Differential degradation of intact polar and core glycerol dialkyl glycerol tetraether lipids upon post-depositional oxidation

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.     

Stable carbon isotopic compositions of intact polar lipids reveal complex carbon flow patterns among hydrocarbon degrading microbial communities at the Chapopote asphalt volcano

Seepage of asphalt forms the basis of a cold seep system at 3000 m water depth at the Chapopote Knoll in the southern Gulf of Mexico. Anaerobic microbial communities are stimulated in the oil-impregnated sediments as evidenced by the presence of intact polar membrane lipids (IPLs) derived from archaea and Bacteria at depths up to 7 m below the seafloor. Detailed investigation of stable carbon isotope composition (δ¹³C) of alkyl and acyl moieties derived from a range of IPL precursors with distinct polar head groups resolved the complexity of carbon metabolisms and utilization of diverse carbon sources by uncultured microbial communities. In surface sediments most of the polar lipid-derived fatty acids with phosphatidylethanolamine (PE), phosphatidylglycerol (PG) and diphosphatidylglycerol (DPG) head groups could be tentatively assigned to autotrophic sulfate-reducing bacteria, with a relatively small proportion involved in the anaerobic oxidation of methane. Derivatives of phosphatidyl-(N)-methylethanolamine (PME) were abundant and could be predominantly assigned to heterotrophic oil-degrading bacteria. Archaeal IPLs with phosphate-based hydroxyarchaeols and diglycosidic glyceroldibiphytanylglyceroltetraethers (GDGTs) were assigned to methanotrophic archaea of the ANME-2 and ANME-1 cluster, respectively, whereas δ¹³C values of phosphate-based archaeols and mixed phosphate-based and diglycosidic GDGTs point to methanogenic archaea. At a 7 m deep sulfate-methane transition zone that is linked to the upward movement of gas-laden petroleum, a distinct increase in abundance of archaeal IPLs such as phosphate-based hydroxyarchaeols and diglycosidic archaeol and GDGTs is observed; their δ¹³C values are consistent with their origin from both methanotrophic and methanogenic archaea. This study reveals previously hidden, highly complex patterns in the carbon-flow of versatile microbial communities involved in the degradation of heavy oil including hydrocarbon gases that would not have been evident from classical compound-specific isotope analyses of either bulk IPL or apolar lipid derivatives.     

Comparison of extraction and work up techniques for analysis of core and intact polar tetraether lipids from sedimentary environments

Glycerol dibiphytanyl glycerol tetraether-based intact polar lipids (IPL GDGTs) are used as biomarkers for living Archaea and are analyzed utilizing a variety of extraction and quantification techniques. Most IPL GDGT studies have used a modified Bligh–Dyer extraction method, but it has been suggested that Soxhlet extraction may be more efficient for environmental samples and biomass. We investigated the impact of three different extractions (Soxhlet, Bligh–Dyer and accelerated solvent extraction, ASE), two IPL quantification methods and two work up techniques (Na₂SO₄ and SiO₂ column) on the amount and distribution of CL (core lipid)- and IPL-derived GDGTs and crenarchaeol-based IPLs in marine sediments from the Arabian Sea and Icelandic shelf, as well as a microbial mat from a Dutch beach. The different extraction procedures gave a similar yield of CL- and IPL-derived GDGTs. Direct analysis of crenarchaeol IPLs showed, however, that, while GDGTs with a monohexose head group were not affected by the extraction method, there was a large effect on IPL GDGTs containing dihexose or hexose, phosphohexose head groups. Quantification of IPL-derived GDGTs by way of either separation over a silica column or by subtraction of CL GDGTs in the total lipid extract before and after hydrolysis gave similar results, but the ‘subtraction-method’ had a relatively large quantification error. However, the silica column, as well as drying over a Na₂SO₄ column, resulted in a loss of the hexose, phosphohexose IPLs by up to 80%. Based on the results, a modified Bligh–Dyer extraction with as little further treatment as possible is recommended to allow measurement of the full range of IPL GDGTs in sediments.     

Identification and distribution of intact polar branched tetraether lipids in peat and soil

Branched glycerol dialkyl glycerol tetraether lipids (GDGTs) are membrane lipids of soil bacteria that occur ubiquitously in soil, but their occurrence as intact polar lipids (IPLs) has not been well studied. Here, we report the identification and distribution of IPL-branched GDGTs throughout a depth profile of a Swedish peat bog. In addition to two reported glycosidic IPL branched GDGTs, we identified IPL branched GDGTs with a hexose-glycuronic acid, phospho-hexose, or hexose-phosphoglycerol head group, based on mass spectrometry. A selected reaction monitoring (SRM) assay was developed to monitor changes in head group distribution with depth. The abundance of the IPL branched GDGTs increased below the water table, suggesting that they were primarily produced in this part of the peat. This was supported by the concentrations of core lipid and IPL-derived branched GDGTs, which also substantially increased below the water table. However, individual IPL trends differed, which may be due to changes in the microbial community composition with depth or to different degradation rates for the different IPL branched GDGTs. The SRM method was also applied to two different soil types, which showed that similar IPL branched GDGTs as those in peat were present, albeit with different distributions.     

Separation of core and intact polar archaeal tetraether lipids using silica columns: Insights into living and fossil biomass contributions

Intact polar lipids (IPLs) are frequently used as biomarkers for living microbial cells and can be separated from core lipids (i.e. lipids without polar headgroups), which are mainly derived from fossil (i.e. dead) cell material, using column chromatography. We have compared the effect of various silica column conditions on the separation and recovery of archaeal glycerol dialkyl glycerol tetraether (GDGT) core lipids, glycolipids and phosphoglycolipids using authentic standards and direct analysis with various high performance liquid chromatography-mass spectrometry (HPLC-MS) techniques. The commonly used procedure to separate these compound classes using dichloromethane, acetone and methanol as eluents, respectively, did not separate core GDGTs from glyco- and phosphoglyco-GDGTs. In contrast, a recently described procedure using hexane:ethyl acetate (3:1, v:v), ethyl acetate and methanol achieved both high recovery and successful separation of core GDGTs from the other IPLs. Application of the method to a geothermally heated soil and suspended particulate matter from the North Sea showed that it separates most of the core GDGTs from the other IPLS and that considerable qualitative and quantitative differences can occur between core and IPL-GDGTs. We conclude that the method is therefore appropriate for the separation of intact archaeal IPLs and their fossil analogues.     

Fossilization and degradation of intact polar lipids in deep subsurface sediments: A theoretical approach

Intact polar membrane lipids (IPLs) are frequently used as markers for living microbial cells in sedimentary environments. The assumption with these studies is that IPLs are rapidly degraded upon cell lysis and therefore IPLs present in sediments are derived from in situ microbial production. We used a theoretical approach to assess whether IPLs in surface sediments can potentially represent fossilized IPLs derived from the upper part of the water column and whether IPLs can be preserved during sediment burial. Previous studies which examined the degradation kinetics of IPLs show that phospholipids, i.e. ester-linked lipids with a phosphor-containing head group, degrade more rapidly than glycosidic ether lipids, i.e. ether-linked lipids with a glycosidically bound sugar moiety. Based on these studies, we calculate that only a minor fraction of phospholipids but a major fraction of glycosidic ether lipids biosynthesized in the upper part of the water column can potentially reach deep-sea surface sediments. Using a simple model and power law kinetic degradation parameters reported in the literature, we also evaluated the degradation of IPLs during sediment burial. Our model predicts a log-log relationship between IPL concentrations and depth, consistent with what has been observed in studies of IPLs in subsurface sediments. Although our results do not exclude production of IPLs in subsurface sediment, they do suggest that IPLs present in the deep biosphere may contain a substantial fossil component potentially masking in situ IPL production.     

Assessing the ratio of archaeol to caldarchaeol as a salinity proxy in highland lakes on the northeastern Qinghai–Tibetan Plateau

The ratio of archaeol to caldarchaeol (the ACE index) has been proposed recently as an index for paleosalinity reconstruction and is based principally on archaeal core lipids (CLs) from coastal salt pans (Turich, C., Freeman, K.H., 2011. Archaeal lipids record paleosalinity in hypersaline systems. Organic Geochemistry 42, 1147–1157). We have examined possible relationships between salinity and ACE in both CLs and intact polar lipids (IPLs) from suspended particulate matter (SPM) and surface sediments of lakes and surrounding soils on the northeastern Qinghai–Tibetan Plateau. Our results showed that ACE values were positively correlated with salinity in all samples; however, CL ACE values were systematically higher than IPL ACE values, probably due to different degradation kinetics of intact polar (IP) archaeol and IP caldarchaeol. On the other hand, surface sediment ACE values from both CLs and IPLs were lower than SPM ACE values, probably due to enhanced production of caldarchaeol relative to archaeol in the sediment. Our results demonstrate that the ACE proxy reflects changes in salinity in diverse environments on the Qinghai–Tibetan Plateau, which is promising for paleosalinity reconstruction; however, caution should be used when applying the salinity proxy before we have a better understanding of degradation kinetics of archaeal IPLs and in situ production of caldarchaeol and archaeol in sediments.     

Fluxes and distribution of tetraether lipids in an equatorial African lake: Constraints on the application of the TEX86 palaeothermometer and BIT index in lacustrine settings

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.     

Influence of lake water pH and alkalinity on the distribution of core and intact polar branched glycerol dialkyl glycerol tetraethers (GDGTs) in lakes

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.     

南海东北部岩芯沉积物磁性特征及对甲烷事件的指示

在甲烷渗漏海域,沉积物磁化率通常表现出异常的低值特征,这与硫酸盐-甲烷转换带（SMTZ）内甲烷厌氧氧化反应（AOM）的发育而导致的自生矿物的形成作用有关。通过测定南海东北部Site DH-CL11、Site 973-2、Site 973-4三个站位400个岩芯沉积物样品的磁化率,并结合三个站位自生黄铁矿丰度和硫同位素等数据探讨了南海北部天然气水合物潜在区沉积物磁化率的变化特征及其对甲烷渗漏事件的指示意义。结果表明:在甲烷异常渗漏海域,上涌甲烷与下渗硫酸盐在SMTZ内发生AOM反应生成了大量的HS-,造成亚铁磁性矿物大量溶解,同时生成大量顺磁性自生黄铁矿,导致沉积物磁化率的异常降低;但是,在HS-不足时,铁硫化物黄铁矿化不充分,会优先生成胶黄铁矿,进而出现二次磁信号。在天然气水合物潜在海域,沉积物磁化率的异常特征可以反映下部甲烷通量的变化,从而指示下伏天然气水合物藏演化,因此能够成为探测天然气水合物藏的一种间接有效的手段,将有助于我国南海北部海域天然气水合物的勘探。     

台西南深海底栖有孔虫及其5万年来冷泉微生境变迁记录

为探讨冷泉区底栖有孔虫组合特征、受控因素及其冷泉微生境随时间的变迁,本文对台西南海盆取自九龙甲烷礁和海洋四号区冷泉区的973-4和973-5两根岩心展开了底栖有孔虫及其壳体氧碳同位素研究。测年结果揭示两根岩心为晚更新世约5万年来海洋氧同位素期(MIS)MIS 3至MIS 1早期的沉积序列。两个站位共识别了底栖有孔虫79属233种,优势类别在973-4组合中为Uvigerina(23.3%)、Bulimina(10.71%)和Cibicidoides(9.87%),在973-5组合中是Bulimina(20.6%),两站位的组合优势和常见属种均以内生类别为主。有孔虫分异度显然同时受到正常深海环境因子TOC和沉积物粒度的影响。总体上,优势和常见类群与TOC相关性较弱,但与δ¹⁸O_{Uvigerina spp.}有不同程度的相关性,说明有孔虫还受冷泉特殊营养物质和流体因子影响。5万年来,973-4和973-5站位底栖有孔虫组合生活的冷泉微生境,经历了由双壳-自生碳酸盐岩(MIS 3至MIS 2早期)向双壳-菌席(MIS 2晚期至MIS 1早期)的变迁。底栖有孔虫的优势类群也随微生境的变迁而演替,如973-4站位MIS 3-MIS 1的优势类群依次为U.peregrina、Cibicidoides-Bulimina、U.vadescens和Cibicides,973-5站位为Chilostomella+ Globobulimina、Cibicidoides、Bulimina。有孔虫壳体氧碳同位素特征也随时间改变,从MIS 3到MIS 2早期,在自生碳酸盐岩水岩交换背景下,具有富δ¹⁸O和亏损δ¹³C特点(3.5‰~4.49‰,-2‰~-0.2‰); MIS 2晚期—MIS 1早期因双壳和菌席的生物地球化学作用影响,具有略富集δ¹⁸O和略微亏损δ¹³C的特征(2.5‰~3.5‰,-1‰~-0.1‰)。自5万年来两个区甲烷渗漏逐渐减弱,其间发生了几次增强事件。973-4站位记录了1次持续时间约10 ka的增强事件(35~25 ka);973-5站位记录3次(45 ka,35 ka,14~12 ka)。其中,45 ka时海底上涌的甲烷通量可能最大,在海底表面形成水合物。35 ka时的甲烷喷溢增强事件可能为区域性事件。     

Absence of seasonal patterns in MBT-CBT indices in mid-latitude soils

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.     

Distribution of intact and core GDGTs in marine sediments

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.     

Advection and diffusion determine vertical distribution of microbial communities in intertidal sediments as revealed by combined biogeochemical and molecular biological analysis

Porewater advection stimulates nutrient exchange and microbial activity in shallow marine sediments, whereas element cycling in deeper diffusion-dominated sediments is comparatively slow due to limited nutrient supply. We studied the vertical distribution of microbial communities and organic matter (OM) cycling in these contrasting porewater regimes down to 5 m depth at an intertidal flat of the southern North Sea. Archaea, Bacteria and Eukarya were targeted, combining intact polar lipid (IPL) analysis with qualitative and quantitative molecular biological techniques. The largely sandy section 1 of the core (<75 cm) is characterized by rapid burial of fresh marine OM and intense porewater advection. This supply fuels heterotrophic microbes, as evident from the ¹³C isotopic composition of total organic carbon and IPL derivatives. Major sources of OM are algae and cyanobacteria, as suggested by the elevated amount of eukaryotic 18S rRNA gene copies and phosphate-free IPLs. The relative abundance of most phospholipids remained largely constant over the entire core, except for diphosphatidylglycerol, which represented about half of total IPL abundance in the lower part of section 1 (>50 cm) and the diffusion-dominated section 2 (75–490 cm). This suggests bacteria adapting their membranes in response to increasing physicochemical stress and starvation in the nutrient limited, fine grained sediments of section 2 with less bioavailable, predominantly terrestrial, OM. Relative amounts of bacterial acyl ether and diether phospholipids increased in this lower section and were assigned to sulfate reducers and yet uncultured myxobacteria. Archaea were an order of magnitude less abundant than Bacteria, and were affiliated mainly with Methanosarcinales and Methanomicrobiales. Accordingly, the archaeal IPL composition was typical for a methanogenic community. IPLs not exclusively derived from in situ microbial production emphasize that these biomarkers have to be interpreted with caution in sediments with complex hydrogeology. Our results demonstrate that contrasting subsurface flow regimes significantly impact on the vertical zonation of biogeochemical properties and microorganisms in marine sediments.     

Seasonal changes in glycerol dialkyl glycerol tetraether concentrations and fluxes in a perialpine lake: Implications for the use of the TEX86 and BIT proxies

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.     

Distribution of bacterial and archaeal ether lipids in soils and surface sediments of Tibetan lakes: Implications for GDGT-based proxies in saline high mountain lakes

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.     

Archaeal lipids in Neogene dolomites (Monterey and Sisquoc Formations, California) - Planktic versus benthic archaeal sources

The distribution of archaeal lipids, including archaeol and glycerol dibiphytanyl glycerol tetraethers (GDGTs), in dolomite concretions and surrounding sediment from the Monterey Formation (Miocene) and the Sisquoc Formation (Miocene-Pliocene) were examined to distinguish planktic from benthic contributions. For this purpose, dolomites with positive δ¹³C values (+7‰ to +13‰) were chosen; such highly positive values point to pronounced methanogenesis of benthic archaea in the sedimentary column. At first glance, distributions and relative abundances of GDGTs in both dolomites and background sediment were similar, resembling patterns of marine planktic crenarchaea. A contribution of benthic euryarchaea to the GDGT pool became evident only from variations in the δ¹³C values of different biphytanes obtained after ether cleavage of GDGTs. Whereas bi- and tricyclic biphytanes had an isotopic signal typical of planktic archaea (δ¹³C −23.6‰ to −20.5‰ and −23.4‰ to −21.2‰, respectively) for both dolomite and background sediment, acyclic and monocyclic biphytanes showed lower values for dolomite samples (−25.1‰ to −22.6‰ and −27.6‰ to −24.7‰, respectively), indicating a contribution of lipids from benthic archaea. The isoprenoid diether archaeol (δ¹³C −23.9‰ to −22.9‰), assigned to euryarchaea, was only detected in dolomite samples, also reflecting additional input from sedimentary archaea, probably autotrophic methanogens. The occurrence of lipids derived from methanogenic archaea agrees with the strong ¹³C-enrichment of dolomites and with mineral formation taking place in the zone of archaeal methanogenesis. This implies that the lipid biomarker inventory of sedimentary strata needs to be interpreted carefully, as it is often not straightforward to discriminate between input from the water column and sedimentary microbial activity.     

Occurrence and distribution of tetraether membrane lipids in soils: Implications for the use of the TEX86 proxy and the BIT index

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.     

Distributions of branched GDGTs in soils and lake sediments from western Uganda: Implications for a lacustrine paleothermometer

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.     

http://www.sciencedirect.com/science/article/pii/S1674987111001332

Isoprenoidal glycerol dialkyl glycerol tetraethers（iGDGTs） from the Gulu hot springs （23-83.6℃,pH>7） and Yangbajing hot springs（80—128℃,pH>7） were analyzed in order to investigate the distribution of archaeal lipids among different hot springs in Tibet.A soil sample from Gulu was incubated at different temperatures and analyzed for changes in iGDGTs to help evaluate whether surrounding soil may contribute to the iGDGTs in hot springs.The sources of bacterial GDGTs（bGDGTs） in these hot springs were also investigated.The results revealed different profiles of iGDGTs between Gulu and Yangbajing hot springs. Core iGDGTs and polar iGDGTs also presented different patterns in each hot spring.The PCA analysis showed that the structure of polar iGDGTs can be explained by three factors and suggested multiple sources of these compounds.Bivariate correlation analysis showed significant positive correlations between polar and core bGDGTs.suggesting the in situ production of bGDGTs in the hot springs.Furthermore,in the soil incubation experiment,temperature had the most significant influence on concentration of bGDGTs rather than iGDGTs.and polar bGDGTs had greater variability than core bGDGTs with changing temperature.Our results indicated that soil input had little influence on the composition of GDGTs in Tibetan hot springs. On the other hand,ring index and TEX₈₆ values were both positively correlated with incubation temperature, suggesting that the structure of archaeal lipids changed in response to varying temperature during incubation.