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1.
Glycerol dialkyl glycerol tetraether (GDGT)-based proxies are increasingly used in modern carbon cycling and palaeoenvironmental investigations. It is therefore crucial to examine the robustness (sources, transport and degradation) of all GDGT-based proxies in continental margins, where sedimentation rates and extent of carbon cycling are high. We have analyzed the distributions of GDGTs in surface sediments from the Lower Yangtze River and East China Sea (ECS) shelf. The results revealed multiple sources and complex shelf processes that govern the distributions. The isoprenoid GDGT-inferred sea surface temperatures (SSTs) are robust and reflect the satellite-derived annual mean SSTs on the shallow ECS shelf, confirming an origin from surface water column-dwelling crenarchaeota. The input from methanogen-sourced, isoprenoid GDGTs is significant in the river surface sediments but they are almost absent from the ECS shelf. Branched GDGTs are also abundant in the river sediments, but ca. 95% are degraded in the Yangtze estuary, a much greater extent than observed for other terrigenous organic matter (OM) proxies. There is also evidence for production of branched GDGTs in the oxic ECS shelf water column and the anoxic sediments/waters of the Lower Yangtze River. As a result, branched GDGT-based proxies in the lower river and ECS surface sediments do not reflect the catchment environmental conditions. The effective degradation in the estuary and widespread aquatic contributions of branched GDGTs improves our understanding of how to use branched GDGT-based proxies in marginal seas.  相似文献   

2.
Glycerol dialkyl glycerol tetraethers (GDGTs) are increasingly used as proxies for paleoclimate studies of marine and lacustrine environments. While GDGT-based proxies have been applied to a number of lake environments globally, little is known about the distribution of GDGTs on the Qinghai-Tibet Plateau. We have investigated the isoprenoid GDGTs (iGDGTs) and branched GDGTs (bGDGTs) in Lake Qinghai sediments and the surrounding surface soil in order to examine their potential use as paleoclimate proxies on the Qinghai-Tibet Plateau. The results show that (i) the values of the iGDGT/bGDGT ratio for surrounding soil were at the higher end among globally distributed soils and consequently BIT values (avg. 0.71) at the lower end, (ii) the TEX86 values decreased while the MBT and CBT values increased along an onshore soil–nearshore sediment–offshore sediment transect, (iii) the TEX86 values for the offshore sediments were almost identical and their inferred temperatures were close to mean summer surface water temperature and (iv) the bGDGT-inferred mean annual air temperature (MAAT) and pH for soil were consistent with measured MAAT and pH. However, the CBT-inferred pH for offshore sediments seemed inconsistent with the pH of lake water or sediment. Our results suggest that (i) the higher pH may be an important factor leading to the higher iGDGT/bGDGT values (and lower BIT values) in surrounding surface soil, (ii) both iGDGTs and bGDGTs may originate from terrestrial input and in situ production for this saline lake, especially for nearshore sediments. However, for offshore sediments, terrestrial iGDGT input seems minor, and TEX86 may be useful for paleoclimate studies of Lake Qinghai.  相似文献   

3.
Bacterially produced branched glycerol dialkyl glycerol tetraethers (GDGTs) are ubiquitous in soils and lake sediments and can potentially be used to reconstruct past temperatures. In lakes, however, it is still unclear if these compounds are derived from eroded soils or if they are produced in situ. To better understand environmental controls on the distributions of these compounds and the sources of branched GDGTs to lake sediments, we compare branched GDGT distributions and concentrations in lake sediments and catchment soils within a 3600 m altitudinal transect in western Uganda. Reconstructed mean annual air temperature (MAAT), determined from the degree of methylation (MBT) and cyclisation (CBT) of branched GDGTs in soils, decreases with increasing altitude, as is expected from the air temperature gradient in our transect. However, we observe significant offsets between observed and reconstructed temperatures in soils from wet, high elevation soils but not in most dry, low elevation soils. Branched GDGT distributions differ significantly between lake sediments and soils at all elevations, with greater differences at low elevations than at high elevations. These data support previous hypotheses that branched GDGTs are produced in situ in lakes and suggest that the abundance of water in soil environments may play a role in controlling the distribution of branched GDGTs. While branched GDGTs in lacustrine sediments can be used to reconstruct temporal temperature variations in lakes, we urge caution in utilizing this proxy in lacustrine systems with high soil loadings, especially if there is evidence of changing clastic fluxes through time.  相似文献   

4.
Bacterial and archaeal lipids, such as glycerol dialkyl glycerol tetraethers (GDGTs) and dialkyl glycerol diethers, are increasingly used as proxies for specific environmental parameters, such as air temperature and soil pH in lacustrine environments. Little is known, however, about the distribution and applicability of bacterial and archaeal lipids on the Tibetan Plateau. We investigated nine different watersheds across the plateau by way of sediments from lakes and rivers, as well as the surrounding soils. Our transect study included a salinity gradient and focused on saline lakes, which are rarely examined. We analyzed archaeal isoprenoid (i) and bacterial branched (b) GDGTs, as well as archaeol to trace their sources and environmental factors, influencing their distributions. We could show that iGDGTs were produced in situ and bGDGTs were primarily soil-derived although we could not exclude in situ production of bGDGTs in the lakes. The most important environmental variables correlating with GDGT distributions were temperature and salinity. Bacterial GDGT distributions correlated mainly with salinity, while archaeal lipid distributions correlated with temperature. Based on the correlation of methylation (MBT′) and cyclisation (CBT) indices of bGDGTs with pH and mean annual air temperature (MAAT), we established local calibrations for the Tibetan lakes. TEX86 could also be applied to reconstruct temperature, which was strongly biased towards measured summer lake water temperature, indicating enhanced production of iGDGTs in the summer months. Existing proxies show, therefore, potential for palaeoclimate reconstruction on the Tibetan Plateau if local calibrations are applied.  相似文献   

5.
To determine where and when glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in lakes are produced, we collected descending particles in Lake Lucerne (Switzerland) using two sediment traps (at 42 and 72 m water depth) with a monthly resolution from January 2008 to late March 2009. Suspended particulate matter (SPM) was monthly filtered from the water column at three different depths. The potential application of GDGTs in palaeoenvironmental and palaeoclimatic reconstructions was investigated by comparing core lipids and their relative GDGT distribution, with lake water temperatures throughout the year. Fluxes of GDGTs and their concentrations in the water column vary according to a seasonal pattern, showing a similar trend in the SPM and sediment traps. Fluxes and concentrations of isoprenoid GDGTs increase with depth, maximum values being observed in the deeper part of the water column, indicating production of isoprenoid GDGTs by Thaumarchaeota in the deep (∼50 m), aphotic zone of Lake Lucerne. The flux-weighted averages of the proxies TEX86 (0.27) and BIT (0.03) based on the total extracted GDGTs are similar at both trap depths. A sediment core from the same location showed that in the first few centimetres of the core TEX86 and BIT values of 0.29 and 0.07, respectively, are similar to those recorded for descending particles and SPM, indicating that the sedimentary TEX86 records the annual mean temperature of deeper waters in Lake Lucerne. TEX86 values are slightly higher below 20 cm in the core. This offset is interpreted to be caused by the present-day trophic state of the lake, which probably resulted in a deeper niche of the Thaumarchaeota. Branched GDGTs represent only a minor fraction of the total GDGTs in the lake and their origin remains unclear. Our data reveal that GDGTs in lakes have a large potential for palaeoclimatic studies but indicate that knowledge of the system is important for accurate interpretation.  相似文献   

6.
The distribution of isoprenoid and branched glycerol dialkyl glycerol tetraether (GDGT) lipids was studied in material from various sources in and around Lake Challa, a crater lake on the southeastern slope of Mt. Kilimanjaro (Tanzania), to examine the provenance of GDGTs in lake sediments and their potential application as palaeoenvironmental and palaeoclimatic proxies. The study material included samples collected at monthly intervals in a sediment trap over one complete annual cycle, particles suspended in the stratified water column, profundal surface sediments, and soils surrounding the lake. The sediment trap time series revealed that crenarchaeol and related isoprenoid GDGTs were predominantly produced in January and February, following the locally prominent short rain season (November-December). The TEX86-inferred temperature derived from sedimenting particles corresponded well with lake surface-water temperature at this time of largest crenarchaeol flux. Molecular ecological analysis showed that Group 1.1a and 1.1b crenarchaeota are the most likely source organisms of these GDGTs. GDGT-0 in the lake sediments does not only originate from lake surface-dwelling crenarchaeota but seems predominantly derived from archaea residing in the deeper, anoxic part of the water column. The main flux of branched GDGTs to the sediment was during the short rain season and is most probably derived from eroded catchment soils in surface run-off. However, a contribution from in-situ production of branched GDGTs in the lake sediment or water, or in groundwater cannot be fully excluded. We conclude that palaeoclimatic reconstruction based on branched GDGT distributions in lake sediments should only be performed when the origin of those branched GDGTs is well constrained.  相似文献   

7.
The TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms) paleothermometer is based on the relative distribution of archaeal lipids, i.e. isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs), and is increasingly used to reconstruct past sea water temperatures. To establish a more extensive, global calibration of the TEX86 paleothermometer, we analyzed GDGTs in 287 (in comparison with 44 in currently used calibration) core-top sediments distributed over the world oceans and deposited at different depths. Comparisons of TEX86 data with (depth-weighted) annual mean temperatures of the overlying waters between 0 m and 4000 m as well as with different seasonal mean temperatures at 0 m water depth showed that the TEX86 proxy reflects mostly annual mean temperatures of the upper mixed layer. The relationship between TEX86 values and sea-surface temperatures (SSTs) was non-linear mainly because below 5 °C the change in TEX86 values was minor with temperature. This suggests that the TEX86 proxy might not be directly applicable for the Polar Oceans. Nevertheless, between 5 °C and 30 °C, the TEX86 proxy has a strong linear relationship with SSTs. Here, we, therefore, propose a new linear calibration model (T = −10.78 + 56.2 ∗ TEX86, r2 = 0.935, n = 223) for past SST reconstructions using the TEX86 palaeothermometer.  相似文献   

8.
Quantifying past continental temperature changes is an important aspect of paleoclimate research as it allows us to constrain the amplitude of natural variability, test predictive climate models, and provide a proper context for changes that may arise in response to anthropogenically-induced climate change. The recently developed biomarker-based methylation index of branched tetraethers/cyclization ratio of branched tetraethers (MBT/CBT) proxy shows potential as a new method for continental temperature reconstruction, but thus far it has only been applied successfully in ocean margin sediments. To assess whether this proxy is also applicable to the sedimentary record in tropical lacustrine systems, we investigated the distribution of branched glycerol dialkyl glycerol tetraethers (GDGTs) in recently deposited sediments from 46 lakes in tropical East Africa. These lakes span a substantial range in surface elevation (770-4500 m above sea level), and thus also a wide gradient of mean annual temperature. We find that, saline lakes excepted, branched GDGTs are universally abundant in the lakes investigated and can be used to predict mean annual air temperature (MAAT) with a high degree of accuracy. However, the existing global MBT/CBT calibration for MAAT based on soils predicts inaccurate temperatures when applied to our African lake dataset. This observation, together with the fact that surface water pH, and to lesser extent, lake depth appear to influence the distribution of branched GDGTs among sites, leads us to conclude that in situ production of branched GDGTs in lakes is likely. The robust relationship between branched GDGT distribution and the temperature and pH of African freshwater lakes makes these compounds suitable for paleoenvironmental reconstruction, however we urge caution in using branched GDGTs in lake sediments to infer past temperatures, unless their exact origin can be determined.  相似文献   

9.
Archaea have unique glycerol dialkyl glycerol tetraether (GDGT) lipids that can be used to develop paleotemperature proxies such as TEX86. This research is to validate proposed GDGT-proxies for paleotemperature determination in the South China Sea (SCS). Samples were collected from core-top sediments (0–5 cm) in the northern SCS. Total lipids were extracted to obtain core GDGTs, which were identified and quantified using liquid chromatography-mass spectrometry (LC-MS). The abundance of isoprenoidal GDGTs (iGDGTs) ranged from 271.5 ng/g dry sediment to 1266.3 ng/g dry sediment, whereas the branched GDGTs (bGDGTs), supposedly derived from terrestrial sources, ranged from 22.2 ng/g dry sediment to 56.7 ng/g dry sediment. The TEX86-derived sea surface temperatures ranged from 20.9 °C in the coast (water depth < 160 m) to 27.9 °C offshore (water depth > 1000 m). TEX86-derived temperatures near shore (<160 m water depth) averaged 23.1 ± 2.5 °C (n = 4), which were close to the satellite-derived winter mean sea surface temperature (average 22.6 ± 1.0 °C, n = 4); whereas the TEX86-derived temperatures offshore averaged 27.4 ± 0.3 °C (n = 7) and were consistent with the satellite mean annual sea surface temperature (average 26.8 ± 0.4 °C, n = 7). These results suggest that TEX86 may record the sea surface mean annual temperature in the open ocean, while it likely records winter sea surface temperature in the shallower water.  相似文献   

10.
Distribution of intact and core GDGTs in marine sediments   总被引：1，自引：0，他引：1  
We conducted a survey of archaeal GDGT (glycerol dibiphytanyl glycerol tetraether) distributions in marine sediments deposited in a range of depositional settings. The focus was comparison of two pools presumed to have distinct geobiological significance, i.e. intact polar GDGTs (IP GDGTs) and core GDGTs (C GDGTs). The former pool has been suggested to be related to living communities of benthic archaea in marine sediments, while the latter is commonly interpreted to consist of molecular fossils from past planktonic archaeal communities that inhabited the surface ocean. Understanding the link between these two pools is important for assessment of the validity of current molecular proxies for sedimentary archaeal biomass and past sea surface temperatures. The relative distributions of GDGTs in the two pools in a core at a CH4 rich site in the Black Sea provide evidence for in situ production of glycosidic IP GDGTs and their subsequent degradation to corresponding C GDGTs on timescales that are short in geological terms. In addition, we monitored the relationship between the IP GDGT and C GDGT pools in a sample set from various ocean basins with subseafloor depth from a few cm to 320 m and 0 to 4 Myr in age. Notable differences between the two pools can be summarized as follows: the GDGT with acyclic biphytanes, GDGT-0, and its analogues with two and three cyclopentane moieties (GDGT-2 and -3) are generally more abundant in the pool of IP GDGTs, while crenarchaeol tends to be more abundant in the C GDGT pool. Consequently, the ring index is generally higher for the C GDGTs while TEX86, a molecular proxy ratio not considering the two major GDGTs, tends to be higher in the IP GDGT pool. These differences in the proportion of individual GDGTs in the two pools are probably due to in situ production of IP GDGTs with distributions differing from those of C GDGTs. Despite these differences, we observed significant correlation of these two ratios between the two pools. Specifically, in both pools TEX86 is high in sediments from warm oceanic regimes and low in cold regimes. We discuss these relationships and suggest that recycling of core GDGTs by benthic archaea is an important mechanism linking both molecular pools.  相似文献   

11.
Absence of seasonal patterns in MBT-CBT indices in mid-latitude soils   总被引：1，自引：0，他引：1  
The degree of methylation and cyclization of bacteria-derived branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in soils depends on temperature and soil pH. Expressed in the methylation index of branched tetraethers (MBT) and cyclization ratio of branched tetraethers (CBT), these relationships are used to reconstruct past annual mean air temperature (MAT) based on the distribution of branched GDGTs in ancient sediments; the MBT-CBT proxy. Although it was shown that the best correlation of this proxy is with annual MAT, it remains unknown whether a seasonal bias in temperature reconstructions could occur, such as towards a seasonal period of ‘optimal growth’ of the, as yet, unidentified soil bacteria which produce branched GDGTs. To investigate this possibility, soils were sampled from eight different plots in the USA (Minnesota and Ohio), The Netherlands (Texel) and the UK (Devon) in time series over 1 year and analyzed for their branched GDGT content. Further analyses of the branched GDGTs present as core lipids (CLs; the presumed fossil pool) and intact polar lipids (IPLs; the presumed extant pool) were undertaken for two of the investigated soil plots. The amount of IPL-derived branched GDGTs is low relative to the branched GDGT CLs, i.e. only 6-9% of the total branched GDGT pool. In all soils, no clear change was apparent in the distribution of branched GDGT lipids (either core or IPL-derived) with seasonal temperature change; the MBT-CBT temperature proxy gave similar temperature estimates year-round, which generally matched the mean annual soil temperature. In addition to a lack of coherent changes in relative distributions, concentrations of the branched GDGTs did not show clear changes over the seasons. For IPL-derived GDGTs these results suggest that their turnover time in soils is in the order of 1 year or more. Thus, our study does not provide evidence for seasonal effects on the distribution of branched GDGTs in soils, at least at mid-latitudes, and therefore, no direct evidence for a bias of MBT-CBT reconstructed temperatures towards a certain season of optimal growth of the source bacteria. If, however, there is a slight seasonal preference of branched GDGT production, which can easily be obscured by natural variability due to the heterogeneity of soils, then a seasonal bias may potentially still develop over time due to the long turnover time of branched GDGTs.  相似文献   

12.
Branched glycerol dialkyl glycerol tetraethers (GDGTs) are bacterial membrane lipids, ubiquitously present in soils and peat bogs, as well as in rivers, lakes and lake sediments. Their distribution in soil is controlled mainly by pH and mean annual air temperature, but the controls on their distribution in lake sediments are less well understood. Several studies have found a relationship between the distribution of branched GDGTs in lake sediments and average lake water pH, suggesting an aquatic source for them, besides that for soil transported to the lake via erosion. We sampled the surface water suspended particulate matter (SPM) from 23 lakes in Minnesota and Iowa (USA), that vary widely in pH, alkalinity and trophic state. The SPM was analyzed for the concentration and distributions of core lipid (presumed fossil origin) and intact polar lipid (IPL, presumed to derive from living cells) branched GDGTs. The presence of substantial amounts (18–48%) of IPL-derived branched GDGTs suggests that branched GDGTs are likely of autochthonous origin. Temperature estimates based on their distribution using lake-specific calibrations agree reasonably with water temperature at time of sampling and average air temperature of the season of sampling. Importantly, a strong correlation between the distribution of branched GDGTs and lake water pH was found (r2 0.72), in agreement with a predominant in situ production. An stronger correlation was found with lake water alkalinity (r2 0.83), although the underlying mechanism that controls the relationship is not understood. Our results raise the potential for reconstructing pH/alkalinity of past lake environments, which could provide important knowledge on past developments in lake water chemistry.  相似文献   

13.
微生物细胞膜脂甘油二烷基甘油四醚（GDGTs）样品在实验室冰箱储存过程中可能会遭受降解,进而对GDGTs各指标应用的准确性产生影响.了解GDGTs各类化合物抗降解能力的差异能够为指标的准确应用提供重要的判别手段.2017年,通过对2012年的石笋样品提取物（GDGTs）进行二次测试,发现GDGTs化合物绝对含量明显减少且各化合物的相对含量变化明显:细菌brGDGTs含量相对于古菌isoGDGTs含量变化较小,对应的干旱化指标Ri/b值略有减小,陆源输入指数BIT值增大,故细菌brGDGTs化合物在保存过程中更稳定;古菌isoGDGTs含环少的化合物变化较小,环化指数CBT值增加,表明少环的化合物在降解过程中更稳定;基于古菌isoGDGTs建立的古温度指标TEX86值显著降低;基于细菌brGDGTs建立的甲基化指数MBT值增加,表明甲基越多的化合物越易降解.   相似文献   

14.
Glycerol dialkyl glycerol tetraether (GDGT) lipids are membrane lipids which were long thought to be synthesized mainly by archaea, organisms thought to be limited to extreme environments. Analysis of environmental samples over the last decade has shown, however, that their structural diversity and sources are much wider than anticipated and that they occur ubiquitously in a wide range of environments, such as oceans and lakes, and their (sub)surface sediments, as well as soils. Several GDGTs have been unambiguously identified and can be used as biomarker lipids, since they are preserved in immature sediments <140 Ma. Close examination of the distributions has led to the discovery that GDGTs might be used as proxies for certain environmental parameters, such as the input of soil organic matter to marine environments, soil pH, air temperature and sea and lake water temperature. Here, we review the progress made over the last decade in the analysis, occurrence and recognition of sources of GDGTs, their applications as biomarker lipids, and the development and application of proxies based on their distributions.  相似文献   

15.
Archaeal and bacterial glycerol dialkyl glycerol tetraether lipids (GDGTs) are used in various proxies, such as TEX86 and the BIT index. In living organism, they contain polar head groups (intact polar lipids – IPLs). IPL GDGTs have also been detected in ancient marine sediments and it is unclear whether or not they are fossil entities or are part of living cells. In order to determine the extent of degradation of IPL GDGTs over geological timescales, we analyzed turbidite deposits, which had been partly reoxidized for several kyr after deposition on the Madeira Abyssal Plain. Analysis of core lipid (CL) and IPL-derived GDGTs showed a reduction in concentration by two orders of magnitude upon post-depositional oxidation, while IPL GDGTs with a mono- or dihexose head group decreased by 2–3 orders of magnitude. The BIT index for CL- and IPL-derived GDGTs increased substantially upon oxidation from 0.1 to up to 0.5. Together with changing MBT/CBT values, this indicates preferential preservation of soil-derived branched GDGTs over marine isoprenoid GDGTs, combined with in situ production of branched GDGTs in the sediment. The TEX86 value for IPL-derived GDGTs decreased by 0.07 upon oxidation, while that of CL GDGTs showed no significant change. Isolation of IPLs revealed that the TEX86 value for monohexose GDGTs was 0.55, while the that for dihexose GDGTs was substantially higher, 0.70. Thus, the decrease in TEX86 for IPL-derived GDGTs was in agreement with the dominance of monohexose GDGTs in the oxidized turbidite, probably caused by a combination of in situ production as well as selective preservation of terrestrial isoprenoid GDGTs. Due to the low amount of IPL GDGTs vs. CL GDGTs, the impact of IPL degradation on CL-based TEX86 paleotemperature estimates was negligible.  相似文献   

16.
This study of five small (<3.0 ha) lakes in southwestern Greenland examines the veracity of branched glycerol dialkyl glycerol tetraethers (br GDGTs) as a temperature proxy in lacustrine systems. The proximity (<5 km) of the lakes suggests that their temperature history, and thus their br GDGT records, should be similar. Distributions of br GDGTs in (i) surface sediments from all five lakes, (ii) 14C-dated sediment cores from two lakes (Upper and Lower EVV Lakes) and (iii) soil samples from the area surrounding the lakes were examined. The temporal records of br GDGT-based temperature for the two cores exhibited both similarities and major discrepancies. The differences between the paleotemperature records for the two lakes suggest that br GDGTs are not solely soil-derived, reflecting air temperature, but also indicate an additional br GDGT contribution from another source. Among the broader suite of lake sediments, there was a strong correlation (R2 0.987) between br GDGT-based surface sediment temperatures and measured summer bottom water temperatures for the four lakes with hypoxic/anoxic bottom waters, including Upper EVV Lake. The correlation suggests production of br GDGTs by anaerobic bacteria within the bottom water and/or sediment–water interface, reflecting environmental temperature for the individual lakes and augmenting the uniform, soil-derived signal. Hence, assessment of br GDGTs in Greenland lake sediments provides evidence for their origin from anaerobic autochthonous bacteria and indicates that interpretation of lacustrine br GDGT-based paleotemperature records requires contextual knowledge of individual lake systems and potential source(s) of sedimentary br GDGTs.  相似文献   

17.
Peatlands are widespread and important natural archives of environmental change. Here we explore the potential of the recently introduced MBT-CBT proxy (methylation index and cyclisation ratio of branched tetraethers) to estimate past annual mean air temperature (MAT) based on the distribution of bacterially-derived branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in peat and coal. To this end, branched GDGTs in an ombrotrophic peat bog from Switzerland and three coal deposits of increasing maturity were analysed.For the surface of the bog, reconstructed annual MAT is higher than both measured annual MAT and measured in situ pore water temperature. Changes in the CBT ratio, considered a proxy for pH, with depth in the bog do not match with present day in situ pore water pH, but coincide with a peat stratigraphic boundary. This indicates that GDGTs down the bog profile are predominantly fossil and not derived from extant biomass. The MBT-CBT derived annual MAT record also shows a large drop at this stratigraphic boundary, which likely relates to past change in trophic status of the bog. Branched GDGTs are abundant in an immature lignite (vitrinite reflectance, Ro 0.25%), but occur in low amount in a slightly more mature coal (Ro 0.32%). Annual MAT could be reconstructed for the lignite alone and is higher than other proxy-based estimates from approximately the same time and location.Our results indicate potential for the application of the MBT-CBT proxy in peat and immature coals, but improved constraints on the effects of different types of peat on branched GDGT distributions as well as improved calibration of MAT estimates are needed before the method can be confidently applied.  相似文献   

18.
The endorheic Lake Van in eastern Anatolia (Turkey) is the world’s largest soda lake and it is an important site in paleoclimate studies to understand past continental conditions in western Asia. In order to gain further insights into the biomarker signatures in Lake Van’s sediments we have analyzed particulate material in sediment traps deployed between August 2006 and July 2007. The biomarkers used were long chain alkenones (LCAs C37-C39, haptophyte lipids), isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs, Archaea membrane lipids) and pigments (chlorins and fucoxanthin). The biomarker fluxes indicate a strong seasonality in export primary productivity and the phytoplankton community structure. The highest total mass and organic carbon fluxes were found in summer, coupled to strong stratification while the lowest mass fluxes occurred in winter at the time of water column mixing. With increasing temperatures in early spring, phytoplankton export productivity grew, coupled with an increase of total mass flux and organic carbon, which might be associated to enhanced nutrient input from snowmelt runoff. The percentage of C37:4 shows some correspondence with observed seasonal changes in Lake Van’s stratification structure. We also evaluated the potential applicability of molecular temperature proxies derived from Archaea and haptophyte lipids. The use of the TEX86 proxy was precluded by low GDGT abundances. Estimated LCA temperatures were consistent with temperatures in the photic zone but no seasonality changes were observed despite the wide annual temperature range measured at Lake Van.  相似文献   

19.
A diverse collection of globally distributed soil samples was analyzed for its glycerol dialkyl glycerol tetraether (GDGT) membrane lipid content. Branched GDGTs, derived from anaerobic soil bacteria, were the most dominant and were found in all soils. Isoprenoid GDGTs, membrane lipids of Archaea, were also present, although in considerably lower concentration. Crenarchaeol, a specific isoprenoid membrane lipid of the non-thermophilic Crenarchaeota, was also regularly detected and its abundance might be related to soil pH. The detection of crenarchaeol in nearly all of the samples is the first report of this type of GDGT membrane lipid in soils and is in agreement with molecular ecological studies, confirming the widespread occurrence of non-thermophilic Crenarchaeota in the terrestrial realm. The fluvial transport of crenarchaeol and other isoprenoid GDGTs to marine and lacustrine environments could possibly bias the BIT index, a ratio between branched GDGTs and crenarchaeol used to determine relative terrestrial organic matter (TOM) input. However, as crenarchaeol in soils is only present in low concentration compared to branched GDGTs, no large effect is expected for the BIT index. The fluvial input of terrestrially derived isoprenoid GDGTs could also bias the TEX86, a proxy used to determine palaeo surface temperatures in marine and lacustrine settings and based on the ratio of cyclopentane-containing isoprenoid GDGTs in marine and lacustrine Crenarchaeota. Indeed, it is shown that a substantial bias in TEX86-reconstructed sea and lake surface temperatures can occur if TOM input is high, e.g. near large river outflows.  相似文献   

20.
Branched glycerol dialkyl glycerol tetraethers (GDGTs) are membrane lipids of unknown bacteria that are ubiquitous in soil and peat. Two indices based on the distribution of these lipids in soils, the Cyclization of Branched Tetraethers (CBT) and the Methylation of Branched Tetraethers (MBT) indices have been shown to correlate with soil pH, and mean annual air temperature (MAT) and soil pH, respectively, and can be used to reconstruct MAT in palaeoenvironments. To verify the extent to which branched GDGTs in marine sediments reflect the distribution pattern on land and whether these proxies are applicable for palaeoclimate reconstruction in high latitude environments with a MAT of <0 °C, we compared the branched GDGT distribution in Svalbard soils and nearby fjord sediments. Although branched GDGT concentrations in the soil are relatively low (0.02–0.95 μg/g dry weight (dw)) because of the cold climate and the short growing season, reconstructed MATs based on the MBT/CBT proxy are ca. ?4 °C, close to the measured MAT (ca. ?6 °C). Concentrations of branched GDGTs (0.01–0.20 μg/g dw) in fjord sediments increased towards the open ocean and the distribution was strikingly different from that in soil, i.e. dominated by GDGTs with one cyclopentane moiety. This resulted in MBT/CBT-reconstructed MAT values of 11–19 °C, well above measured MAT. The results suggest that at least part of the branched GDGTs in marine sediments in settings with a low soil organic matter (OM) input may be produced in situ. In these cases, the application of the MBT/CBT palaeothermometer will generate unrealistic MAT reconstructions. The MBT/CBT proxy should therefore only be used at sites with a substantial input of soil OM relative to the amount of marine OM, i.e. at sites close to the mouth of rivers with a catchment area where sufficient soil formation takes place and the soil thus contains substantial amounts of branched GDGTs.  相似文献   

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