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1.
We studied the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) in suspended particulate matter from the water column of Lake Tanganyika (East Africa), where sediment studies had shown the applicability of the TEX86 proxy for reconstructing surface lake water temperature. GDGTs, in particular crenarchaeol, showed maximum abundance within the suboxic zone (100–180 m), suggesting that this is the preferred niche of ammonia-oxidizing Thaumarchaeota. Despite evidence for anaerobic methane oxidation in deep anoxic water (300–1200 m) no unambiguous evidence for an imprint of methanotrophic archaea on GDGT distribution was found. Comparison of TEX86 and BIT indices with those of surface sediments suggests that the sedimentary GDGTs are derived predominantly from the oxic zone and suboxic zone of the lake.  相似文献   

2.
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   

3.
Isoprenoid and branched glycerol dialkanol diethers (iso GDDs and br GDDs) have recently been detected in various environments, including sediment, soil and peat deposits. Their structures strongly resemble those of glycerol dialkyl glycerol tetraethers (GDGTs). Nevertheless, the origin of GDDs and their link to GDGTs remain unclear. Here we examined the concentration and distribution of iso GDDs and br GDDs, together with those of iso GDGTs and br GDGTs along a 4 m peat core from Tanzania. Whereas br GDDs have only been detected to date in their core lipid (CL) form, we report here, for the first time, br GDDs in both CL and intact polar lipid (IPL) forms, suggesting a biosynthetic origin for br GDDs. Concentrations of br GDGTs and br GDDs on one hand and of iso GDGTs and iso GDDs on the other hand, were observed to significantly co-vary (R2 0.49–0.58; p < 0.05), both for the CL and IPL fractions. Moreover, the fractional abundance of each GDD correlated significantly with that of its GDGT analogue (R2 0.33–0.97; p < 0.05). Taken together, the data show that GDDs and GDGTs are closely related and likely take part in common metabolic pathways, although the hypothesis of GDDs being degradation products of GDGTs cannot be excluded.  相似文献   

4.
We evaluate the application of the branched glycerol dialkyl glycerol tetraether (br GDGT) based palaeotemperature and palaeoenvironmental proxy to a hyper-eutrophic, polluted and shallow oxic lake. Lake Zeekoevlei is the largest freshwater lake in South Africa, located close to Cape Town. We use published lake-based and soil-based calibration equations, and compare the reconstructed mean annual air temperature (MAT) with regional (South African) and local (Cape Town) instrumental temperature records. The distribution of br GDGTs in the lake sediments is influenced by air temperature. The lake-based calibration equation, which uses the methylation index of branched tetraethers/cyclisation ratio of branched tetraethers (MBT/CBT), formulated for African lakes (Tierney et al., 2010), fits well with the instrumental temperature records. Moreover, the CBT-derived pH likely reflects historic socioeconomic changes in catchment. Our results suggest that a polluted/hyper-eutrophic status and shallow water urban setting do not preclude application of the MBT/CBT-MAT proxy. However, further research is necessary to understand the behaviour of br GDGT–producing bacteria in polluted and highly productive lakes.  相似文献   

5.
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   

6.
Quantitative climate reconstructions are fundamental to understand long-term trends in natural climate variability and to test climate models used to predict future climate change. Recent advances in molecular geochemistry have led to calibrations using glycerol dialkyl glycerol tetraethers (GDGTs), a group of temperature-sensitive membrane lipids found in Archaea and bacteria. GDGTs have been used to construct temperature indices for oceans (TEX86 index) and soils (MBT/CBT index). The aim of this study is to examine GDGT-temperature relationships and assess the potential of constructing a GDGT-based palaeo-thermometer for lakes. We examine GDGT-temperature relationships using core top sediments from 90 lakes across a north-south transect from the Scandinavian Arctic to Antarctica including sites from Finland, Sweden, Siberia, the UK, Austria, Turkey, Ethiopia, Uganda, Chile, South Georgia and the Antarctic Peninsula. We examine a suite of 15 GDGTs, including compounds used in the TEX86 and MBT/CBT indices and reflecting the broad range of GDGT inputs to small lake systems.GDGTs are present in varying proportions in all lakes examined. The TEX86 index is not applicable to our sites because of the large relative proportions of soil derived and methanogenic components. Similarly, the MBT/CBT index is also not applicable and predicts temperatures considerably lower than those measured. We examine relationships between individual GDGT compounds and temperature, pH, conductivity and water depth. Temperature accounts for a large and statistically independent fraction of variation in branched GDGT composition. We propose a GDGT-temperature regression model with high accuracy and precision (R2 = 0.88; RMSE = 2.0 °C; RMSEP = 2.1 °C) for use in lakes based on a subset of branched GDGT compounds and highlight the potential of this new method for reconstructing past temperatures using lake sediments.  相似文献   

7.
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   

8.
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   

9.
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   

10.
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   

11.
Lake sediments from four sites in the southwest Yukon Territory, Canada, provided paleotemperature records for the past 2000 yr. An alpine and a forest site from the southeastern portion of the study area, near Kluane Lake, and another alpine-forest pair of lakes from the Donjek River area located to the northwest yielded chironomid records that were used to provide quantitative estimates of mean July air temperature. Prior to AD 800, the southwest Yukon was relatively cool whereas after AD 800 temperatures were more variable, with warmer conditions between ~ AD 1100 and 1400, cooler conditions during the Little Ice Age (~ AD 1400 to 1850), and warming thereafter. These records compare well with other paleoclimate evidence from the region.  相似文献   

12.
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13.
In ocean margin sediments both marine and terrestrial organic matter (OM) are buried but the factors governing their relative preservation and degradation are not well understood. In this study, we analysed the degree of preservation of marine isoprenoidal and soil-derived branched glycerol dialkyl glycerol tetraethers (GDGTs) upon long-term oxygen exposure in OM-rich turbidites from the Madeira Abyssal Plain by analyzing GDGT concentrations across oxidation fronts. Relative to the anoxic part of the turbidites ca. 7-20% of the soil-derived branched GDGTs were preserved in the oxidized part while only 0.2-3% of the marine isoprenoid GDGT crenarchaeol was preserved. Due to these different preservation factors the Branched Isoprenoid Tetraether (BIT) index, a ratio between crenarchaeol and the major branched GDGTs that is used as a tracer for soil-derived organic matter, substantially increases from 0.02 to 0.4. Split Flow Thin Cell (SPLITT) separation of turbidite sediments showed that the enhanced preservation of soil-derived carbon was a general phenomenon across the fine particle size ranges (<38 μm). Calculations reveal that, despite their relatively similar chemical structures, degradation rates of crenarchaeol are 2-fold higher than those of soil-derived branched GDGTs, suggesting preferential soil OM preservation possibly due to matrix protection.  相似文献   

14.
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   

15.
《Chemical Geology》2007,236(3-4):181-198
Variations in molybdenum isotopic composition, spanning the range of ∼ 2.3‰ in the terms of 97Mo/95Mo ratio, have been measured in sediment cores from three lakes in northern Sweden and north-western Russia. These variations have been produced by both isotopically variable input of Mo into the lakes due to Mo isotopic heterogeneity of bedrock in the drainage basins and fractionation in the lake systems due to temporal variations in limnological conditions. Mo isotope abundances of bedrock in the lake drainage basins have been documented by analysis of Mo isotope ratios of a suite of molybdenite occurrences collected in the studied area and of detrital fractions of the lake sediment cores. The median δ97Mo value of the investigated molybdenites is 0.26‰ with standard deviation of 0.43‰ (n = 19), whereas the median δ97Mo value of detrital sediment fractions from two lakes is − 0.40‰ with standard deviation of 0.36‰ (n = 15).The isotopic composition of Mo in the sediment cores has been found to be dependent on redox conditions of the water columns and the dominant type of scavenging phases. Hydrous Fe oxides have been shown to be an efficient scavenger of Mo from porewater under oxic conditions. Oxidative precipitation of Fe(II) in the sediments resulted in co-precipitation of Mo and significant authigenic enrichment at the redox boundary. In spite of a pronounced increase in Mo concentration associated with Fe oxides at the redox boundary the isotopic composition of Mo in this zone varies insignificantly, suggesting little or no isotope fractionation during scavenging of Mo by hydrous Fe oxides. In a lake with anoxic bottom water a chironomid-inferred reconstruction of O2 conditions in the bottom water through the Holocene indicates that increased O2 concentrations are generally associated with low δ97Mo/95Mo values of the sediments, whereas lowered O2 contents of the bottom water are accompanied by relatively high δ97Mo/95Mo values, thus confirming the potential of Mo isotope data to be a proxy for redox conditions of overlying waters. However, it is pointed out that other processes including input of isotopically heterogeneous Mo and Mn cycling in the redox-stratified water column can be a primary cause of variations in Mo isotopic compositions of lake sediments.  相似文献   

16.
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   

17.
《Quaternary Science Reviews》2007,26(19-21):2420-2437
Lateglacial environments at Hijkermeer, northwest Netherlands, were reconstructed by means of chironomid, diatom and pollen analyses. Diatom assemblages indicate that Hijkermeer was a shallow, oligo- to mesotrophic lake during this period. Pollen assemblages reflect the typical northwest European Lateglacial vegetation development and provide an age assessment for the record from the beginning of the Older Dryas (ca 14 000 calibrated 14C yr BP) into the early Holocene (to ca 10 700 calibrated 14C yr BP). The chironomid record is characterized by several abrupt shifts between assemblages typically found in mid-latitude subalpine to alpine lakes and assemblages typical for lowland environments. Based on the chironomid record, July air temperatures were reconstructed using a chironomid-temperature transfer-function from central Europe. Mean July air temperatures of ca 14.0–16.0 °C are inferred before the Older Dryas, of ca 16.0–16.5 °C during most of the Allerød, of ca 13.5–14.0 °C during the Younger Dryas, and of ca 15.5–16.0 °C during the early Holocene. Two centennial-scale decreases in July air temperature were reconstructed during the Lateglacial interstadial which are correlated with Greenland Interstadial events (GI)-1d and -1b. The results suggest that vegetation changes in the Netherlands may have been promoted by the cooler climate during GI-1d, immediately preceding the Older Dryas biozone, and GI-1b. The Hijkermeer chironomid-inferred temperature record shows a similar temperature development as the Greenland ice core oxygen isotope records for most of the Lateglacial and a good agreement with other temperature reconstructions available from the Netherlands. This suggests that chironomid-based temperature reconstruction can be successfully implemented in the Northwest European lowlands and that chironomids may provide a useful alternative to oxygen isotopes for correlating European lake sediment records during the Lateglacial.  相似文献   

18.
Late-Holocene environmental and climatic conditions were reconstructed from diatom assemblages in sediment cores from four western Montana lakes: Crevice Lake, Foy Lake, Morrison Lake, and Reservoir Lake. The lakes show synchroneity in timing of shifts in diatom community structure, but the nature of these changes differs among the lakes. Two of the sites provide highly resolved records of hydrologic balance, while the other two stratigraphic sequences primarily record temperature impact on lake thermal structure. All four lakes show significant change in five discrete intervals: 2200–2100, 1700–1600, 1350–1200, 800–600, and 250 cal yr BP. The similarities in the timing of change suggest overlying regional climatic influences on lake dynamics. The 800–600 cal yr BP shift is evident in other paleorecords throughout the Great Plains and western US, associated with the transition from the Medieval Climate Anomaly to the Little Ice Age. Large-scale climatic mechanisms that influence these lake environments may result from atmospheric circulation patterns that are driven by interactions between Pacific and Atlantic sea-surface temperatures, which are then locally modified by topography.  相似文献   

19.
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   

20.
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   

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