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Abstract

A multiproxy approach, focusing on biological proxies, was undertaken to determine the influence of sea-level changes along the southern Cape coast and the variability of water masses over the central Agulhas Bank region during the Holocene. A 30.47 m sedimentary core extracted from the coastal lake, Eilandvlei, shows continuous deposition for the last 9000 years. Microfossils were classified based on salinity and habitat, diatoms were further defined by their temperature preferences as either warm- or cold-water species. The composition of the assemblages is strongly linked to fluctuations in marine inflow into the system and the extent of freshwater inputs to the lake as recorded at the core site. Paleoproductivity improved during periods of equatorward migration of the westerlies, notably from 5800 to 5000 cal yr B.P., when surface water mixing increased. Spectral analysis revealed periodicities associated with solar cycles, namely Gleissberg and De Vries. The system remained predominantly marine and submerged in response to an early-mid Holocene transgression until 4700 cal yr B.P., when sea-level fluctuations and landscape modification restricted marine incursions, modified the exposed landscape and shifted the system toward a lagoon state. This multiproxy record provides insights into oceanic exchanges, surface water conditions and the mechanisms that govern them during the Holocene.

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... South Africa, especially the southern Cape coast, is known to have experienced distinct environmental changes related to relative sea level fluctuations during the Holocene (Cooper et al., 2018;Kirsten et al., 2018b;Marker and Miller, 1993;Reddering, 1988;Wündsch et al., 2018Wündsch et al., , 2016a. Voëlvlei can contribute to our understanding of sea level changes during the Holocene due to its present location at an elevation of 5 m a.s.l. and the high temporal resolution of its Early and Middle Holocene depositional record. ...
... Compton, 2006;Cooper et al., 2018). However, in comparison to local/regional records the results of this study are in line with the findings from Eilandvlei (Kirsten et al., 2018b;Wündsch et al., 2018) and Groenvlei (Wündsch et al., 2016a), which indicate a rising sea level during the Early Holocene and a high stand during the Middle Holocene. The proxies used in this study generally support the assumed sea-level evolution at the southern Cape coast of South Africa (Kirsten et al., 2018b;Marker and Miller, 1993;Reddering, 1988;Wündsch et al., 2018Wündsch et al., , 2016a. ...
... However, in comparison to local/regional records the results of this study are in line with the findings from Eilandvlei (Kirsten et al., 2018b;Wündsch et al., 2018) and Groenvlei (Wündsch et al., 2016a), which indicate a rising sea level during the Early Holocene and a high stand during the Middle Holocene. The proxies used in this study generally support the assumed sea-level evolution at the southern Cape coast of South Africa (Kirsten et al., 2018b;Marker and Miller, 1993;Reddering, 1988;Wündsch et al., 2018Wündsch et al., , 2016a. In (supra-)regional comparison, these results are also in line with studies from the west coast of South Africa (Baxter and Meadows, 1999;Carr et al., 2015;Kirsten et al., 2020) and Namibia (Compton, 2006), as reviewed in Cooper et al. (2018), showing a rapid sea level rise to a maximum of +3.8 m a.s.l. ...
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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.
... The YRZ has been the focus of most paleoenvironmental and associated -paleoclimatic research. There, the southern Cape coast and especially the Wilderness area with its numerous coastal lakes including such as Bo Langvlei (du Plessis et al., 2020), Eilandvlei (Kirsten et al., 2018a;Kirsten et al., 2018b;Quick et al., 2018;Reinwarth et al., 2013;Wündsch et al., 2018;Wündsch et al., 2016b), Groenvlei (Martin, 1959(Martin, , 1968Wündsch et al., 2016a) and Swartvlei (Birch et al., 1978;Haberzettl et al., 2019) has yielded multiple paleoenvironmental records ( Fig. 1 B). These coastal lakes have formed between large coastal 50 dune cordons that lie parallel to the coast. ...
... South Africa, especially the southern Cape coast, is known to have experienced distinct environmental changes related to relative sea level fluctuations during the Holocene (Cooper et al., 2018;Kirsten et al., 2018b;Wündsch et al., 2018;Wündsch et al., 2016a). Voёlvlei provides the possibility to contribute to our understanding of sea level changes during the Holocene due to its present location at an elevation of 5 m a.s.l. and the high temporal resolution in the Early and Mid-Holocene. ...
... Overall, the results of this study only provide evidence of marine water intrusion in the Voёlvlei system rather than sea level index points and thus cannot be used to generate or corroborate an exact relative sea level curves (e.g., Compton, 2006;Cooper et al., 2018). However, in local/regional comparison the results of this study are in line with the findings from Eilandvlei 465 (Kirsten et al., 2018b;Wündsch et al., 2018) and Groenvlei (Wündsch et al., 2016a) which indicate a rising sea level during the Early Holocene and a high-stand during the Mid Holocene. They therefore, support the assumed sea level evolution at the southern Cape coast of South Africa (Kirsten et al., 2018b;Marker and Miller, 1993;Reddering, 1988;Wündsch et al., 2018;Wündsch et al., 2016a). ...
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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.
... Also the opinion that it happened in isolation from the interior Highveld-bound blesbok ( Figure 10). This area, received rain all year round of which about 65% fell through the winter period between April and October [31,94,124,163,164]. With the last sea-level rise and closing of the EAST-Portal of the P-AP between 9-7 Ka BP [14,[32][33][34][35][36]94,103] (Figure 11); came the last true isolation between the bontebok and the blesbok and the last P-AP migration of large grazers. ...
... This area, received rain all year round of which about 65% fell through the winter period between April and October [31,94,124,163,164]. With the last sea-level rise and closing of the EAST-Portal of the P-AP between 9-7 Ka BP [14,[32][33][34][35][36]94,103] (Figure 11); came the last true isolation between the bontebok and the blesbok and the last P-AP migration of large grazers. Several significant post-LGM (LGM was 20-18 Ka BP) climate and vegetation changes happened after 20 Ka BP in the south-western Cape [27,37,143]. ...
... The bontebok was one of few remaining large grazer species in the southern WC to have survived the post-LGM drying environmental shift 18-13 Ka BP and the cooling YDC (12.8 Ka BP) which was followed by successive climate oscillations after [86,164]. After the last sea-level rise and resultant closure of the migration portal south of present Port Elizabeth 9-7 Ka BP [94,163], the western bontebok population became trapped in the C 3 /CAM winter-rainfall vegetation of the SWC-LRV [38]. These bontebok entered a bottleneck of long-term diversity decline [127] with the reduced C 4 grazing resources to the detriment of the species. ...
Article
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Re-evaluation of bontebok following a multidisciplinary approach indicate its native habitat as the now submerged Palaeo-Agulhas Plain off the southern South African coast, and dominated by C 4 grasslands and savannah. Large grazer populations migrated across the plain and around the eastern end of the Cape Folded Belt into the Eastern Cape interior following climate oscillations and geographic-shifts of the winter-/summer-/all-year rainfall isohyets. Presently 77% of all bontebok are found on private farmland with grassland bioregions in the Eastern Cape, Free State and other. Bontebok showed enhanced performance in these grasslands if compared with poor performance on southwestern Cape Lowland Renosterveld (SWC-LRV). Renosterveld (RV) was previously perceived as the bontebok's native habitat of origin. We argue that bontebok became trapped in RV due to sea-level rises and consequent multiple species congestion. Bontebok meta-population management on private farms showed significant species improvement when compared with government conservation actions in SWC-LRV. Geographic habitat constraints appear to have been the greatest factor limiting bontebok integrity. IUCN recognizes a global population size of 1,618 as reported by the Non-Detriment Finding of the Scientific Authority of South Africa, whereas actual population size is more than 7,000. We quantify post-1930s bontebok performance against phylogeographic and palaeoclimate proxies.
... In the YRZ there are several palaeoenvironmental and eclimatic records. The southern Cape coast, with its numerous coastal lakes including Eilandvlei (Kirsten et al., 2018a(Kirsten et al., , 2018bQuick et al., 2018;Reinwarth et al., 2013;Wündsch et al., 2016bWündsch et al., , 2018, Swartvlei (Birch et al., 1978;Haberzettl et al., 2019) and Groenvlei (Martin, 1959(Martin, , 1968Wündsch et al., 2016a) has already been investigated with respect to palaeoenvironmental reconstructions ( Fig. 1 B). These coastal lakes formed in dune cordons parallel to the coast and have all been directly influenced by marine intrusions caused by sea level changes during the Holocene (Martin, 1959(Martin, , 1968Reinwarth et al., 2013;Wündsch et al., 2016a) complicating their palaeoenvironmental interpretation. ...
... Therefore, apart from relative changes in moisture sources, wind-driven evapotranspiration offers a plausible explanation for the observed trends in dD wax . The Vankervelsvlei dD wax record also exhibits a similar pattern to the d 15 N composite record from Seweweekspoort (SWP) (Fig. 6), Chase et al. (2017), although (as outlined above) it contradicts the multi-proxy moisture reconstructions of the Wilderness lakes (e.g., Kirsten et al., 2018b;Quick et al., 2018;Wündsch et al., 2018;Wündsch et al., 2016a). As discussed by Quick et al. (2018) the interpretation of the SWP composite record gives an inconsistent regional climatic signal compared to the AFT pollen and supporting other hydrological indicators from the Wilderness area. ...
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.
... Recent research initiatives on the southern Cape coast have been directed at addressing this knowledge gap, focusing on sea level, climate and vegetation dynamics during the Holocene (Haberzettl et al., 2019;Kirsten et al., 2018;Quick et al., 2018;Reinwarth et al., 2013;Strobel et al., 2019;Wündsch et al., 2016aWündsch et al., , 2016bWündsch et al., , 2018. The climate along the southern Cape coast is influenced by both tropical and temperate climate systems, and the region hosts a highly diverse vegetation including fynbos and thicket elements and includes the Knysna Afrotemperate Region -the most extensive forest complex in southern Africa (Geldenhuys, 1993;Midgley et al., 1997). ...
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This paper presents continuous, high resolution fossil pollen and microcharcoal records from Bo Langvlei, a lake in the Wilderness Embayment on South Africa’s southern Cape coast. Spanning the past ~1300 years and encompassing the Medieval Climate Anomaly (MCA; c. AD 950–1250) and the Little Ice Age (LIA; c. AD 1300–1850), these records provide a rare southern African perspective on past temperature, moisture and vegetation change during these much debated periods of the recent geological past. Considered together with other records from the Wilderness Embayment, we conclude that conditions in the region during the MCA chronozone were – in the context of the last 1300 years – likely relatively dry (reduced levels of Afrotemperate forest pollen) and perhaps slightly cooler (increased percentages of Stoebe-type pollen) than present. The most significant phase of forest expansion, and more humid conditions, occurred during the transition between the MCA and the most prominent cooling phase of the LIA. The LIA is clearly identified at this locality as a period of cool, dry conditions between c. AD 1600 and 1850. The mechanisms driving the changes observed in the Bo Langvlei pollen record appear to be generally linked to changes in temperature, and changes in the influence of tropical circulation systems. During warmer periods, moisture availability was higher at Bo Langvlei, and rainfall was perhaps less seasonal. During colder periods, precipitation resulting from tropical disturbances was more restricted, resulting in drier conditions. While increased precipitation has been reported during the LIA from Verlorenvlei in the Western Cape as a result of an equatorward displacement of the westerly storm-track at this time, the opposing response at Bo Langvlei suggests that any increased influence of westerlies was insufficient to compensate for the concurrent reduction in tropical/local rainfall in the region.
... Palaeoenvironmental investigations in the southwestern Cape and adjacent oceanic environment have revealed that the region has experienced some major fluctuations in climate, sea level and ocean dynamics during the Holocene (Compton, 2001;Granger et al., 2018;Kirsten and Meadows, 2016;Miller et al., 1995). Climate variability is primarily attributed to the strength and positioning of the SWW and southeast trade winds, but also to external forcing, such as from solar variability (Kirsten et al., 2018). The coastal lake of Verlorenvlei has been subjected to a host of Quaternary palaeoenvironmental studies (Fig. 1C). ...
Article
We present a diatom record from a well‐dated 15.25 m composite sedimentary core from Verlorenvlei, a shallow coastal lake on the west coast of South Africa. We show that fluctuations in the diatom record occur in response to changes in sea level, ocean–atmosphere interactions and latitudinal shifts in the wind belts. During the early to mid‐Holocene, the system primarily responds to sea level changes. A marine community that favours high nutrients is evident, particularly during 9200–8000, 7420–7000 and 6200–5600 cal a bp, corroborating periods of Benguela upwelling linked to fluctuations in the southeast trade winds. Increases in bioproductivity (%TOC, C/N) and fresher‐water diatoms are associated with wetter conditions over the region and the northward migration of the southern westerly wind belt, most notably between 8000 and 7500 cal a bp and over the last 700 years. The latter trends are concomitant with changes in the extent of Antarctic sea ice and availability of moisture in southern South America. During the late Holocene, as sea levels stabilised to modern levels, climate variability is more strongly evident. The body of evidence further reveals the sensitivity of the region to high‐latitude atmospheric mechanisms, but also showcases the significance of the southeast trade winds.
... Neumann et al. (2011) interpreted this moist period by the increase of Euclea, Ericaceae, and Anthospermae, which is evident in core PV11.3 as well (Fig. 9). This moist phase has been identified in various other coastal lakes of the Western Cape (Meadows and Baxter, 1999;Martin, 1968;Carr et al., 2006Carr et al., , 2015Wündsch et al., 2016Wündsch et al., , 2018Kirsten et al., 2018;Quick et al., 2018). During most of this moist period, charcoal concentrations indicate high incidence of fires (Fig. 10), due likely to increased biomass produced by the favorably wet conditions. ...
Article
A multi-proxy approach conducted on a sediment core from a small lake in the Cape Flats (Princessvlei, South Africa), supported by five AMS dates, reveals the paleoenvironments over the last 3900 years. Despite some gaps in the records, phytoliths, diatoms, δ¹⁸Odiatom, pollen, coprophilous fungus spores, microscopic charred particles (micro-charcoal), and burnt-grass phytoliths, indicate vegetation disturbances caused by climatic changes, anthropogenic influences, fire, and herbivore activity. Pollen spectra indicates a moist period (3600-2600 cal yr BP), which co-occurs with an increase in fires, possibly due to greater biomass fuel loads coupled with the moderate presence of large herbivores. Subsequently, a dry period (2600-1900 cal yr BP) saw a rapid increase of large herbivores probably congregating around the lake, a contention supported also by the occurrence of nutrient-rich waters. This dry period saw reduced fires and a decline of C3 grasses in favor of C4 grasses. The arrival of herders in the Cape after 2000 cal yr BP is not immediately apparent in the multiple records, except for minor vegetation changes and regional fires c. 1200–1400 cal yr BP. However, a more consistent presence of livestock in the immediate area of Princessvlei occurs only after c. 600 cal yr BP, when peak frequencies of coprophilous spores coincide with changes in vegetation composition and occurrence of more eutrophic waters in the lake. The introduction of exotic flora, fire suppression, and a reduction of herding activities, characterizes the period of European settlement (c. 300 BP to present).
... Species composition of sessile epiphytic diatom communities is a direct result of the particular combination of long-term environmental factors prevailing in their ecosystems. Therefore, it is likely that they could be used effectively as proxies for reconstructing past climatic conditions (Kirsten et al. 2018), and for monitoring present multidimensional changes recently detected in South African coastal waters (Schlegel et al. 2017a(Schlegel et al. , 2017b. ...
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Kelp forests are believed to host a large biomass of epiphytic fauna and flora, including diatoms, which constitute the base of aquatic food webs and play an important role in the transfer of energy to higher trophic levels. Epiphytic diatom assemblages associated with two common species of South African kelps, Ecklonia maxima and Laminaria pallida, were investigated in this study. Primary blades of adult and juvenile thalli of both kelp species were sampled at False Bay in July 2017 and analysed using scanning electron microscopy. Our findings showed that both kelp species hosted relatively low densities of diatoms (ranging from 7 [SD 5] cells mm–2 on adult specimens of L. pallida to 43 [SD 66] cells mm–2 on blades of juvenile E. maxima), with Amphora and Gomphoseptatum reaching the highest absolute abundances. Although non-metric multidimensional scaling showed overlapping and largely scattered sample sets, a significant relationship between the diatom communities and the species and age of the host macroalga was detected by two-way PERMANOVA. In general, more abundant and diverse diatom communities were observed on juvenile thalli than on adult thalli, with species belonging to Navicula and Rhoicosphenia contributing significantly to the observed dissimilarity. Due to a significant interaction between species and age effects, however, the overall ability of kelp species, their age, and their interaction to explain the variation in diatom community structure was limited. We suggest that the low densities of epiphytic diatoms were directly related to the sloughing of epithelial cells observed in both kelp species. We further speculate that on such unstable substrata some diatom taxa might adapt to an endophytic life to avoid the antifouling mechanisms developed by their hosts.
... Species composition of sessile epiphytic diatom communities is a direct result of the particular combination of long-term environmental factors prevailing in their ecosystems. Therefore, it is likely that they could be used effectively as proxies for reconstructing past climatic conditions (Kirsten et al. 2018), and for monitoring present multidimensional changes recently detected in South African coastal waters (Schlegel et al. 2017a(Schlegel et al. , 2017b. ...
Article
Full-text available
Kelp forests are believed to host a large biomass of epiphytic fauna and flora, including diatoms, which constitute the base of aquatic food webs and play an important role in the transfer of energy to higher trophic levels. Epiphytic diatom assemblages associated with two common species of South African kelps, Ecklonia maxima and Laminaria pallida, were investigated in this study. Primary blades of adult and juvenile thalli of both kelp species were sampled at False Bay in July 2017 and analysed using scanning electron microscopy. Our findings showed that both kelp species hosted relatively low densities of diatoms (ranging from 7 [SD 5] cells mm–2 on adult specimens of L. pallida to 43 [SD 66] cells mm–2 on blades of juvenile E. maxima), with Amphora and Gomphoseptatum reaching the highest absolute abundances. Although non-metric multidimensional scaling showed overlapping and largely scattered sample sets, a significant relationship between the diatom communities and the species and age of the host macroalga was detected by two-way PERMANOVA. In general, more abundant and diverse diatom communities were observed on juvenile thalli than on adult thalli, with species belonging to Navicula and Rhoicosphenia contributing significantly to the observed dissimilarity. Due to a significant interaction between species and age effects, however, the overall ability of kelp species, their age, and their interaction to explain the variation in diatom community structure was limited. We suggest that the low densities of epiphytic diatoms were directly related to the sloughing of epithelial cells observed in both kelp species. We further speculate that on such unstable substrata some diatom taxa might adapt to an endophytic life to avoid the antifouling mechanisms developed by their hosts.
... This record contains 563 high-precision dates and has an average resolution of 120 yr, providing new insight into analyzing multiscale climate change and its driving mechanisms. Conventional spectral analysis methods are mostly based on Fourier transforms, which require stationary data (Cooper et al., 2000;McDermott et al., 2001;Rousse et al., 2006;Kirsten et al., 2018). The Fourier transforms lack the time localization of the spectral components, although it can capture high resolution in the frequency domain. ...
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The empirical mode decomposition (EMD) method is used to re-analyse the high-resolution and precisely-dated stalagmite record from Chinese caves over the past 640 ka. Results show that (1) the variation in the Asian Monsoon can be completely decomposed into ten quasiperiod oscillations, among which the precession and semiprecession band oscillations are the most prominent periodicities, with contribution rates of 31.1% and 30.7%, respectively; (2) the cross-spectrum analysis of the semiprecession component and bi-hemisphere insolation (BHI) are strongly correlated, indicating an amplified response of precipitation and temperature variability to the interhemispheric insolation in the low-latitude regions, thus further affecting the intensity of the Asian Monsoon; (3) on millennial timescales, obvious oscillations at the 5 ka and 1–2 ka bands roughly correspond to the classical Bond and Dansgaard-Oeschger (DO) cycles. Additionally, a strong correlation is found between the detrended stalagmite δ18O records and Ca/Sr sequence from the North Atlantic (especially at the 5 ka band). This result means that the 5 ka cycle is characteristic of the glacial-interglacial cycle since the middle and late Pleistocene and may imply that climate change on the millennial timescale is the result of an interaction between global ice volume and insolation.
... Other methods based on microfossils have been used to reconstruct taxon-specific productivity. Diatom abundance, spores and species composition have been used to study diatom productivity changes in a variety of oceanic setting, such as upwelling off NW Africa area (Abrantes, 1991), the Peruvian upwelling system (Fleury et al., 2015), the southern Cape coast during the Holocene (Kirsten et al., 2018), and in the Okhotsk Sea (Artemova et al., 2017). Coccolithophore assemblages and abundances, more specifically the abundance of Florisphaera profunda, were used to reconstruct coccolithophore productivity and to infer total productivity and nutricline changes. ...
Article
The ocean's biological pump strongly influences atmospheric CO 2 and regulates Earth's climate. Determining the contribution of different phytoplankton groups to the biological pump over geological timescales remains an important, yet elusive, goal in biogeochemistry and organic geochemistry. Towards that end, source-specific lipid biomarkers can be used, but this approach requires the quantification of the biomarker-to-carbon ratio in different phytoplankton species, and under differing environmental conditions. We investigated responses of brassicasterol, dinosterol and C 37 alkenones to three temperatures (15, 20 and 25 °C) and three N:P supply ratios (10:1, 24:1 and 63:1 mol mol ⁻¹ ) in three diatoms, three dinoflagellates and one coccolithophore, in laboratory experiments. Brassicasterol was produced by one diatom species, three dinoflagellates and the coccolithophore Emiliania huxleyi, while dinosterol and C 37 alkenones were produced by dinoflagellates and E. huxleyi, respectively. Overall, carbon-normalized contents of lipid biomarkers varied by about a factor of three over the wide ranges of temperature and N:P supply ratios, in all species. Within the factor of three, brassicasterol was highest under the balanced N:P condition in diatoms, but under N and P deficiency in dinoflagellates. Brassicasterol in E. huxleyi was highest at lower temperatures. Dinosterol in dinoflagellates and C 37 alkenones in E. huxleyi varied with temperature and N:P supply ratios, but not systematically. Compared to those in our experiments, smaller ranges are expected of N:P ratios and temperature and hence carbon-normalized biomarker contents at individual locations over time. Thus, our results imply that lipid biomarkers can be used to estimate taxon-specific carbon fluxes through time.
... In comparison to previously published pollen records from the southern Cape coastal margin (e.g. Martin, 1968;Scholtz, 1986;Quick et al., 2015Quick et al., , 2016 (Fig. 1), this record represents the first continuously sub-centennial record of Holocene vegetation change, and provides an opportunity for the first detailed assessment of past climate gradients between the coast and the continental interior, complementing the other proxy data derived from the same core (W€ undsch et al., 2016bKirsten et al., 2018). (red) and winter/temperate (blue) rainfall dominance. ...
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The southern Cape is a particularly dynamic region of South Africa in terms of climate change as it is influenced by both temperate and tropical circulation systems. This paper presents pollen and microcharcoal data generated from a sediment core extracted from the coastal lake Eilandvlei spanning the last $8900 years. With an average sample resolution of 57 years, this record represents the highest resolution record of Holocene vegetation change from the region. The data indicate that cool, seasonal and moderately dry conditions characterized the Wilderness Embayment from $8900 to 8000 cal a BP. Afrotemperate forests expanded from $8000 cal a BP until 4700 cal a BP. This humid period is followed by indications of more arid and seasonal conditions until 3500 cal a BP. A long-term increase in forest taxa suggests steadily increasing moisture availability across the late Holocene. Strong affinities are noted with records from more tropical regions of South Africa, suggesting that tropical systems are of importance in maintaining higher moisture availability in the region. An important mechanism of climate change is the Agulhas Current, which transmits what appears to be a localized signal of tropical variability to the southern Cape coast.
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The southwestern Cape of South Africa is a particularly dynamic region in terms of long-term climate change. We analysed fossil pollen from a 25,000 year sediment core taken from a near-coastal wetland at Pearly Beach that revealed that distinct changes in vegetation composition occurred along the southwestern Cape coast. From these changes, considerable variability in temperature and moisture availability are inferred. Consistent with indications from elsewhere in southwestern Africa, variability in Atlantic Meridional Overturning Circulation (AMOC) was identified as a strong determinant of regional climate change. At Pearly Beach, this resulted in phases of relatively drier conditions (~24–22.5 cal ka BP and ~22–18 cal ka BP) demarcated by brief phases of increased humidity from ~24.5–24 cal ka BP and 22.5–22 cal ka BP. During glacial Termination I (~19–11.7 ka), a marked increase in coastal thicket pollen from ~18.5 to 15.0 cal ka BP indicates a substantial increase in moisture availability, coincident, and likely associated with, a slowing AMOC and a buildup of heat in the southern Atlantic. With clear links to glacial and deglacial Earth system dynamics and perturbations, the Pearly Beach record represents an important new contribution to a growing body of data, providing insights into the patterns and mechanisms of southwestern African climate change.
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Today's knowledge about paleomagnetic secular variations in (southern) Africa is extremely limited. Here, we derive paleomagnetic records from independently radiocarbon dated sediment records from three maars in Madagascar: Andraikiba and Amparihibe are terrestrial maars located on the main island of Madagascar and on a small island in the Northwest, whereas Crater Lake is a maar which has an open connection to the Indian Ocean. Studied through alternating field demagnetization of u-channel samples, characteristic remanent magnetization (ChRM) directions document similar inclination and declination patterns in two of the archives for the past 2,500 years (except for the last 400 cal BP). These new data are the first allowing to test the reliability of previous records which often suffer from low resolution and other obstacles. This will further allow to start to distinguish between robustly confirmed paleomagnetic secular variation data for this region from potentially problematic data. Considering a much lower resolution and a shorter covered time interval of archaeomagnetic data from La Réunion, South Africa, Botswana and Zimbabwe the temporal successions of maxima and minima reveal a coherent picture. Although slightly shifted in time, similarities also exist to the global geomagnetic field reconstruction model SHAWQ2k (Campuzano et al., 2019) which includes the above mentioned data. Surprisingly also similarities, though only in declination, are observed to the CALS3K.4 model (Korte and Constable, 2011) which is used as Northern Hemisphere biased model example. In contrast to this, no declination similarities but remarkable inclination similarities are observed to a lake record from Lake Malawi. An analog inclination pattern is also observed in a record from the Makran Accretionary Wedge which is ∼5,000 km to the north of the investigated sites. Interestingly the spatial distribution of archives showing these inclination similarities resembles the spatial distribution of inclination anomalies detected in model predictions. PSV similarities over such a large area are suggestive of a large-scale core dynamic origin independent of westward drift of non-dipole field components often associated with PSV records. This study emphasizes the potential of maar lakes in Madagascar for paleo reconstructions but also suggests that shallow marine and marine-brackish systems should be avoided if possible when trying to expand the (South) African paleomagnetic database.
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The southern Cape is a particularly dynamic region of South Africa in terms of climate change as it is influenced by both temperate and tropical circulation systems. This paper presents pollen and microcharcoal data generated from a sediment core extracted from the coastal lake Eilandvlei spanning the last $8900 years. With an average sample resolution of 57 years, this record represents the highest resolution record of Holocene vegetation change from the region. The data indicate that cool, seasonal and moderately dry conditions characterized the Wilderness Embayment from $8900 to 8000 cal a BP. Afrotemperate forests expanded from $8000 cal a BP until 4700 cal a BP. This humid period is followed by indications of more arid and seasonal conditions until 3500 cal a BP. A long-term increase in forest taxa suggests steadily increasing moisture availability across the late Holocene. Strong affinities are noted with records from more tropical regions of South Africa, suggesting that tropical systems are of importance in maintaining higher moisture availability in the region. An important mechanism of climate change is the Agulhas Current, which transmits what appears to be a localized signal of tropical variability to the southern Cape coast.
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Recent studies of the South African climatic system revealed a highly complex interplay of forcing factors on precipitation regimes. This includes the influence of the tropical easterlies, the strength of the Southern Hemispheric Westerlies as well as sea surface temperatures along the coast of the subcontinent. This study of a sediment core at the terrestrial-marine interface spanning a time interval of ~ 4 ka provides insights on the highly dynamic climatic system in southernmost South Africa. Several organic proxies sensitive to changes in climatic parameters like the distribution and isotopic composition of plant-wax lipids as well as indicators for sea surface temperatures and soil input give information on climatic changes during the investigated time period. Moreover, the micropaleontology, mineralogical and elemental composition of the sediments reflects the variability of the terrigenous input to the core site. The combination of downcore sediment signatures and a catchment-wide provenance study indicate that the Little Ice Age was characterized by relatively warm sea surface temperatures in Mossel Bay and arid climatic conditions favorable to torrential flood events sourced in the Gouritz headlands. In contrast, the so-called Medieval Climate Anomaly is expressed by humid conditions in the Gouritz River catchment with lower, but highly variable sea surface temperatures in the Mossel Bay area. The coincidence between humid climatic conditions and cooler sea surface temperatures has been attributed to a strengthened and more southerly anticyclonic circulation. In this climatic setting strong tropical easterlies supply Indian Ocean moisture to South Africa and at the same time Agulhas Bank upwelling pulses become more common due to an increase in Agulhas Current transport as well as alongshore southeasterly winds. These processes resemble the modern day oceanography in summer and can be conceptualized in a regional climate model.
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Long-term (20+ years) water quality datasets for estuaries are rare, especially for smaller systems. Monitoring of salinity, temperature, pH, dissolved oxygen and turbidity has been undertaken since 1991 in the intensively utilised, modified, and managed temporarily open/closed Touw Estuary and its associated three interconnected estuarine lakes, Eilandvlei, Langvlei and Rondevlei, of the Wilderness Lakes System, South Africa, a national park and Ramsar site. Spatial variability of salinity, pH and turbidity was pronounced in the Touw Estuary but largely absent in the lakes. Significant differences in median salinity, pH, dissolved oxygen and turbidity occurred between lakes, with reverse salinity and pH gradients frequently occurring. Seasonal variability in temperature and dissolved oxygen occurred in all waterbodies. Significant long-term declines in salinity have occurred in the more inland lakes, with decreases in turbidity and pH also occurring in some waterbodies. Water chemistry of the Wilderness Lakes is changing from that of an estuarine to a lacustrine system. Both biological and physical features were driving water quality changes, including reductions in river inflow, reduced marine connectivity, constriction of flow between waterbodies, and declines in submerged plant biomass. Management actions are proposed relating specifically to addressing the apparent causes for water quality changes.
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Salt marshes are highly productive and biologically diverse coastal wetlands that are threatened by rising sea-level. Salt marsh habitats within the Swartkops Estuary were examined to determine their structure along an elevation gradient and how this structure has changed over the past seven decades, what the primary drivers of this structure were and whether the salt marsh surface is stable, rising or declining relative to current and future sea-level rise. Relative sea-level has been rising by 1.82 mm·year− 1 over the past 36 years, with a short-term trend of 7.48 mm·year− 1 measured during the study period. GIS analyses showed that during the last 70 years, losses of floodplain, intertidal and supratidal salt marsh are mainly attributed to developmental pressure. The main environmental drivers influencing salt marsh distribution were soil moisture and elevation. Elevation dictates tidal inundation periodicity and frequency, and thus acts to influence all edaphic factors influencing vegetation distribution. Rod Surface Elevation Table results for the past six years indicate that the salt marsh surface elevation is keeping pace (2.98 ± 2.34 mm·year−1) with historic relative sea-level rise (RSLR), but at an accelerated RSLR, only two of the eight RSET stations show an elevation rate surplus. These results should be interpreted with caution though because of the short time-series (RSET and RSL) and the high likelihood that the current ratio of sediment elevation change will be accelerated in response to the increased sea-level rise.
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The ‘Cape Flats’ region, situated in the winter rainfall zone of South Africa, is a low-lying tombolo underlain by recent fluvial and aeolian sands and characterised by numerous small lakes and wetlands. One of these is Princessvlei, a eutrophic, freshwater coastal lake. The lake lies in an inter-dunal depression encroached on in the more recent past by high-density residential, industrial and agricultural land uses. A 210-cm core extracted from the lake periphery yielded high diatom fossil concentrations for the upper 174 cm. Princessvlei appears to oscillate between two ecologically stable states, namely, a state characterised by clear water, oligotrophic, benthic communities and a turbid state dominated by eutrophic, planktonic species. The two stable ecological states are interpreted to be a function of the relative dominance of catchment precipitation or groundwater influx which augments the open water conditions. From 2600 to 1500 cal. BP (173–135 cm), the system is predominantly turbid with greater moisture availability before a relatively rapid development of oligotrophic and dilute conditions from 1300 to 610 cal. BP (135–30 cm). A brief period of deeper water depths and meso-eutrophic conditions is observed between 550 and 445 cal. BP (23–13 cm). Following a short-lived hiatus, poly-hypertrophic, alkaline species are abundant in the top 10 cm coinciding with European colonisation in the region from the 17th century.
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Introduction Historically, limnological and paleolimnological research has focused on large and typically deep lakes but in the last two decades there has been a growing interest in smaller and shallower water bodies. Shallow lakes are justifiably considered as a separate lake type, distinguished physically from deeper waters by the fact that they are permanently mixed (polymictic) with a consequent lack of stratification of temperature or oxygen and with increased potential for nutrient recycling and redistribution of seston by physical water circulation patterns (Carrick et al., 1994). Whilst this is a useful distinction, there is no single definition of a shallow lake (Padisák & Reynolds, 2003). Scheffer (1998), in his classic text book, acknowledged a fundamental difference in the behavior, ecological functioning, and biotic communities of shallow waters and arbitrarily selected a mean depth of less than 3 m to define shallowness. For the purposes of this chapter we have chosen to adopt this definition and thereby to focus on lakes where, under a favorable light climate, benthic algae and/or rooted submerged macrophytes may occupy the majority of the lakebed (see also Jeppesen et al., 1997). Under enriched conditions, however, the mechanisms that stabilize the macrophyte communities of shallow lakes may often break down and a transition to pelagic production with phytoplankton dominance occurs (Scheffer et al., 1993; Vadeboncoeur et al., 2003). Importantly, because of these characteristics, shallow lakes are, for the most part, more vulnerable to a given pollutant load than large lakes.
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The import of relatively salty water masses from the Indian Ocean to the Atlantic is considered to be important for the operational mode of the Atlantic Meridional Overturning Circulation (AMOC). However, the occurrence and the origin of changes in this import behaviour on millennial and glacial/interglacial timescales remains equivocal. Here we reconstruct multiproxy paleosalinity changes in the Agulhas Current since the Last Glacial Maximum and compare the salinity pattern with records from the Indian- Atlantic Ocean Gateway (I-AOG) and model simulations using a fully coupled atmosphere–ocean general circulation model. The reconstructed paleosalinity pattern in the Agulhas Current displays coherent variability with changes recorded in the wider I-AOG region over the last glacial termination. We infer that salinities simultaneously increased in both areas consistent with a quasi-inter-hemispheric salt-seesaw response, analogous to the thermal bipolar seesaw in response to a reduced cross-hemispheric heat and salt exchange during times of weakened AMOC. Interestingly, these hydrographic shifts can also be recognised in the wider Southern Hemisphere, which indicates that salinity anomalies are not purely restricted to the Agulhas Current System itself. More saline upstream Agulhas waters were propagated to the I-AOG during HS1. However the salt-flux into the South Atlantic might have been reduced due to a decreased volume transport through the I-AOG during the AMOC slowdown associated with HS1. Hence, our combined data-model interpretation suggests that intervals with higher salinity in the Agulhas Current source region are not necessarily an indicator for an increased salt import via the I-AOG into the South Atlantic.
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We present an 8000-year biomarker and stable carbon isotope record from the Verlorenvlei Estuary, South Africa. We assessed how leaf wax lipids, insoluble macromolecular organic matter, bulk C/N data and compound-specific stable carbon isotopes were linked to the site’s palynological record and to evidence for regional sea level and environmental change. Down-core trends in bulk δ13C are closely coupled to trends in pollen types from saline-tolerant taxa. These trends are mirrored by variations in the incorporation of reduced sulphur into macromolecular organic matter. This process, quantified with the thiophene ratio, is closely associated with periods of higher sea level 8,000–4,300 cal yr BP. We propose the thiophene ratio is a proxy for relative marine influence within (peri) estuarine sediments. All measured variables indicate differences between early-middle Holocene (8,000–4,300 cal BP) and late Holocene conditions at Verlorenvlei. The former period was more saline and preserves more labile macromolecular organic matter. Marine influence declined after 4,300 cal yr BP, and although the abundance of short-chain-length n-alkanes suggests continued presence of wetland flora until 2,500 cal yr BP, organic matter preservation became poorer and a drying trend was inferred, most notably for the interval 2,500–900 cal BP. Increasing freshwater inundation is apparent during the last 700 cal yr, consistent with several records from this region. Leaf wax n-alkane distributions are largely uncorrelated with bulk organic matter variables, with the exception of the abundance of C31 and C33 n-alkanes, which are negatively correlated with δ13CTOC. Furthermore, C31–C33 n-alkane δ13C values are uncorrelated with C23–C29 δ13C and δ13CTOC. They are also higher than our newly measured terrestrial (C3) vegetation C29 and C31 end-member values of −35 ± 2 and −34 ± 1 ‰, respectively. These patterns are best explained by a dominant contribution of local riparian vegetation to the C23–C29 n-alkanes, but time-varying contributions of non-local leaf waxes to the C31–C33 signals. This renders inferences concerning regional environmental change from long-chain leaf waxes potentially challenging in this setting.
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The major cause of sea-level change during ice ages is the exchange of water between ice and ocean and the planet's dynamic response to the changing surface load. Inversion of ∼1,000 observations for the past 35,000 y from localities far from former ice margins has provided new constraints on the fluctuation of ice volume in this interval. Key results are: (i) a rapid final fall in global sea level of ∼40 m in <2,000 y at the onset of the glacial maximum ∼30,000 y before present (30 ka BP); (ii) a slow fall to -134 m from 29 to 21 ka BP with a maximum grounded ice volume of ∼52 × 10(6) km(3) greater than today; (iii) after an initial short duration rapid rise and a short interval of near-constant sea level, the main phase of deglaciation occurred from ∼16.5 ka BP to ∼8.2 ka BP at an average rate of rise of 12 m⋅ka(-1) punctuated by periods of greater, particularly at 14.5-14.0 ka BP at ≥40 mm⋅y(-1) (MWP-1A), and lesser, from 12.5 to 11.5 ka BP (Younger Dryas), rates; (iv) no evidence for a global MWP-1B event at ∼11.3 ka BP; and (v) a progressive decrease in the rate of rise from 8.2 ka to ∼2.5 ka BP, after which ocean volumes remained nearly constant until the renewed sea-level rise at 100-150 y ago, with no evidence of oscillations exceeding ∼15-20 cm in time intervals ≥200 y from 6 to 0.15 ka BP.
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Satellite imagery combined with coastal measurements of wind and sea surface temperature has been used to investigate the upwelling. It is found that the prominent capes in the eastern half of the region are important in initiating the upwelling, and a simple theory is advanced to explain the observations.-from Authors
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Sediment traps were placed in St Helena Bay and Elands Bay in the southern Benguela system in December 1984 for 2–3 days. Comparisons of abundance of selected phytoplankton species in the water column, from the traps and in the sediment/water interface revealed differences in the sinking behaviour of the selected species. These differences are most likely exaggerated by the formation of resting spores. Although the water column was dominated by vegetative cells, the flux to the sediment occurred largely in the form of resting spores. The results suggest rapid formation and settling of resting spores from the water column. These transient events may be missed by typical field sampling procedures, thereby underestimating frequency of spore formation. High concentrations of resting spores dominated the phytoplankton assemblages of the sediment/water interface, suggesting that seeding by spores from the sediment is possibly an important factor resulting in the success of Chaetoceros species and the consistently high phytoplankton standing crops in this region.
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The Southern Hemisphere SHCal04 radiocarbon calibration curve has been updated with the addition of new data sets extending measurements to 2145 cal BP and including the ANSTO Younger Dryas Huon pine data set. Outside the range of measured data, the curve is based upon the ern Hemisphere data sets as presented in IntCal13, with an interhemi-spheric offset averaging 43 ± 23 yr modeled by an autoregressive process to represent the short-term correlations in the offset.
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Introduction Atmospheric temperature records from central Greenland and Antarctic ice cores reveal a dramatic shift between the Late Pleistocene and the Holocene in terms of estimates and amplitudes of temperature change. Following recovery of these long, high-resolution ice cores in the early 1990s, initially it was believed that this shift into the Holocene involved a change from low mean temperatures with large, rapid oscillations on decadal to millennial timescales, to high mean temperatures with relatively little variability. More recently, other records from different regions of the world, together with our increased understanding of external climate forcings and feedbacks, have shown that this ice-core-derived picture of Holocene climate stability is not the case (Maslin et al., 2001; Wanner et al., 2008). Holocene climate variability appears to exhibit relatively regular patterns of change. However, these patterns of change are complex; not all changes are observed globally or synchronously (Mayewski et al., 2004). And, although the oscillations in climate during the Holocene are of lower amplitudes than those of the Late Pleistocene, they are of sufficient magnitude to cause significant perturbations to our contemporary climate and to have had an impact on human civilizations. The primary goal of this chapter is to present a detailed view of the contribution of diatom analysis from marine sedimentary records to our understanding of climatic and environmental change during the Holocene. This chapter will provide a link between other chapters in this book that deal with diatoms as indicators of recent changes in oceanographic condition (Romero and Armand, this volume) and diatoms as indicators of paleoceanographic events (Jordan and Stickley, this volume).
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Current research indicates an increase in Agulhas leakage for the past and coming decades. This change potentially alters the strength of the Atlantic meridional overturning circulation, in particular, through advection of positive density anomalies into the North Atlantic. To explore the fate of Agulhas leakage, results from a Lagrangian analysis were evaluated, with virtual floats advected within an eddy-permitting ocean model (ORCA025). A considerable fraction of Agulhas leakage reached the subtropical North Atlantic: of a mean Agulhas leakage transport of 15.3 Sv entering the South Atlantic, 9.7, 7.7, and 6.1 Sv crossed sections at 6°S, 6°N, and 26°N, respectively. The most probable transit time of leakage to reach the respective latitudes is one to two decades. We suggest that changes in Agulhas leakage could manifest in the Gulf Stream regime most probably within two decades. These results were supported by an eddy-resolving implementation of the ocean model (INALT01).
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The inter-ocean exchange of warm and salt-enriched waters around South Africa (Agulhas leakage), may have played an important role in the mechanism of deglaciations. Paleoceanographic reconstructions from the Agulhas leakage corridor show that leakage maxima occurred during glacial terminations. Therefore enhanced leakage has been suggested as a forcing mechanism to shift the Atlantic Meridional Overturning Circulation into the interglacial mode of circulation. At present, studies have not considered that upstream changes in the properties of the Agulhas Current itself may, in part, explain the observed variability in the Agulhas leakage and play an important role in defining the upper ocean hydrography of the South Atlantic. Here, we present a multi-proxy record from a marine sediment core (CD154 17-17K) located in the main trajectory of the Agulhas Current that spans the past 100 kyr. The record shows considerable variability in reconstructed upper ocean temperatures and salinity. We also find that the relative abundance of tropical and sub-tropical planktic foraminifera, previously used as a proxy for Agulhas leakage fauna, shows considerable upstream variability, likely reflecting changes in the hydrography of the southwest Indian Ocean sub-gyre (SWIOSG) and upper ocean temperatures. Idealised numerical model simulations demonstrate that both a shifting and an intensification of the Southern Hemisphere westerlies modify the vigour of the SWIOSG. These changes also drive increased kinetic and eddy variability in the Agulhas Return Current that potentially enhances cross frontal mixing of southern sourced waters into the SWIOSG system. Our results suggest that variability in the upstream Agulhas Current hydrography is strongly linked to the dynamics of the Agulhas Return Current and strength of the SWIOSG and that downstream variability in the leakage area (Atlantic sector) at least partly reflects regional variations of the Agulhas Current as a whole.
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The Atlantic Ocean receives warm, saline water from the Indo-Pacific Ocean through Agulhas leakage around the southern tip of Africa. Recent findings suggest that Agulhas leakage is a crucial component of the climate system and that ongoing increases in leakage under anthropogenic warming could strengthen the Atlantic overturning circulation at a time when warming and accelerated meltwater input in the North Atlantic is predicted to weaken it. Yet in comparison with processes in the North Atlantic, the overall Agulhas system is largely overlooked as a potential climate trigger or feedback mechanism. Detailed modelling experiments - backed by palaeoceanographic and sustained modern observations - are required to establish firmly the role of the Agulhas system in a warming climate.
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The Agulhas Cur rent plays a crucial role in the thermohaline circulation through its leakage into the South Atlantic. Under both past and present climates, the trade winds and westerlies could have the ability to modulate the amount of Indian - Atlantic inflow. Compelling arguments have been put forward suggesting that trade winds alone have little impact on the magnitude of Agulhas l eakage. Here, employing three ocean models for robust analysis - a global coarse resolution, a regional eddy - permitting and a nested high - resolution eddy - resolving configuration - and systematically altering the position and intensity of the westerly wind belt in a series of sensitivity experiments, it is shown that the westerlies, in particular their intensity, control the leakage. Leakage resp onds proportionally to the westerlies intensity up to a certain point. Beyond this, through the adjustment of the large - scale circulation, energetic interactions occur between the Agulhas Return Current and the Antarctic Circumpolar Current that result in a state where leakage no longer increases. This adjustment takes place within 1 to 2 decades. Contrary to previous assertions, our results further show that an equatorward (poleward) shift in westerlies increases (decreases ) leakage. This occurs due to the redistribution of momentum input by the winds. It is concluded that the reported present - day leakage increase could therefore reflect an unadjusted oceanic response mainly to the strengthening westerlies over the last few decades.
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The IntCal09 and Marine09 radiocarbon calibration curves have been revised utilizing newly available and updated data sets from 14C measurements on tree rings, plant macrofossils, speleothems, corals, and foraminifera. The calibration curves were derived from the data using the random walk model (RWM) used to generate IntCal09 and Marine09, which has been revised to account for additional uncertainties and error structures. The new curves were ratified at the 21st International Radiocarbon conference in July 2012 and are available as Supplemental Material at www.radiocarbon.org. The database can be accessed at http://intcal.qub.ac.uk/intcal13/. © 2013 by the Arizona Board of Regents on behalf of the University of Arizona.
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Satellite imagery over a 3-year period is used to provide statistical data on the position of the Agulhas Current, frontal features, and meanders off the southeast coast of South Africa. Data recorded during a cruise undertaken over the continental shelf off Algoa Bay in May 1987 provided subsurface temperature data of a large frontal feature and is used to correlate in situ measurements with satellite imagery. It is found that the inshore thermal front of the Agulhas Current lay, on average, at the shelf break northeast of Algoa Bay and seaward of it to the west. Its thermal core was about 26 km further out. The Agulhas Current influences the ocean structures over the shelf area by longer-term Natal Pulses and shorter term warm surface frontal plume fluctuations. A case study of a rapidly propagating meander (average speed, 42 km/day) showed that an inshore plume formed a thin wedge over the cooler shelf water, and it illustrated that the surface signature as seen by satellite imagery does not necessarily correspond to the subsurface structure. The formation of a large shear-edge eddy could also be followed and was possibly accentuated by a severe storm.
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An investigation of phytoplankton production and physiology was undertaken during two research cruises on the southeastern shelf of southern Africa. The data set included photosynthesis-irradiance and active fluorescence parameters, phytoplankton absorption coefficients and HPLC pigment concentrations. Primary production was estimated to vary over a similar range for both cruises within 0.27–3.69gCm−2d−1. Pigment indices indicated that diatoms were dominant on the first cruise and the communities were subject to conditions where the mixed layer was deeper than the euphotic zone and they optimized their photosynthesis to very low light intensities at the bottom and below the euphotic zone. Mixed diatom-flagellate populations were observed during the second cruise where the euphotic zone was deeper than the mixed layer and the populations adapted to irradiances higher in the euphotic zone. In response to a mean lower water column PAR, it was found that these mixed communities increased the proportion of chlorophyll a in the pigment pool and had a higher quantum yield of photochemistry and higher light-limited photosynthetic efficiency.
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Ostracoda from Quaternary coastal environments in the south-western Cape.
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The diverse lagoons and coastal lakes along the east coast of South Africa occupy incised valleys that were flooded during the rise and subsequent stabilisation of relative sea-level during the Holocene. Sedimentary deposits contained within these waterbodies provide an opportunity to investigate complex hydrological and sedimentological processes, and examine sea-level controls governing system geomorphic evolution. In this paper, we combine diatom and sulfur isotope analyses from two sediment cores extracted from the northern sub-basins of Lake St Lucia, a large shallow estuarine lake that is today largely isolated from direct ocean influence behind a Holocene-Pleistocene barrier complex. Analyses allow the reconstruction of hydrological changes associated with the geomorphic development of the system over the mid-to late Holocene. The sedimentary sequences indicate that St Lucia was a shallow, partially enclosed estuary/embayment dominated by strong tidal flows prior to ∼6200 cal. BP. Infilling was initiated when sea-level rise slowed and stabilised around present day levels, resulting in the accumulation of fine-grained sediment behind an emergent proto-barrier. Diatom assemblages, dominated by marine benthic and epiphytic species, reveal a system structured by marine water influx and characterised by marsh and tidal flat habitats until ∼4550 cal. BP. A shift in the biological community at ∼4550 cal. BP is linked to the development of a back-barrier water body that supported a brackish community. Marine planktonics and enrichments in δ³⁴S suggest recurrent, large-scale barrier inundation events during this time, coincident with a mid-Holocene sea-level highstand. Periodic marine incursions associated with episodes of enhanced storminess and overwash remained prevalent until ∼1200 cal. BP, when further barrier construction ultimately isolated the northern basins from the ocean. This study provides the first reconstruction of the palaeohydrological environment at Lake St Lucia and highlights the long-term geomorphic controls that have shaped the recent evolution and natural dynamics of the system. Unlike most coastal lake systems, this system is particularly effective as an archive of geomorphological change. Systems driven by back-barrier modifications, such as Lake St Lucia, highlight how geomorphological changes driven by sediment-supply, climate and sea level can be distributed unevenly over several isolated back-barrier basins.
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A 30.5 m sediment core was recovered from the coastal lake Eilandvlei (EV13), which represents a unique high-resolution record of environmental change for southern Africa. For the establishment of a robust chronology, special emphasis was placed on the calibration of radiocarbon (¹⁴C) ages obtained from the dating of different material. However, the reliability of ¹⁴C ages can be problematic since coastal lakes interact with different source pools providing ¹⁴C-depleted (“old”) carbon thus causing reservoir effects. The origin of old carbon affecting the EV13 samples was most likely sourced from the Indian Ocean. Two pre-bomb marine molluscan shells were therefore analysed to determine the regional marine reservoir offset (ΔR), with obtained ΔR values of 134 ± 38 and 161 ± 38 ¹⁴C yrs providing the first available data for the south coast of South Africa. However, the application of the resulting average ΔRmean = 148 ± 27 ¹⁴C yrs for the calibration of the entire EV13 record underestimates the variable reservoir effects throughout the Holocene. These were possibly caused by past changes in the connectivity between the present lake system and the ocean as well as a varying degree of upwelling in this area. To solve this problem, three sample pairs (each consisting of wood fragments and bulk organic sediment from the same core depth) were dated to calculate the variable past reservoir effects. This approach provided a median basal age of 8920 ⁺²⁰⁰/-250 cal BP. Palaeomagnetic secular variation stratigraphy was used to corroborate the chronology for the topmost 1.5 m of the record (past millennium), thus providing the first Holocene sediment based inclination and declination data from South Africa.
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Detrended seasonal mean sea surface temperature records for 5° × 5° grid squares in the South Atlantic and south-western Indian oceans are subjected to spectral analysis. Inter-annual variability of sea surface temperatures in the frequency domain is identified. The dominant wavelengths of many of the sea surface temperature oscillations are shown to correspond with some of the rainfall oscillations of Southern Africa. It is concluded that longitudinal displacements and changes in the intensity of sea surface temperature gradients in the mid-latitudes of the South Atlantic Ocean may play an important role in controlling temperate circulation systems which influence Southern African rainfall variability. -from Author
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Introduction Although studied less than aquatic diatoms, aerial diatoms are discussed in an extensive literature. Most publications on the topic consist merely of floristic lists. Thus, our understanding of aerial diatom ecology is meager. Given the brevity of the current chapter, it is not possible to list all of the pertinent literature. This paper will summarize aerial diatom studies based on floristic literature and my own work. The most important pioneer worker on aerial diatoms was probably Johannes Boye Petersen. Unlike many early soil phycologists, he treated diatoms with both detail and taxonomic accuracy. Petersen (1915, 1928, 1935) examined numerous aerial samples from Denmark, Iceland, and east Greenland. In all, he found 196 diatom taxa from soils, wet rocks, wet tree bark, and mosses, many of which were new to science at that time. Other important early floristic works are those of Beger (1927, 1928), Krasske (1932, 1936, 1948), Hustedt (1942, 1949), Lund (1945), and Bock (1963). More recent studies report diatom floras associated with limestone caves, sandstone cliff faces, wet rocks, mosses, and soils. Added to these studies are numerous papers on aerial algae, which discuss diatoms to some extent. Reviews on terrestrial algae have generally slighted the diatoms, although none have ignored them (Novichkova-Ivanova, 1980; Metting, 1981; Starks et al., 1981; Hoffmann, 1989; Johansen, 1993). Petersen (1935) defined a number of categories for aerial algae based on their habitat type. Euaerial algae inhabit raised, prominent objects that receive moisture solely from the atmosphere.
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Deep-water Quaternary Ostracoda from the continental margin off south-western Africa (SE Atlantic Ocean)
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In this 2006 volume John Murray investigates the ecological processes that control the distribution, abundance and species diversity of benthic foraminifera in environments ranging from marsh to the deepest ocean. To interpret the fossil record it is necessary to have an understanding of the ecology of modern foraminifera and the processes operating after death leading to burial and fossilisation. This book presents the ecological background required to explain how fossil forms are used in dating rocks and reconstructing past environmental features including changes of sea level. It demonstrates how living foraminifera can be used to monitor modern-day environmental change. Ecology and Applications of Benthic Foraminifera presents a comprehensive and global coverage of the subject using all the available literature. It is supported by a website hosting a large database of additional ecological information (www.cambridge.org/0521828392) and will form an important reference for academic researchers and graduate students in Earth and Environmental Sciences. © Cambridge University Press, 2009 and John Murray 2006. All rights reserved.
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A paleolimnological record from the central Messenian plain (southwestern Peloponnese, southern Greece) indicates rapid changes in the water level and chemistry of a transient lake on the flanks of the Taygetos Mountains during the last c. 6000 years. The analyses are based on diatoms as well as carbon and nitrogen isotopes from bulk sediments in a 7.5-m-long sediment core retrieved from the drained fen of Agios Floros, at the northern banks of the ancient River Pamisos. The sequence consists of fen peat in the uppermost section underlain by lacustrine sediments, which are punctuated by two layers of clay with diatomaceous silt bands. High accumulation rate is recorded in the oldest part of the section (up to 23 mm/yr), particularly during two decadal-long periods centered at c. 5700 and c. 5300 cal. BP. The diatom record reveals pronounced peaks in the planktonic taxon Cyclotella distinguenda, which correspond to the laminated sequences, reflecting the rapid development of a deep lake with an open water environment during these two time periods. Another two events with intermediate water levels are inferred at c. 5200 and c. 4600 cal. BP. These short-lived phases were probably, to a large extent, caused by local tectonic processes and the consequent hydrological anomalies of the nearby karst springs, although abrupt climatic changes with enhanced precipitation might have also played a role. At c. 4500 cal. BP, our data suggest the development of terrestrial conditions in this area, which can be attributed to the decreasing activity/dry up of springs, probably associated with more arid climate. After c. 2500 cal. BP, the diatom record infers a return to wetter conditions, probably as a response to more humid climate with marked seasonality and human activities, developing the present-day environment with cultivated and seasonally semi-flooded fields.
Article
Important aspects of lower-atmosphere behavior are associated with solar phenomena ranging from short-lived events such as solar flares, through 27-day solar rotations to the 11-year, 22-year, and even longer solar cycles. The discussion touches upon the influence of the sunspot cycle on annual rainfall and other climatic factors, the response of the lower atmosphere to short-lived solar phenomena, the role of the earth's magnetic field in controlling the morphology of the lower atmosphere, and the geophysical phenomena affected by the earth's magnetic field - notably cosmic-ray-induced ionization and total ozone content of the atmosphere.
Article
Cyclonic frontal eddies accompanying the sinuous meanders of the Agulhas Current along the south-eastern edge of the Agulhas Bank force both warm Subtropical Surface water and cold, less-saline Indian Ocean Central water onto the continental shelf. The advective juxtaposition of these water masses establishes an arealy extensive vertical thermocline of remarkable intensity over the shelf. Temperature differentials of 8–10°C over depth intervals of as little as 10 m are not uncommon. Several manifestations of upwelling, viz. coastal, midshelf and shelf-edge, are evident in the results of three CTD-surveys made since 1982. Advective maintenance of the thermocline appears to compete with turbulent breakdown, a seasonal change of dominance occurring in parts remote from Agulhas Current influence. The gravitationally unstable salinity distribution could facilitate double-diffusive diapycnal mixing as well as ease turbulent mixing with winter cooling of the surface mixed layer.
Article
Long-term (20+ years) water quality datasets for estuaries are rare, especially for smaller systems. Monitoring of salinity, temperature, pH, dissolved oxygen and turbidity has been undertaken since 1991 in the intensively utilised, modified, and managed temporarily open/closed Touw Estuary and its associated three interconnected estuarine lakes, Eilandvlei, Langvlei and Rondevlei, of the Wilderness Lakes System, South Africa, a national park and Ramsar site. Spatial variability of salinity, pH and turbidity was pronounced in the Touw Estuary but largely absent in the lakes. Significant differences in median salinity, pH, dissolved oxygen and turbidity occurred between lakes, with reverse salinity and pH gradients frequently occurring. Seasonal variability in temperature and dissolved oxygen occurred in all waterbodies. Significant long-term declines in salinity have occurred in the more inland lakes, with decreases in turbidity and pH also occurring in some waterbodies. Water chemistry of the Wilderness Lakes is changing from that of an estuarine to a lacustrine system. Both biological and physical features were driving water quality changes, including reductions in river inflow, reduced marine connectivity, constriction of flow between waterbodies, and declines in submerged plant biomass. Management actions are proposed relating specifically to addressing the apparent causes for water quality changes.
Article
Tropical paleoclimate records provide important insights into the response of precipitation patterns and the Hadley circulation to past climate changes. Paleo-records are commonly interpreted as indicating north–south shifts of the Intertropical Convergence Zone (ITCZ), with the ITCZ’s mean position moving toward the warmer hemisphere in response to changes in cross-equatorial temperature gradients. Though a number of records in tropical Central and South America, North Africa, Asia and the Indo-Australian region are consistent with this interpretation, the magnitudes and regional variability of past ITCZ shifts are poorly constrained. Combining estimates of past tropical sea surface temperature (SST) gradients with the strong linear relationship observed between zonally averaged ITCZ position and tropical SST gradients in the modern seasonal cycle and in models of past climates, we quantify past shifts in zonally averaged ITCZ position. We find that mean ITCZ shifts are likely less than 1◦ latitude during the Last Glacial Maximum (LGM), Heinrich Stadial 1 (HS1) and mid-Holocene (6 ka) climates, with the largest shift during HS1. The ITCZ’s position is closely tied to heat transport between the hemispheres by the atmosphere and ocean; accordingly, these small mean ITCZ shifts are associated with relatively large (∼0.1–0.4 PW) changes in cross-equatorial atmospheric heat transport (AHTEQ). These AHTEQ changes point to changes in cross-equatorial ocean heat transport or net radiative fluxes of the opposite sign. During HS1, the increase in northward AHTEQ is large enough to compensate for a partial or total shutdown in northward heat transport by the Atlantic Ocean’s meridional overturning circulation. The large AHTEQ response for small changes in mean ITCZ position places limits on the magnitude of past shifts in the globally averaged ITCZ. Large (>=5◦) meridional displacements of the ITCZ inferred from regional compilations of proxy records must be limited in their zonal extent, and ITCZ shifts at other longitudes must be near zero, for the global mean shift to remain <=1◦ as suggested by our results. Our examination of model results and modern observations supports variable regional and seasonal changes in ITCZ precipitation. This work thus highlights the importance of a dense network of tropical precipitation reconstructions to document the regional and seasonal heterogeneity of ITCZ responses to past climate changes.
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This paper provides a review of different particle size scales, size class terminology and particle size distribution (‘textural’) classification schemes which are widely used in sedimentology, geomorphology, soil science, aquatic ecology and civil engineering. It is concluded that a revised system of size class nomenclature, based on the Udden (1898) and Wentworth (1922) schemes, provides the most logical and consistent framework for use with sediments and a wide range of other particulate materials. A refined scheme is proposed which has five first‐order size classes (boulder, gravel, sand, silt and clay), each of which has five second‐order subdivisions with limits defined at one phi intervals. The scheme is simple and intuitively easy to understand. The paper also provides a review of previous schemes that have been proposed to describe and classify sediments on the basis of the proportions of gravel, sand and mud, or sand, silt and clay using trigons (also termed ternary diagrams). Many of these schemes do not have a logical basis and provide limited or uneven resolution. New gravel, sand and mud and sand, silt and clay classification systems are proposed that are both more logical and provide greater discriminatory power than previous schemes; they are therefore more suitable for use in environmental and forensic investigations. A new Microsoft Excel® program, freely available to download from http://www.kpal.co.uk, allows rapid classification of sediments based on the proportions of gravel, sand and mud and sand, silt and clay proportions and graphical comparison of the data for different sample groups.
Article
Two 9400-year long 10Be data records from the Arctic and Antarctic and a 14C record of equal length were used to investigate the periodicities in the cosmic radiation incident on Earth throughout the past 9400 years. Fifteen significant periodicities between 40 and 2320 years are observed in the 10Be and 14C records, there being close agreement between the periodicities in each record. We found that the periodic variations in the galactic cosmic radiation are the primary cause for periods 250 years. The spectral line for the Gleissberg (87-year) periodicity is narrow, indicating a stability of ≈ 0.5 %. The 9400-year record contains 26 Grand Minima (GM) similar to the Maunder Minimum, most of which occurred as sequences of 2 – 7 GM with intervals of 800 – 1200 years in between, in which there were no GM. The intervals between the GM sequences are characterised by high values of the modulation function. Periodicities
Article
Models of southern African palaeoclimate implicate surface atmospheric circulation anoma lies as forcing large-scale changes during the Late Quaternary. The available proxy data are insufficient to test the models since they provide information about temperature and rainfall rather than surface circulation. A conceptual model is proposed which links coastal ocean temperatures with atmospheric circulation and allows a history of surface circulation to be inferred from sea-temperature data. A Holocene sea-surface temperature (SST) record was constructed by measuring the oxygen isotope composition of marine mollusc shells preserved in an archaeological cave deposit on the coast of the eastern Agulhas Bank, southern Africa. Radiocarbon-dating of individual shells allowed definition of the timing and timespan of events in the record. By serially sampling along the growth axis of each shell, information was obtained about intra-annual variability as well as millennial-scale trends. During the early Holocene, the sea surface on the eastern Agulhas Bank was colder than it is at present. Maximum summer and winter temperatures obtained 5800 years ago, exceeding by more than 2°C those recorded in the region today during non-El Nino years. On average, temperatures remained high for the following 1500 years but dropped again during the Late Holocene. At 650 BP, at the start of the 'Little Ice Age' in southern Africa, the surface waters on the eastern Agulhas Bank were colder during both winter and summer.
Article
Borings on the fen at Groenvlei show that the earliest sediment was a diatomaceous nekron mud in a small freshwater lake. This filled in and fen peat formed over it. Subsequently, marine mud was spread over almost the whole basin, the course of the transgression being followed by diatom analysis. A change in composition of the marine diatom flora, apparently due to an unexplained change of temperature of the sea, is recorded. The marine incursion graded into brackish lagoon stages, towards the end of which conditions were rather unfavourable to diatom growth, followed by increasingly freshwater conditions, during which calcareous muds were laid down. The eastern basin of the lake became more or less separated from the main lake by a sand bar. The calcareous lake mud was overgrown by the latest reedswamp and sedge fen. Radiocarbon dating gave an age of 6870 years for a sample from the upper part of the submerged peat bed at Groenvlei, and a lapse of little under 5000 years before freshwater deposits again occupied the eastern basin of the lake (14C age of gyttja sample, 1905 years). This dating of the Groenvlei transgression is compatible both with the main post-Glacial eustatic rise of sea-level in the North Temperate latitudes, and with similar transgressions on the coasts of south-eastern Australia, New Zealand, and Fuego–Patagonia. The maximum height of the sea-level is considered to have been about 1.5 m higher than at present.
Article
The taphonomy of neritic diatoms entrained into, and transported offshore by, the Peru and South Equatorial currents may represent fluctuations in surface current and southeast trade wind strength in the eastern equatorial Pacific (EEP). This proposition was investigated through the construction of a neritic/pelagic diatom ratio (NPDR) which is compared with other palaeoceanographic proxies analysed through the Olduvai timeslab (ca. 2.0–1.75 Ma) at Ocean Drilling Program sites 677, 847 and 851: a new sea-surface temperature (SST) record for all the sites is derived from a diatom transfer function, and upwelling and bioproductivity records are provided by previously published radiolarian and CaCO3 data, respectively, for site 677 only. Results indicate a consistent relationship between proxy records, suggesting that the presence of neritic diatoms in pelagic sediments are most likely attributable to surface current and trade wind variability. The NPDR has the potential of becoming an extremely useful micropalaeontological tool in multi-proxy palaeoceanographic studies of coastal upwelling systems. The palaeoceanographic history of the Olduvai timeslab in the EEP, as indicated by previous studies, is supported and enhanced by the NPDR and new SST data reported here. The early and late periods of the Olduvai timeslab are characterised by strong upwelling, low SST, and high bioproductivity, and increased trade wind strengths inferred from the NPDR. The mid-Olduvai (1.80–1.90 Ma) appears to have been a period of weaker upwelling, higher SSTs, lower bioproductivity and decreased trade wind strength as indicated by the NPDR. The NPDR is important as it increases our understanding of the relationship between palaeoceanographic processes and trade wind strength in the region.
Article
Eocene to middle Miocene stratigraphic changes in species richness, abundance and valve size of Chaetoceros resting spores in the Norwegian Sea (DSDP Site 338) were investigated in order to understand past productivity and paleoenvironmental changes in upwelling regions. As a result, drastic resting spore events were recognized in a ∼6 myr interval across the Eocene/Oligocene boundary (EO Event), the Oligocene/Miocene boundary (OM Event) and in the early middle Miocene (emM Event). The EO Event was characterized by explosive diversification at both the morpho-generic and specific levels, an increase in abundance, and a decrease in valve size from the upper Eocene through the lowest Oligocene. The OM Event was defined by a two-fold increase in species richness. During the emM Event spore abundance decreased rapidly, and species richness and valve size decreased gradually. These changes may indicate changes in the nutrient supply, especially in upwelling regions. The increased species richness suggests a change from a stable water column with a constant nutrient supply in the Eocene to an unstable one with a sporadic nutrient supply by increased vertical mixing in the Oligocene, based on evaluation of the ecologic differences between dinoflagellate cysts and Chaetoceros resting spores. The role of main primary producer might have switched from dinoflagellates and/or nannoplankton in the Eocene to diatoms, especially Chaetoceros, in the Oligocene in the Norwegian Sea. Increased resting spore species richness during the OM Event may show that environmental changes such as global cooling and nutrient mixing led to a diversification of the spore producing genus Chaetoceros. The emM Event might have been affected by changes in paleoceanographic conditions, perhaps a decrease in nutrient supply. This study presents the first paleoceanographic analysis using not only the total resting spore abundance but also the abundances of individual species, and establishes the value of spore taxonomy and diatom analysis including spores.
Article
One hundred and sixty-five surface sediment samples from the Southern Ocean were examined for distribution and relative abundance of Chaetoceros resting spores. The contribution of resting spores to the total diatom assemblage ranges from 0% in the Subantarctic Zone to 95% in the Antarctic Peninsula sector. On the basis of both absolute and relative abundances four ‘biogeographic’ zones are distinguished: (1) the Antarctic Peninsula sector, (2) the Embayment Systems (Ross Sea and Weddell Sea), (3) the Continental Shelf zone (water depth 2000 m).Chaetoceros resting spores abundance reaches up to 900 × 106 valves/g of dry sediment in the Gerlache Strait, southwest of the Antarctic Peninsula. The hydrology of this region is characterized by an intense stratification of the water column due to sea-ice meltwater inputs, continental glacial runoffs and thermal warming of the surface water layer. The availability of nutrients, the lack of vertical mixing in those surface waters having low salinity ( 2.4 °C) is thought to be the main pre-condition for development of large Chaetoceros species blooms. We propose that increased relative abundances of Chaetoceros resting spores in fossil diatom assemblages from the Southern Ocean can therefore be used as tracers of water-column stratification due to glacial melt water.