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Abstract

Reconstructing past responses of coastal wetlands to climate change contextualizes ongoing and future developments in these globally important ecosystems. The molecular distributions and stable isotope ratios (δ²H and δ¹³C) of sedimentary plant wax n-alkanes are frequently used to infer past vegetation and hydroclimate changes in wetland systems. However, there is limited modern information available about these compounds in subtropical wetlands. Here we analyzed mature leaves from 30 typical plant species and roots from 6 plant species collected in the Florida Everglades, including tree island plants, freshwater wetland plants, mangroves, and seagrass. The n-alkane abundance (2 to 884 µg/g dry weight), percent of aquatic plants ratio (Paq, 0 to 1), average chain length (ACL23-33, 24.0 to 30.7), concentration weighted average (CWA) δ²H (-231 to -78‰) and δ¹³C values (-38.9 to -14.4‰) spanned wide ranges with plant growth habit. Significant differences in n-alkane abundances, Paq, ACL23-33, CWA δ²H and δ¹³C values were found to exist between the leaves and roots of some emergent aquatic plants. Simple mass balance calculations of wetland aquatic plants suggest that long chain n-alkanes (e.g., C29n-alkanes) are predominantly derived from leaves rather than roots in wetland surface sediments/soils. However, the contribution from mid-chain n-alkanes (e.g., C23n-alkane) from roots may be equal to or greater than those from leaves. This implies that the differences in the isotopic compositions between root and leaf derived material need to be taken into account when interpreting down core changes in mid-chain n-alkane δ²H and d¹³C values, which may be derived from variable contributions from leaves and roots rather than a change in hydroclimate or vegetation. Considering the large variation in both n-alkane distribution proxies and isotopic composition, no single molecular index or stable isotope ratio can capture multivariate changes of wetland ecosystems in the past. Nevertheless, principal component analysis shows promising potential to resolve different plant functional types. Paleo-reconstruction of subtropical aquatic ecosystems using n-alkanes will be most useful if the full molecular and isotopic distribution information of plant waxes are used.

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... enriched relative to those in monocots, especially for non-woody dicots (Liu et al., 2017). These results were also supported by various observations ranging from local to global scales (Daniels et al., 2017;Zhang et al., 2017;Pedentchouk and Zhou, 2018;He et al., 2020;Bai et al., 2020). So far, the plant type (dicots vs. monocots) is the well-recognized classification that exerts a critical role in determining δ 2 H wax values in modern plants, and thus the effect of plant types on δ 2 H wax values persists from the catchment-level to global scale. ...
... Considering the rapidly increasing importance of leaf wax n-alkanes in paleoclimate and paleoenvironmental studies, considerable efforts should be made to investigate leaf wax n-alkane biomarkers in modern plants at high-resolution spatial and temporal scales because the modern plant analysis builds a foundation for biomarker-based reconstructions in the future (e.g., Chikaraishi and Naraoka, 2003;Hou et al., 2007;Sachse et al., 2012;Garcin et al., 2014;Feakins et al., 2016a;Zhang et al., 2017;Diefendorf and Freimuth, 2017;Liu andAn, 2019, 2020;He et al., 2020) and examine the controls and relationships between leaf wax biomarkers and hydroclimate factors apart from the effect of plant types on leaf wax biomarkers. The dual-proxy or multiproxy coupling provides an effective tool to reduce the ambiguity and uncertainties derived from a single proxy and disentangle hydroclimate signals from plant type effects in sedimentary leaf wax biomarkers, but it needs to be firstly tested in in-situ modern plants. ...
... The δ 2 H wax data were fromBai et al., 2011;Bai et al., 2014;Bai et al., 2019;Bai et al., 2020;Balascio et al., 2018;Berke et al., 2019;Bi et al., 2005;Chikaraishi and Naraoka, 2003;Daniels et al., 2017;Dion-Kirschner et al., 2020;Duan et al., 2010;Duan and He, 2011a;Duan et al., 2011b;Duan et al., 2014;Duan et al., 2018.;Eley et al., 2014;Feakins and Sessions, 2010a;Feakins et al., 2016a;Freimuth et al., 2017;Freimuth et al., 2019;Gao et al., 2014b;Griepentrog et al., 2019;He et al., 2020;Hou et al., 2007;Kahmen et al., 2013b;Krull et al., 2006;Liu and Yang, 2008;Liu et al., 2015a;Liu et al., 2017;Liu et al., 2018b;Liu et al., 2019b;Mȕgler et al., 2008;Oakes and Hren, 2016;O'Connor et al., 2020;Pedentchouk et al., 2008;Romero and Feakins, 2011;Sachse et al., 2006;Sachse et al., 2009;Sachse et al., 2010;Sessions, 2006;Smith and Freeman, 2006;Tipple and Pagani, 2013;Wang et al., 2018a;Yan et al., 2020;Yang and Huang, 2003;Yang et al., 2011aYang et al., , 2011bZhang et al., 2017;Zhao et al., 2018. ...
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Terrestrial leaf wax n-alkane biomarkers provide considerable insights into paleoenvironmental reconstruction. Over decades, a substantial number of field investigations were performed to constrain hydroclimatic factors that influence leaf wax n-alkane biomarkers to improve their utility for paleoenvironmental applications. However, a critical issue, the plant type effects, does exist which potentially affects the fidelity of leaf wax n-alkane biomarkers for paleohydroclimate calibration. Here we review the effects of plant types on terrestrial leaf wax n-alkane biomarkers from three aspects: leaf wax n-alkane distribution (wood vs. non-wood), hydrogen isotope (δ2Hwax; dicot vs. monocot) and carbon isotope (δ13Cwax; C3 vs. C4) biomarkers. Then we demonstrate the relationships between three forms of leaf wax n-alkane biomarkers, and provide examples of the cross-calibration among them in paleo-applications. The in-depth review of plant type effects on leaf wax n-alkane biomarkers will be helpful to interpret the hydroclimate and vegetation signals in the geologic past.
... Constraining ε 2 H wax/ precip is challenging because sediment archives represent a mixture of waxes derived from the catchment vegetation, where plants in the catchment make different amounts of waxes and various plant taphonomic controls result in only some waxes making it into the lake (Freimuth et al., 2019). Modern calibrations exploring the relationship between δ 2 H precip values and δ 2 H wax values at local and regional scales where vegetation and climate are unique and understudied are needed for accurate reconstructions of δ 2 H precip values and past hydroclimate (Feakins and Sessions, 2010;Feakins et al., 2016;Daniels et al., 2017;Berke et al., 2019;Liu and An, 2019;He et al., 2020). To date, multiple studies have examined ε 2 H wax/precip between δ 2 H wax and source water δ 2 H values across the tropics , mid-latitudes in the Northern Hemisphere (Sachse et al., 2004;Smith and Freeman, 2006;Kahmen et al., 2013;Tipple and Pagani, 2013;Freimuth et al., 2019;Freimuth et al., 2020) and the Arctic (Wilkie et al., 2013;Daniels et al., 2017;Berke et al., 2019;Dion-Kirschner et al., 2020;O'Connor et al., 2020). ...
... On the Falkland Islands we find similar results, with each plant growth form being significantly different from the others (ANOVA, p < 0.05 for all chain lengths). The δ 2 H wax values observed for each of the growth forms studied here are within the range of δ 2 H wax values observed in other terrestrial plants found in the mid and high latitudes (Sachse et al., 2012;Freimuth et al., 2017;Berke et al., 2019;He et al., 2020). On the Falkland Islands, we observe similar trends in graminoids and ferns to other regions, with graminoids being more 2 H-depleted and ferns being more 2 H-enriched than other plant growth forms (Fig. 4b) (Sachse et al., 2012;Daniels et al., 2017;Berke et al., 2019;He et al., 2020). ...
... The δ 2 H wax values observed for each of the growth forms studied here are within the range of δ 2 H wax values observed in other terrestrial plants found in the mid and high latitudes (Sachse et al., 2012;Freimuth et al., 2017;Berke et al., 2019;He et al., 2020). On the Falkland Islands, we observe similar trends in graminoids and ferns to other regions, with graminoids being more 2 H-depleted and ferns being more 2 H-enriched than other plant growth forms (Fig. 4b) (Sachse et al., 2012;Daniels et al., 2017;Berke et al., 2019;He et al., 2020). Differences in n-alkane δ 13 C wax and δ 2 H wax values between and within plant growth forms found on the Falkland Islands can be further used to assess which species are contributing the greatest amounts of plant waxes to sediment archives (see Section 4.4). ...
Article
The hydrogen isotopic composition of terrestrial plant waxes (δ²Hwax) is widely used to reconstruct past hydroclimate. δ²Hwax values reflect plant source water or precipitation δ²H (δ²Hprecip) values, and when extracted from sediment archives, records of past δ²Hprecip values can be generated. In order to better interpret these δ²Hwax records, modern calibrations between plant waxes and source water are required when vegetation and location diverge from plant calibrations in other regions. To date, no modern study has examined how δ²Hwax values and source water δ²H values relate in the southern mid- and high-latitude maritime climatic regions where the climate is affected by the Southern Hemisphere Westerly Wind Belt. We present the first modern calibration of δ²Hwax values on the Falkland Islands by analyzing n-alkane plant wax concentrations, δ²H and δ¹³C values from 11 of the most common plant species, one lichen species, and surface lake sediment samples from four sites on Mount Usborne on East Falkland. Based on plant wax concentrations, the most commonly observed plants on the landscape, Empetrum rubrum and Cortaderia pilosa, are contributing the most to the waxes in sediment archives. We calculate the fractionation between the n-C29 alkane δ²Hwax and δ²Hprecip values (ε²Hwax/precip) for all plant species to be –110 ± 17‰ (1σ, n = 22), which is similar to the global average ε²Hwax/precip. Observed and modelled monthly δ²Hprecip values indicate that δ²Hwax values can be interpreted as mean annual δ²Hprecip values, ultimately establishing the framework for utilizing plant wax-based paleoreconstructions from the mid-latitude maritime climatic regions.
... There have been a growing number of studies assessing how d 13 C and d 2 H values of mangrove leaf wax n-alkane and other lipid compounds respond to salinity increase either along a spatial transect within a single site, with time-series observations, and in lab culture experiments (Ladd and Sachs, 2012He et al., 2017He et al., , 2020Park et al., 2019). These studies have shown that: (a) 2 H/ 1 H fractionation between surface water and lipids (a 2 H lipid-sw = (1000 + d 2 H lipid )/(1000 + d 2 H surface water ) of mangroves increases with salinity, resulting in lower d 2 H lipid values at high salinity in both Indo-West Pacific and Americas-East Atlantic mangroves (He et al., 2017); (b) although net 2 H/ 1 H fractionation for n-C 31 -alkane increased by 0.8‰, 1.4‰, and 1.8‰/ppt in Rhizophora mangle, Avicennia germinans and Laguncularia. ...
... Although n-alkanes are resistant to degradation and thus widely detected in geological records, they can be derived from both terrestrial and aquatic sources (Bianchi and Canuel, 2011). Even when focusing on long-chain n-alkanes, they could be derived from diverse origins in mangrove systems including from terrestrial plants associated with the mangrove ecosystem and adjacent wetlands, seagrass beds, and others (Ladygina et al., 2006;Nelson et al., 2018;He et al., 2020;Chen et al., 2021). Similar ambiguity of source-tracing also exists for most sterols and fatty acids (e.g., Bianchi and Canuel, 2011). ...
... Compound-specific d 13 C values were obtained by gas chromatography-isotope ratio mass spectrometry (GC-IRMS), using an HP 6890 GC equipped with a DB-5 fused silica capillary column, a combustion interface (Finnigan GC combustion IV), and a Finnigan MAT delta Plus V mass spectrometer, following a previously published procedure (He et al., 2020). Briefly, three external standard mixtures containing n-heptadecane and squalane at different concentrations, with known d 13 C values of À21.3‰ and À29.5‰, respectively, were used to check instrument performance during the entire analysis period and for correction purposes. ...
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Reconstructing past climate change in mangrove swamps contextualizes ongoing and future developments in these globally important ecosystems. Taraxerol, a well-recognized lipid biomarker for mangroves, is a promising target compound for calibration since it is relatively refractory and well preserved in sediments and since mangrove lipid δ2H and δ13C values have been shown to respond to salinity changes. Here we investigate the δ2H and δ13C values of taraxerol in leaves of two mangrove species (Rhizophora mangle and Laguncularia racemosa) and three dated mangrove cores along a spatial transect from the Shark River Estuary of South Florida, USA, to constrain its applicability for hydroclimate reconstructions. The net 2H discrimination between surface water and taraxerol increased by 1.0‰ ppt-1 over a salinity range of 0.7-32 ppt for both R. mangle and L. racemosa. Although the δ13C values of taraxerol showed a significant positive correlation with salinity in L. racemosa, the inverse trend was observed in R. mangle. The isotopic signature and spatial trends of taraxerol observed in mangrove leaves were well imprinted in mangrove surface sediments. In addition, we further tested if the isotopic signal of taraxerol from mangrove leaves could be preserved in sediment cores on a time scale of ca. 300 yrs. No strong evidence of significant diagenetic alteration was observed for δ2H values of taraxerol. In contrast, an increase up to ∼1.1‰ was observed for δ13C, excluding the Suess effect. Considering the consistent salinity-dependent discrimination of 2H to salinity, and no significant diagenetic alteration of taraxerol δ2H values on centennial time scales, taraxerol H isotopes are a promising proxy for hydroclimate reconstruction in mangrove and mangrove-adjacent systems. However, the interpretation of δ13C values of taraxerol should be treated with caution since its correlation with salinity may be species-specific and a slight diagenetic enrichment in 13C may occur.
... δ 13 C n-alkanes values in mangrove leaves of different species and locations may provide relevant information on the relationship between local environmental stressors, plant physiology and carbon uptake in these ecosystems (Ladd and Sachs, 2013;He et al., 2020He et al., , 2021. Studies on the isotopic composition of mangrove species have already been developed in Brazil (Giarrizzo et al., 2011;Soares et al., 2015;Larcher et al., 2016;Tognella et al., 2016), but focusing only in the isotopic characterization of the bulk leaf material. ...
... This predominance of odd-numbered long-chain compounds is reflected in the average chain length (ACL 25-35 ; modified after Poynter et al., 1989) and carbon preference index (CPI 24-36 ; modified after Cooper and Bray, 1963) results. The ACL 25-35 showed a mean value of 30.2 ± 1.3 for all 19 samples, ranging from 28.0 to 32.1, while the CPI 24-36 showed a This study Shark River estuary (He et al., 2020) p Table S1). Mean values of total n-alkane concentration, ACL 25-35 and CPI 24-36 for each species are presented in Fig. 3. ...
... However, the rationale behind this mechanism is unclear and this correlated trend between ACL and CPI might be circumstantial. In the mangrove leaves from the Shark River estuary (Florida Everglades, USA) (He et al., 2020), L. racemosa also presented the lowest mean ACL when compared to A. germinans and R. mangle (Fig. 3b). The CPI values, however, showed the opposite trend (Fig. 3c). ...
Article
The carbon isotopic composition (δ¹³C) of selected specific biomarkers provides important information on the relationship between plant species and the environment in which they developed. Here we present baseline data of δ¹³C values of individual n–alkanes (δ¹³Cn-alkanes) from leaf waxes of mangrove species – Avicennia spp., Rhizophora mangle and Laguncularia racemosa – typical of mangrove forests collected from equatorial to sub-tropical regions in Brazil. The objective was to evaluate the influence of tree physiology, local hydrology and climatic factors upon the molecular and isotopic signature of the leaf wax n-alkanes. Total n-alkanes concentration, average chain length (ACL25-35) and carbon preference index (CPI24-36) are higher for Avicennia spp. contrasting with the lower values found for R. mangle and L. racemosa. This seems to derive from the physiological characteristics of each species with respect to the intrinsic water use efficiency under the specific tidal flooding regime of mangrove forests. For Avicennia spp. inverse correlations between δ¹³Cn-alkanes values (δ¹³C27, δ¹³C29, δ¹³C31, δ¹³C33 and δ¹³CWA) and mean annual precipitation suggests plant response to local environmental conditions. For R. mangle species, relationships between ACL25-35 and mean annual potential evapotranspiration, and between CPI24-36 and mean annual temperature were observed. These results indicate that δ¹³Cn-alkanes values in Avicennia spp. as well as ACL25-35 and CPI24-36 in R. mangle may be useful in tracing environmental changes and, as long as preservation of reliable record in sediments is verified, may also be applied in the reconstruction of past environmental conditions. The different molecular and isotopic footprints of the mangrove species presented here have potential in future interpretations of n-alkanes as biomarkers in biogeochemical studies in mangrove-dominated coastal regions.
... Abundant OM input is proposed to originate from plants living close to or in the water. Paq > 0.4 indicates a major input of submerged/floating species (Ficken et al., 2000;He et al., 2020) for organic facies A samples, which probably grew in freshwater Pr = pristane; Ph = phytane; CPI = 2 × ∑ odd n-C 23 -29 /( ∑ even n-C 22 -28 + ∑ even n-C 24 -30 ); TAR = (n-C 27 + n-C 29 + n-C 31 )/(n-C 15 + n-C 17 + n-C 19 ); Paq = (n-C 23 + n-C 25 )/(n-C 23 + n-C 25 + n-C 29 + n-C 31 ); HD/D = 8β-(H)-homodrimane/8β-(H)-drimane. environments at the lake margin, close to land. ...
... n-C 27 ; Fig. 4a). Abundant middle-chain odd-numbered nalkanes correspond not only to Sphagnum mosses (Nichols et al., 2006;Bingham et al., 2010), Chara (Mead et al., 2005) and freshwater algae (Riboulleau et al., 2007), but also aquatic angiosperms, such as freshwater Nymphaea (Coetzee, 1967) and Ruppia (Mead et al., 2005;He et al., 2020). The odd-numbered long-chain n-alkanes leading to high CPI values (up to 4.7) might indicate higher land plants. ...
... Terrestrial plant input is probably not derived mainly from trees/ shrubs/ferns and mangroves, as they typically produce very low Paq values of <0.3 (Ficken et al., 2000;Mead et al., 2005;He et al., 2020) and a distribution of long chain n-alkanes maximizing at C 29 , C 31 or C 33 (Mead et al., 2005;Volkman et al., 2015;He et al., 2020); such a pattern is not observed in the studied samples. The absences of diploptene and fernenes argue also for the lack of ferns (Bottari et al., 1972). ...
Article
Thick Pleistocene fine-grained rocks in the central Qaidam Basin, China, are regarded as the principal source rocks for microbial gas there. Here, for the first time, a detailed investigation on the organic geochemistry and petrology of this sedimentary sequence is presented. Two different, immature, lacustrine source rocks facies are present in the study area. Organic facies A samples with high TOC (4.1–25.3 wt%) and TS (1.5–3.7 wt%) contents mainly developed under a suboxic to anoxic freshwater column at the lake margins. This shallow water fluvio-lacustrine facies, accounts for only a small proportion of the whole sedimentary sequence, but has the highest petroleum generation potential. In contrast, organic facies B samples having lower TOC (0.5–1.1 wt%) and TS (0.14–1.0 wt%) contents were deposited in a more oxic, brackish-lacustrine water body. However, exceptionally, anoxic conditions were present in sediments that were sealed by algae mats. The majority of the interval is comprised of gas prone, mixed type II-III kerogen derived mainly from aquatic plants. Organic facies A is interpreted to be derived from abundant macrophytes (e.g. non-marine algae, submerged angiosperms) around the lake margin. By contrast, the organic matter in organic facies B mainly stems from saltwater algae, with some additional bacterial contribution. Terrestrial higher plants is subordinate in both organic facies. The organic carbon accumulation rate was high compared to organic matter-rich Quaternary marine sediments. Cold and dry climate conditions and high burial rates favored methanogenesis via carbonate reduction in the sediments.
... Additionally, biosynthetic fractionation between leaf water and waxes can vary seasonally (Newberry et al., 2015), with environmental stresses (Ladd & Sachs, 2015), and with changes in plant metabolism (Cormier et al., 2018). Large differences in α Wax−P can also exist among plant species growing at the same site (Eley et al., 2014;Feakins & Sessions, 2010;He et al., 2020;Sachse et al., 2012). With the current tropical Pacific data set we can only examine factors that might relate to differences among types of plants, and not factors that can occur within a single plant, such as metabolic state. ...
... Because mangroves discriminate more against 2 H as salinity increases (He et al., 2017;Ladd & Sachs, 2012, 2017, they should have lower δ 2 H Wax values than nearby freshwater plants. This relationship was observed in the Florida Everglades, where mangroves have δ 2 H Wax values ∼50 ‰ lighter than those from nearby freshwater trees, despite equivalent δ 2 H P values (He et al., 2020). Significant contributions of mangrove leaf waxes in coastal areas in the tropical Pacific could result in sedimentary δ 2 H Wax values that fall below the global calibration line. ...
... Some of the lakes included in the tropical Pacific survey were partially or completely covered by floating aquatic vegetation (Table 1). Since aquatic plants at diverse sites tend to have lower alkane δ 2 H Wax values than nearby terrestrial plants (Chikaraishi & Naraoka, 2003;Dion-Kirschner et al., 2020;Gao et al., 2011;He et al., 2020), differing relative contributions of leaf waxes from aquatic plants could also reduce sedimentary δ 2 H Wax values. There are a few reasons why aquatic plants may have relatively low δ 2 H Wax values. ...
Article
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Hydrogen isotope ratios of sedimentary leaf waxes (δ2HWax values) are increasingly used to reconstruct past hydroclimate. Here, we add δ2HWax values from 19 lakes and four swamps on 15 tropical Pacific islands to an updated global compilation of published data from surface sediments and soils. Globally, there is a strong positive linear correlation between δ2H values of mean annual precipitation (δ2HP values) and the leaf waxes n-C29-alkane (R2 = 0.74, n = 665) and n-C28-acid (R2 = 0.74, n = 242). Tropical Pacific δ2HWax values fall within the predicted range of values based on the global calibration, and the largest residuals from the global regression line are no greater than those observed elsewhere, despite large uncertainties in δ2HP values at some Pacific sites. However, tropical Pacific δ2HWax values in isolation are not correlated with estimated δ2HP values from isoscapes or from isotope-enabled general circulation models. Palynological analyses from these same Pacific sediment samples suggest no systematic relationship between any particular type of pollen distribution and deviations from the global calibration line. Rather, the poor correlations observed in the tropical Pacific are likely a function of the small range of δ2HP values relative to the typical residuals around the global calibration line. Our results suggest that δ2HWax values are currently most suitable for use in detecting large changes in precipitation in the tropical Pacific and elsewhere, but that ample room for improving this threshold exits in both improved understanding of δ2H variability in plants, as well as in precipitation.
... Additionally, biosynthetic fractionation between leaf water and waxes can vary seasonally (Newberry et al., 2015), with environmental stresses (Ladd & Sachs, 2015), and with changes in plant metabolism (Cormier et al., 2018). Large differences in α Wax−P can also exist among plant species growing at the same site (Eley et al., 2014;Feakins & Sessions, 2010;He et al., 2020;Sachse et al., 2012). With the current tropical Pacific data set we can only examine factors that might relate to differences among types of plants, and not factors that can occur within a single plant, such as metabolic state. ...
... Some of the lakes included in the tropical Pacific survey were partially or completely covered by floating aquatic vegetation (Table 1). Since aquatic plants at diverse sites tend to have lower alkane δ 2 H Wax values than nearby terrestrial plants (Chikaraishi & Naraoka, 2003;Dion-Kirschner et al., 2020;Gao et al., 2011;He et al., 2020), differing relative contributions of leaf waxes from aquatic plants could also reduce sedimentary δ 2 H Wax values. There are a few reasons why aquatic plants may have relatively low δ 2 H Wax values. ...
... Aquatic plants may have minimal influence on sedimentary δ 2 H Wax values because submerged plants are not at risk of desiccation and have little need for the moisture barrier provided by long-chain leaf waxes. They therefore tend to have low concentrations of these compounds (Dion-Kirschner et al., 2020;Ficken et al., 2000;He et al., 2020). Overall, our results suggest that increased presence of aquatic plants does not unequivocally result in decreased δ 2 H Wax values in tropical Pacific lake sediments. ...
... This also explains the inverse relationship between α n-alkane-Water and salinity for the mangrove A. corniculatum and surface sediment (Fig. 5), because the inverse α nalkane-Water -salinity relationship in other terrigenous plants has not previously been reported. And mangrove trees generally show depleted δ 2 H n-alkanes values up to 50‰ compared to nearby freshwater trees and seagrasses (He et al., 2020). This certainly does not exclude the possibility that other mangrove species (e.g., A. marina) contribute greatly to surface sediment. ...
... The loadings on PC3 are characterized by a positive correlation with Paq and δ 2 H n-C27 (Fig. 7). Thus, PC3 might represent organic matter input from phytoplankton sources including seagrasses (Ficken et al., 2000;He et al., 2020). ...
Article
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Hydrogen isotope ratios of mangrove lipids have recently gained increasing interest because of their potential for reconstructing past water salinity and paleohydrology conditions. Although it has been suggested that the deuterium fractionation in mangrove leaf lipids appears to be primarily affected by water salinity, it remains an important research topic if this inverse relationship between salinity and hydrogen fractionation in leaf wax can be universally applied. In this regard, we have investigated the hydrogen isotopic composition of n-alkanes in leaves of Aegiceras corniculatum and surface sediments and the environmental water (surface water; αn-alkane-Water), collected along a 30 ppt salinity gradient in the Zhanjiang mangrove estuary of China, aiming to evaluate the impact of water salinity on the hydrogen isotopic fractionation of leaf wax derived n-alkanes. δ2H values of A. corniculatum leaf wax n-alkanes were between −183‰ and − 127‰ for C27, C29, and C31 n-alkanes, and their αn-alkane-Water values range from 0.823 to 0.887. Sedimentary δ2H values range from −171‰ to −132‰ for C27, C29, and C31 n-alkanes, and their αn-alkane-Water values were between 0.832 and 0.887. Inverse relationships between αn-alkane-Water and salinity were observed in mangrove A. corniculatum and surface sediment. This relationship for mangrove A. corniculatum may result from the selective utilization of freshwater during the rain event, changes in biosynthetic fractionation, and the relatively high humidity and/or larger changes in water vapour isotopic composition. For surface sediment, the inverse relationship could be mainly attributed to the mangrove-derived organic matter input, as the principal component analysis (PCA) results demonstrated a strong connection between the studied mangrove tree and the surface sediment. The consistent negative correlations between αn-alkane-Water and salinity for mangrove leaf waxes and surface sediments provide encouraging evidence for applying the hydrogen isotopic composition of mangrove leaf wax for paleohydrological reconstruction.
... For instance, soil water was normally more enriched in heavy isotopes than lake water (Schefuß et al., 2005). Additionally, the isotopic difference between waxes from sediment and from soil, not only because of different degradation processes but also because of the integration by soils over seasons, species (e.g., grass and tree), plant organs (e.g., leaf and root), and other heterogeneities within vegetation canopies He et al., 2020). The ε rain-soil may be influenced by temperature, relative humidity, altitude, biosynthesis, and vegetation type (Chikaraishi et al., 2004;Krull et al., 2006;Jia et al., 2008;Bai et al., 2011;Herrmann et al., 2017;Wang et al., 2017b). ...
... The wetland was not the main land type we focused on because most studies in wetlands showed n-alkane properties in the sediment rather than in the topsoil. Some reports also showed a significant isotopic difference between roots and leaves; however, the long-chain n-alkanes are predominantly derived from leaves rather than roots (He et al., 2020). Given the influences of RH and δ 2 H rain in a specific year on the δ 2 H value in recent leaves, a deviation of recent leaves from the long-term average recorded by soils is likely to be observed. ...
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The average chain length (ACL), carbon preference index (CPI), and hydrogen isotope composition (δ²H) of long-chain n-alkanes in sediments have been used to retrieve information about the paleoclimate. Despite their importance as in-between media from leaves to sediments, n-alkanes of surface soils have not been systematically analyzed at large scale. Such an investigation of the spatial variation of n-alkane properties in soil and their dependence on climatic and botanic (e.g., vegetation type) factors could provide a rationale for a better estimation of the past environment. We synthesized the patterns and δ²H of long-chain n-alkanes in soil (δ²Hn-alkanes) with regard to vegetation types (cropland, grassland, shrubland, and woodland) and environmental factors using data from peer-reviewed papers. Our results showed that the ACL and CPI of soil C27–C33 n-alkanes were not suitable indicators for differentiating vegetation types at large scale; instead, ACL significantly correlated with water conditions such as mean annual precipitation (MAP) and Palmer drought severity index (PDSI), and CPI significantly correlated with temperature without significant influence of vegetation type. The variation (i.e., standard deviation) of fractionation between the δ²H values in annual precipitation and in soil n-alkanes (ɛrain-soil) was smaller than that reported in leaves; therefore, soils were better suited to quantifying the general growing conditions of plants at a certain site. The fractionation ɛrain-soil correlated with climatic conditions as described by the PDSI and relative humidity (RH). This correlation agreed with the change in leaf water enrichment with changing RH taken from the literature and was independent of the vegetation type at large scale. This meta-analysis may provide useful information for the variations of the patterns and δ²Hn-alkanes values in surface soils.
... However, the compounds n-C23 and n-C25 were also abundant and the Paq value found for this species agreed with values reported by Ficken et al. (2000) for macrophytes. In present study, this index grouped monocot (Paq > 0.1) and dicot species (Paq < 0.1), which are in accordance with other studies (Ficken et al., 2000;He et al., 2020;Yu et al. 2021). The same was found for the CPI, which was higher in the latter group (CPI > 10, except for R. mangle) and lower in the monocot species (CPI ≤ 6). ...
Article
The aim of the present study was to characterize the distribution and sources of terrigenous organic matter (OM) in surface sediments of a subtropical estuarine-lagoon system surrounded by the Atlantic Rainforest. n-Alkanes and their isotopic composition were evaluated in 20 surface sediment samples, as well as in leaves from nine representative plant species collected from the Cananéia-Iguape Estuarine-Lagoon System (CIELS; Brazil). According to the calculated n-alkanes indices, the analyzed plants could be grouped as monocotyledonous (aquatic productivity index [Paq] > 0.1; carbon preference index [CPI] < 6), dicotyledonous (Paq < 0.1; CPI > 10), mangrove (average chain length [ACL23−33] < 29; ratio of n-C31 over n-C29 concentration [Norm31] < 0.1) and riparian (ACL23−33 > 29; Norm31 > 0.2) vegetation. δ13C n-alkane values were higher (> −26‰) in the C4 aquatic macrophyte analyzed (Spartina alterniflora), with intermediate values (−30 to −27‰) for the pteridophyte (Pteridium aquilinum), and lower values for the C3 species (< −31‰). In sediment, total n-alkanes, total organic carbon, and total nitrogen concentrations ranged from 510 to 10,000 ng g⁻¹ in dry sediment, 0.15 to 6.37% and not detected to 0.74%, respectively, being governed by the mud composition. ACL23−33 and the Norm31 values ranged from 29.0 to 30.4 and 0.39 to 0.57, respectively. Paq and CPI values ranged from 0.06 to 0.25 and 4.2 to 6.6, respectively. These indexes showed that the main terrigenous OM source to CIELS sediment was the riparian vegetation, coming from the rivers that discharge into the system. The n-alkanes δ13C composition, ranging from −36.0 ± 0.0 to −24.8 ± 0.3‰, also showed contributions of macrophytes input to the deposited OM. The influence of mangrove trees is not dominant but increases in the CIELS eastern region, where this kind of vegetation is more developed.
... Those were used to retrospectively predict the whereabouts of migrating fish (Brennan et al., 2019;Torniainen et al., 2017;Trueman et al., 2012), and to assign fish to discrete, geographically segregated stocks (Matta et al., 2010). With respect to aquatic plants or algae, their cellular lipid compounds have been shown to track the hydrogen isotopic signature of the ambient water, but with a potential additional influence of varying salinity (Aichner et al., 2017;Häggi et al., 2015;He et al., 2020;Ladd & Sachs, 2015Sachs & Schwab, 2011;Schouten et al., 2006). These dependencies have frequently been applied in paleoclimatic studies, for reconstruction of past hydrological conditions and salinities (e.g. ...
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River estuaries are characterized by mixing processes between marine water masses and freshwater inflows. Since the latter are depleted in heavier stable isotopes compared to the marine realm, estuaries often show a linear correlation between salinity and water isotopes (δ18O and δ2H values). In this study we evaluated spatial and seasonal isotope dynamics along three estuarine lagoon transects, located at the northern German Baltic Sea coast. The data show strong seasonality of isotope values, even at locations located furthest from the river mouths. They further reveal a positive and linear salinity‐isotope correlation in spring, but in two of the three studied transects hyperbolic and partially reverse correlations in summers. We conclude that additional hydrological processes partially overprint the two‐phase mixing correlation during summers: aside from the isotope seasonality of the riverine inflows, the shallow inner lagoons in the studied estuaries are influenced by evaporation processes. In contrast the estuarine outflow regions are under impact of salinity and isotope fluctuations of the Baltic Sea. Understanding those processes is crucial to understand water isotope and salinity dynamics. This is also of relevance in context of ecological studies, for example, when interpreting oxygen and hydrogen isotope data in aquatic organisms, that depend on ambient estuarine waters. This article is protected by copyright. All rights reserved.
... Applications of δ 13 C values, and to a lesser extent δ 2 H of individual n-alkanes, have been used in plant wax, soil and sediment studies and have been shown to be suitable for identification of OM sources since stable isotope values vary with the type of plants from which they originate (gymnosperms, angiosperms, aquatic plants) Naraoka, 2003, 2007;Hou et al., 2007;Katrantsiotis et al., 2018;He et al., 2020). They have also been used to determine anthropogenic inputs and to identify petroleum derived nalkanes by exploiting the small differences in stable isotope values originating from the initial sources of OC, kinetic fractionation processes, mixing, biodegradation, or 2 H/ 1 H exchange with water and minerals Pedentchouk and Turich, 2018). ...
Article
Sediments comprise a multitude of inorganic and organic components, with much of the composition of the organics still not fully characterized. Our research targeted n-alkanes, to determine whether compound specific carbon and hydrogen isotope analysis allows for their source identification in coastal sediments. Here, we map the current abundances and sources of straight chain n-alkanes in sediments of the St. Lawrence Estuary and Gulf using molecular (diagnostic ratios) and isotopic fingerprinting (δ¹³C, δ²H). n-Alkane abundances (117.11 ± 1.61 to 418.64 ± 70.20 µg/g OC), carbon preference index (CPI; 1.95 ± 0.05 to 5.09 ± 0.10), average chain length (ACL; 28.36 ± 0.02 to 28.97 ± 0.01), proportion of aquatic submerged plants and terrestrial plant inputs (Paq; 0.295 ± 0.003 to 0.377 ± 0.002), terrigenous aquatic ratio (TAR; 3.43 ± 0.16 to 7.99 ± 0.05), and n-alkane ratio (NAR; 0.169 ± 0.011 to 0.584 ± 0.011) values varied along the terrestrial-marine continuum. Large differences in the concentration weighted average (WA) δ¹³C and δ²H for odd and even n-alkanes were found, with WA δ¹³C ranging from -30.9 ± 0.3 to -33.4 ± 0.09 ‰ and -28.8 ± 0.01 to -32.3 ± 0.2 ‰, respectively, and 165.6 ± 3.6 to -200.8 ± 2.4 ‰ and -96.0 ± 2.8 to -158.7 ± 2.1 ‰ for δ²H. The diagnostic ratios were shown to misrepresent the input sources of organic matter (OM) and were inaccurate as source indicators when more than one OM source was present. With the addition of compound specific δ¹³C and δ²H analysis of n-alkanes, it was determined that the n-alkanes were predominantly derived from natural, rather than anthropogenic sources, with variations being driven by geographic changes in vegetation type and differing ratios of terrestrial and marine OM inputs. Importantly, compound specific isotope analysis of the even numbered n-alkanes would permit identification and tracking of petroleum-derived contaminants. Molecular data alone are ineffective for this, owing to the similarity in CPI values for petroleum-derived contaminants and highly degraded OM which is discharged by the St. Lawrence River into the estuary.
... Par ailleurs, Eglinton & Hamilton (1967) ont également mis en évidence un lien entre la longueur de chaîne des n-alcanes dans les cires cuticulaires des embryophytes et leur espèce. Ces travaux ont inspiré de nombreuses études (e.g., Barnes & Barnes, 1978 ;Kawamura & Ishiwatari, 1985 ;Ficken et al., 2000 ;He et al., 2020) montrer que les n-alcanes courts (moins de 21 atomes de carbone) dérivent principalement de cyanobactéries, tandis que les n-alcanes longs (plus de 27 atomes de carbone) sont principalement attribués aux embryophytes. Les n-alcanes de longueur de chaine intermédiaire (n-C 22 à n-C 26 ) peuvent dériver de diverses sources, incluant les cyanobactéries et les embryophytes, mais également les macrophytes aquatiques. ...
Thesis
Le lac Dziani Dzaha est un lac de maar salé, alcalin, stratifié, en partie euxinique et caractérisé par une forte productivité primaire cyanobactérienne. Par le passé, des environnements similaires ont révélé un fort potentiel à l’accumulation de matière organique (MO) sédimentaire, favorable à la formation de certaines roches-mères particulièrement recherchées par l’exploration pétrolière. Dans cette thèse, nous avons cherché à décrire le lac Dziani comme analogue de ces environnements et à caractériser les mécanismes et les processus propices à l’accumulation de MO sédimentaire dans ce type d’écosystèmes. Nous avons pour cela combiné des descriptions sédimentologiques et géochimiques d’une carotte représentative de l’histoire sédimentaire du lac, depuis peu après sa mise en place (il y a moins de 9000 ans) jusqu’à l’actuel. Des analyses globales (analyses Rock-Eval et élémentaires en C, N, S), isotopiques (δ13C, δ15N, δ18O, et une approche multi-phasique inédite du δ34S, Δ33S et Δ36S), et moléculaires de la MO et/ou des phases minérales associées ont permis de décrire plusieurs changements dans le fonctionnement biogéochimique du lac Dziani (e.g., structure de la colonne d’eau, nature des producteurs primaires, processus microbiens dominants, mécanismes de préservation de la MO impliquant le soufre) depuis son remplissage marin initial. Cette thèse apporte donc de nouveaux éléments de compréhension sur l'accumulation de MO sédimentaire en milieu salé alcalin, contrôlée notamment par des processus diagénétiques précoces où le soufre joue un rôle central, et sur les signatures géochimiques qui lui sont associées
... The character of OM can be assessed using a range of approaches, such as OM bulk properties (e.g., total carbon content and isotopic compositions) and the molecular compositions (e.g., Chmura and Aharon, 1995;Volkman et al., 2000;Drenzek et al., 2009;He et al., 2020). Biomarkers are of indispensable advantage in reconstructing the information of origin and transformation of OM due to their stability of chemical structure from the bio-precursor molecules, even after decomposition and diagenesis (Eglinton et al., 1964;Meyers, 2003). ...
Article
Transport and transformation of organic matter (OM) from the river to the marginal sea is a significant part of the global carbon cycle. Biomarkers are of indispensable advantage in precisely identifying the origin of OM that is crucial to understand the organic carbon cycle. Application of more biomarker molecules with mutually confirmable information commonly implies stricter constraint of the source but also brings challenges to the data analysis and interpretation due to a large amount of molecular information. Here we used random forest (RF) classification models to analyze 123 polar lipid biomarkers of six categories, including fatty alcohols, fatty acids, alkan-2-ones, steroids, triterpenoids, and 1-O-monoalkylglycerol ethers (MAGEs) from the sediments and soils in the Yellow River and the Bohai Sea of eastern China. The environmental specificity of biomarkers was assessed based on the effective distinguishment of samples from different habitats by RF models. The sources of polar lipid biomarkers were constrained according to their environmental specificity, and four genetic classifications, i.e., bacteria, algae and zooplankton, terrestrial higher plants, and anthropogenic input were identified. The spatial distribution of OM sources provides a reasonable scheme of the sink for biospheric OM in this typical "land-river-ocean" system. A type of MAGEs as the most important variables for the RF models was effectively used to be a potential bottom-water oxygen proxy to assess the preservation of OM, and ca. 37% of marine in-situ fresh OM was estimated to decompose under varying redox conditions in the surface sediments of Bohai Sea.
... Samples in the lower part of the stratigraphic level (between 0 and 300 m) of the Tista valley show a sharp decrease (65% to 11%) in HMW n-alkane abundance, suggesting a significant reduction in contribution from vascular land plants (Table S3). Notably, such a small amount of HMW n-alkane may be contributed by green algae or seagrasses, which proliferate in marginal marine conditions and produce several orders of magnitude less long-chain n-alkanes than vascular land plants (Chevalier et al., 2015;He et al., 2020;Liu et al., 2015). In addition, high P aq value (≥0.5) and dominance in mid-chain n-alkanes at the lowermost stratigraphic level (∼30 m) suggests significant OM contribution from the aquatic sources (Table S4; Figure 2a; Figure 3) (Cranwell et al., 1987;Ficken et al., 2000;Freeman & Colarusso, 2001). ...
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The final stages of evolution of the Himalayan foreland basin (HFB) are preserved in the Siwalik Group of rocks deposited by meandering and/or braided rivers in the western and central regions of HFB. However, the time-equivalent deposits in the eastern part of the foreland provide contradictory evidence of both terrestrial and marine environments. To address the ambiguity, molecular level characterization and stable isotopic composition of organic matter (OM) have been employed in the late Miocene-Pliocene sequence of the eastern HFB. The n-alkane distribution, carbon isotopic (13C) signature of n-alkanes and distribution of hopane and sterane isomers suggest OM contributions from both marine and terrestrial sources during the late Miocene period. Increase in short-chain n-alkane abundance and gammacerane index, low pristane/phytane ratio, higher 13C values, higher regular sterane/17-hopane, C31R/C30-hopane and C27/C29 steranes ratios and presence of C30 sterane provides substantial evidence of stratified anoxic conditions and marine influences at specific stratigraphic intervals. During the late Miocene period, mixing of marine OM sources with terrestrial sources argue for marginal marine depositional conditions amid fluvial-dominating environments. The entry of marine waters in the eastern HFB through the pre-existing cratonic troughs possibly resulted from eustatic or relative sea-level rise. No further evidence of marine incursions is observed in the younger Pliocene sediments. The higher detrital influx from the rising Himalayas, the onset of Northern Hemisphere Glaciation and evolution of physical barriers such as Shillong Massifs and Barind Tracts altogether led to the cessation of marine incursions into the HFB.
... We further obtained other n-alkane-based proxies, such as the total normal alkanes (TNA), C 27 /C 31 , carbon preference index (CPI) and the ''P-aqueous'' ratio (P aq ), which are usually applied to reconstruct the paleovegetation and paleoclimate . For example, the TNA was used to estimate or predict the variations in historical vegetation (He et al., 2020) and other organisms in sediments. The C 27 /C 31 was applied to infer the vegetation (tree/grass) changes (Ling et al., 2017), because nalkanes from grasses and woods are preferentially dominated by C 31 and C 27 (Cranwell, 1973;Cranwell et al., 1987), respectively. ...
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Understanding the relationship between historical vegetation and bacteria is critical for disentangling spatiotemporal variations in microbial communities. However, the utility of historical vegetation as indicated by the reconstruction proxies like n-alkanes to explain microbial succession has been understudied, especially regarding aquatic microbes living under contrasting climates. Here, we studied bacterial and n-alkane succession in sediment cores from Kusai Lake and Lugu Lake under contrasting climates, that is, the drier and colder climates and the warmer and wetter conditions, respectively, and further explored how bacterial communities are affected by historical vegetation. In both lakes, the Shannon diversity of bacteria and n-alkanes consistently and significantly (P
... Our results revealed no significant difference in ACL and CPI across seasons, except CPI between spring and autumn (Fig. 2). This observation is consistent with the recent surveys of modern plants on the central Chinese Loess Plateau and in the Florida Bay in the United States (He et al., 2020), which suggests that the effect of seasonality on leaf wax n-alkane distribution is minimal. No significant differences in δ 2 H wax and δ 13 C wax values were observed in the species analyzed in the present study across seasons (Fig. 3). ...
Article
Reconstructing paleoelevation allows the temporal evolution of biogeochemical processes and hydroclimate regimes to be understood and quantified. A dual-isotope biomarker of clumped hydrogen and carbon isotopes of leaf wax n-alkanes was recently proposed in humid tropical forests, and it was proven to be superior to a single-isotope proxy that was previously reported. However, it remains unknown whether the dual-isotope biomarker is suitable in arid conditions. The present study investigated leaf wax n-alkane distribution, hydrogen (δ²Hwax) and carbon (δ¹³Cwax) isotopes in terrestrial plants along an arid mountainous transect. We found that the effects of seasonality on n-alkane distribution, δ²Hwax and δ¹³Cwax were minimal for all species (p > 0.05), and that species-specific δ²Hwax values remained almost unchanged for most species, in contrast to δ¹³Cwax values. Significant correlations between altitude and δ²Hwax values (R² = 0.54, 0.58, and 0.75 for spring, summer, and autumn, respectively), instead of δ¹³Cwax values (R² = 0.08, 0.43, and 0.12 with p = 0.24, 0.01, and 0.19 for spring, summer, and autumn, respectively), were observed, suggesting that δ²Hwax values, but not δ¹³Cwax values, can be reliably used as a proxy for reconstructing paleoelevation in arid conditions. Therefore, it will be necessary to identify other proxies to supplement δ²Hwax values under a dual-isotope approach in future research.
... A close correlation occurs between leaf wax n-alkane δ 2 H values (δ 2 H wax ) from modern plants/sediments and precipitation δ 2 H values worldwide (Sachse et al., 2006(Sachse et al., , 2012Liu and Yang, 2008;Collins et al., 2013;Liu et al., 2016;He et al., 2020), which facilitates δ 2 H wax as a good indicator for paleo-environmental conditions such as precipitation and altitude (Pagani et al., 2006;Niedermeyer et al., 2010;Schefuβ et al., 2011;Seki et al., 2011;Zhang et al., 2017). Almost all of the above reports of δ 2 H wax values focused on the analysis of the entire leaf, regardless of intra-leaf δ 2 H wax variability. ...
Article
Several recent studies showed that leaf wax n-alkane δ²H values (δ²Hwax) within a leaf were heterogeneous in a small number of species. It still remains unclear whether the heterogeneity of intra-leaf δ²Hwax values is general for various species, how δ²Hwax values vary spatially and temporally, and whether there is a common explanation for the intra-leaf δ²Hwax heterogeneity in higher plants. Here we compared the hydrogen isotope compositions of leaf wax and corresponding leaf water (δ²Hlw) across leaf sections among a variety of monocot and dicot plant species. There is significant and consistent heterogeneity for both δ²Hwax and δ²Hlw, i.e., base-to-tip ²H-enrichment for monocots (except Hemerocallis citrina, and Dactylis glomerata) whereas base-to-tip and center-to-edge increases in δ²Hwax and δ²Hlw for dicots. The consistent occurrence of variations of δ²Hlw and δ²Hwax values within a leaf imply that δ²Hwax values probably inherit point-to-pint from in-situ δ²Hlw values, and thus the intra-leaf δ²Hwax heterogeneity mainly results from the spatial pattern of intra-leaf δ²Hlw values associated with veinal structures between dicots and monocots. The general heterogeneity of intra-leaf δ²Hwax values further intensifies that it is necessarily needed for in-depth understanding leaf wax biomarker.
... Table 4) related to the concentrations of mid molecular weight n-alkanes (n-C 21-25 ) and the second-largest peak of n-C 23 and n-C 25 in part of the samples (Fig. 6a, b) indicate that submerged macrophytes indeed contributed to the accumulated OM. Fig. 6a and b indicates that emergent aquatic plants (typically producing C 25 , C 27 or C 29 n-alkanes during diagenesis) and conifers (C 27 or C 29 n-alkanes) are relatively more important than other woody terrestrial vegetation and mangroves (typically producing C 29 , C 31 or C 33 n-alkanes; He et al. 2020). However, these conclusions drawn by the evaluation of GC-FID data should be taken with caution, since the n-alkane distribution is easily affected by maturity (Peters et al. 2005). ...
Article
A comprehensive geochemical and petrographic study was performed on 23 clay-rich shale samples collected from 18 wells drilled in the mid-western part of the Ordos Basin to investigate the characteristics and thermal maturity of organic matter as well as the paleo-depositional environment of the Chang 7 Member within the Triassic Yanchang Formation. The samples are characterized by moderate to very high organic carbon (TOC) contents ranging from 0.24% to more than 24.83% (2.72% on average) and total sulfur (TS) contents of 0.06% to 6.51%. Rock-Eval S1 and S2 values range from 0.13 to 3.70 mg HC/g rock and 0.62 to 92.50 mg HC/g rock, respectively, implying good to very good hydrocarbon generation potential for most of the samples. Rock-Eval data and petrographical observations reveal that the kerogen is mainly composed of type II to type III, consisting of a mixture of aquatic and terrigenous organic matter. The aliphatic fractions show variable monomodal to bimodal distributions of normal alkanes. Different parameters such as TOC/TS and molecular biomarker data imply that the Chang 7 member is a typical clay-rich source rock deposited in a sulphate-poor lacustrine to fluvio-deltaic environment under oxic to sub-oxic conditions with substantial input of organic matter derived from higher land plants, mainly conifers. Low values of gammacerane further indicate a low salinity of the paleolake. Average Tmax values of 445 ◦C, vitrinite reflectance values between 0.68% and 0.88%, biomarker parameters, such as sterane ratios of C29 ααα 20S/(20S + 20R) and C29 ββ/(ββ + αα), and maturity indices based on aromatic compounds, indicate a thermal maturity within the oil window for the studied samples, showing some regional variability within the mid-western Ordos Basin.
... Marine OM from organisms in the water column contains predominantly proteins and carbohydrates, whereas terrestrial components such as lignocellulose derived from vascular plants are generally more nitrogen-depleted (Hedges et al., 1997). It has been observed that marine OM is broadly more reactive than terrestrial OM (Prahl et al., 1997;Aller and Blair, 2004;Zhang et al., 2018;He et al., 2020) and thus remineralization of terrestrial OM is much less efficient than marine OM. ...
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Archaea are widespread in marine sediments and play important roles in the cycling of sedimentary organic carbon. However, factors controlling the distribution of archaea in marine sediments are not well understood. Here we investigated benthic archaeal communities over glacial-interglacial cycles in the northern South China Sea and evaluated their responses to sediment organic matter sources and inter-species interactions. Archaea in sediments deposited during the interglacial period Marine Isotope Stage (MIS) 1 (Holocene) were significantly different from those in sediments deposited in MIS 2 and MIS 3 of the Last Glacial Period when terrestrial input to the South China Sea was enhanced based on analysis of the long-chain n-alkane C31. The absolute archaeal 16S rRNA gene abundance in subsurface sediments was highest in MIS 2, coincident with high sedimentation rates and high concentrations of total organic carbon. Soil Crenarchaeotic Group (SCG; Nitrososphaerales) species, the most abundant ammonia-oxidizing archaea in soils, increased dramatically during MIS 2, likely reflecting transport of terrestrial archaea during glacial periods with high sedimentation rates. Co-occurrence network analyses indicated significant association of SCG archaea with benthic deep-sea microbes such as Bathyarchaeota and Thermoprofundales in MIS 2 and MIS 3, suggesting potential interactions among these archaeal groups. Meanwhile, Thermoprofundales abundance was positively correlated with total organic carbon (TOC), along with n-alkane C31 and sedimentation rate, indicating that Thermoprofundales may be particularly important in processing of organic carbon in deep-sea sediments. Collectively, these results demonstrate that the composition of heterotrophic benthic archaea in the South China Sea may be influenced by terrestrial organic input in tune with glacial-interglacial cycles, suggesting a plausible link between global climate change and microbial population dynamics in deep-sea marine sediments.
... Surrogate was not spiked before extraction for each sample, but mean recoveries of the compounds added into typical estuarine samples were > 85% for C 36 n-alkane-d10. According to triplicate analyses of multiple typical rivers and estuarine sediment samples, the standard deviations of most biomarker compound concentrations were < 15% (He et al. 2014(He et al. , 2018b(He et al. , 2020. Reported concentrations of each compound in this study were considered semi-quantitative for not being recovery-corrected. ...
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Purpose: Variations in hydrocarbon composition and concentration are essential for the assessment of its source and environmental health in estuarine environments. This study aims to reveal the spatial and temporal distribution of multiple hydrocarbons in surface sediments of the Pearl River Estuary, and to further determine various sources of these hydrocarbons. Materials and methods: Surface sediments were taken monthly from four sites along a spatial transect from the Pearl River Estuary in a whole year by using a gravitational bottom sampler. A series of bulk parameters including the grain size, total organic carbon content, and total nitrogen content were measured. Hydrocarbons in sediments were extracted and analyzed by the gas chromatography-mass spectrometry. Results and discussion: The abundances of higher plants and phytoplankton-derived n-alkanes, petrogenically derived pristane, phytane, hopanes and steranes with geological configuration, anthropogenically derived hydrocarbons, including linear alkylbenzenes, polycyclic aromatic hydrocarbons all showed decreasing trends towards the coastal ocean. In contrast, the diatom-derived C25 highly branched isoprenoids increased towards saline sites. Monthly fluctuations of aliphatic and aromatic hydrocarbons at the estuarine mouth were smaller than the upstream freshwater sites, where high monthly variation (up to 5 folds) was observed for most of the hydrocarbons detected. Conclusions: This study showed substantial monthly and spatial variations of hydrocarbons in the surface sediments of the Pearl River Estuary. Distribution and composition of sedimentary hydrocarbons indicate mainly anthropogenic inputs (e.g., petroleum residues and synthetic detergents) and terrestrially higher plants, followed by aquatic inputs.
Article
Compound-specific δ²H values of leaf wax n-alkanes are increasingly being used to infer past hydroclimates. However, differences in n-alkane production and apparent fractionation factors (εapp) among different plant groups complicate the relationships between n-alkane δ²H values and those of environmental water. Mid- and long-chain n-alkanes in sedimentary archives (i.e., n-C23 and n-C29) are thought to derive from aquatic and terrestrial plants, respectively, and track the isotopic composition of either lake water or precipitation. Yet, the relationship between n-C23 δ²H values and lake water δ²H values is not well constrained. Moreover, recent studies show that n-alkane production is greater in terrestrial plants than in aquatic plants, which has the potential to obscure n-alkane aquatic inputs to sedimentary archives. Here, we investigated n-alkane contributions to sedimentary archives from both aquatic and terrestrial plants by analyzing n-alkane δ²H values in plants and lake sediments at 29 sites across mid-latitude North America. We find that both aquatic and terrestrial plants synthesize n-C23 and that sedimentary n-C23 δ²H values parallel those of terrestrial plants and differ from those of aquatic plants. Our results indicate that across mid-latitude North America, both mid- and long-chain n-alkanes in lake sediments commonly derive from terrestrial higher plants challenging the assumption that submerged aquatic plants produce the n-C23-alkane preserved in lake sediments. Moreover, angiosperm and gymnosperm plants exhibit similar εapp values between n-C29 and mean annual precipitation (MAP) δ²H values across North America. Therefore, vegetation shifts between angiosperm and gymnosperm plants do not strongly affect εapp values between n-C29 and MAP. Our results show that both mid- and long-chain n-alkanes track the isotopic composition of MAP in temperate North America.
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The ability of tidal marshes to maintain their elevation despite rising sea levels depends on the accumulation of organic matter (OM) from estuarine particulate matter and in situ marsh plant production. Although previous studies have examined carbon in marshes, few have used the source composition of OM in marsh soils to provide insight into the processes controlling the delivery, transformation, and fate of carbon within marshes. This study used multiple geochemical tools (i.e., n-alkanes, fatty acids (FA), sterols, and stable carbon and nitrogen isotopes) to characterize the sources of OM in surface (0–10 cm) and subsurface (30–40 cm) soils collected along transects from the marsh edge to the low marsh/high marsh transition in tidal marshes around the Chesapeake Bay. Four study sites were selected for their different physical settings and soil types, allowing us to characterize the dominant OM sources in the soils and identify factors that contributed to differences in marsh OM composition. Contributions of OM from estuarine (i.e., short-chain FA, brassicasterol, cholesterol), marsh plant (i.e., long-chain FA, mid- and long-chain n-alkanes, sitosterol, taraxerol), and microbial (i.e., branched fatty acids, ergosterol) sources were identified in all soils. Results from a stable isotope mixing model allowed us to quantify the relative contributions of these OM sources to the marsh soils and indicated that soil OM was comprised of 29.0 ± 9.0% estuarine POM, 22.7 ± 5.5% riverine POM, 22.1 ± 11.3% C3 marsh plant OM, and 37.9 ± 13.8% C4 marsh plant OM. Relative contributions from these sources varied depending on location in the marsh with estuarine OM contributing a greater fraction of OM near the marsh edge and to surface soils while marsh plants contributed larger amounts of OM to soils in the marsh interior and in the sub-surface. Our results suggest that long-term carbon sequestration in marsh soils in controlled by the marsh plant community, and that changes to a marsh's physical setting or plant community in response to changing climate or human activity could alter the sequestration of carbon in marshes.
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The hydrogen isotope composition (δD) of paleoclimatic archives is increasingly being used as a proxy for the reconstruction of Quaternary hydroclimatic variability, but the factors controlling δD in arid/semi-arid regions remain poorly understood. We compared the hydrogen isotope composition of individual n-alkanes in the surface sediments of a series of lakes and puddles in Tibet with local meteorological and hydrological data. The results show that the δD of short-chain and long-chain n-alkanes is sensitive to hydroclimatic conditions, with δDp being the predominant control. δD becomes less depleted where precipitation amount and humidity are lower or evaporation is stronger. In addition, temperature is a significant factor affecting δD of mid-chain n-alkanes, with a significant negative correlation, and the temperature in spring, autumn and winter is an important influence on it. We believe that the δD of long-chain n-alkanes is still a good paleohydrological index in the arid/semi-arid region; δD indices of mid-chain and short-chain n-alkanes are also useful, and more mechanism research is needed.
Article
Leaf wax n-alkane compositions have been widely applied to reconstruct paleoclimate histories in peat deposits, yet understanding of how the n-alkanes vary during seasonal plant growth remains limited. Here we report variations in the molecular and wax-derived n-alkane hydrogen isotope (δ2Halk) in the three dominant vascular plant species (Sanguisorba officinalis, Carex argyi, Euphorbia esula) and surface peat deposits nearby from the Dajiuhu peatland over a growing season. All three species show a relatively high carbon preference index (CPI) in the beginning of the growing season, with the CPI values reaching as high as 50 in two of the three species. Two species (S. officinalis, E. esula) display relatively stable average chain length (ACL) values over the four sampling intervals, with standard derivations of 0.2–0.3. In contrast, C. argyi exhibits a significant fluctuation of ACL values (averaging 28.1 ± 1.4) over the growing season. The δ2Halk in all three species decreased during leaf growth. In the final stage of growth, the δ2Halk values of the three species are similar to those in the surface peats collected from the peatland. Combining the results of our measurements of alkane concentration and δ2H values, it is likely that de novo synthesis of leaf wax n-alkanes in the peat-forming plant species is mainly at the early stage of leaf development. In the following months, the removal process exceeds renewal, resulting in a general decrease of the concentration of the total n-alkanes and the integrated δ2Halk values. Thus the δ2Halk values probably integrate the environmental variations at the end of the plant growth period rather than the whole period or the early growth period. These results are significant and have the potential to improve the utility of δ2Halk values in paleoenvironmental reconstructions.
Article
Leaf wax n-alkanes in lacustrine sequences have been widely applied for reconstructions of Late Quaternary paleoclimate histories. Such applications depend on knowledge of the factors controlling the characterization of the n-alkane paleoproxies of aquatic plants. This study analyzed the molecular distributions and carbon and hydrogen isotopic compositions of n-alkanes of aquatic plant samples collected from the middle and lower reaches of the Yangtze River (MLYR). In this collection, the n-alkane distributions of submersed/floating plants and emersed plants are distinctly different. By combining their Paq and n-alkane δ¹³C values, the n-alkane contributions from submersed plants and emersed plants in the MLYR can be differentiated. It is also noteworthy that the hydrogen isotope fractionation (εalk/water; avg. −166‰) between the n-C23 alkane of submersed plants in the MLYR and in precipitation agrees well with previous studies, suggesting a nearly constant εalk/water for the submersed plants. Moreover, compared to C3 dicots growing in a similar climate, submersed plants in the humid MLYR have lower n-alkane δ²H values. This feature may arise from isotopic differences in the evapotranspiration of lake water versus leaf water and in the biosynthetic fractionations of leaf waxes of these two plant groups. These results aid interpretations of the hydrogen isotope offset of the δ²H values of n-alkanes between aquatic and terrestrial plants in humid climates.
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Continental margin seas are the main sites for deposition and burial of organic carbon from different sources with different properties, and they play an important role in the global biogeochemical carbon cycle. The inner shelf of the East China Sea (ECS) accepts a large amount of terrestrial organic carbon with high primary productivity and is an ideal site for the study of the source, transport and burial of sedimentary organic carbon. This comprehensive analysis of the literature gives a brief review of related sedimentological studies, with the aim of providing references for future research. Bulk analyses (TOC/TN, δ13C, etc.) and biomarkers (n-alkanes, sterols, lignin, etc.), among other methods, have indicated that the source of organic carbon in this location is affected by the sedimentary environment. The terrestrial components increase significantly landward from the sea, and have seasonal macroscopic transport characteristics best described as “storing in Summer and transporting in Winter” along the coast. This dynamic process affects the coastal transport path and final destination of terrestrial organic carbon, producing a lingulate, or tongue-shaped, cross-shelf zone of transported terrigenous organic carbon near latitude 29°N in the ECS. This may be accompanied by cross-shelf transport of terrestrial organic carbon, affecting the source-to-sink process of deep-sea organic carbon. In addition, human activity and extreme climatic events have also significantly affected the records of the deposition processes of the sediment and organic carbon; such factors need further study. The inner shelf of the ECS is an important area of buried terrigenous organic carbon. Its burial efficiency has been influenced by the source, the content and the sedimentation rate, and is related to the mineralization path of the organic matter during the early diagenetic process. The geochemical characteristics of the organic carbon contained in the sediments are also useful for reconstructing long-term sea-level change, primary productivity, paleoceanography and paleoclimate evolution, thus providing a basis for clarifying the environmental evolution of the ECS inner shelf during its geological history.
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Wetland ecosystems are often characterized by self-organized landscape patterning, driven by abiotic and biotic factors. In the Florida Everglades, natural sheet flow is hypothesized to have distributed sediments to form the pattern of linear emergent ridges and submerged sloughs. Drainage and barriers to flow have degraded these microtopographic features. As part of the Comprehensive Everglades Restoration Plan, the Decompartmentalization Physical Model is a landscape-scale experiment to evaluate ecosystem responses to restored sheet flow by increasing freshwater inputs and removing barriers to flow. To test the proposed mechanism that flow rebuilds ridge-slough microtopography by remobilizing slough sediments into ridges, four molecular markers capable of distinguishing ridge, slough, and microbial sources were evaluated in flocculent benthic sediments (floc) and advected sediments (collected in traps) during preflow, high-flow, and postflow conditions over 4 years. The combined use of the four biomarkers, namely, the aquatic proxy (Paq), C20 highly branched isoprenoids, kaurenes, and botryococcenes, showed compositional patterns that clearly distinguished ridge and slough organic matter. Of these molecular parameters, the Paq was the most reliable in distinguishing among organic matter sources. Long-term patterns in floc Paq at ridge and slough sites indicate a general increase, indicative of preferential mobilization of slough material. The Paq values for advected sediments are also strongly associated with slough environments, supporting temporal trends in floc samples. Our results tentatively confirm the hypothesis that increased flow in degraded ridge-and-slough wetlands, and associated sediment transport, is a potentially viable mechanism to restore historic patterns of microtopography.
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Hydrogen and carbon isotope values (δD & δ¹³C) were measured on lipid biomarkers from a sediment core collected in Lake Tulane, Florida, USA, to infer shifts in climate and hydrologic variables during the Last Glacial. Isotopic trends from 24 samples correlate with plant community shifts evaluated in a previous pollen study by Grimm et al. (2006). We observe maxima in Δleaf values and minima in δD values concurrent with peaks in Pinus pollen abundances and Heinrich Events 4-2. Increased Δleaf values during North Atlantic cold spells indicate lower water-use-efficiency among angiosperms around Lake Tulane. Combined δD values from terrestrial and aquatic lipids, confirm that aridity decreased during cold, stadial periods (Heinrich Events), and increased during warm, interstadials. Furthermore, lower δD values in aquatic lipids during stadials are attributed to warming, as well as changing moisture sources. The anti-phase relationship between temperatures and aridity derived from our subtropical lacustrine record and those at high latitude in the North Atlantic is likely the result of complex ocean-atmosphere teleconnections that resulted from the collapse of Atlantic Meridional Overturning Circulation during Heinrich Events in the North Atlantic.
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The hydrogen isotope values of n-alkanes (δ2Hn-alkane) reflect a plant's water source and water relations, while the carbon isotope values (δ13Cn-alkane) relate to a plant's carbon metabolism and response to environmental conditions. However, the isotopic dynamics of the transition from heterotrophic to autotrophic metabolism during foliar development on δ2Hn-alkane and δ13Cn-alkane remain unclear. Here, we monitored δ2Hn-alkane and δ13Cn-alkane across a growing season from Betula occidentalis, Populus angustifolia, and Acer negundo. In addition, we compiled δ2H values of atmospheric vapor, leaf water, xylem water, and stream water as well as δ13C values of bulk leaf tissue (δ13Cbulk). We found δ2Hn-alkane and δ13Cn-alkane varied with leaf development and indicated that the majority of wax development occurred during the initial growing season. The patterns in δ2Hn-alkane were broadly consistent between species and with previous studies; however, each species had a unique final δ2Hn-alkane value. The δ13Cbulk for all species demonstrated a characteristic 13C-enrichment during the initial growing season, followed by 13C-depletion, while δ13Cn-alkane did not exhibit a consistent trend between the species. These δ13C data suggested a decoupling of the isotopic inputs between n-alkanes and photosynthetic leaf tissue. When coupled with δ2Hn-alkane, these data suggested that the precursor compounds utilized in initial production of n-alkanes might be variable and possibly indicated that the stored precursors used for initial leaf tissue and wax production originated from different sources. Nonetheless, these data indicated that the isotopic signatures of n-alkanes relate to a mixture of precursors, but only during a distinct period of leaf ontogeny.
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Here we report the molecular biomarker co-occurrence of three different races of Botryococcus braunii (B. braunii) in the freshwater wetland ecosystem of the Florida Everglades, USA. Thespecific biomarkers include C32-C34 botryococcenes for race B, C27-C32 n-alkadienes and n-alkatrienes for race A, and lycopadiene for race L. The n-alkadienes and n-alkatrienes were present up to 3.1 and 69.5 µg/g dry weight (dw), while lycopadiene was detected in lower amounts up to 3.0 and 1.5 µg/g dw in periphyton and floc samples, respectively. Nutrient concentrations (P and N) did not significantly correlate with the abundances of these compounds. In contrast, n-alkadienes and n-alkatrienes were present in wider diversity and higher abundance in the floc from slough (deeper water and longer hydroperiod) than ridge (shallower water and shorter hydroperiod) locations. n-Alkadienes, n-alkatrienes, and lycopadiene, showed lower δ13C values from -40.0 to -35.5‰, suggesting that the source organisms B. braunii at least partially utilize recycled CO2 (13C depleted) produced from OM respiration rather than atmospheric CO2 (13C enriched) as the major carbon sources.
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The application of lipids in soils as molecular proxies, also often referred to as biomarkers, has dramatically increased in the last decades. Applications range from inferring changes in past vegetation composition, climate and/or human presence to unraveling input and turnover of soil organic matter (SOM). Molecules used include extractable and ester-bound lipids as well as their carbon or hydrogen isotopic composition. While holding great promise, the application of soil lipids as molecular proxies comes with several constraining factors the most important of which are: i) variability in the molecular composition of plant-derived organic matter plant-internally and in between plant individuals; ii) variability in (relative contribution of) input pathways into the soil; and iii) transformation and/or (selective) degradation of (some of) the molecules once present in the soil. Unfortunately, the information about such constraining factors and their impact on the applicability of molecular proxies is fragmented and scattered. The purpose of this study is to provide a critical review of the current state of knowledge with respect to the applicability of molecular proxies in soil science, specifically focusing on the factors constraining such applicability. Variability in genetic, ontogenetic and environmental factors influence plant n-alkane patterns in the way that no unique compounds or specific molecular proxies pointing to e.g. plant-community differences or environmental influences, exist. Other components such as n-alcohols, n-fatty acids, cutin- and suberin-derived monomers have received far less attention in this respect. Furthermore, there is a high diversity of input pathways offering both opportunities and limitations for the use of molecular proxies at the same time. New modelling approaches might offer a possibility to unravel such mixed input signals. Finally, transformation and turnover of SOM offer opportunities when tracing such processes is the purpose of applying a molecular proxy, whilst posing limitations when they obliterate molecular proxy signals linked to other phenomena. For n-alkanes several modelling approaches have recently been developed to compensate for (selective) degradation. Still such techniques are in their infancy and information about their applicability to other classes of components than n-alkanes is lacking yet. All constraining factors considered can have a significant influence on the applicability of molecular proxies in soil science. The degree of influence strongly depends on the type of molecular proxy as well as the environmental context in which it is applied. However, the potential impact of the constraining factors should always explicitly be addressed whenever molecular proxies are applied in a soil scientific context. More importantly, there is still a serious lack of available information in particular for compound classes other than the n-alkanes. Therefore, we urgently call for the consideration of more holistic approaches determining various parameters during sampling as well as using as many compound classes as possible.
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Significance The equatorial Pacific is centrally important in Earth’s climate system. Changes there cause disruptions to global economies and food and water security. Yet, projected tropical precipitation changes in response to higher greenhouse gas concentrations remain uncertain, due in part to a scarcity of paleoclimate records to validate models. We therefore applied a new method in Galápagos lakes for reconstructing rainfall and salinity using sedimentary lipids from microalgae and mangrove trees. Our data revise the understanding of changes over the Common Era to provide a spatially and temporally coherent view of tropical Pacific rainfall. We separate convergence zone from El Niño-driven changes and observe substantial fluctuations even in the absence of perturbations as large as current anthropogenic forcing.
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A series of iso- and anteiso-monomethylalkanes (MMAs) with carbon numbers from C23 to C35 and C14 to C34, respectively, were detected in Avicennia germinans. These compounds were present in varying amounts up to 54.1, 1.0 and 3.4 µg/g dw in leaves, bark, and the crustose lichens attached on the bark of A. germinans, respectively. These MMAs were not detected in leaf waxes of Rhizophora mangle and Laguncularia racemosa, but were detected in significantly lower abundances (2 ~ 6% of that in A. germinans leaf wax) in bark and lichen of R. mangle. Significant odd-carbon number distributions and even-carbon number distributions were observed for long chain (C ≥ 25) iso- (maximizing at C31) and anteiso-MMAs (maximizing at C32), respectively, in A. germinans leaf wax. However, no obvious carbon number preferences were detected for bark and lichen. The long chain (LC) iso- and anteiso-MMAs in A. germinans leaf waxes were found to be enriched in 13C by 0.3 ~ 4.3‰ and 0.7 ~ 4.2‰ compared to the n-alkanes with same carbon numbers, respectively, across the salinity gradient of 19.7 ~ 32.0 PSU. In comparison, the LC iso- and anteiso- MMAs were found to be more depleted in D by 6.1 ~ 55.1‰ and 7.3 ~ 57.0‰ compared to the n-alkanes with same carbon numbers, respectively. The results imply that A. germinans could be another important source of iso- and anteiso-alkanes in sediments and soils, and that these compounds could potentially be used as biomarkers for this species in mixed mangrove environments.
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Peatlands form important landscape elements in many parts of the world and play significant roles for biodiversity and global carbon balance. This new edition has been revised and updated, documenting the latest advances in areas such as microbial processes and relations between biological processes and hydrology. As well as referencing the latest research, this book exposes a rich older literature where an immense repository of natural history has accumulated. It starts with an overview of the main peatland types before examining the entire range of biota present, together with their specific adaptations to peatland habitats. Detailed coverage is devoted to the genus Sphagnum, the most important functional plant group in northern peatlands, although tropical and southern hemisphere peatlands are also covered. Throughout the book the interactions between organisms and environmental conditions (especially wetness, availability of oxygen, and pH) are emphasised, with chapters on the physical and chemical characteristics of peat, the role of peat as an archive of past vegetation and climate, and peatland succession and development. Several other key factors and processes are then discussed, including hydrology and nutrient cycling. The fascinating peatland landforms in different parts of the world are described, together with theories on how they have developed. Human interactions with peatlands are considered in terms of management, conservation, and restoration. A final chapter focuses on the role of peatlands as sources or sinks for the greenhouse gases carbon dioxide and methane, and the influences of climate change on peatlands.
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Hydrogen isotope ratios (2H/1H or δ2H) of plant leaf waxes typically covary with those of precipitation, and are therefore used as a proxy for past hydrologic variability. Mangroves present an important exception to this relationship, as salinity can strongly influence 2H fractionation in leaf lipids. To better understand and calibrate this effect, δ2H values of taraxerol and n-alkanes were measured in the leaves of Rhizophora spp. (red mangroves) from three estuaries and four brackish lakes on the Micronesian islands of Pohnpei and Palau, and compared to the δ2H and δ18O values of leaf water, xylem water and surface water. Net 2H discrimination between surface water and taraxerol increased by 0.9 ± 0.2‰ per part per thousand (ppt−1) over a salinity range of 1–34 ppt. Xylem water was always depleted in 2H relative to surface water, and the magnitude of this depletion increased with salinity, which is most likely due to a combination of greater 2H discrimination by roots during water uptake and opportunistic use of freshwater. Changes in the 2H content of xylem water can account for up to 43% of the change in net taraxerol fractionation with salinity. Leaf water isotopes were minimally enriched relative to xylem water and there was not significant variability in leaf water enrichment with salinity, which is consistent with a Péclet-modified Craig–Gordon model of leaf water enrichment. As leaf water enrichment is therefore unlikely to be responsible for increased 2H/1H fractionation in mangrove leaf lipids at elevated salinities, the majority of this signal is most likely explained either by changes in biosynthetic fractionation in response to salt stress or by salinity influenced changes in the timing of water uptake and lipid synthesis.
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Hydrogen isotopic ratios of terrestrial plant leaf waxes (δD) have been widely used for paleoclimate reconstructions. However, underlying controls for the observed large variations in leaf wax δD values in different terrestrial vascular plants are still poorly understood, hampering quantitative paleoclimate interpretation. Here we report plant leaf wax and source water δD values from 102 plant species grown in a common environment (New York Botanic Garden), chosen to represent all the major lineages of terrestrial vascular plants and multiple origins of common plant growth forms. We found that leaf wax hydrogen isotope fractionation relative to plant source water is best explained by membership in particular lineages, rather than by growth forms as previously suggested. Monocots, and in particular one clade of grasses, display consistently greater hydrogen isotopic fractionation than all other vascular plants, whereas lycopods, representing the earlier-diverging vascular plant lineage, display the smallest fractionation. Data from greenhouse experiments and field samples suggest that the changing leaf wax hydrogen isotopic fractionation in different terrestrial vascular plants may be related to different strategies in allocating photosynthetic substrates for metabolic and biosynthetic functions, and potential leaf water isotopic differences.
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A litterbag method was used for studying the variability in chemical and carbon isotopic compositions of four grasses during litter decomposition. After the 300 d degradation, > 90% of litter mass was lost for three C4 species (Setaria viridis, Eleusine indica, Amaranthus retroflexus) and one C3 species (Erigeron speciosus). The solid state 13C NMR spectra showed that mean proportion of aromatic and alkyl carbon increased from ca. 10% to 15% and ca. 10% to 20%, respectively, whereas that of O-alkyl carbon substantially decreased from ca. 70% to 50%. The carbon preference index and average chain length of n-alkanes remained relatively constant, whereas the carbon isotopic compositions of long chain n-alkanes varied < 2‰. Our results demonstrate that the degradation of litters alone does not significantly change the n-alkane chemical and carbon isotopic proxies. Compared to open plant-soil systems, our litterbag experiments present much less variability in chemical and carbon isotopic compositions of n-alkanes. Based on these facts, we recommend a combined measurement of chemical and carbon isotopic properties in evaluation of carbon sources, dynamics and paleoenvironments.
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Long chain (C21 to C37) n-alkanes are among the most long-lived and widely utilized terrestrial plant biomarkers. Dozens of studies have examined the range and variation of n-alkane chain-length abundances in modern plants from around the world, and n-alkane distributions have been used for a variety of purposes in paleoclimatology and paleoecology as well as chemotaxonomy. However, most of the paleoecological applications of n-alkane distributions have been based on a narrow set of modern data that cannot address intra- and inter-plant variability. Here, we present the results of a study using trees from near Chicago, IL, USA, as well as a meta-analysis of published data on modern plant n-alkane distributions. First, we test the conformity of n-alkane distributions in mature leaves across the canopy of 38 individual plants from 24 species as well as across a single growing season and find no significant differences for either canopy position or time of leaf collection. Second, we compile 2093 observations from 86 sources, including the new data here, to examine the generalities of n-alkane parameters such as carbon preference index (CPI), average chain length (ACL), and chain-length ratios for different plant groups. We show that angiosperms generally produce more n-alkanes than do gymnosperms, supporting previous observations, and furthermore that CPI values show such variation in modern plants that it is prudent to discard the use of CPI as a quantitative indicator of n-alkane degradation in sediments. We also test the hypotheses that certain n-alkane chain lengths predominate in and therefore can be representative of particular plant groups, namely, C23 and C25 in Sphagnum mosses, C27 and C29 in woody plants, and C31 in graminoids (grasses). We find that chain-length distributions are highly variable within plant groups, such that chemotaxonomic distinctions between grasses and woody plants are difficult to make based on n-alkane abundances. In contrast, Sphagnum mosses are marked by their predominance of C23 and C25, chain lengths which are largely absent in terrestrial vascular plants. The results here support the use of C23 as a robust proxy for Sphagnum mosses in paleoecological studies, but not the use of C27, C29, and C31 to separate graminoids and woody plants from one another, as both groups produce highly variable but significant amounts of all three chain lengths. In Africa, C33 and C35 chain lengths appear to distinguish graminoids from some woody plants, but this may be a reflection of the differences in rainforest and savanna environments. Indeed, variation in the abundances of long n-alkane chain lengths may be responding in part to local environmental conditions, and this calls for a more directed examination of the effects of temperature and aridity on plant n-alkane distributions in natural environments.
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Leaf waxes (i.e., n-alkyl lipids or n-alkanes) are land-plant biomarkers widely used to reconstruct changes in climate and the carbon isotopic composition of the atmosphere. There is little information available, however, on how the production of leaf waxes by different kinds of plants might influence the abundance and isotopic composition of n-alkanes in sedimentary archives. This lack of information increases uncertainty in interpreting n-alkyl lipid abundance and δ13C signals in ancient settings. We provide here n-alkyl abundance distributions and carbon isotope fractionation data for deciduous and evergreen angiosperm and gymnosperm leaves from 46 tree species, representing 24 families. n-Alkane abundances are significantly higher in angiosperms than gymnosperms; many of the gymnosperm species investigated did not produce any n-alkanes. On average, deciduous angiosperms produce 200 times more n-alkanes than deciduous gymnosperms. Although differences between angiosperms and gymnosperms dominate the variance in n-alkane abundance, leaf life-span is also important, with higher n-alkane abundances in longer-lived leaves. n-Alkanol abundances covary with n-alkanes, but n-alkanoic acids have similar abundances across all plant groups. Isotopic fractionation between leaf tissue and individual alkanes (εlipid) varies by as much as 10‰ among different chain lengths. Overall, εlipid values are slightly lower (−4.5‰) for angiosperm than for gymnosperm (−2.5‰) n-alkanes. Angiosperms commonly express slightly higher Δleaf (photosynthetic discrimination) relative to gymnosperms under similar growth conditions. As a result, angiosperm n-alkanes are expected to be generally 3–5‰ more depleted in 13C relative to gymnosperm alkanes for the same locality. Differences in n-alkane production indicate the biomarker record will largely (but not exclusively) reflect angiosperms if both groups were present, and also that evergreen plants will likely be overrepresented compared with deciduous ones. We apply our modern lipid abundance patterns and εlipid results to constrain the magnitude of the carbon isotope excursion (CIE) at the onset of the Paleocene–Eocene Thermal Maximum (55.8Ma). When Bighorn Basin (WY) sediment n-alkanes are interpreted in context of floral changes and modern n-alkane production estimates for angiosperms and gymnosperms, the CIE is greater in magnitude (−5.6‰) by ∼1‰ compared to previous estimates that do not take into account n-alkane production.
Book
The fourth edition is a major expansion and revision of the previous edition. New material includes coverage of the various computer models used in Everglades science, incorporated into a revised Chapter 21, which is now called "Synthesis: Ecological Relationships, Processes, and Models for the Everglades." Also included is coverage of four major improvements in our understanding of the Everglades: 1) the role of water flow, 2) the origin and evolution of the larger tree islands, 3) the role of sulfur in the biogeochemistry of mercury, and 4) quantified economic benefits of restoration. Updates and extensive improvements of maps are included throughout as well as incorporation of new Everglades science literature.
Article
Sedimentary plant wax distributions and isotopic compositions are powerful, widely applied paleoenvironmental proxies. However, there is conflicting evidence on the behavior of these proxies at high-latitude sites, where extreme climate and light conditions may uniquely influence plant physiology and growth. Here, we present modern sedimentary n-alkane and n-alkanoic acid abundances and compound-specific (δ²H and δ¹³C) isotope values from a 22-lake transect extending from northwest to southernmost Greenland, covering a large latitudinal and climatic gradient. Sedimentary plant waxes are similar in abundance and carbon isotopic composition across the transect, suggesting no major differences in biologic sources. There are strong correlations (r = 0.8–0.9) between δ²H values of many long-chain sedimentary waxes and those of modelled precipitation, with n-alkanes more tightly correlated to precipitation than n-alkanoic acids. Data presented here also demonstrate that δ²H values of mid-chain sedimentary waxes do not strongly correlate to the δ²H values of lake water when it decouples isotopically from precipitation (i.e. in glacier-fed and evaporatively-enriched lakes). This calls into question the common interpretation that mid-chain sedimentary waxes can be ascribed to aquatic plants. We contextualize our Greenland data with an updated global dataset of δ²H values of modern sedimentary waxes and precipitation. This update adds 100 + lakes from recently published literature to the seminal review presented by Sachse et al. (2012). This large new compilation suggests a global average apparent fractionation including Arctic data between n-C28 alkanoic acids and annual precipitation (εC28/ANN) of −99‰, and between n-C29 alkanes and annual precipitation (εC29/ANN) of −121‰. The latter value is remarkably consistent with the value first reported by Sachse et al. (2012).
Article
The origin of mid-chain (C23 and C25) and C27n-alkanes in sedimentary archives can be ambiguous. In coastal Portugal, a peat deposit representing a mid-Holocene paleowetland (PRC-South) that was initially dominated by Sphagnum presents an ideal setting for assessing methods of mid-chain n-alkane source elucidation. In particular, we examine a comprehensive record of the difference in δD between mid-chain (δDmid) and C29n-alkanes (ΔDmid-C29) in relation to n-alkane molecular distributions and compound-specific δ¹³C values in order to improve interpretations of paleoenvironmental change and evaluate the reliability of δDmid values for hydrologic reconstructions. Mid-chain n-alkane production remains significant at PRC-South after a substantial Sphagnum decline, yet an increase in ΔDmid-C29 values indicates a primarily terrestrial origin of mid-chain n-alkanes in the upper 50 cm of the deposit. This progression towards more positive ΔDmid-C29 values coincides with a relative increase in n-C27 abundance and potentially represents the replacement of Sphagnum moss by tree species that produce abundant mid-chain n-alkanes, such as Betula, Quercus, or Fagus spp. According to our analysis, ΔDmid-C29 displays promise as a tool for mid-chain n-alkane source attribution in Sphagnum paleoenvironments, but interspecies variability of hydrogen isotope fractionation in Sphagnum mosses and terrestrial vegetation could critically hinder its application. Our use of ΔDmid-C29 at PRC-South ultimately exemplifies the importance of accounting for vegetation composition when qualitatively or quantitatively interpreting sedimentary δDn-alkane values, particularly in Sphagnum wetlands.
Article
Sedimentary δDn-alkane values have been widely used as a valuable proxy for paleoenvironmental reconstruction. A number of studies have focused on δDn-alkane values that derived exclusively from leaves, while less attention has been paid to the root-derived n-alkanes and their impact on sedimentary δDn-alkane values. In this study, we sampled modern plant leaf and root materials from different growth contexts (slopes and seasons) on the Chinese Loess Plateau to compare leaf-derived n-alkanes with root-derived n-alkanes. Our results demonstrated that total n-alkane (C27–C33) concentrations varied substantially between leaf and root materials, with average values of 209 and 29.5 μg/g observed in leaves and roots respectively. The results suggest that ca. 12% of the n-alkane concentrations in sediments may be derived from plant roots. Furthermore, leaf-derived δDn-alkane values for Stipa bungeana (grass), Artemisisa vestita (shrub) and Bothriochloa ischaemum (grass) averaged −184‰, −152‰ and −198‰, compared with their root-derived δDn-alkane values of −199‰, −179‰ and −163‰, respectively. These statistically significant differences in concentrations and δD values between leaf-derived and root-derived n-alkanes suggest that the contribution of n-alkanes derived from plant roots is important for evaluating the resultant n-alkane compositions of sediments for paleoenvironmental reconstruction. Our findings indicates that the effects of root-derived n-alkanes on total sedimentary δDn-alkane values should be considered carefully in future paleoenvironmental reconstruction efforts.
Article
The hydrogen isotope compositions (δD) of n-alkanes and fatty acids (FAs) are widely applied in palaeoclimatic reconstructions, and the determinations of their hydrogen isotope fractionation factor values (ε) are vital for quantitatively reconstructing past precipitation variations. Currently, studies on n-alkane and FA ε values focus on terrestrial plants, which, however, show large uncertainties because of the influence of evapotranspiration. Therefore, in this study, we analysed the ε values of algae and submerged plants immersed in lakes, which are not affected by evapotranspiration, to understand the hydrogen isotope fractionation of plant lipid synthesis. By investigating the δD values of lipids (n-alkanes and FAs) in algae and submerged plants and the δD values of co-existing water (including lake bottom water, surface sediment water, and leaf water of algae and submerged plants) fromfive Tibetan Plateau lakes, we find that the n-alkane ε values of algae and submerged plants show narrow changes, ranging from−176 to−159‰and−167 to−142‰, respectively. The FA ε values of algae and submerged plants also show small variations, ranging from −160 to −121‰ (except Chara) and −161 to −138‰, respectively. Therefore, the average biosynthetic hydrogen isotope fractionation of these plants is −162‰for n-alkanes and−145‰for FAs, and the small ε differences between FAs and n-alkanes can be related to the different magnitudes of FA utilization in n-alkane synthesis. Finally,we find that the biosynthetic hydrogen isotope fractionation factors of aquatic plants are close to those of terrestrial grasses but slightly more negative than those of terrestrial woody plants. Thus, our results are helpful for understanding the hydrogen isotope fractionation variations in terrestrial plant lipids, which is beneficial for palaeohydrological reconstructions.
Article
Sedimentary plant waxes and their hydrogen and carbon (δ²H and δ¹³C) isotopes are important proxies for past hydrologic and vegetation change. However sedimentary waxes accumulate from diverse sources, integrating uncertainties from: (i) variable isotope fractionation among plant species, and (ii) unresolved processes controlling the transport of waxes from plants to sediments. We address these uncertainties by comparing the molecular and isotopic composition of n-alkanes and n-alkanoic acids in recent bog sediments with all major plant species growing in the catchment of Browns Lake Bog (BLB) in Ohio, USA. There are two distinct plant assemblages at BLB, including a forest dominated by trees and a bog shoreline composed of shrubs, woody groundcover, herbs and graminoids. n-Alkane concentrations in trees were 10–300 times higher than in shoreline plants, while n-alkanoic acid concentrations were generally lower and comparable across all species. The overall range of wax δ²H values among individual plants (77‰ for n-C29 alkane and 84‰ for n-C28 alkanoic acid) was likely driven by interspecies differences in biosynthetic δ²H fractionation as well as source water differences between forest and shoreline plants. A considerably smaller range of δ²Hwax values in the bog sediments (9‰ for n-C29 alkane and 11‰ for n-C28 alkanoic acid) suggests that sediments are either biased toward specific plants, or that signal averaging processes during or after deposition are constant. The combined δ²H and δ¹³C signatures of plant sources and sediments indicate a sediment bias mainly toward trees, with contributions from woody shrubs and groundcover growing in the bog shoreline. Within trees and woody shrubs, we observed δ²Hwax–δ¹³Cwax relationships of opposite sign for n-C29 alkane and n-C28 alkanoic acid, which we speculate may reflect contrasting seasonal timing of synthesis and plant metabolic status between compound classes. The net apparent δ²H fractionation between precipitation and wax (εapp) was approximately 30‰ larger for n-alkanes (−133‰) than for n-alkanoic acids (−103‰), both at the plant level and in sediments. These results demonstrate the sensitivity of sediments in a hydrologically closed basin to woody plants growing in the associated catchment and can guide εapp estimates for sedimentary records from similar depositional settings.
Article
Tropical montane regions tend to have high rates of precipitation, biological production, erosion, and sediment export, which together move material off the landscape and toward sedimentary deposits downstream. Plant wax biomarkers can be used to investigate sourcing of organic matter and are often used as proxies to reconstruct past climate and environment in sedimentary deposits. To understand how plant waxes are sourced within a wet, tropical montane catchment, we measure the stable C and H isotope composition (δ¹³C and δD) of n-alkanes and n-alkanoic acids in soils along an elevation transect and from sediments within the Madre de Dios River network along the eastern flank of the Peruvian Andes, draining an area of 75,400 km² and 6 km of elevation. Soils yield systematic trends in plant wax δ¹³C (+1.75 and +1.31‰ km⁻¹, for the C29 n-alkanes and C30 n-alkanoic acids respectively in the mineral horizon) and δD values (−10 and −12‰ km⁻¹, respectively) across a 3.5 km elevation transect, which approximates trends previously reported from canopy leaves, though we find offsets between δ¹³C values in plants and soils. River suspended sediments generally follow soil isotopic gradients defined by catchment elevations (δ¹³C: +1.03 and +0.99‰ km⁻¹ and δD: −10 to −7‰ km⁻¹, for the C29 n-alkanes and C30 n-alkanoic acids respectively) in the wet season, with a lowering in the dry season that is less well-constrained. In a few river suspended sediments, petrogenic contributions and depth-sorting influence the n-alkane δ¹³C signal. Our dual isotope, dual compound class and seasonal sampling approach reveals no Andean-dominance in plant wax export, and instead that the sourcing of plant waxes in this very wet, forested catchment approximates that expected for spatial integration of the upstream catchment, thus with a lowland dominance on areal basis, guiding paleoenvironmental reconstructions in tropical montane regions. The dual isotope approach provides a cross-check on the altitudinal signals and can resolve ambiguity such as might be associated with vegetation change or aridity in paleoclimate records. Further, the altitude effect encoded within plant waxes presents a novel dual-isotope biomarker approach to paleoaltimetry.
Article
Sedimentary δDn-alkane value is widely utilized as a reliable proxy for paleo-hydrological reconstruction. Applications of this proxy must be based upon a globally clear understanding of the relationship between leaf wax δDn-alkane values and precipitation δD(δDp), defined as apparent fractionation (εapp). However, there is a critical concern about whether relatively constant εapp values exist across different latitudes. In this study, we systematically analyzed the variations of available εapp with latitudes based upon two compiled-new databases of higher plants and sediments over the world. We found that the total average εapp was relatively constant, i.e., −116 ± 5‰ (n = 941), in higher plants across different latitudes without consideration of plant types (e.g., dicots, monocots, gymnosperms), and was still constant but slightly lower average εapp, i.e., −125 ± 6‰ (n=460), in sediments across the latitudes. The slightly lower average εapp in sediments relative to higher plants probably derived from the contribution of aquatic plants with isotopically D-depleted εapp in lake sediments. Interestingly, with consideration of plant types, average εapp increased in dicots but decreased inmonocots slightly from low to high latitudes. The counteraction of these competing trends generates relatively constant average εapp values in higher plants, and resultantly constant average εapp values occur in sediments at the global scale. It is important to elaborate relatively constant εapp values from higher plants and sediments across different latitudes when sedimentary δDn-alkane is utilized as a proxy for paleohydrological reconstruction.
Article
We investigated the seasonal variations of the molecular and hydrogen isotopic compositions of leaf cuticular n-alkanes (δDalk) in two subtropical deciduous tree species (Quercus chenii, Liquidambar formosana) over a three-year period. Average chain length values of long chain n-alkanes in both species are lower in young leaves than in mature ones. The δDalk values in both tree species show seasonal fluctuations that can be as large as 78‰ in a single year. The seasonal changes in molecular and isotopic compositions suggest that leaf wax n-alkanes in these subtropical deciduous tree species integrate environmental information over a large portion of the annual growth cycle in response to fluctuations in environmental stresses (e.g. storms, droughts). In addition, the changes reveal that seasonal patterns of δDalk can differ between different deciduous species at the same location and between different years. Spring leaves collected in 2014 had more negative δDalk values than those collected in 2015 and 2016, possibly responding to differences in annual precipitation D/H ratios and associated atmospheric circulation. These preliminary results highlight that extended seasonal monitoring of leaf wax δDalk values can improve their application to reconstructing paleohydrological histories.
Article
Sedimentary plant wax ²H/¹H ratios are important tools for understanding hydroclimate and environmental changes, but large spatial and temporal uncertainties exist about transport mechanisms from ecosystem to sediments. To assess atmospheric pathways, we collected aerosol samples for two years at four locations within a ∼60 km radius in northern Switzerland. We measured n-alkane distributions and ²H/¹H ratios in these samples, and from local plants, leaf litter, and soil, as well as surface sediment from six nearby lakes. Increased concentrations and ²H depletion of long odd chain n-alkanes in early summer aerosols indicate that most wax aerosol production occurred shortly after leaf unfolding, when plants synthesize waxes in large quantities. During autumn and winter, aerosols were characterized by degraded n-alkanes lacking chain length preferences diagnostic of recent biosynthesis, and ²H/¹H values that were in some cases more than 100‰ higher than growing season values. Despite these seasonal shifts, modeled deposition-weighted average ²H/¹H values of long odd chain n-alkanes primarily reflected summer values. This was corroborated by n-alkane ²H/¹H values in lake sediments, which were similar to deposition-weighted aerosol values at five of six sites. Atmospheric deposition rates for plant n-alkanes on land were ∼20% of accumulation rates in lakes, suggesting a role for direct deposition to lakes or coastal oceans near similar production sources, and likely a larger role for deposition on land and transport in river systems. This mechanism allows mobilization and transport of large quantities of recently produced waxes as fine-grained material to low energy sedimentation sites over short timescales, even in areas with limited topography. Widespread atmospheric transfer well before leaf senescence also highlights the importance of the isotopic composition of early season source water used to synthesize waxes for the geologic record.
Article
Sedimentary lipid biomarkers have become widely used tools for reconstructing past climatic and ecological changes due to their ubiquitous occurrence in lake sediments. In particular, the hydrogen isotopic composition (expressed as δD values) of leaf wax lipids derived from terrestrial plants has been a focus of research during the last two decades and the understanding of competing environmental and plant physiological factors influencing the δD values has greatly improved. Comparatively less attention has been paid to lipid biomarkers derived from aquatic plants, although these compounds are abundant in many lacustrine sediments. We therefore conducted a field and laboratory experiment to study the effect of salinity and groundwater discharge on the isotopic composition of aquatic plant biomarkers. We analyzed samples of the common submerged plant species, Potamogeton pectinatus (sago pondweed), which has a wide geographic distribution and can tolerate high salinity. We tested the effect of groundwater discharge (characterized by more negative δD values relative to lake water) and salinity on the δD values of n-alkanes from P. pectinatus by comparing plants (i) collected from the oligotrophic freshwater Lake Stechlin (Germany) at shallow littoral depth from locations with and without groundwater discharge, and (ii) plants grown from tubers collected from the eutrophic Lake Müggelsee in nutrient solution at four salinity levels. Isotopically depleted groundwater did not have a significant influence on the δD values of n-alkanes in Lake Stechlin P. pectinatus and calculated isotopic fractionation factors εl/w between lake water and n-alkanes averaged −137 ± 9‰ (n-C23), −136 ± 7‰ (n-C25) and −131 ± 6‰ (n-C27), respectively. Similar ε values were calculated for plants from Lake Müggelsee grown in freshwater nutrient solution (−134 ± 11‰ for n-C23), while greater fractionation was observed at increased salinity values of 10 (163 ± 12‰) and 15 (−172 ± 15‰). We therefore suggest an average ε value of −136 ± 9‰ between source water and the major n-alkanes in P. pectinatus grown under freshwater conditions. Our results demonstrate that isotopic fractionation can increase by 30–40‰ at salinity values 10 and 15. These results could be explained either by inhibited plant growth at higher salinity, or by metabolic adaptation to salt stress that remain to be elucidated. A potential salinity effect on δD values of aquatic lipids requires further examination, since this would impact on the interpretation of downcore isotopic data in paleohydrologic studies.
Article
The effect of salinity on hydrogen isotope fractionation during the production of leaf wax n-alkanes was assessed for Laguncularia racemosa (white mangrove), Rhizophora mangle (red mangrove), and Avicennia germinans (black mangrove) along a 31 ppt (parts per thousand) salinity gradient in the Shark River estuary, Florida, USA. Significant variation in hydrogen isotope ratios were observed among these three Atlantic-East Pacific (AEP) species, with increasing leaf wax n-alkane ²H/¹H fractionation with increasing salinity. Net ²H/¹H fractionation for hentriacontane (n-C31) increased by 0.8, 1.4 and 1.8‰/ppt in R. mangle, A. germinans and L. racemosa, respectively. The observations are consistent with published δ²HnC31 data from 5 species of Indo-West Pacific (IWP) mangroves, which increased with salinity by 0.7 - 1.5‰/ppt. Although all measured species from both the AEP and IWP regions have more ²H/¹H fractionation at high salinity, differences in slope and intercepts of these are relationships are observed among genera. These differences may result from variation in the composition of compatible solutes, reliance on storage carbohydrates, and/or physiological responses to salt. However, no statistically significant difference in the sensitivity of δ²HnC31 to salinity was observed in four Rhizophora species from both Indo-West Pacific and Americas-East Atlantic regions, which makes sedimentary Rhizophora lipids a promising target for paleohydroclimatic reconstruction.
Article
The hydrogen isotopic composition of leaf waxes (δDwax) primarily reflects that of plant source water. Therefore, sedimentary δDwax records are increasingly used to reconstruct the δD of past precipitation (δDp) and to investigate paleohydrologic changes. Such reconstructions rely on estimates of apparent fractionation (εapp) between δDp and the resulting δDwax. However, εapp values are modified by numerous environmental and biological factors during leaf wax production. As a result, εapp can vary widely among plant species and growth forms. This complicates estimation of accurate εapp values and presents a central challenge to leaf wax paleohydrology. During the 2014 growing season, we examined εapp in the five deciduous angiosperm tree species (Prunus serotina, Acer saccharinum, Quercus rubra, Quercus alba, and Ulmus americana) that dominate the temperate forest at Brown’s Lake Bog, Ohio, USA. We sampled individuals of each species at weekly to monthly intervals from March to October and report δD values of n-C29 alkanes (δDn-C29 alkane) and n-C28 alkanoic acids (δDn-C28 acid), as well as xylem (δDxw) and leaf water (δDlw). n-Alkane synthesis was most intense 2-3 weeks after leaf emergence and ceased thereafter, whereas n-alkanoic acid synthesis continued throughout the entire growing season. During bud swell and leaf emergence, δDlw was a primary control on δDn-C29 alkane and δDn-C28 acid values, which stabilized once leaves became fully expanded. Metabolic shifts between young and mature leaves may be an important secondary driver of δDwax changes during leaf development. In mature autumn leaves of all species, the mean εapp for n-C29 alkane (-107‰) was offset by approximately -19‰ from the mean εapp for n-C28 alkanoic acid (-88‰). These results indicate that in temperate settings n-alkanes and n-alkanoic acids from deciduous trees are distinct with respect to their abundance, timing of synthesis, and εapp values.
Article
Hydrogen isotope ratios (²H/¹H or D/H) of sedimentary mangrove lipid biomarkers can be exploited as a quantitative proxy of past salinity and water isotopes. This approach is based on the observation that apparent ²H/¹H fractionation between surface water and mangrove leaf lipids increases with surface water salinity. In order to better understand the mechanisms responsible for this empirical relationship, we analyzed the isotopic composition of surface water, xylem water, leaf water and leaf lipids from Bruguiera gymnorhiza mangroves growing around eleven marine lakes and a lagoon on the rock islands of Palau, spanning a salinity range of 5–32 parts per thousand (ppt). Net fractionation increased with increasing salinity for both nC31-alkane (0.7 ± 0.1‰ ppt⁻¹) and for the pentacyclic triterpenoid lupeol (0.5 ± 0.2‰ ppt⁻¹). These trends could not be attributed to changes in biosynthetic fractionation with salinity, but seem more likely to be due to increasing disequilibrium between xylem water and water vapor as salinity increases. In Palau's humid climate, this most likely causes leaf water to become less ²H-enriched relative to surface water and to xylem water as salinity increases. This supposition is supported not only by measurements of leaf water ²H enrichment, but also by the correlation (R² = 0.66) between leaf water isotopes and those of rain water, which are assumed to be in equilibrium with water vapor isotopes, and by the dependence of leaf water isotopes on water vapor isotopes in a Péclet-modified Craig-Gordon model. These results should inform the application of sedimentary mangrove lipid hydrogen isotope ratios to infer past hydroclimatic changes.
Article
Terrestrial plant biomarkers and their carbon isotopes provide insights into carbon cycling, paleovegetation and paleoclimate, ranging in scale from local to global. Over the past decade, considerable efforts have been made to constrain the factors that influence plant biomarkers and their carbon isotope composition to improve their utility for paleo applications. Global and regional replication of time intervals of great interest, such as during carbon cycle perturbations, has increased the need to compare among sites, but doing so has also complicated interpretation of carbon cycle perturbations due to the differences among records. This has led to questions regarding the fidelity of isotope records, the sensitivity of the isotope record to climate, and the best practices for reconciling records. But, at the same time, it has led to new exciting information on ecosystem responses to climate change. By removing competing influences of climate, ecosystem and biology, modern biomarker and isotope calibrations provide a means of reconciling and improving paleorecords and placing quantitative constraints on their interpretation. Here, we review the factors that influence the concentration of plant biomarkers and their carbon isotope composition and provide best practice for reconciling biomarker carbon isotope records for interpreting climate, ecosystem, and carbon cycling in the geologic past.
Article
Long chain n-alkanes (C-27-C-33) in lake sediment records are commonly considered to be terrestrial plant biomarkers when reconstructing paleoclimatic and paleolimnological history. However, the extent to which their accumulation is influenced by n-alkanes originating from algae and submerged plants is largely unclear. Furthermore, to our knowledge, few studies have systematically analyzed the variation in n-alkane concentration or distributions between different submerged plant and algal species. We systematically investigated the n-alkane distributions of 13 algae (including 10 Cladophora and 3 Spirogyra), 68 submerged plants (including 37 Potamogeton, 7 Myriophyllum, 4 Ruppiaceae and 20 Chara) and 13 terrestrial plants (including 7 grasses and 6 shrubs) from 16 Qinghai -Tibetan Plateau lakes. The results indicate that the total n-alkane (C-21-C-33) concentration varied between different submerged plants. Potamogeton, Myriophyllum and Ruppiaceae exhibited high concentration, with average values 235.8, 295.9 and 275.9 mu g/g, respectively. These values were slightly higher than the concentrations found in terrestrial plant leaves (avg. 206.4 mu g/g), whereas the average concentration in Chara was only 2.0 mu g/g, significantly lower than that of other submerged plants. Similarly, the concentration in algae was also very low, with average values of 2.0 mu g/g and 4.0 mu g/g for Cladophora and Spirogyra, respectively. Submerged plant and algal long chain (C-27-C-33) alkanes accounted for a large proportion of the total C-21-C-33 n-alkanes, with average ratios (long chain vs. total n-alkanes) of 20, 3, 22 and 27% for Potamogeton, Myriophyllum, Ruppiaceae and Chara, respectively. Cladophora and Spirogyra exhibited average ratios of 34% and 65%, respectively. Therefore, submerged plant long chain n-alkane contributions to lacustrine sediments, especially those of Potamogeton and Ruppiaceae, should not be considered negligible due to their high n-alkane concentration. Conversely, some algae, such as Cladophora and Spirogyra, minimally contributed n-alkanes to the lake sediments.
Article
The hydrogen isotopic composition of terrestrial plant leaf waxes is widely used as a proxy for the isotopic composition of ambient water at the time of plant growth, yet there is considerable uncertainty about how environmental or plant-specific factors impact apparent hydrogen isotope fractionation. We sampled leaves from four riparian plant species (Pinus strobus, Tsuga canadensis, Phalaris arundinacea and Corylus americana) during the 2013 growing season (April to October) to evaluate the controls of hydrogen isotope fractionation in plant-derived n-alkanes. Our data show that plants from different taxonomic classes produce distinct seasonal patterns of isotopic change that could reflect differences in stomatal regulation, soil versus groundwater uptake, and timescales of wax synthesis. These patterns are distinct even for plants from the same functional group (i.e. angiosperms versus gymnosperms). P. strobus (gymnosperm tree) and P. arundinacea (angiosperm grass) are characterized by a significant increase in apparent fractionation during the growing season (> 50‰ change). In contrast, n-alkanes from C. americana (angiosperm) show a decrease in ε value from –145 ± 8‰ during early season growth to –111 ± 6‰ late in the season. T. canadensis (gymnosperm) exhibits a constant ε of –123 ± 10‰ throughout the growing season with minimal temporal change. Sedimentary n-alkanes collected from the stream channel adjacent our plant sample locality show minor changes over the sampling period with a mean ε value of –130 ± 6‰. Apparent fractionation differences between plant species greatly exceed variations in stream composition throughout the growing season, indicating that differences in the timing of wax production or groundwater δD cannot alone explain the variation in ε between different plant types. In contrast to individual plant leaves, fluvial sediments represent a time-integrated ecosystem average of n-alkane δD that yields a more consistent isotopic record of changes in ambient ecosystem water than individual plants.
Article
The hydrogen isotopic compositions of sedimentary hydrocarbon molecules are now being used to address a range of scientific questions, from paleoclimate to environmental reconstruction to understanding of petroleum systems. Here I review the environmental, biological, and physical/chemical factors that influence the H isotopic compositions of sedimentary hydrocarbons. A hierarchy of four main controls can be recognized: i) the composition of environmental water that serves as ultimate hydrogen source to the biosphere ii) physiologic and metabolic processes in organisms that fix water hydrogen into organic molecules, iii) hydrogen exchange processes that alter D/H ratios slowly over time under geologic conditions, and iv) kinetic fractionations that arise during the thermal conversion of sedimentary organic matter to liquid hydrocarbons. Variations in the terrestrial hydrologic cycle, and in biologic fractionations, create lipids with δD values spanning a huge range, and carrying abundant information about source organisms and environments. This information is gradually lost over geologic timescales due to hydrogen exchange, although the rates of this process appear to vary by orders of magnitude in clastic versus carbonate sediments. The H isotopes of hydrocarbons in many sediments of low to moderate thermal maturity may only be partially exchanged. Finally, additional kinetic fractionations are imposed during thermal cracking to generate mobile hydrocarbons. The δD values of sedimentary hydrocarbons are thus concluded to generally reflect a complicated combination of processes. Although it is tempting to view this as a failure of the isotopic record, there are numerous opportunities for understanding environment, diagenetic, and catagenetic processes if we can learn to quantitatively disentangle the competing fractionations.
Article
Compound specific carbon and deuterium stable isotope values (δ13C and δD) and the relative abundance of mid-chain n-alkanes (Paq) were determined for a series of dominant wetland plants, a surface slough-to-ridge soil transect, and slough and ridge soil cores, to assess historical vegetation successions induced by hydrological modification in an anthropogenically impacted, subtropical wetland, the Florida Everglades, USA. A difference of as much as 3.6 and 130 ‰ in their δ13C and δD values was observed between the two most abundant emergent macrophyte species (Cladium and Eleocharis), respectively. A clear n-alkane δD value depletion (−130 to −167 ‰) and decreasing Paq was observed along the slough-to-ridge soil transect, likely the result of an eco-hydrological transition from slough-to-ridge dominated vegetation (Eleocharis to Cladium). In agreement with the relatively constant Paq values, the lack of significant changes in the δD depth profile for the slough core, suggest a consistent slough type of vegetation composition over time at that location. In contrast, changes of both n-alkane δ13C and δD values for the ridge core, especially after ~1960 AD, coincide with the expected plant successions from historically long hydroperiod (>8 months), slough type plants (Eleocharis, Utricularia, Nymphaea) to present day, shorter hydroperiod (<8 months), ridge type plants (Cladium). These δ13C and δD changes seem to be driven by vegetation shifts associated with hydrological change. The application of the compound-specific stable isotope determinations may strongly complement the biomarker approach for paleo-hydrological assessments in wetland ecosystems.
Article
Eight C25 highly branched isoprenoid (HBI) alkenes were detected in the freshwater wetland of the Florida Everglades and tentatively assigned as a C25 diene, three C25trienes, two C25tetraenes and two C25 pentaenes based on their mass spectra, retention index and literature reports. One diene and one triene were observed more frequently than the others. The HBIs were present in varying amount of up to 1000, 19000, 780 and 150 ng/g dry weight in the periphyton, floc, surface soils and deeper soils (ca. 1900 AD), respectively. Compound specific carbon isotope analysis of the two most dominant HBIs (one diene and one triene) showed them to be highly depleted in 13C (δ13C -40.0 to -38.5‰) in freshwater floc, suggesting recycled CO2 produced from the decomposition of organic matter as an important C source in the biosynthesis of these compounds. HBIs were present across this freshwater wetland in greater diversity and abundance at locations with higher surface water N concentration and longer hydroperiod (inundation). Historical variation in the occurrence of the HBIs in soil cores from multiple slough and ridge environments was assessed. Their increased abundance after ca. 1960 AD in almost all cores suggests proliferation of their precursor biota (freshwater diatoms) over the past five decades.
Article
Long-chain n-alkyl compounds are major components of epicuticular waxes from the leaves of vascular plants. These compounds include n-alkanes, n-alkanols, n-alkanoic acids, and wax esters and are associated with cuticle, a multilayered structure on the exterior of leaves and fruits. Cuticle has multiple functions that include a primary role protecting against moisture loss. It also provides exterior strength and stiffness and these properties can vary with composition and environmental conditions, such as temperature and humidity.
Article
The Everglades ecosystem evolved under conditions of relatively low phosphorus inputs. Phosphorus inputs to the Water Conservation Areas have increased nearly threefold (from 129 tonnes under predrainage conditions to 376 tonnes currently) due to inflows of agricultural runoff water. Everglades National Park appears to have experienced smaller increases in annual P input, from 88 tonnes predrainage to 89 tonnes. Cattail Typha has become established in sawgrass Cladium jamaicense in Everglades marsh areas near anthropogenic P inputs. Contrasting responses of the two species to nutrient enrichment characterize sawgrass as a low nutrient status species that is competitive in infertile habitats, in contrast to cattail as a high nutrient status species that is competitive when nutrient supply increases. Cattail density has increased relative to sawgrass density at eutrophic sites. Calcareous blue-green, green, diatom-rich, and desmid-rich periphyton communities are either eliminated or replaced by a community dominated by the filamentous algae Microcoleus lyngbyaceus and other pollution indicator species, apparently with adverse effects on food webs and oxygen budgets. Colony counts of facultative bacteria and fungi that colonize leaf litter are almost an order of magnitude lower at eutrophic sites in comparison to oligotrophic sites. The number of macroinvertebrate taxa that colonize leaf litter is reduced, Diptera numbers are reduced, snails are nearly eliminated, isopods are eliminated, and density of annelid worms is more than doubled at eutrophic compared to oligotrophic sites. -from Authors
Article
Compound-specific δ2H values of leaf wax n-alkanes have emerged as a potentially powerful paleohydrological proxy. Research suggests terrestrial plant n-alkane δ2H values are strongly correlated with meteoric water δ2H values, and may provide information on temperature, relative humidity, evaporation, and precipitation. This is based upon several assumptions, including that biosynthetic fractionation of n-alkanes during synthesis is constant within a single species. Here we present a multi-isotope study of the n-alkanes of riparian Salix viminalis growing in Norwich, UK. We measured n-alkane δ2H, leaf water δ2H, xylem water δ2H, and bulk foliar δ13C and evaluated the variability of n-alkane δ2H values and net biosynthetic fractionation (εlw-wax) over a whole growing season. S.viminalis n-alkane δ2H values decreased by 40‰ between the start of the growing season in April and the time when they stabilized in July. Variation in leaf and xylem water δ2H did not explain this variability. εlw-wax varied from -116‰ during leaf expansion in April to -156‰ during the stable phase. This suggests that differential biosynthetic fractionation was responsible for the strong seasonal trends in S.viminalis n-alkane δ2H values. We suggest that variability in εlw-wax is driven by seasonal differences in the carbohydrate source and thus the NADPH used in n-alkane biosynthesis, with stored carbohydrates utlized during spring and recent ocurring growing season assimilates used later in the season. This is further supported by bulk foliar δ13C values, which are 13C-enriched during the period of leaf flush, relative to the end of the growing season. Our results challenge the assumption that biosynthetic fractionation is constant for a given species, and suggest that 2H-enriched stored assimilates are an important source for n-alkane biosynthesis early in the growing season. These findings have implications for the interpretation of sedimentary n-alkanes and call for a careful design of calibration studies using contemporary samples.
Article
n-Alkanes are long-chained hydrocarbons contained in the cuticle of terrestrial plants. Their hydrogen isotope ratios (δ2H) have been used as a proxy for environmental and plant ecophysiological processes. Calibration studies designed to resolve the mechanisms that determine the δ2H values of n-alkanes have exclusively focused on n-alkanes derived from leaves. It is, however, unclear in which quantities n-alkanes are also produced by other plant organs such as roots or inflorescences, or whether different plant organs produce distinct n-alkane δ2H values. To resolve these open questions, we sampled leaves, sheaths, stems, inflorescences and roots from a total of 15 species of European C3 grasses in an alpine and a temperate grassland in Switzerland. Our data show slightly increased n-alkane concentrations and n-alkane δ2H values in the alpine compared to the temperate grassland. More importantly, inflorescences had typically much higher n-alkane concentrations than other organs while roots had very low n-alkane concentrations. Most interestingly, the δ2H values of the carbon autonomous plant organs leaves, sheaths and stems were in general depleted compared to the overall mean δ2H value of a species, while non-carbon autonomous organs such as roots and inflorescences show δ2H values that are higher compared to the overall mean δ2H value of a species. We attribute organ-specific δ2H values to differences in the H-NADPH biosynthetic origin in different plant organs as a function of their carbon relationships. Finally, we employed simple mass balance calculations to show that leaves are in fact the main source of n-alkanes in the sediment. As such, studies assessing the environmental and physiological drivers of n-alkanes that focus on leaves produce relationships that can be employed to interpret the δ2H values of n-alkanes derived from sediments. This is despite the significant differences that we found among the δ2H values in the different plant organs. Our study brings new insights into the natural variability of n-alkane δ2H values and has implications for the interpretation of n-alkane δ2H values in ecological and paleohydrological research.
Article
The coastal vegetation of southern Florida is undergoing dramatic changes due to the instability of the ocean water-freshwater boundary. These vegetation changes will be determined by the response of each particular species to saline ocean water, particularly whether it can use ocean water or not. In this study, isotopic data were used to determine the relative usage of freshwater or ocean water by plants in the Florida keys. The results indicate that, with some exceptions, plants toward the interior of the keys were using freshwater while those toward the edge were using ocean water. A plot of the hydrogen and oxygen isotopic composition of the plant water yielded a mixing line between typical freshwater values and those of ocean water. In general, the isotopic ratios of stem water for species found in hardwood hammocks were confined to the freshwater end of the line, followed by values of stem water from mangrove margin species. found in mangroves, however, had water with extremely variable isotopic ratios, ranging from values typical of ocean water to values typical of freshwater. This variability is consistent with the hypothesis that mangroves are fully capable of growing in freshwater, but are limited to saline habitats because of competitive exclusion by fast-growing glycophilic plants.
Article
Paleoecological reconstructions of environmental changes provide important information for Everglades restoration targets. Traditionally this has been achieved using a combination of biological and physical indicators. However, as microfossils may be sporadically abundant in Everglades soils, organic geochemical methods can provide information at the molecular level. To reconstruct vegetation trends over the last century, soil cores from Shark to Taylor Sloughs, the primary flowpaths of the southern Everglades, were examined using several geochemical proxies. The n-alkane derived biomarker Paq effectively distinguished organic inputs from sawgrass and slough habitats. Other proxies examined include Kaurenes, cyclic diterpenoids unique to sawgrass roots; biomarkers of algae (highly branched isoprenoids (C20HBIs) and Botryococcenes); lignin phenols as vascular plant indicators; and macrofossils. At all sites, soil profiles from sawgrass marshes showed vegetation had shifted over the last 100 years, from sloughs to sawgrass-dominated marshes, reflecting decreased water levels (shorter hydroperiods) induced by water management. Paleo-assessments of modern sloughs, however, indicate these habitats remained deeper water habitats throughout the period of record, though shifts toward shorter hydroperiod vegetation were observed. In Taylor Slough, evidence of increasingly dry conditions in sloughs was confirmed by seed inputs from woody species. At 3 of the 5 sites, recent increases in C20HBIs and Botryococcene concentrations indicated greater periphyton abundance, coincidental with increased mineral concentrations observed in surface waters during the mid-20th Century. Bulk proxies such as organic content and carbon:nitrogen ratios also supported findings of changes in relative contributions of microbial and higher plants in this ecosystem.
Article
Gannan Gahai Lake, in the northeast of the Tibetan Plateau, is the largest freshwater lake in the Gannan plateau, and is rarely influenced by human activity. Aquatic plants in the lake and terrestrial plants in the surrounding area were systematically collected, and the n-alkane distributions and hydrogen isotopic composition investigated for the first time. The distributions exhibited a unimodal/bimodal distribution with maxima at C17 or C21 and at C23 to C31, and were dominated by odd numbered components with carbon preference index (CPI) values of 2.3 to 14.7. Average chain length (ACL) values, ranging from 25.7 to 29.6, correlated with the dominant n-alkane carbon number. The average δD values of the n-alkanes ranged from –246‰ to –130‰. In general, the δD values were higher for woody plants, lower for herbaceous plants and in between for aquatic plants. The data show that the hydrogen isotopic composition of the plant n-alkanes was influenced by species, leaf surface size, plant water source and environment. The δD values tended to be lower with increasing ACL values, which results from the reduction in leaf water evaporation. The isotopic fractionation during n-alkane synthesis was larger for herbaceous plants, smaller for woody plants and in between for aquatic plants.