research, and numerous studies have reported that leaf wax dDn-alkane values were affected by a lot of factors. However, the current knowledge regarding the constructive relationships among these factors is still unclear. Based upon a limited compilation of available dDn-alkane records from published literature across the world, we proposed a hierarchical framework of three-level controls on leaf wax dDn-alkane values by using an Interpretive Structural Modeling (ISM). Precipitation dD, plant types and evapotranspiration are the first-level controls that serve as direct co-controls on leaf wax dDn-alkane values in higher plants, on average contributing 47 ± 22%, 20 ± 9.8% and 33%, respectively. The proportions of different controls on leaf wax dDn-alkane values vary significantly over the world, probably responsible for the across-site substantial variability of isotopic fractionation (εapp) between leaf wax dDn-alkane values and precipitation dD. Moreover, extra factors (e.g., salinity, aridity, seasonality etc.) and climate parameters (e.g., temperature, relative humidity, light intensity etc.) execute the second-level and third-level controls on leaf wax dDn-alkane values, respectively. They play indirect controls, through affecting the first-level controls, and thus are translated into leaf wax dDn-alkane values. To quantitatively disentangle these controls on leaf wax dDn-alkane values is important for better understanding the spatial-temporal variability of sedimentary dDn-alkane values.
... The hydrogen from rainfall is incorporated into terrestrial plant epicuticular waxes, resulting in a demonstrable correlation between terrestrial plant δD wax and precipitation δD values (e.g., Huang et al., 2004;Sachse et al., 2004), despite an offset in δD wax due to isotopic fractionations from biosynthesis and evaporative processes. In the tropics, there is a strong relationship between δD values and precipitation amount (Dansgaard, 1964;Rozanski et al., 1993), and plant waxes from south Africa predominantly record the δD values of their source waters (Griepentrog et al., 2019;Herrmann et al., 2017). As such, we interpret more positive (negative) values to indicate more (less) arid conditions due to tropical rainfall amount. ...
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Efforts to understand long‐term Indian Ocean dynamics and land‐sea linkages in southeast Africa during periods of significant global and regional climate change have been inhibited by a lack of high‐resolution climate records, particularly during the Plio‐Pleistocene. Here we present new biomarker and pollen records from International Ocean Discovery Program (IODP) Site U1478, located at the Upper Agulhas Confluence near the Limpopo River mouth, to establish environmental conditions at the southeast African margin between 4 and 1.8 Ma and address this spatiotemporal gap. Compound‐specific hydrogen isotopes of terrestrial leaf waxes (δDwax) and TEX86, using marine archaeal lipids, document hydroclimate variability and sea surface temperature (SST), respectively, permitting an onshore‐offshore climate comparison. The U1478 records establish the Limpopo catchment response to the switch in Indonesian Throughflow source waters, the mid‐Pliocene Warm Period, and intensification of Northern Hemisphere glaciations at ∼2.7 Ma. Broad coherence between the δDwax and SST records supports a linkage between Indian Ocean temperatures and southeast African hydroclimate. We hypothesize that additional mechanisms including Indian Ocean cross‐basin SST gradients (ΔSST) and high latitude glaciation acted as hydroclimate controls during the Plio‐Pleistocene. We use ΔSST to evaluate ocean‐atmosphere patterns similar to the Indian Ocean Dipole (IOD) and establish generally wetter conditions in the region associated with positive IOD‐like phases. Additionally, an obliquity signal evident in the δDwax record indicates that glacial‐interglacial variability likely influenced the tropical rain belt position and also controlled rainfall. Hydroclimate and environmental conditions across the Plio‐Pleistocene in southeast Africa may have important implications for regional hominin evolution.
... This in-phase relationship persists until the onset of the Holocene, when high latitude ice sheets retreated and direct local insolation forcing once again became the dominant driver of southeast African rainfall variability (Chase et al., in press). Additionally, interpretation of the Cape Fold speleothem d 18 O record pre-dating the transition at~70 ka is generally consistent with changes in rainfall amount associated with the "amount effect" (Dansgaard, 1964;Herrmann et al., 2017), consistent with an expanded summer rainfall zone. After~70 ka, when an obliquity cycle becomes apparent, speleothem d 18 O likely reflects changes in regional temperatures, suggesting a change in rainfall regimes and a restriction of the zone of tropical dominance (Chase et al., in press). ...
Article
Southern Africa hosts regions of exceptional biodiversity and is rich with evidence for the presence and activities of early humans. However, few records exist of the concurrent changes in climate that may have shaped the region's ecological evolution and the development and dispersal of our ancestors. This lack of evidence limits our ability to draw meaningful inferences between important changes in the global and regional climate systems and their potential influence in shaping the region's natural and cultural history. This paper synthesises the data currently available to define a general empirically-based conceptual model of the spatio-temporal dynamics of climate change as they relate to changes in the earth's orbital configurations. The goal is to identify mechanistic links between orbital forcing, which can be calculated continuously over the past several million years, and environmental responses to related changes in the major atmospheric and oceanic circulation systems influencing southern Africa. Once identified, these relationships can be used to infer the most likely trends and patterns of climate variability for periods and regions for which proxy evidence is not available. Findings indicate that coherent patterns of change can be observed at wavelengths associated with ∼400-kyr and ∼100-kyr cycles of orbital eccentricity. In southeastern Africa, the ∼2400-kyr grand cycle in eccentricity may have had an influence long-term patterns of aridification and humidification, and the stronger ∼400-kyr eccentricity cycle has a significant influence across inter-tropical Africa, through changes in hydroclimate and monsoon circulation. The attribution of the ∼100-kyr cycle to specific orbital controls depends on location, as it can be determined by eccentricity-modulated direct insolation forcing or through the combined orbital parameters and earth system responses that drive the evolution of Pleistocene glacial-interglacial cycles. Following the onset of the mid-Pleistocene transition (c. 1250–700 ka), the increasing development of substantial polar ice sheets influence the nature of high-latitude drivers in southern Africa. In southwestern Africa, records indicate an evolution in climate and circulation systems strongly correlated with the global benthic δ¹⁸O record, suggesting a particular sensitivity to high latitude forcing. The close correlation between ∼100-kyr eccentricity and glacial-interglacial cycles makes it difficult to determine whether high- or low-latitude drivers dominate in southeastern Africa, but the spatio-temporal patterning of environmental variability in many records are generally considered to indicate a degree of high-latitude influence. Records from southeastern and southernmost Africa also indicate that the influence of low latitude forcing, expressed through the local precessional cycle, is – at least over the last glacial-interglacial cycles - dependent on eccentricity. Periods of reduced eccentricity, particularly during periods of extensive high-latitude ice sheet development, result in diminished influence in direct forcing and an increase in the expression of high latitude forcing, and an increasingly positive correlation between the northern and southern tropics at these wavelengths. In general, the records available allow for a simple conceptual model of the relationship between orbital parameters and regional climates to be defined, with the strongest relationships existing at longer timescales, such as the ∼400-kyr eccentricity cycle. At finer spatio-temporal timescales, the data indicate degrees of complexity that are not readily predicted, but the expansion of the regional dataset will continue to allow for refinements to the conceptual model described.
... Long-chain n-alkanes are leaf waxes produced by higher terrestrial plants and serve as valuable biomarkers as they remain well preserved in soils and sediments over millennia because of their low water solubility and high resistance to degradation (Eglinton and Eglinton, 2008;Sachse et al., 2012;Sessions, 2016). In South Africa, the δ 2 H n-alkane signal shows potential to reconstruct the isotopic signal of precipitation and thus directly refers to the precipitation source (Herrmann et al., 2017;Strobel et al., 2020). However, δ 2 H n-alkane has rarely been used in terrestrial archives at the southern Cape coast (Strobel et al., 2019). ...
Article
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South Africa is a key region to reconstruct and understand past changes in atmospheric circulation, i.e. temperate westerlies and tropical easterlies. However, due to the scarcity of natural archives, South Africa's environmental evolution during the late Quaternary remains highly debated. Many available sediment archives are peri-coastal lakes and wetlands; however, the paleoenvironmental signals in these archives are often overprinted by sea-level changes during the Holocene. This study presents a new record from the coastal wetland Voëlvlei, which is situated in the year-round rainfall zone of South Africa on the southern Cape coast. It presents an ideal sedimentary archive to investigate both sea level and environmental changes. A 13 m long sediment core was retrieved and analysed using a multi-proxy approach. The chronology reveals a basal age of 8440 +200/-250 cal BP. Paleoecological and elemental analyses indicate marine incursions from ca. 8440 to ca. 7000 cal BP with a salinity optimum occurring at 7090 +170/-200 cal BP. At ca. 6000 cal BP, the basin of Voëlvlei was in-filled with sediment resulting in an intermittent (sporadically desiccated) freshwater lake similar to present. In contrast to previous investigations which used indirect proxies for hydrological reconstructions, here we apply a combined biomarker–sedimentological approach that allows the potential identification of precipitation sources, in combination with relative estimates of moisture availability. Increasing moisture is observed throughout the record starting from 8440 +200/-250 cal BP with contributions from both westerlies and easterlies from ca. 8440 to ca. 7070 cal BP. Westerly-derived rainfall dominates from ca. 7070 to ca. 6420 cal BP followed by a distinct shift to an easterly dominance at ca. 6420 cal BP. An overall trend to westerly dominance lasting until ca. 2060 cal BP is followed by a trend towards an easterly dominance to the present, but both phases show several intense, short-term variations. These variations are also evident in other regional studies, highlighting that the source and seasonality of precipitation has varied distinctly on the southern Cape during the Holocene. Comparison of the Voëlvlei record with other regional studies suggests a coherent trend in the overall moisture evolution along the southern Cape coast during the past 8500 years.
... Feakins et al., 2016;Sachse et al., 2006), soils (e.g. Herrmann et al., 2017;Jia et al., 2008;van der Veen et al., 2020), and sediments (e.g. Hou et al., 2008;Ladd et al., 2021;Sachse et al., 2004) are significantly correlated with δ 2 H p . ...
Article
To understand the cause of known isotopic differences between grasses and trees, their seasonal variability and environmental drivers, we present a parallel study of a perennial grass, Stipa bungeana, and an evergreen tree, Quercus aquifolioides, in the monsoon-dominated southeastern Tibetan Plateau (TP), by conducting fortnightly paired δ²H analyses of precipitation (δ²Hp), leaf water, and n-alkanes (δ²Hwax) over a period of 2 y. Similar to previous investigations, we show that δ²Hwax in both species are correlated to δ²Hp throughout two growing seasons. However, for the first time, we identified a time lag of 2 months in variation of δ²Hwax relative to δ²Hp in new Q. aquifolioides leaves, whereas nearly no lagged phenomena were observed in S. bungeana and old mature Q. aquifolioides leaves. The δ²Hwax in new Q. aquifolioides leaves correlated significantly with the total amount of n-alkanes. These findings suggest that the turnover rate of n-alkanes in new Q. aquifolioides leaves is possibly changing with their synthesis, and that is a possible mechanism of the observed delayed phenomenon. A paired t-test showed that the difference of δ²Hwax between trees and grasses becomes insignificant after time-lag correction. Moreover, the apparent fractionation in Q. aquifolioides was significantly correlated with weekly or fortnightly precipitation amount, temperature, relative humidity, and wind velocity, while in S. bungeana only weekly or fortnightly precipitation amount. Our high temporal resolution observations hence confirm that climate variables are important drivers of plant δ²Hwax and, give significant implications for δ²Hwax-base paleoclimate reconstructions in global monsoonal regions.
... There is a strong relationship between leaf wax dD and dD in precipitation, even with secondary influences from vegetation type and evapotranspiration (e.g. Garcin et al., 2012;Herrmann et al., 2017). Less negative (more positive) dD reflects aridity, based upon the inference that dD within low-latitude precipitation is controlled by amount effects, and continentality, so that lower rainfall amounts have a higher (more positive) dD (Dansgaard, 1964). ...
Article
The Namib Desert and the Kalahari constitute the drylands of southern Africa, with the current relatively humid portions of the latter having experienced periodically drier conditions during the Late Quaternary. This study explores the range of dryland archives and proxies available for the past ~190 ka. These include classic dryland geomorphological proxies, such as sand dunes, as well as water-lain sediments within former lakes and ephemeral fluvial systems, lake shorelines, sand ramps, water-lain calcrete and tufa sediments at the interface of surface hydrological and hydrogeological, speleothems and groundwater hydrogeological records, and hyrax middens. Palaeoenvironmental evidence can also be contained within geoarchaeological archives in caves, overhangs and rockshelters. This integration of records is undertaken with the aim of identifying a (or a number of) terrestrial regional chronostratigraphic framework(s) for this time period within southern Africa, because this is missing from the Quaternary stratigraphy lexicon. Owing to a lack of long, near-continuous terrestrial sequences in these drylands, the correspondence between nearby terrestrial records are explored as a basis for parasequences to build this chronostratigraphy. Recognising the modern climatological diversity across the subcontinent, four broad spatial subdivisions are used to explore potential sub-regional parasequences, which capture current climatic gradients, including the hyper-arid west coast and the decrease in aridity from the southwest Kalahari toward the north and east. These are the Namib Desert, the northern Kalahari, the southern Kalahari and the eastern fringes of the southern Kalahari. Terrestrial chronostratigraphies must start from premise that climate-driven environmental shifts may have occurred independently to those in other terrestrial locations and may be diachronous compared to the marine oxygen isotope stratigraphy (MIS), which serves as a global-scale master climatostratigraphy relating to global ice volume. The fragmented nature of preserved evidence means that we are still some way from producing unambiguous parasequences. There is however, a rich record to consider, compile and compare, within which seven broad wetter intervals are identified, with breaks between these inferred to be relatively drier, and some also have proxy evidence for drying. The onset and cessation of these wetter intervals does not align with MIS: they occur with greater frequency, but not with regular periodicity. Precession-paced insolation forcing is often invoked as a key control on southern African climate, but this does not explain the pacing of all of the identified events. Overall, the pattern is complex with some corresponding wetter intervals across space and others with opposing west-east trends. The evidence for drying over the past 10 ka is pronounced in the west (Namib Desert), with ephemerally wet conditions in the south (southern Kalahari). The patterns identified here provide a framework to be scrutinised and to inspire refinements to proposed terrestrial chronostratigraphies for southern Africa. Considering changes across this large geographic area also highlights the complexity in environmental responses across space as we continue to test a range of hypotheses about the nature of climatic forcing in this region.
... As a result, the isotopic signature of the source water is recorded in plant-derived lipids, indirectly reflecting the precipitation δ 2 H in the plant δ 2 H wax (Sauer et al., 2001;Sachse, Radke and Gleixner, 2004;Smith and Freeman, 2006;Hou, D'Andrea and Huang, 2008). Numerous studies on δ 2 H wax in soils that traverse altitudinal transects have been published with the aim to explore the suitability of δ 2 H wax as a paleo-elevation proxy (Jia et al., 2008;Peterse et al., 2009;Luo et al., 2011;Bai, Fang and Tian, 2012;Ernst et al., 2013;Nieto-Moreno et al., 2016;Coffinet et al., 2017;Herrmann et al., 2017). The general conclusion is that relatively stable climate conditions are favorable for an optimal functioning of these proxies and serve as a promising tool for the reconstruction of paleo-elevation. ...
Thesis
Anthropogenic climate change alters the hydrological cycle. While certain areas experience more intense precipitation events, others will experience droughts and increased evaporation, affecting water storage in long-term reservoirs, groundwater, snow, and glaciers. High elevation environments are especially vulnerable to climate change, which will impact the water supply for people living downstream. The Himalaya has been identified as a particularly vulnerable system, with nearly one billion people depending on the runoff in this system as their main water resource. As such, a more refined understanding of spatial and temporal changes in the water cycle in high altitude systems is essential to assess variations in water budgets under different climate change scenarios. However, not only anthropogenic influences have an impact on the hydrological cycle, but changes to the hydrological cycle can occur over geological timescales, which are connected to the interplay between orogenic uplift and climate change. However, their temporal evolution and causes are often difficult to constrain. Using proxies that reflect hydrological changes with an increase in elevation, we can unravel the history of orogenic uplift in mountain ranges and its effect on the climate. In this thesis, stable isotope ratios (expressed as δ2H and δ18O values) of meteoric waters and organic material are combined as tracers of atmospheric and hydrologic processes with remote sensing products to better understand water sources in the Himalayas. In addition, the record of modern climatological conditions based on the compound specific stable isotopes of leaf waxes (δ2Hwax) and brGDGTs (branched Glycerol dialkyl glycerol tetraethers) in modern soils in four Himalayan river catchments was assessed as proxies of the paleoclimate and (paleo-) elevation. Ultimately, hydrological variations over geological timescales were examined using δ13C and δ18O values of soil carbonates and bulk organic matter originating from sedimentological sections from the pre-Siwalik and Siwalik groups to track the response of vegetation and monsoon intensity and seasonality on a timescale of 20 Myr. I find that Rayleigh distillation, with an ISM moisture source, mainly controls the isotopic composition of surface waters in the studied Himalayan catchments. An increase in d-excess in the spring, verified by remote sensing data products, shows the significant impact of runoff from snow-covered and glaciated areas on the surface water isotopic values in the timeseries. In addition, I show that biomarker records such as brGDGTs and δ2Hwax have the potential to record (paleo-) elevation by yielding a significant correlation with the temperature and surface water δ2H values, respectively, as well as with elevation. Comparing the elevation inferred from both brGDGT and δ2Hwax, large differences were found in arid sections of the elevation transects due to an additional effect of evapotranspiration on δ2Hwax. A combined study of these proxies can improve paleoelevation estimates and provide recommendations based on the results found in this study. Ultimately, I infer that the expansion of C4 vegetation between 20 and 1 Myr was not solely dependent on atmospheric pCO2, but also on regional changes in aridity and seasonality from to the stable isotopic signature of the two sedimentary sections in the Himalaya (east and west). This thesis shows that the stable isotope chemistry of surface waters can be applied as a tool to monitor the changing Himalayan water budget under projected increasing temperatures. Minimizing the uncertainties associated with the paleo-elevation reconstructions were assessed by the combination of organic proxies (δ2Hwax and brGDGTs) in Himalayan soil. Stable isotope ratios in bulk soil and soil carbonates showed the evolution of vegetation influenced by the monsoon during the late Miocene, proving that these proxies can be used to record monsoon intensity, seasonality, and the response of vegetation. In conclusion, the use of organic proxies and stable isotope chemistry in the Himalayas has proven to successfully record changes in climate with increasing elevation. The combination of δ2Hwax and brGDGTs as a new proxy provides a more refined understanding of (paleo-)elevation and the influence of climate.
... Additionally, the isotopic difference between waxes from sediment and from soil, not only because of different degradation processes but also because of the integration by soils over seasons, species (e.g., grass and tree), plant organs (e.g., leaf and root), and other heterogeneities within vegetation canopies He et al., 2020). The ε rain-soil may be influenced by temperature, relative humidity, altitude, biosynthesis, and vegetation type (Chikaraishi et al., 2004;Krull et al., 2006;Jia et al., 2008;Bai et al., 2011;Herrmann et al., 2017;Wang et al., 2017b). For instance, some observations indicated that ε rain-soil became more negative with altitude (e.g., in the Gongga Mountain), which may be related to changes in plant transpiration and evaporation of soil water with altitude (Jia et al., 2008); however, other studies have proposed that this trend is reversed at lower altitudes (e.g., in the Longmen Mountains below 2 800 m) (Bai et al., 2017). ...
Article
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The average chain length (ACL), carbon preference index (CPI), and hydrogen isotope composition (δ²H) of long-chain n-alkanes in sediments have been used to retrieve information about the paleoclimate. Despite their importance as in-between media from leaves to sediments, n-alkanes of surface soils have not been systematically analyzed at large scale. Such an investigation of the spatial variation of n-alkane properties in soil and their dependence on climatic and botanic (e.g., vegetation type) factors could provide a rationale for a better estimation of the past environment. We synthesized the patterns and δ²H of long-chain n-alkanes in soil (δ²Hn-alkanes) with regard to vegetation types (cropland, grassland, shrubland, and woodland) and environmental factors using data from peer-reviewed papers. Our results showed that the ACL and CPI of soil C27–C33 n-alkanes were not suitable indicators for differentiating vegetation types at large scale; instead, ACL significantly correlated with water conditions such as mean annual precipitation (MAP) and Palmer drought severity index (PDSI), and CPI significantly correlated with temperature without significant influence of vegetation type. The variation (i.e., standard deviation) of fractionation between the δ²H values in annual precipitation and in soil n-alkanes (ɛrain-soil) was smaller than that reported in leaves; therefore, soils were better suited to quantifying the general growing conditions of plants at a certain site. The fractionation ɛrain-soil correlated with climatic conditions as described by the PDSI and relative humidity (RH). This correlation agreed with the change in leaf water enrichment with changing RH taken from the literature and was independent of the vegetation type at large scale. This meta-analysis may provide useful information for the variations of the patterns and δ²Hn-alkanes values in surface soils.
... The data were compilation for higher plants (Chikaraishi and Naraoka, 2003;Liu et al., 2006;Smith and Freeman, 2006;Sachse et al., 2006Sachse et al., , 2009Sachse et al., , 2010Liu and Yang, 2008;Pedentchouk et al., 2008;Yang et al., 2011aYang et al., , 2011bBai et al., 2011Bai et al., , 2014Duan and He, 2011;Duan et al., 2014;Tipple and Pagani, 2013;Eley et al., 2014;Gao et al., 2014;Liu et al., 2015Liu et al., , 2017bFeakins et al., 2016;Zhang et al., 2017;Daniels et al., 2017;Freimuth et al., 2017;Bi et al., 2005;Krull et al., 2006;Sessions, 2006;Yang and Huang, 2003;Kahmen et al., 2013b). For the sediment database, we compiled the available sediment leaf wax δD n-alkane /ε app records from various locations around the world to form a surface sediment database (n = 460), which distributed from Asia (Chikaraishi and Naraoka, 2006;Jia et al., 2008;Aichner et al., 2010;Bai et al., 2011Bai et al., , 2014Huang et al., 2016;Wang et al., 2017;Seki et al., 2010), the Americas (Polissar and Freeman, 2010;Douglas et al., 2012;Pautler et al., 2014), Europe (Sachse et al., 2004;Leider et al., 2013), Africa (Peterse et al., 2009;Garcin et al., 2012;Schwab et al., 2015;Vogts et al., 2016;Herrmann et al., 2017), Australia (Krull et al., 2006) and some regions in the high latitudes of the Arctic (Daniels et al., 2017;Pautler et al., 2014). These two databases provide relatively more data in low latitudes or high latitudes for robust analysis. ...
Article
Sedimentary δDn-alkane value is widely utilized as a reliable proxy for paleo-hydrological reconstruction. Applications of this proxy must be based upon a globally clear understanding of the relationship between leaf wax δDn-alkane values and precipitation δD(δDp), defined as apparent fractionation (εapp). However, there is a critical concern about whether relatively constant εapp values exist across different latitudes. In this study, we systematically analyzed the variations of available εapp with latitudes based upon two compiled-new databases of higher plants and sediments over the world. We found that the total average εapp was relatively constant, i.e., −116 ± 5‰ (n = 941), in higher plants across different latitudes without consideration of plant types (e.g., dicots, monocots, gymnosperms), and was still constant but slightly lower average εapp, i.e., −125 ± 6‰ (n=460), in sediments across the latitudes. The slightly lower average εapp in sediments relative to higher plants probably derived from the contribution of aquatic plants with isotopically D-depleted εapp in lake sediments. Interestingly, with consideration of plant types, average εapp increased in dicots but decreased inmonocots slightly from low to high latitudes. The counteraction of these competing trends generates relatively constant average εapp values in higher plants, and resultantly constant average εapp values occur in sediments at the global scale. It is important to elaborate relatively constant εapp values from higher plants and sediments across different latitudes when sedimentary δDn-alkane is utilized as a proxy for paleohydrological reconstruction.
Article
Diepkloof Rock Shelter (DRS) represents a site of major interest for reconstructing early human behaviours during the Middle Stone Age (MSA). Rock shelters such as DRS also potentially preserve information concerning the environmental context for such behaviours. In this respect the organic matter composition of rock shelter sediments has rarely been investigated in detail, particularly at the molecular level. Here, we used pyrolysis-gas chromatography/mass spectrometry (py-GC/MS) to systematically assess the organic matter composition of bulk sediments within the MSA and Later Stone Age (LSA) sequence at DRS. From this we sought to gain insights into site usage, taphonomy and burning practices. Additionally, we analysed the chain length distribution of leaf-wax n-alkanes as well as their hydrogen and carbon isotopic compositions (δDwax and δ¹³Cwax) to investigate their potential as hydroclimate and vegetation indicators. This constitutes the first leaf-wax isotopic data in a terrestrial context of this antiquity in South Africa.
Thesis
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This thesis aims to advance the understanding of Holocene climate changes and their environmental consequences in South Africa using plant wax derived n-alkanes and their compound-specific stable carbon and hydrogen isotopes (d13Cwax, dDwax). The first part deals with a source-to-sink approach of terrestrial organic material (TOM) deposited in the mudbelt offshore western South Africa. It is shown that TOM in the northern mudbelt is mainly delivered by the Orange River and contains an integrated homogenous catchment signal. The second part deals with the applicability of dDwax as a hydrological recorder using soil samples from across South Africa. It is shown that dDwax suits well as qualitative hydrological proxy in the summer rainfall zone (SRZ) but not in the winter rainfall zone due to more complex environmental conditions. The third part deals with the Holocene hydrological and vegetation reconstruction in South Africa using a marine sediment core. The investigation reveals that towards the middle Holocene increasingly C4 plants abundance and wetter conditions occurred in the SRZ. During the last 4,000 years the SRZ experienced a gradual drying.
Article
Sedimentary organic matter (OM) analyses along a 130 km long transect of the Mkhuze River from the Lebombo Mountains to its outlet into Lake St Lucia, Africa's most extensive estuarine system, revealed the present active trapping function of a terminal freshwater wetland. Combining bulk OM analyses, such as Rock-Eval®, and source-specific biomarker analyses of plant-wax n-alkanes and their stable carbon (δ13C) and hydrogen (δD) isotopic composition showed that fluvial sedimentary OM originating from inland areas is mainly deposited in the floodplain and swamp area of the wetland system but not in the downstream lake area. A distinctly less degraded OM signature, i.e., a considerably lower degree of transformation of unstable components (higher I index) and lower contribution of refractory and persistent fractions (lower R index) as well as recognizably higher δD values compared to samples from upstream sub-environments, characterizes surface sediments of Lake St Lucia. The offset in δD indicates that the contributing vegetation, although similar to upstream vegetation inputs in terms of photosynthetic pathway (δ13C) and alkane distribution pattern, experienced different hydrological growth conditions. The results suggest that under current conditions hinterland sedimentary OM is deposited throughout the wetland system up to the Mkhuze Swamps, which ultimately captures the transported OM. Consequently, samples from the downstream located Lake St Lucia show locally derived signals instead of integrated signals encompassing the river catchment. This finding raises important constraints for future environmental studies as the assumption of watershed-integrated signals in sedimentary archives retrieved from downstream lakes or offshore might not hold true in certain settings.
Chapter
Climate changes during the Quaternary (last ~2.6 million years) were extremely important in shaping the land surfaces, environments and ecosystems that are present today, and in providing the conditions under which human evolution could take place. Although this period was well marked in the northern hemisphere by cycles of cold ice ages and warmer interglacials, the late Quaternary record in South Africa is more subdued but can be elucidated from a wide range of palaeoenvironmental and palaeoclimatic proxies including landforms and sediments, fossils of biological remains such as pollen, and from isotopes. These records indicate that regional changes in sea level and fluctuations in moisture availability were the primary consequences of the global scale climatic changes that took place throughout this period.
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Leaf-wax n-alkane hydrogen isotope composition (δD alk ) is widely applied as a proxy for paleohydroclimatic changes, but the factors controlling δD alk remain to be clarified. In this study, in order to determine the dominant controls, we measured δD alk in 54 surface soils along a southeast–northwest transect across a steep climatic gradient in China. δD alk is positively correlated with annual precipitation δD (δD pann ) and better correlated with growing season (April–October) precipitation δD (δD pgs ). However, the variability of δD pann (38‰) and δD pgs (35‰) does not explain all of the variance in δD alk (84‰). The apparent fractionation of hydrogen isotopes between n-alkanes and precipitation (ε alk/p ) is significantly correlated with variables related to aridity: relative humidity, rainfall amount, and aridity index (defined as the ratio of average precipitation to average potential evapotranspiration). This result demonstrates the important control of aridity on δD alk , which may result from the increasing soil evaporation and plant transpiration, as well as decreasing biosynthetic fractionation, in arid climates. We compiled published δD alk data from surface lake/soil sediments in China, and found that aridity effects are pronounced in regions with mean annual precipitation (MAP) < 800 mm. Therefore, we suggest that soil δD alk reflects precipitation δD modified by aridity, with a greater degree of modification in arid environments (e.g., MAP < 800 mm). Future work to quantitatively determine the effects of aridity on δD alk is needed to improve the reliability of the application of δD alk in paleoenvironmental studies.
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The hydrogen isotopic compositions (δD) of long-chain plant leaf waxes can reflect changes of continental hydrology and thus have been increasingly utilized for paleoclimate reconstruction. One of the unresolved major issues is whether variations of leaf wax δD (δDwax) signals along a precipitation gradient reflect changes of precipitation δD, precipitation amount, and/or evapo-transpiration. This ambiguity limits our interpretation of δDwax in geological records as well as the quantitative reconstruction of paleohydrological variability. Here we systematically investigated δD of soil water and waxes extracted from soil and plant leaves in the Chinese Loess Plateau (CLP) and its surrounding areas along a precipitation gradient with mean annual precipitation (MAP) varying from 140 mm to 676 mm. The results showed that while the variation of modeled precipitation δD has no significant correlation with MAP, soil water δD, soil δDwax, and individual plant δDwax exhibit negative correlations with MAP. In relatively arid areas, the δD values of soil water and plant leaf waxes are significantly more positive due to much lower precipitation relative to evapo-transpiration, suggesting that effective precipitation (P:E ratio) has played a crucial role in the D-enrichment of soil and plant leaf waxes in this region. While parallel decreasing trends in δDwax are found along the increase of precipitation gradient among dominant plants (including Artemisia spp., Aster hispidus, Stipa bungeana, and Cleistogenes squarrosa) in the region with similar slopes, the large δD offsets between plant groups suggest that plant type is an important factor in controlling plant hydrogen isotope fractionations. Our results indicate that δDwax preserved in paleosols can be used to infer past conditions of water availability in arid and semi-arid inland regions, but with a mechanism that is different from the influence of “amount effect” in humid areas. Moreover, vegetation changes should be constrained by independent paleobotanical data before monsoon related paleohydrology in the CLP and its surrounding areas can be quantitatively reconstructed using the plant wax δD proxy.
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South Africa is a key region for paleoclimate studies reconstructing and understanding past changes in atmospheric circulation, i.e., temperate Westerlies and tropical Easterlies. However, due to the scarcity of natural archives, the environmental evolution during the late Quaternary remains highly debated. Many archives that are available are peri-coastal lakes and wetlands and sea level changes during the Holocene often overprinted the paleoenvironmental signals in these archives. This study presents a new record from the coastal wetland Voёlvlei, which is an intermittent lake situated in the year-round rainfall zone (YRZ) of South Africa at the southern Cape coast. It presents an ideal archive to investigate both sea level and environmental changes. A 13 m-long sediment core was retrieved from Voёlvlei and analysed using a multi-proxy approach. The chronology reveals a basal age of 8,440+200/−250 cal BP. Paleoecological and elemental analyses indicate marine intrusions from 8,440 to 7,000 cal BP with a salinity optimum at 7,030+150/−190 cal BP. Since 6,000 cal BP, silting up has been causing an intermittent freshwater lake. Inferred from changes in allochthonous input, δ13Cn-alkane and δ2Hn-alkane increasing moisture is observed from 8,440+200/−250 cal BP. The δ2Hn-alkane record provides new evidence in contribution of different precipitation sources throughout the record with contributions from both Westerlies and Easterlies from 8,440 to 7,070 cal BP. Westerlies dominate from 7,070 to 6,420 cal BP followed by a distinct shift to an Easterly-dominance at 6,420 cal BP. An overall trend to a Westerly-lasting until 2,060 cal BP is followed by a trend towards an Easterlies-dominance, but both phases show several climatic spikes. Those spikes are also evident in other regional studies highlighting that the source and seasonality of precipitation has a mayor role for the hydrological balance. By comparing the Voёlvlei record with other regional studies, a similar trend in the overall moisture evolution along the southern Cape coast is inferred during the past 8.500 yrs.
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Variation in δ18O and δ13C values in a speleothem from the Cango Caves in southernmost South Africa enable the construction of coherent regional composite records spanning the past 113,500 yr. Novel for the region in terms of both their length and detail, these records indicate environmental and climatic changes that both are consistent with records from the wider region and show a clear evolution from low- to high-latitude forcing dominance across the last glacial period. Prior to ca. 70 ka, the influence of direct low-latitude insolation forcing is expressed through increases in summer rainfall during austral summer insolation maxima. With the onset of Marine Isotope Stage 4, cooler global conditions and the development of high-latitude ice sheets appear to have supplanted direct insolation forcing as the dominant driver pacing patterns of environmental change, with records from the Southern and Northern Hemisphere tropics exhibiting a positive relationship until after the Last Glacial Maximum. These results highlight the complexity of South African climate change dynamics as a response to changing global boundary conditions and provide a critical reference for regional and global comparisons.
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This paper aims to identify chronostratigraphic palaeo-climatic boundaries based on proxy indications from mountain- and coastal wetlands in eastern South Africa and Lesotho. Phase boundaries were identified from timing of climate change inferred by proxies, as well as regime shifts in climate variability. Sometimes magnitude and/or frequency of change was also considered. Summarizing the common palaeo-climatic indications suggest the following chronostratigraphic climate phases: 25 to 18 ka, 18 to 15 ka, 15 to 11.5, 11.5 to 8 ka, 8 to 5.5 ka, 5.5 to 2 ka and 2 to 0 ka. The most robust boundaries were identified at 18 ka, 15 ka and 2 ka, i.e. these boundaries were supported by several proxies/sites. The other boundaries were less clearly detected from available proxies/sites and should be regarded tentative. The timing of a climate shift often coincides at coast and mountain sites. However, the climate conditions within each chronostratigraphic phase sometimes vary between coast and inland sites. The 25 to 18 ka phase was cool and dry with strong and frequent storms, followed by the ca. 18 to 15 ka period when conditions were less severe but still generally cool and dry. At ca. 15 to 11.5 ka several proxies infer warmer climate, with less winter rains. During 11.5 to 8 ka a general increase in wetness is inferred, followed by warming over the 8 to 5.5 ka phase. Between 5.5 and 2 ka a successive change towards wetter is indicated, although timing differ between sites. After 2 ka generally a more variable climate is seen, often with high magnitude shifts between dry and wet. The data resolution, i.e. the number of available wetland records, increases with time from very low during glacial times, to highest resolution during late Holocene. Geographically, sites in the mountain region are overrepresented compared to coastal sites. A comparison with coastal lake records suggests a more variable climate at coastal sites compared to mountain sites during mid- and late Holocene, although different proxy resolution and methodology cannot be ruled out as an explanation. A case study compares multiproxy records from Drakensberg (Sekhokong, Ntsikeni) and the coast (Mfabeni), discussing advantages and problems associated with proxy-comparisons within and between sites.
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Where coastal topography is low and flat, the clouds dissipate inward over broad areas with little biological impact, but where isolated mountains or steep coastal slopes intercept the clouds, a fog-zone develops. This moisture allows the development of plant communities termed lomas formations. These floristic assemblages function as islands separated by hyperarid habitat devoid of plant life. Since growth is dependent upon available moisture, an understanding of climatic patterns is essential in efforts to interpret present-day plant distributions. Topography and substrate combine to influence patterns of moisture availability. The ecological requirements and tolerances of individual species ultimately determines community composition. Species endemism exceeds 40% and suggests that the lomas formations have evolved in isolation from their nearest geographic neighbors in the Andes. While the arid environment is continuous, there appears to be a significant barrier to dispersal between 18° and 22°S latitude in extreme N Chile, Less than 7% of a total flora, estimated at nearly 1000 species, occur on both side of this region. -from Authors
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The hydrogen isotopic composition (δ2H) of leaf waxes, especially of n-alkanes (δ2Hn-alkanes), is increasingly used for paleohydrological and paleoclimate reconstructions. However, it is challenging to disentangle past changes in the isotopic composition of precipitation and changes in evapotranspirative enrichment of leaf water, which are both recorded in leaf wax δ2H values. In order to overcome this limitation, Zech M. et al. (2013) proposed a coupled δ2Hn-alkanes–δ18Osugar biomarker approach. This coupled approach allows for calculating (i) biomarker-based "reconstructed" δ2Hδ18O values of leaf water (δ2Hδ18Oleaf water), (ii) biomarker-based reconstructed deuterium excess (d-excess) of leaf water, which mainly reflects evapotranspirative enrichment and which can be used to reconstruct relative air humidity (RH) and (iii) biomarker-based reconstructed δ2Hδ18Oprecipitation values. Here we present a climate transect validation study by coupling new results from δ2H analyses of n-alkanes and fatty acids in topsoils along a climate transect in Argentina with previously measured δ18O results obtained for plant-derived sugars. Accordingly, both the reconstructed RH and δ2Hδ18Oprecipitation values correlate highly significantly with actual RH and δ2Hδ18Oprecipitation values. We conclude that compared to single δ2Hn-alkane or δ18Osugar records, the proposed coupled δ2Hn-alkane–δ18Osugar biomarker approach will allow more robust δ2Hδ18Oprecipitation reconstructions in future paleoclimate research. Additionally, the proposed coupled δ2Hn-alkane–δ18Osugar biomarker approach allows for the establishment of a "paleohygrometer", more specifically, the reconstruction of mean summer daytime RH changes/history.
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Hydrogen isotopic ratios of terrestrial plant leaf waxes (δD) have been widely used for paleoclimate reconstructions. However, underlying controls for the observed large variations in leaf wax δD values in different terrestrial vascular plants are still poorly understood, hampering quantitative paleoclimate interpretation. Here we report plant leaf wax and source water δD values from 102 plant species grown in a common environment (New York Botanic Garden), chosen to represent all the major lineages of terrestrial vascular plants and multiple origins of common plant growth forms. We found that leaf wax hydrogen isotope fractionation relative to plant source water is best explained by membership in particular lineages, rather than by growth forms as previously suggested. Monocots, and in particular one clade of grasses, display consistently greater hydrogen isotopic fractionation than all other vascular plants, whereas lycopods, representing the earlier-diverging vascular plant lineage, display the smallest fractionation. Data from greenhouse experiments and field samples suggest that the changing leaf wax hydrogen isotopic fractionation in different terrestrial vascular plants may be related to different strategies in allocating photosynthetic substrates for metabolic and biosynthetic functions, and potential leaf water isotopic differences.
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Leaf tissues of vascular plant species from the arid Richtersveld of northern Namaqualand (Succulent Karoo), South Africa, show a great range of variation in carbon isotope discrimination. Analyses of Δ values for species with obligate C_3 metabolism demonstrated that the water use efficiency indicated by these values varies between species and within a species population both between seedlings and mature shrubs and between sites along an aridity gradient. An expected pattern of increasing water use efficiency with greater longevity in these species was not present, however. Only among ephemerals were there taxa with high Δ values associated with low water use efficiency, but these high values were balanced by other ephemeral species with low Δ values comparable to those typical of moderate to long-lived shrubs. The extent to which putative CAM species in the succulent flora rely entirely on nocturnal carbon fixation is highly variable. Species of Crassulaceae and Aloe (Asphodelaceae) relied almost exclusively on CAM photosynthesis, although Crassulaceae from more mesic environments may be CAM-flexible. Succulent-leaved species of Senecio and leaves of stem-succulent Euphorbia were CAM-flexible in their Δ values, while shrubby species of Ceraria (Portulacaceae) appeared to rely primarily on C_3 photosynthesis. Variable patterns of reliance on CAM photosynthesis were present in the ecologically dominant Mesembryanthema of the Aizoaceae. Shrubby species of the subfamily Mesembryanthemoideae, although capable of CAM-flexibility in less xeric habitats, relied largely on CAM fixation of carbon in our field sites. Within the subfamily Ruschioideae, there was a wide range of Δ values indicating species with CAM, CAM flexibility, and primary reliance on C_3 photosynthesis. Low Δ values indicative of typical CAM fixation were associated with species with greater longevity, suggesting that a positive correlation may exist between the degree of reliance on CAM photosynthesis and plant life span.
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Stable isotopes in water (δ2H and δ18O) are important indicators of hydrological and ecological pattern and process. δ2H and δ18O of water are incorporated into geological and biological systems in a predictable manner and have been used extensively as tracers in hydrological, ecological and forensic studies. Physical processes result in spatial variation of δ2H, δ18O in water across the landscape (so-called “isoscapes”) and provide the basis for hydrological, ecological, archaeological and forensic studies. Southern Africa is a globally important meeting point for ocean and climate systems, biological diversity and human societies, yet there is little information on the spatial variability in δ2H and δ18O in water across this important region. Here we present the first ground water and tap water isoscapes for southern Africa. We compare and contrast these two water resources, and consider how well global models of precipitation isotopes capture isotopic variation across South Africa. Ground water and tap water samples were collected from across South Africa, analysed for δ2H and δ18O, and used to generate interpolated δ2H, δ18O and deuterium-excess (d = δ2H – 8*δ18O) isoscapes. We found coherent spatial structure in δ2H, δ18O and d of ground water and tap water that could be predicted by a geostatistical model based on simple environmental parameters (elevation, mean annual precipitation, precipitation minus potential evaporation, distance to coast and modeled isotope ratio of precipitation). This spatial structure resulted in considerable differences in isotopic composition of water in many of the major wildlife reserves in South Africa, indicating a good potential for wildlife forensics in this region. δ2H and δ18O of ground water, and to a lesser extent tap water, reflected the δ2H and δ18O of long-term weighted annual precipitation at the two GNIP stations in South Africa. However, large discrepancies between modelled isotopic composition of precipitation and our ground water and tap water isoscapes, particularly at higher elevations, highlighted uncertainty in the accuracy of modelled precipitation isoscapes for this region. Increased spatial sampling of precipitation, especially for high elevation regions, and temporal sampling of ground and tap water would considerably aid isotopic studies in this region.
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The International Atomic Energy Agency (IAEA), in cooperation with the World Meteorological Organization (WMO), has been conducting a world-wide survey of hydrogen (2H/1H) and oxygen (18O/16O) isotope composition of monthly precipitation since 1961. At present, 72 IAEA/WMO network stations are in operation. Another 82 stations belonging to national organizations continue to send their results to the IAEA for publication. The paper focuses on basic features of spatial and temporal distribution of deuterium and 18O in global precipitation, as derived from the IAEA/WMO isotope database. The internal structure and basic characteristics of this database are discussed in some detail. The existing phenomenological relationships between observed stable isotope composition of precipitation and various climate-related parameters such as local surface air temperature and amount of precipitation are reviewed and critically assessed. Attempts are presented towards revealing interannual fluctuations in the accumulated isotope records and relating them to changes of precipitation amount and the surface air temperature over the past 30 years.
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1] We present a dust plume source inventory for southern Africa. In order to locate and track the local, short-lived plume events, source and frequency data have been derived from Meteosat Second Generation (MSG) thermal infrared composite data (4 km data using 8.7, 10.8, and 12.0 μm) and Moderate Resolution Imaging Spectroradiometer (MODIS) visible composite data (0.25 km data utilizing 0.620 – 0.670 μm, 0.545 – 0.565 μm, and 0.459 – 0.479 μm). Between January 2005 and December 2008, a total of 328 distinct daytime dust plumes more than 10 km in length were detected. These plumes were attributed to 101 distinct point sources, consisting largely of ephemeral inland lakes, coastal pans as well as dry river valleys in Namibia, Botswana, and South Africa. These data also provided sub-basin scale source observations for large basins such as Etosha and Makgadikgadi Pans. Citation: Vickery, K. J., F. D. Eckardt, and R. G. Bryant (2013), A sub-basin scale dust plume source frequency inventory for southern Africa,
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Long chain (C21 to C37) n-alkanes are among the most long-lived and widely utilized terrestrial plant biomarkers. Dozens of studies have examined the range and variation of n-alkane chain-length abundances in modern plants from around the world, and n-alkane distributions have been used for a variety of purposes in paleoclimatology and paleoecology as well as chemotaxonomy. However, most of the paleoecological applications of n-alkane distributions have been based on a narrow set of modern data that cannot address intra- and inter-plant variability. Here, we present the results of a study using trees from near Chicago, IL, USA, as well as a meta-analysis of published data on modern plant n-alkane distributions. First, we test the conformity of n-alkane distributions in mature leaves across the canopy of 38 individual plants from 24 species as well as across a single growing season and find no significant differences for either canopy position or time of leaf collection. Second, we compile 2093 observations from 86 sources, including the new data here, to examine the generalities of n-alkane parameters such as carbon preference index (CPI), average chain length (ACL), and chain-length ratios for different plant groups. We show that angiosperms generally produce more n-alkanes than do gymnosperms, supporting previous observations, and furthermore that CPI values show such variation in modern plants that it is prudent to discard the use of CPI as a quantitative indicator of n-alkane degradation in sediments. We also test the hypotheses that certain n-alkane chain lengths predominate in and therefore can be representative of particular plant groups, namely, C23 and C25 in Sphagnum mosses, C27 and C29 in woody plants, and C31 in graminoids (grasses). We find that chain-length distributions are highly variable within plant groups, such that chemotaxonomic distinctions between grasses and woody plants are difficult to make based on n-alkane abundances. In contrast, Sphagnum mosses are marked by their predominance of C23 and C25, chain lengths which are largely absent in terrestrial vascular plants. The results here support the use of C23 as a robust proxy for Sphagnum mosses in paleoecological studies, but not the use of C27, C29, and C31 to separate graminoids and woody plants from one another, as both groups produce highly variable but significant amounts of all three chain lengths. In Africa, C33 and C35 chain lengths appear to distinguish graminoids from some woody plants, but this may be a reflection of the differences in rainforest and savanna environments. Indeed, variation in the abundances of long n-alkane chain lengths may be responding in part to local environmental conditions, and this calls for a more directed examination of the effects of temperature and aridity on plant n-alkane distributions in natural environments.
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Leaf waxes (i.e., n-alkyl lipids or n-alkanes) are land-plant biomarkers widely used to reconstruct changes in climate and the carbon isotopic composition of the atmosphere. There is little information available, however, on how the production of leaf waxes by different kinds of plants might influence the abundance and isotopic composition of n-alkanes in sedimentary archives. This lack of information increases uncertainty in interpreting n-alkyl lipid abundance and δ13C signals in ancient settings. We provide here n-alkyl abundance distributions and carbon isotope fractionation data for deciduous and evergreen angiosperm and gymnosperm leaves from 46 tree species, representing 24 families. n-Alkane abundances are significantly higher in angiosperms than gymnosperms; many of the gymnosperm species investigated did not produce any n-alkanes. On average, deciduous angiosperms produce 200 times more n-alkanes than deciduous gymnosperms. Although differences between angiosperms and gymnosperms dominate the variance in n-alkane abundance, leaf life-span is also important, with higher n-alkane abundances in longer-lived leaves. n-Alkanol abundances covary with n-alkanes, but n-alkanoic acids have similar abundances across all plant groups. Isotopic fractionation between leaf tissue and individual alkanes (εlipid) varies by as much as 10‰ among different chain lengths. Overall, εlipid values are slightly lower (−4.5‰) for angiosperm than for gymnosperm (−2.5‰) n-alkanes. Angiosperms commonly express slightly higher Δleaf (photosynthetic discrimination) relative to gymnosperms under similar growth conditions. As a result, angiosperm n-alkanes are expected to be generally 3–5‰ more depleted in 13C relative to gymnosperm alkanes for the same locality. Differences in n-alkane production indicate the biomarker record will largely (but not exclusively) reflect angiosperms if both groups were present, and also that evergreen plants will likely be overrepresented compared with deciduous ones. We apply our modern lipid abundance patterns and εlipid results to constrain the magnitude of the carbon isotope excursion (CIE) at the onset of the Paleocene–Eocene Thermal Maximum (55.8Ma). When Bighorn Basin (WY) sediment n-alkanes are interpreted in context of floral changes and modern n-alkane production estimates for angiosperms and gymnosperms, the CIE is greater in magnitude (−5.6‰) by ∼1‰ compared to previous estimates that do not take into account n-alkane production.
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The O- and H-isotope composition of rainfall collected over a variety of time periods at the University of Cape Town (UCT) between 1996 and 2008 has been determined. A continuous record of monthly rainfall from 1996 to 2008 has a range in delta D and delta(18)O values from -57 to +18 parts per thousand and -8.1 to +3.5 parts per thousand, respectively. These data show limited but discernable temperature and amount effects. Daily rainfall between June 2000 to September 2001 ranges in delta D and delta(18)O from -57 to + 27 parts per thousand, and, -9.0 to +6.4 parts per thousand respectively, showing only a marginally greater temperature and amount effect than the monthly data. Rainfall collected at < hourly intervals during two storms in July and August 2000 showed changes in delta D and delta(18)O value of 26 and 3.3 parts per thousand, respectively in as little as 30 minutes during the passage of a cold front. Unusual events such as snow and hail in the area have much lower delta D and delta(18)O values (-60 and -10.9 parts per thousand). The deuterium excess values were significantly higher in the storm samples (average 20.0) and the hail and snow samples (average 30.1) than in the monthly rain samples (average 12.4). The UCT monthly samples. define a meteoric water line whose equation is delta = 6.41*delta(18)O + 8.66 (r = 0.88). The daily and hail/snow samples show a better correlation (r = 0.93) due to the greater spread of values, with an equations of the line of best fit of delta D = 6.64 delta(18)O + 11.89. Rain water collected during two storms in July and August show an excellent correlation between delta D and delta(18)O (r = 0.97) with an equation of best fit of delta D = 7.89 delta(18)O + 19.35. There has been no systematic change in annual amount, mean temperature or weighted mean isotope composition from 1996 to 2008, but 1996 (16.1 degrees C) was, on average, 1.3 degrees C colder than 1997 to 2008 (ave. 17.3 degrees C). Annual rainfall at UCT has varied from similar to 1000 mm (1997) to 1700 mm (2001), and the weighted mean annual delta D and delta(18)O values calculated from the monthly samples varies from -16 to -7 parts per thousand and -3.8 to -2.6 parts per thousand, respectively. The years 2005 and 2006 showed a significantly lower deuterium excess (similar to 6) compared to typical values of about 16, which suggests that these years experienced less frontal rain. This difference in isotope composition of rainfall is detectable in the isotope composition of water collected from springs on the lower slopes of Table Mountain and suggests that the Table Mountain aquifer(s) are recharged by approximately 50% within three years.
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In order to define criteria for long-term climate change models in Southern Africa, an overview of the available pollen data during the Late Quaternary is needed. Here we reassess the paleo-climatic conditions in southern Africa by synthesising available fossil pollen data that can provide new insights in environmental change processes. The data considered here include the latest as well as previously published information that has been difficult to assess. Available calibrated pollen sequences spanning the Late Pleistocene and Holocene were subjected to Principal Components Analysis (PCA) to monitor taxa sensitive to moisture and temperature fluctuations. The PCA values are presented graphically as indicators of climate variability for the region. The results cover different biomes that include the summer-rain region in the north and east, the winter-rain area in the south and the dry zone in the west. The PCA plots directly reflect major changes of terrestrial environments due to variations in temperature and moisture. Mostly sub-humid but fluctuating conditions are indicated during the cold Marine Isotope Stage (MIS) 2, which were followed by a dry phase soon after the beginning of the Holocene but before the middle Holocene in the northern, central and eastern parts of the sub-continent. Marked but non-parallel moisture changes occurred in different subregions during the Holocene suggesting that climatic forcing was not uniform over the entire region. Some events seemed to have had a more uniform effect over the sub-continent, e.g., a relatively dry summer rain event at c. two thousand years ago, which can possibly be related to the ENSO phenomenon. The role of anthropogenic activities in some of the most recent vegetation shifts is likely.
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Leaf wax n-alkane δ(2) H values carry important information about environmental and ecophysiological processes in plants. However, the physiological and biochemical drivers that shape leaf wax n-alkane δ(2) H values are not completely understood. It is particularly unclear why n-alkanes in grasses are typically (2) H-depleted compared to plants from other taxonomic groups such as dicotyledonous plants and why C3 grasses are (2) H-depleted compared to C4 grasses. To resolve these uncertainties we quantified the effects of leaf water evaporative (2) H-enrichment and biosynthetic hydrogen isotope fractionation on n-alkane δ(2) H values for a range of C3 and C4 grasses grown in climate-controlled chambers. We found that only a fraction of leaf water evaporative (2) H-enrichment is imprinted on the leaf wax n-alkane δ(2) H values in grasses. This is interesting, as previous studies have shown in dicotyledonous plants a nearly complete transfer of this (2) H-enrichment to the n-alkane δ(2) H values. We thus infer that the typically observed (2) H-depletion of n-alkanes in grasses (as opposed to dicots) is because only a fraction of the leaf water evaporative (2) H-enrichment is imprinted on the δ(2) H values. Our experiments also show that differences in n-alkane δ(2) H values between C3 and C4 grasses are largely the result of systematic differences in biosynthetic fractionation between these two plant groups, which was on average -198‰ and -159‰ for C3 and C4 grasses, respectively.
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Various studies have demonstrated that the stable hydrogen isotopic composition (δD) of terrestrial leaf waxes tracks that of precipitation (δDprecip) both spatially across climate gradients and over a range of different timescales. Yet, reconstructed estimates of δDprecip and corresponding rainfall typically remain largely qualitative, due mainly to uncertainties in plant ecosystem net fractionation, relative humidity, and the stability of the amount effect through time. Here we present δD values of the C31 n-alkane (δDwax) from a marine sediment core offshore the Northwest (NW) African Sahel covering the past 100 years and overlapping with the instrumental record of rainfall. We use this record to investigate whether accurate, quantitative estimates of past rainfall can be derived from our δDwax time series. We infer the composition of vegetation (C3/C4) within the continental catchment area by analysis of the stable carbon isotopic composition of the same compounds (δ13Cwax), calculated a net ecosystem fractionation factor, and corrected the δDwax time series accordingly to derive δDprecip. Using the present-day relationship between δDprecip and the amount of precipitation in the tropics, we derive quantitative estimates of past precipitation amounts. Our data show that a) vegetation composition can be inferred from δ13Cwax, b) the calculated net ecosystem fractionation represents a reasonable estimate, and c) estimated total amounts of rainfall based on δDwax correspond to instrumental records of rainfall. Our study has important implications for future studies aiming to reconstruct rainfall based on δDwax; the combined data presented here demonstrate that it is feasible to infer absolute rainfall amounts from sedimentary δDwax in tandem with δ13Cwax in specific depositional settings.
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Plant leaf waxes have been found to record the hydrogen isotopic composition of precipitation and are thus used to reconstruct past climate. To assess how faithfully they record hydrological signals, we characterize leaf wax hydrogen isotopic compositions in forest canopy trees across a highly biodiverse, 3 km elevation range on the eastern flank of the Andes. We sampled the dominant tree species and assessed their relative abundance in the tree community. For each tree we collected xylem and leaf samples for analysis of plant water and plant leaf wax hydrogen isotopic compositions. In total, 176 individuals were sampled across 32 species and 5 forest plots that span the gradient. We find both xylem water and leaf wax D values of individuals correlate (R2 = 0.8 and R2 = 0.3 respectively) with the isotopic composition of precipitation (with an elevation gradient of −21‰ km−1). Minimal leaf water enrichment means that leaf waxes are straightforward recorders of the isotopic composition of precipitation in wet climates. For these tropical forests we find the average fractionation between source water and leaf wax for C29 n-alkanes, −129 ± 2‰ (s.e.m., n = 136), to be indistinguishable from that of temperate moist forests. For C28 n-alkanoic acids the average fractionation is −121 ± 3‰ (s.e.m., n = 102). Sampling guided by community assembly within forest plots shows that integrated plant leaf wax hydrogen isotopic compositions faithfully record the gradient of isotopes in precipitation with elevation (R2 = 0.97 for n-alkanes and 0.60 for n-alkanoic acids). This calibration study supports the use of leaf waxes as recorders of the isotopic composition of precipitation in lowland tropical rainforest, tropical montane cloud forests and their sedimentary archives.
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The arid regions of South Africa are characterized by definite climatic gradients. The increase in rainfall from the Atlantic coast to the interior is particularly noteworthy. In this eastward direction the rainfall season also changes from winter to summer, via an intermediate uniform rainfall zone.
Article
We use the Total Ozone Mapping Spectrometer (TOMS) sensor on the Nimbus 7 satellite to map the global distribution of major atmospheric dust sources with the goal of identifying common environmental characteristics. The largest and most persistent sources are located in the Northern Hemisphere, mainly in a broad "dust belt" that extends from the west coast of North Africa, over the Middle East, Central and South Asia, to China. There is remarkably little large-scale dust activity outside this region. In particular, the Southern Hemisphere is devoid of major dust activity. Dust sources, regardless of size or strength, can usually be associated with topographical lows located in arid regions with annual rainfall under 200-250 mm. Although the source regions themselves are arid or hyperarid, the action of water is evident from the presence of ephemeral streams, rivers, lakes, and playas. Most major sources have been intermittently flooded through the Quaternary as evidenced by deep alluvial deposits. Many sources are associated with areas where human impacts are well documented, e.g., the Caspian and Aral Seas, Tigris-Euphrates River Basin, southwestern North America, and the loess lands in China. Nonetheless, the largest and most active sources are located in truly remote areas where there is little or no human activity. Thus, on a global scale, dust mobilization appears to be dominated by natural sources. Dust activity is extremely sensitive to many environmental parameters. The identification of major sources will enable us to focus on critical regions and to characterize emission rates in response to environmental conditions. With such knowledge we will be better able to improve global dust models and to assess the effects of climate change on emissions in the future. It will also facilitate the interpretation of the paleoclimate record based on dust contained in ocean sediments and ice cores.
Article
The use of hydrogen isotope ratios (δ(2)H) of sedimentary n-alkanes from leaf waxes has become an important tool for reconstructing paleoenvironmental and ancient hydrologic conditions. Studies of modern plant waxes can elucidate driving ecological mechanisms behind geologic deposits. Here, we used a transect across the North American Monsoon region of the western USA from Tucson, Arizona to Salt Lake City, Utah to study variations in leaf wax δ(2)H among co-occurring plants. Three co-occurring life forms were selected: perennial shrub (rabbit brush, Chrysothamnus nauseosus; sagebrush, Artemisia tridentata); tree (Gambel's oak tree, Quercus gambelii); and annual (sunflower, Helianthus annuus). Our results showed that the distributions and abundances of n-alkanes in perennial plants were similar across all sites and generally did not vary with environmental conditions (e.g., precipitation and temperature). In contrast, variations in n-alkane δ(2)H were significantly correlated with the fraction of the annual precipitation coming during the summer monsoon period. We use a modified Craig-Gordon model to speculate on the possible drivers of the δ(2)H values of leaf wax n-alkanes of plants across the region. The model results suggest that the most likely explanation for variation in wax δ(2)H values was a combination of seasonal source water usage and subsequent environmental conditions.
Article
Theory (Craig and Gordon 1965; Dongmann et al. 1974; Sternberg et al. 1986; Farquhar and Lloyd 1993) suggests that the oxygen isotope ratio (δ18O) of plant material should reflect the evaporative conditions under which the material was formed, so that differences in stomatal conductance should show up in plant δ18O. To test this theory we measured the oxygen isotope ratio of organic matter from flag leaves at anthesis and grain at harvest from eight cultivars of spring wheat (Triticum aestivum L.) grown under irrigation in each of three seasons in Mexico. The cultivars ranged widely in stomatal conductance and in average grain yield, with which conductance was positively correlated. Supporting theory, the oxygen isotope ratio of flag leaves (δ18O1) was found to correlate negatively with stomatal conductance for two of the three seasons. The significant correlations are consistent with high conductance cultivars having lower leaf temperatures and kinetic fractionation factors, and higher vapour pressure fractionation factors and Peclet numbers, all of which combine to result in less enriched δ18O1. Yield (grain weight per square meter) was also found to be significantly negatively correlated with δ18O1 in two of the three seasons. δ18O1 was as good a predictor of yield as stomatal conductance, and significantly better than carbon isotope discrimination. Correlations between grain δ18O(δ18O(g)) and physiological parameters were less clear. Significant negative correlations between δ18O(g) and stomatal conductance, leaf temperature and yield were found only during the first season. By measuring the oxygen isotope ratio of cellulose extracted from leaf samples, the difference in fractionation factors (ε(cp)) for cellulose and whole leaf tissue was assessed. ε(cp) was found to be variable, and more negative when δ18O(c) and δ18O1 were lower. Cultivar means for δ13C and δ18O of whole leaf material were found to be significantly positively related, and the factors required to produce such a relationship are discussed.
Article
Hydrogen and carbon isotope ratios of terrestrial plant leaf wax are powerful paleoclimate and paleoenvironmental proxies. However, further research on the underlying controls of the chemical and isotopic composition is needed in order to more quantitatively infer past climate and environmental conditions. There are systematic molecular and isotopic trends along leaves of a C4 grass Miscanthus, but whether such patterns also occur in other plant types is unclear. Here we determined intra-leaf variability in leaf wax abundance and isotope ratios for five terrestrial C3 plants. All species showed a general increasing base-to-tip trend in leaf wax abundance, suggesting a physiological need for more leaf wax in outer leaf sections. The hydrogen isotope values showed base-to-tip deuterium enrichment in all plants, but carbon isotope values showed base-to-tip 13C depletion in most species. Such intra-leaf isotopic distributions are similar to those in the C4 grass Miscanthus, suggesting similar underlying controls. While leaf wax hydrogen isotopic ratio values in general followed similar trends to corresponding leaf water, we found a significant reduction in hydrogen isotopic fractionation at the base of leaves (most pronounced for Dactylis glomerata). We attribute this to the utilization of isotopically enriched glucose transported from outer leaf sections for leaf wax synthesis at the base. The transported sugars were enriched in deuterium because of preferential consumption of 2H-depleted resources during metabolism in outer sections, in addition to biosynthesis using 2H-enriched leaf water.
Article
The observation that the hydrogen isotope composition (δD) of leaf wax lipids is determined mainly by precipitation δD values, has resulted in the application of these biomarkers to reconstruct paleo-climate from geological records. However, because the δD values of leaf wax lipids are additionally affected by vegetation type and ecosystem evapotranspiration, paleo-climatic reconstruction remains at best semi-quantitative. Here, we have used published results for the carbon isotope composition (δ13C) of n-alkanes in common plants along a latitudinal gradient in C3/C4 vegetation and relative humidity in Cameroon and demonstrate that pentacyclic triterpene methyl ethers (PTMEs) and n-C29 and n-C31 in the same soil, derived mainly from C4 graminoids (e.g. grasses) and C3 plants (e.g. trees and shrubs), respectively. We found that the δD values of soil n-C27, n-C29 and n-C31, and PTMEs correlated significantly with surface water δD values, supporting previous observations that leaf wax lipid δD values are an effective proxy for reconstructing precipitation δD values even if plant type changes significantly. The apparent fractionation (εapp) between leaf wax lipid and precipitation δD values remained relatively constant for C3-derived long chain n-alkanes, whereas εapp of C4-derived PTMEs decreased by 20‰ along the latitudinal gradient encompassing a relative humidity range from 80% to 45%. Our results indicate that PTME δD values derived from C4 graminoids may be a more reliable paleo-ecohydrological proxy for ecosystem evapotranspiration within tropical and sub-tropical Africa than n-alkane δD values, the latter being a better proxy for surface water δD values. We suggest that vegetation change associated with different plant water sources and/or difference in timing of leaf wax synthesis between C3 trees of the transitional class and C3 shrubs of the savanna resulted in a D depletion in soil long chain n-alkanes, thereby counteracting the effect of evapotranspiration Denrichment along the gradient. In contrast, evaporative D enrichment of leaf and soil water was significant enough to be recorded in the δD values of PTMEs derived from C4 graminoids, likely because PTMEs record the hydrogen isotopic composition of the same vegetation type.
Article
Trees and shrubs in tropical Africa use the C3 cycle as a carbon fixation pathway during photosynthesis, while grasses and sedges mostly use the C4 cycle. Leaf-wax lipids from sedimentary archives such as the long-chain n -alkanes (e.g., n -C27 to n -C33) inherit carbon isotope ratios that are representative of the carbon fixation pathway. Therefore, n -alkane δ 13C values are often used to reconstruct past C3/C4 composition of vegetation, assuming that the relative proportions of C3 and C4 leaf waxes reflect the relative proportions of C3 and C4 plants. We have compared the δ 13C values of n -alkanes from modern C3 and C4 plants with previously published values from recent lake sediments and provide a framework for estimating the fractional contribution (areal-based) of C3 vegetation cover (fC3fC3) represented by these sedimentary archives. Samples were collected in Cameroon, across a latitudinal transect that accommodates a wide range of climate zones and vegetation types, as reflected in the progressive northward replacement of C3-dominated rain forest by C4-dominated savanna. The C3 plants analysed were characterised by substantially higher abundances of n -C29 alkanes and by substantially lower abundances of n -C33 alkanes than the C4 plants. Furthermore, the sedimentary δ 13C values of n -C29 and n -C31 alkanes from recent lake sediments in Cameroon (−37.4‰ to −26.5‰) were generally within the range of δ 13C values for C3 plants, even when from sites where C4 plants dominated the catchment vegetation. In such cases simple linear mixing models fail to accurately reconstruct the relative proportions of C3 and C4 vegetation cover when using the δ 13C values of sedimentary n -alkanes, overestimating the proportion of C3 vegetation, likely as a consequence of the differences in plant wax production, preservation, transport, and/or deposition between C3 and C4 plants. We therefore tested a set of non-linear binary mixing models using δ 13C values from both C3 and C4 vegetation as end-members. The non-linear models included a sigmoid function (sine-squared) that describes small variations in the fC3fC3 values as the minimum and maximum δ13C values are approached, and a hyperbolic function that takes into account the differences between C3 and C4 plants discussed above. Model fitting and the estimation of uncertainties were completed using the Monte Carlo algorithm and can be improved by future data addition. Models that provided the best fit with the observed δ13C values of sedimentary n-alkanes were either hyperbolic functions or a combination of hyperbolic and sine-squared functions. Such non-linear models may be used to convert δ13C measurements on sedimentary n-alkanes directly into reconstructions of C3 vegetation cover.
Article
We analysed leaves from 42 plants from the South African Succulent Karoo. Whole leaf d13C values clearly differentiated 3 different populations, consisting of plants operating under obligate CAM (crassulacean acid metabolism), facultative CAM and C3 modes. In contrast, the leaf wax n-alkane d13C data from these metabolic groups showed a broader overlapping distribution. CAM plants operating under full CAM mode produced a wide range of apparent 13C fractionation. Succulent/CAM plant wax yield was considerable (up to 23 mg/g in our plants), so its contribution to soil composition and sedimentary leaf wax composition should not be dismissed. The presence of CAM plant wax n-alkanes in sedimentary n-alkanes could therefore be a problem for accessing ecosystem scale C3–C4 proportion.
Article
The distribution of pollen in marine sediments is used to reconstruct pathways of terrigenous input to the oceans and provides a record of vegetation change on adjacent continents. The wind transport routes of aeolian pollen is comprehensively illustrated by clusters of trajectories. Isobaric, 4-day backward trajectories are calculated using the modelled wind-field of ECHAM3, and are clustered on a seasonal basis to estimate the main pathways of aeolian particles to sites of marine cores in the south-eastern Atlantic. Trajectories and clusters based on the modelled wind-field of the Last Glacial Maximum hardly differ from those of the present-day. Trajectory clusters show three regional, and two seasonal patterns, determining the pathways of aeolian pollen transport into the south-eastern Atlantic ocean. Mainly, transport out of the continent occurs during austral fall and winter, when easterly and south-easterly winds prevail. South of 25°S, winds blow mostly from the west and southwest, and aeolian terrestrial input is very low. Generally, a good latitudinal correspondence exists between the distribution patterns of pollen in marine surface sediments and the occurrence of the source plants on the adjacent continent. The northern Angola Basin receives pollen and spores from the Congolian and Zambezian forests mainly through river discharge. The Zambezian vegetation zone is the main source area for wind-blown pollen in sediments of the Angola Basin, while the semi-desert and desert areas are the main sources for pollen in sediments of the Walvis Basin and on the Walvis Ridge. A transect of six marine pollen records along the south-western African coast indicates considerable changes in the vegetation of southern Africa between glacial and interglacial periods. Important changes in the vegetation are the decline of forests in equatorial Africa and the north of southern Africa and a northward shift of winter rain vegetation along the western escarpment.
Article
The environmental controls on leaf wax n-alkane distributions and associated interpretation of such distributions in geological archives have long remained rather enigmatic. Studies of contemporary vegetation often conflate changing environmental conditions and species differences between biomes, making it difficult to assess the extent to which variation is driven by plant adaptation to prevailing environmental conditions and/or more fixed chemotaxonomic patterns. We present a case study of arid and semi-arid regions of South Africa that considers these issues. We anticipate that such conditions may impart strong controls upon leaf wax synthesis.
Article
Precipitation over and near mountains is not caused by topography but, rather, occurs when storms of a type that can occur anywhere (deep convection, fronts, tropical cyclones) form near or move over complex terrain. Deep convective systems occurring near mountains are affected by channeling of airflow near mountains, capping of moist boundary layers by flow subsiding from higher terrain, and triggering to break the cap when low-level flow encounters hills near the bases of major mountain ranges. Mesoscale convective systems are triggered by nocturnal downslope flows and by diurnally triggered disturbances propagating away from mountain ranges. The stratiform regions of mesoscale convective systems are enhanced by upslope flow when they move over mountains. In frontal cloud systems, the poleward flow of warm-sector air ahead of the system may rise easily over terrain, and a maximum of precipitating cloud occurs over the first rise of terrain, and rainfall is maximum on ridges and minimum in valleys. If the low-level air ahead of the system is stable, blocking or damming occurs. Shear between a blocked layer and unblocked moist air above favors turbulent overturning, which can accelerate precipitation fallout. In tropical cyclones, the tangential winds encountering a mountain range produce a gravity wave response and greatly enhanced upslope flow. Depending on the height of the mountain, the maximum rain may occur on either the windward or leeward side. When the capped boundary layer of the eye of a tropical cyclone passes over a mountain, the cap may be broken with intense convection resulting.
Article
Here we present δ13C and δ2H data of long-chained, even-numbered (C27–C31) n-alkanes from C3 (trees) and C4 (grasses) plants and from the corresponding soils from a grassland–woodland vegetation sequence in central Queensland, Australia. Our data show that δ13C values of the C4 grassland species were heavier relative to those of C3 tree species from the woodland (Acacia leaves) and woody grassland (Atalaya leaves). However, n-alkanes from the C4 grasses had lighter δ2H values relative to the Acacia leaves, but showed no significant difference in δ2H values when compared with C3 Atalaya leaves. These results differ from those of previous studies, showing that C4 grasses had heavier δ2H values relative to C3 grasses and trees. Those observations have been explained by C4 plants accessing the more evaporation-influenced and isotopically heavier surface water and tree roots sourcing deeper, isotopically lighter soil water (“Two-layered soil–water system”). By comparison, our data suggest that ecosystem changes (vegetation “thickening”) can significantly alter the soil hydrological characteristics. This is shown by the heavier δ2H values in the woodland soil compared with lighter δ2H values in the grassland soil, implying that the recent vegetation change (increased tree biomass) in the woodland had altered soil hydrological conditions. Estimated δ2H values of the source-water for vegetation in the grassland and woodland showed that both trees and grasses in open settings accessed water with lighter δ2H values (avg. −46‰) compared with water accessed by trees in the woodland vegetation (avg. −7‰). These data suggest that in semi-arid environments the “two-layer” soil water concept might not apply. Furthermore, our data indicate that compound-specific δ2H and δ13C analyses of n-alkanes from soil organic matter can be used to successfully differentiate between water sources of different vegetation types (grasses versus trees) in natural ecosystems.
Article
Leaf-wax hydrogen isotope composition (δDwax) is increasingly applied as a proxy for hydroclimate variability in tropical paleoclimate archives, but the factors controlling δDwax in the tropics remain poorly understood. We measured δDwax and the stable carbon isotope composition of leaf-waxes (δ13Cwax), including both n-alkanes and n-alkanoic acids, from modern lake sediments and soils across a marked aridity gradient in southeastern Mexico and northern Central America to investigate the importance of aridity and vegetation composition on δDwax. In this region the estimated hydrogen isotope composition of meteoric water (δDw) varies by only 25‰, and variability in δDw does not explain the relatively large variance in δDwax (60‰). Instead, the aridity index, defined as the ratio of mean annual precipitation to mean annual potential evapotranspiration (MAP/PET), explains much of the variability in the hydrogen isotope fractionation between leaf-waxes and meteoric water (εwax/w). Aridity effects are more evident in lake sediments than in soils, possibly because integration of leaf-waxes across a broad catchment masks small-scale variability in εwax/w that is a consequence of differences in vegetation and microclimates. In angiosperm-dominated environments, plant ecology, inferred from δ13Cwax, provides a secondary control on εwax/w for n-alkanoic acids (εn-acid/w). Low δ13Cn-acid values are associated with high εn-acid/w values, most likely reflecting differences in biosynthetic hydrogen isotope fractionation between C4 grasses and C3 trees and shrubs. A similar relationship between δ13Cn-alkane and εn-alkane/w is not observed. These results indicate that changes in either aridity or vegetation can cause large variability in δDwax that is independent of the isotopic composition of precipitation, and these effects should be accounted for in paleoclimate studies.
Article
Leaf wax n-alkanes are long-chain hydrocarbons that can persist in sedimentary records over geological timescales. Since their hydrogen isotopic composition (expressed as a δD value) can be correlated to the δD values of precipitation, leaf wax n-alkane δD values have been advocated as new and powerful proxies for paleohydrological research. The exact type of hydrological information that is recorded in the δD values of leaf wax n-alkanes remains, however, unclear. In a companion paper we provide experimental evidence showing that the δD values of leaf wax n-alkanes of angiosperm plants grown under controlled environmental conditions not only reflect δD values of precipitation – as has often been assumed – but that evaporative deuterium (D)-enrichment of leaf water has an additional critical effect on their δD values. Here we present a detailed observational study that illustrates that evaporative D-enrichment of leaf water also affects the δD values of leaf wax n-alkanes in plants from natural ecosystems along a 1500 km climate gradient in Northern Australia. Based on global simulations of leaf water D-enrichment we show that the effects of evaporative D-enrichment of leaf water on leaf wax n-alkane δD values is relevant in all biomes but that it is particularly important in arid environments. Given the combined influence of precipitation δD values and leaf water D-enrichment we argue that leaf wax n-alkane δD values contain an integrated signal that can provide general hydrological information, e.g. on the aridity of a catchment area. We also suggest that more specific hydrological information and even plant physiological information can be obtained from leaf wax n-alkanes if additional indicators are available to constrain the plant- and precipitation-derived influences on their δD values. As such, our findings have important implications for the interpretation of leaf wax n-alkane δD values from paleohydrological records. In addition, our investigations open the door to employ δD values of leaf wax n-alkanes as new ecohydrological proxies in contemporary plant and ecosystem sciences.
Article
Leaf wax n-alkanes of terrestrial plants are long-chain hydrocarbons that can persist in sedimentary records over geologic timescales. Since meteoric water is the primary source of hydrogen used in leaf wax synthesis, the hydrogen isotope composition (δD value) of these biomarkers contains information on hydrological processes. Consequently, leaf wax n-alkane δD values have been advocated as powerful tools for paleohydrological research. The exact kind of hydrological information that is recorded in leaf wax n-alkanes remains, however, unclear because critical processes that determine their δD values have not yet been resolved. In particular the effects of evaporative deuterium (D)-enrichment of leaf water on the δD values of leaf wax n-alkanes have not yet been directly assessed and quantified. Here we present the results of a study where we experimentally tested if and by what magnitude evaporative D-enrichment of leaf water affects the δD of leaf wax n-alkanes in angiosperm C3 and C4 plants. Our study revealed that n-alkane δD values of all plants that we investigated were affected by evaporative D-enrichment of leaf water. For dicotyledonous plants we found that the full extent of leaf water evaporative D-enrichment is recorded in leaf wax n-alkane δD values. For monocotyledonous plants we found that between 18% and 68% of the D-enrichment in leaf water was recorded in the δD values of their n-alkanes. We hypothesize that the different magnitudes by which evaporative D-enrichment of leaf water affects the δD values of leaf wax n-alkanes in monocotyledonous and dicotyledonous plants is the result of differences in leaf growth and development between these plant groups. Our finding that the evaporative D-enrichment of leaf water affects the δD values of leaf wax n-alkanes in monocotyledonous and dicotyledonous plants – albeit at different magnitudes – has important implications for the interpretation of leaf wax n-alkane δD values from paleohydrological records. In addition, our finding opens the door to employ δD values of leaf wax n-alkanes as new ecohydrological proxies for evapotranspiration that can be applied in contemporary plant and ecosystem research.
Article
Hydrogen isotope values (δD) of sedimentary aquatic and terrestrial lipid biomarkers, originating from algae, bacteria, and leaf wax, have been used to record isotopic properties of ancient source water (i.e., precipitation and/or lake water) in several mid- and high-latitude lacustrine environments. In the tropics, however, where both processes associated with isotope fractionation in the hydrologic system and vegetation strongly differ from those at higher latitudes, calibration studies for this proxy are not yet available. To close this gap of knowledge, we sampled surface sediments from 11 lakes in Cameroon to identify those hydro-climatological processes and physiological factors that determine the hydrogen isotopic composition of aquatic and terrestrial lipid biomarkers. Here we present a robust framework for the application of compound-specific hydrogen isotopes in tropical Africa. Our results show that the δD values of the aquatic lipid biomarker n-C17 alkane were not correlated with the δD values of lake water. Carbon isotope measurements indicate that the n-C17 alkane was derived from multiple source organisms that used different hydrogen pools for biosynthesis. We demonstrate that the δD values of the n-C29 alkane were correlated with the δD values of surface water (i.e., river water and groundwater), which, on large spatial scales, reflect the isotopic composition of mean annual precipitation. Such a relationship has been observed at higher latitudes, supporting the robustness of the leaf-wax lipid δD proxy on a hemispheric spatial scale. In contrast, the δD values of the n-C31 alkane did not show such a relationship but instead were correlated with the evaporative lake water δD values. This result suggests distinct water sources for both leaf-wax lipids, most likely originating from two different groups of plants. These new findings have important implications for the interpretation of long-chain n-alkane δD records from ancient lake sediments. In particular, a robust interpretation of palaeohydrological data requires knowledge of the vegetation in the catchment area as different plants may utilise different water sources. Our results also suggest that the combination of carbon and hydrogen isotopes does help to differentiate between the metabolic pathway and/or growth form of organisms and therefore, the source of hydrogen used during lipid biosynthesis.
Article
Transfer of organic carbon (OC) from the terrestrial to the oceanic carbon pool is largely driven by riverine and aeolian transport. Before transport, however, terrigenous organic matter can be retained in intermediate terrestrial reservoirs such as soils. Using compound-specific radiocarbon analysis of terrigenous biomarkers their average terrestrial residence time can be evaluated.Here we show compound-specific radiocarbon (14C) ages of terrigenous biomarkers and bulk 14C ages accompanied by geochemical proxy data from core top samples collected along transects in front of several river mouths in the Black Sea. 14C ages of long chain n-alkanes, long chain n-fatty acids and total organic carbon (TOC) are highest in front of the river mouths, correlating well with BIT (branched and isoprenoid tetraether) indices, which indicates contribution of pre-aged, soil-derived terrigenous organic matter. The radiocarbon ages decrease further offshore towards locations where organic matter is dominated by marine production and aeolian input potentially contributes terrigenous organic matter. Average terrestrial residence times of vascular plant biomarkers deduced from n-C29+31 alkanes and n-C28+30 fatty acids ages from stations directly in front of the river mouths range from 900±70years to 4400±170years. These average residence times correlate with size and topography in climatically similar catchments, whereas the climatic regime appears to control continental carbon turnover times in morphologically similar drainage areas of the Black Sea catchment. Along-transect data imply petrogenic contribution of n-C29+31 alkanes and input via different terrigenous biomarker transport modes, i.e., riverine and aeolian, resulting in aged biomarkers at offshore core locations. Because n-C29+31 alkanes show contributions from petrogenic sources, n-C28+30 fatty acids likely provide better estimates of average terrestrial residence times of vascular plant biomarkers. Moreover, sedimentary n-C28 and n-C30 fatty acids appear clearly much less influenced by autochthonous sources than n-C24 and n-C26 fatty acids as indicated by increasing radiocarbon ages with increasing chain-length and are, thus, more representative as vascular plant biomarkers.
Article
In the tropics, the proportion of heavier water isotopes in precipitation is anticorrelated with the precipitation amount. The physical processes underlying this so-called amount effect are still poorly understood and quantified. In the present study, stable water isotopes (H2 18O and HDO) have been introduced in a single column model including the Emanuel convection parameterization. We investigate the physical processes underlying the amount effect and propose a methodology to quantify their relative contributions. We focus on convective processes, since the idealized framework of the single column models does not allow us to consider the effects of large-scale horizontal advections of air masses of different isotopic signatures. We show that two kinds of processes predominantly explain the amount effect: first, the reevaporation of the falling rain and the diffusive exchanges with the surrounding vapor; and second, the recycling of the subcloud layer vapor feeding the convective system by convective fluxes. This highlights the importance of a detailed representation of rain evaporation processes to simulate accurately the isotopic composition of precipitation in the tropics. The variability of the isotopic composition on different timescales (from days to months) is also studied using a unidimensional simulation of the Tropical Ocean-Global Atmosphere-Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE) campaign. The amount effect is best observable at intraseasonal or longer timescales. The period of time over which convective activity significantly affects the isotopic composition of precipitation is related to the residence time of water within atmospheric reservoirs.
Article
The stable isotopes of water, measured in melt samples taken from snow pits and cores at locations between 1750- and 5930-m altitude on Mount Logan (5951 m) and between 2900 and 4900 m on Mount Steele (5079 m), in the Saint Elias Mountains, Yukon, show a distinctive altitudinal distribution. Several delta O-18 and delta-D versus altitude profiles indicate the general persistence of a nearly iso-delta step, or staircase structure, separating a lower region of altitude-dependent isotopic fractionation between 1750 and 3350 m from another apparent fractionation sequence appearing above about 5300 m. Both of these sequences, but especially the lower one, indicate orderly processes. On the one hand, postdepositional changes to isotope ratios in snow at different altitudes may cause distortions to an otherwise nearly monotonic isotope fractionation sequence, but the main anomaly appears to be far too large to be explained in this way. On the other hand, an explanation linked to processes occurring in the lower and midtroposphere is based on established meteorological principles as well as on upper air data. This hypothesis is proposed as the primary one to explain the gross features of the observed isotope profiles. It is compatible with the concept of secondary-source moisture arriving via the upper troposphere, and it does not exclude the effects of postdepositional stratigraphic and stable isotope ratio changes.
Article
13C/12C ratios in plants depend on factors like temperature, evaporation or seasonal moisture distribution. Fluctuations of 13C/12C in Procavia capensis (hyrax) dung samples from different vegetation zones and various ages over the last 20000 years indicate variations in the amounts of C4 and CAM, or C3 plants consumed by these herbivores. Potentially they also indicate vegetation changes that may have occurred. 13C/12C values for a series of hyrax middens of Late Pleistocene/Holocene age, from a variety of biomes across Southern Africa, show that hyraxes favour mainly C3 plants in their diets but they do incorporate CAM or C4 plants under certain circumstances. In the eastern mountainous summer-rain area around Clarens with C3 woodland and unpalatable “sour” grassland consisting mainly of C4 grasses and fewer of the C3 type, hyraxes seem to avoid at least the C4 component of grass and rely mainly on leaves of the woody plants. Isotopic data for hyrax dung in the western Cape Cederberg region indicate diets composed almost exclusively of C3 plants during the last 20000 years. Slight shifts towards more enriched values occur, e.g., around 420 and 2100 years ago, which may indicate slight increase in CAM or C4 plants. Interestingly no enrichment occurs during the Last Glacial Maximum when a shortage of atmospheric CO2 may have favoured C4 plants. During the late Holocene some CAM and/or C4-plant ingestion by hyraxes is suggested in the dry western and southern areas which receive more summer rains, probably reflecting the availability of some palatable (or “sweet”) summer grasses. Although slight, a comparable pattern of isotope change is observed in three areas viz., the Cederberg, the Karoo and the Namib Desert, suggesting that plant cover is responding to regional climate mechanism ca. 2100 years BP. This does not necessarily imply similar seasonal rainfall shifts over the whole of this wide area.