Dialkyl Glycerol Tetraethers Research Articles

Decadal to interdecadal variability in TEX86 temperature on the inner shelf of the East China Sea over the last 3.7 kyr

To understand the long-term environmental evolution of the East China Sea (ECS) and adjacent areas, the variation in TEX86H-derived sea surface temperature (SST) at site T08 collected from the mud area on the inner continental shelf of the ECS over the last 3725 years was reconstructed. The core-top TEX86H SST is 23.2 °C, close to the annual mean SST (22.9 °C) at site T08. The branched and isoprenoid tetraether (BIT) index, which is a proxy for soil organic matter, ranges from 0.029 to 0.059, indicating that the influence of terrestrial input on the TEX86H SST proxy is negligible. The ratio between glycerol dialkyl glycerol tetraethers (GDGTs) GDGT-0 and GDGT-5 (R0/5), which is used to indicate the contribution of methanogenic archaea to the GDGT pool, varies between 0.04 and 0.62, suggesting a minor influence of methanogens on the TEX86H proxy. During the past 3725 years, variation in TEX86H SST can be divided into three periods with an overall increasing trend. During Period I (3725–2500 yr BP), TEX86H SST ranged from 19.0 to 20.6 °C with a mean of 20.1 °C. The values were generally relatively low but with marked fluctuation, attributed to increased input of cold coastal water caused by a strengthened East Asian winter monsoon (EAWM) and a weakened Kuroshio Current (KC). During Period II (2500–750 yr BP), after a low-temperature period at 2400–2200 yr BP, TEX86H SST increased gradually from 19.2 to 23.5 °C. During this period, an intensified East Asian summer monsoon (EASM) and a strengthened KC and Taiwan Warm Current (TWWC) transported warm water from low-latitude areas to ECS coastal areas. During Period III (750 yr BP to present), TEX86H SST increased gradually and reached a maximum (24.2 °C) at ∼565 yr BP. Subsequently, TEX86H SST decreased and reached a low value (21.9 °C) at ∼279 yr BP. During the period (<750 yr BP), higher TEX86H SST values were caused mainly by intensified KC, whereas lower values corresponded to the Little Ice Age. Spectral analysis reveals that variation in TEX86H SST exhibits periodicities of 175, 80, and 68 years, consistent with the periodicities of solar activity.

Environmental fluctuation impacted the evolution of Early Pleistocene non-human primates: Biomarker and geochemical evidence from Mohui Cave (Bubing, Guangxi, southern China)

Quaternary climate variability, profoundly impacting the evolution and distribution of plants and animals across the globe, has been reconstructed by many sediment archives. Here, we track past changes of temperature, rainfall and plant type by using the n-alkane and glycerol dialkyl glycerol tetraethers (GDGTs), as well as weathering and pedogenesis by mineralogical and geochemical proxies from a 250 cm deep soil sediment profile from the Early Pleistocene deposits in Mohui Cave, southern China. Indices for weathering intensity such as mineral composition (quartz, feldspar and calcite), major element proxies (Ti/Na, Al/Na, Al/Mg, Ti/Mg, CIW, PIA, CPA, CIA), trace element proxies (Ba/Sr and Rb/Sr), and biomarker proxies (OEP, LSR and CPI) show similar secular trends, revealing a three-stage historical accumulation. The high long-chain n-alkane ratios (C31/C29 and (C31+C33)/(C27+C29)) indicate that the vegetation in the region had increases in percentage of woody plants and decreases in herbs during periods of relatively lower temperature and less precipitation, which evidenced from GDGTs, consistent with a weak weathering process influenced by summer monsoon strength. Further, we compared the distribution of these plant types with the taxonomic variation in the vertebrate paleontological fossils excavated from the same profile in Mohui Cave. The distribution of Lufengpithecus appears in only one interval but Gigantopithecus blacki appears throughout the profile, despite an environment that experienced transitional stages from grassland to woodland in relatively cooler and drier conditions. We infer that in face of these climatic changes Lufengpithecus may have preferred to emigrate from the region, while Gigantopithecus blacki may have stayed given that it was limited to staying in its core area.

Evaluation of bacterial glycerol dialkyl glycerol tetraether and &amp;lt;sup&amp;gt;2&amp;lt;/sup&amp;gt;H–&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O biomarker proxies along a central European topsoil transect

Abstract. Molecular fossils, like bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs), and the stable isotopic composition of biomarkers, such as δ2H of leaf wax-derived n-alkanes (δ2Hn-alkane) or δ18O of hemicellulose-derived sugars (δ18Osugar), are increasingly used for the reconstruction of past climate and environmental conditions. Plant-derived δ2Hn-alkane and δ18Osugar values record the isotopic composition of plant source water (δ2Hsource-water and δ18Osource-water), which usually reflects mean annual precipitation (δ2Hprecipiation and δ18Oprecipiation), modulated by evapotranspirative leaf water enrichment and biosynthetic fractionation (εbio). Accuracy and precision of respective proxies should be ideally evaluated at a regional scale. For this study, we analysed topsoils below coniferous and deciduous forests as well as grassland soils along a central European transect in order to investigate the variability and robustness of various proxies and to identify effects related to vegetation. Soil pH values derived from brGDGTs correlate reasonably well with measured soil pH values but are systematically overestimated (ΔpH = 0.6±0.6). The branched vs. isoprenoid tetraether index (BIT) can give some indication whether the pH reconstruction is reliable. Temperatures derived from brGDGTs overestimate mean annual air temperatures slightly (ΔTMA=0.5 ∘C ± 2.4). Apparent isotopic fractionation (εn-alkane/precipitation and εsugar∕precipitation) is lower for grassland sites than for forest sites due to signal damping; i.e. grass biomarkers do not record the full evapotranspirative leaf water enrichment. Coupling δ2Hn-alkane with δ18Osugar allows us to reconstruct the stable isotopic composition of the source water more accurately than without the coupled approach (Δδ2H = ∼-21 ‰ ± 22 ‰ and Δδ18O = ∼-2.9 ‰ ± 2.8 ‰). Similarly, relative humidity during daytime and the vegetation period (RHMDV) can be reconstructed using the coupled isotope approach (ΔRHMDV=∼-17±12). Especially for coniferous sites, reconstructed RHMDV values as well as source water isotope composition underestimate the measured values. This can likely be explained by understorey grass vegetation at the coniferous sites contributing significantly to the n-alkane pool but only marginally to the sugar pool in the topsoils. Vegetation-dependent variable signal damping and εbio (regarding 2H between n-alkanes and leaf water) along our European transect are difficult to quantify but likely contribute to the observed underestimation in the source water isotope composition and RH reconstructions. Microclimate variability could cause the rather large uncertainties. Vegetation-related effects do, by contrast, not affect the brGDGT-derived reconstructions. Overall, GDGTs and the coupled δ2Hn-alkane–δ18Osugar approach have great potential for more quantitative paleoclimate reconstructions.

Simultaneous extraction and chromatographic separation of n-alkanes and alkenones from glycerol dialkyl glycerol tetraethers via selective Accelerated Solvent Extraction

A method is described for simultaneous extraction and chromatographic separation of saturated hydrocarbons (n-alkanes) and unsaturated long-chain ketones (alkenones) from glycerol dialkyl glycerol tetraethers (GDGTs) using selective accelerated solvent extraction (ASE). We show that the method can be used to successfully separate n-alkanes and alkenones from GDGTs in marine sediment samples and n-alkanes from GDGTs in lake sediments and soil samples. A comparison of our method with analysis of samples purified using classical silica column chromatography demonstrates that the new method does not affect the alkenone unsaturation index (U37K'), the TetraEther indeX (TEX86), the Methane Index (MI), the Ring Index (RI), the branched-GDGT-based paleotemperature and soil pH indices, the Branched and Isoprenoid Tetraether (BIT) index, the n-alkane average chain length (ACL) or the carbon preference index (CPI). The new procedure results in higher sample recoveries for n-alkanes and more precise U37K' estimates at low concentrations, and contributes to reduced analyst laboratory time.

Climatic and human impact on the environment: Insight from the tetraether lipid temperature reconstruction in the Beibu Gulf, China

Organic proxies have been widely used to reconstruct temperature and terrestrial input variation at the interface between land and sea. However, the impact of human activities on these proxies has been little constrained, which might cause the misinterpretation of the sediment profile. In order to study the climatic and human impact on the environment, proxies in coastal regions, glycerol dialkyl glycerol tetraethers (GDGTs) in a sediment core spanning 115 years was analyzed from Beibu Gulf, China. The results indicate that isoprenoid GDGTs in the Beibu Gulf were mainly derived from Thaumarchaeota and the branched GDGTs originated mainly from terrestrial soil bacteria. The TEX86 index was applied to reconstruct the temporal SST variation. The results show that variation of TEX86-derived SST is controlled mainly by East Asian Monsoon system and partly by ENSO events. The methylation index of branched tetraethers or the cyclization index of branched tetraethers (MBT/CBT) derived mean annual air temperature (MAAT) agreed with MAAT record at the Qinzhou Station before 1982 AD. However, the population growth and economic development after 1982 AD caused the reduction of terrestrial organic matter input and increase of nutrient, leading to the thriving of Thaumarchaeota and increasing of in situ branched GDGTs proportion, which created the scatter of the MBT/CBT-reconstructed MAAT after 1982 AD. Therefore, human influence on GDGT proxies can be constrained in this way to avoid misinterpretation of paleoclimate and paleoenvironment records.

Distribution of Glycerol Dialkyl Glycerol Tetraethers (GDGTs) in Microbial Mats From Holocene and Miocene Sabkha Sediments

Sabkhas are important settings for understanding early earth biological environments, and the algal mats associated with them are thought to be potential source rocks for hydrocarbon production. In this study we compare the sedimentological facies and distribution of branched, and isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) in modern and mid-Holocene sabkha sediments from Abu Dhabi with well-preserved Miocene (12–13 Ma) sabkha sediments recently recovered at IODP site U1464 off the north coast of Australia. We show that the facies of the Miocene sediments is very similar to Holocene and modern sabkhas. Furthermore, we show that there are distinct patterns of isoprenoid GDGT distributions in the modern sabkha and that these patterns are well preserved in both the buried Holocene and the Miocene algal mats. The sabkha sediments analyzed are characterized by high %GDGT-0 and methane index, dominance of branched GDGTs over crenarchaeol (high BIT index), an unusual distribution of isoprenoid GDGTs 1–3, and also low abundance of archeol. The distribution of branched GDGT differs between modern and Miocene sabkhas and is similar in Miocene sabkha and non-sabkha sediments suggesting that they may be of limited use to distinguish paleo-sabkha sediments. Overall, isoprenoid GDGT distributions appear to be different from those found in modern soils, lakes and marine sediments, as well as from those found in Miocene shallow water sediments right below the sabkha, therefore they could be used in combination with facies analysis to identify paleo-sabkha environments in sedimentary sequences.

Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments

Large fluvial systems, such as the Nile River, allow a basin-scale integrated view of climatological and environmental changes. In this study, we reconstructed the Nile discharge history for the last 20 ka using molecular ratios of glycerol dialkyl glycerol tetraethers (GDGTs) and neodymium (Nd) radiogenic isotopes. By characterizing both the organic and inorganic fractions, we assessed the relevance of the GDGT-based proxies in deltaic environments as tracers of terrigenous origin. A large increase in Nile discharge is documented from 14.8 to 8.4 ka BP reflecting enhanced physical erosion and transport processes from the Ethiopian Traps. We confirmed the primary control of insolation on precipitation on North East Africa through the last 20 ka. The centennial time resolution reached on the sedimentary sequence revealed a step-wise onset and termination of the African Humid Period (AHP) starting at 14.8 ka and ending at 8.4 ka BP, respectively. Our centennial-millennial records allowed furthermore pinpointing the abrupt periods of arid conditions corresponding to the Younger Dryas. These data illustrate the linkage between low and high latitude hydrological variability.

Membrane Lipid Composition of the Moderately Thermophilic Ammonia-Oxidizing Archaeon "Candidatus Nitrosotenuis uzonensis" at Different Growth Temperatures.

"Candidatus Nitrosotenuis uzonensis" is the only cultured moderately thermophilic member of the thaumarchaeotal order Nitrosopumilales (NP) that contains many mesophilic marine strains. We examined its membrane lipid composition at different growth temperatures (37°C, 46°C, and 50°C). Its lipids were all membrane-spanning glycerol dialkyl glycerol tetraethers (GDGTs), with 0 to 4 cyclopentane moieties. Crenarchaeol (cren), the characteristic thaumarchaeotal GDGT, and its isomer (cren') were present in high abundance (30 to 70%). The GDGT polar headgroups were mono-, di-, and trihexoses and hexose/phosphohexose. The ratio of glycolipid to phospholipid GDGTs was highest in the cultures grown at 50°C. With increasing growth temperatures, the relative contributions of cren and cren' increased, while those of GDGT-0 to GDGT-4 (including isomers) decreased. TEX86 (tetraether index of tetraethers consisting of 86 carbons)-derived temperatures were much lower than the actual growth temperatures, further demonstrating that TEX86 does not accurately reflect the membrane lipid adaptation of thermophilic Thaumarchaeota As the temperature increased, specific GDGTs changed relative to their isomers, possibly representing temperature adaption-induced changes in cyclopentane ring stereochemistry. Comparison of a wide range of thaumarchaeotal core lipid compositions revealed that the "Ca Nitrosotenuis uzonensis" cultures clustered separately from other members of the NP order and the Nitrososphaerales (NS) order. While phylogeny generally seems to have a strong influence on GDGT distribution, our analysis of "Ca Nitrosotenuis uzonensis" demonstrates that its terrestrial, higher-temperature niche has led to a lipid composition that clearly differentiates it from other NP members and that this difference is mostly driven by its high cren' content.IMPORTANCE For Thaumarchaeota, the ratio of their glycerol dialkyl glycerol tetraether (GDGT) lipids depends on growth temperature, a premise that forms the basis of the widely applied TEX86 paleotemperature proxy. A thorough understanding of which GDGTs are produced by which Thaumarchaeota and what the effect of temperature is on their GDGT composition is essential for constraining the TEX86 proxy. "Ca Nitrosotenuis uzonensis" is a moderately thermophilic thaumarchaeote enriched from a thermal spring, setting it apart in its environmental niche from the other marine mesophilic members of its order. Indeed, we found that the GDGT composition of "Ca Nitrosotenuis uzonensis" cultures was distinct from those of other members of its order and was more similar to those of other thermophilic, terrestrial Thaumarchaeota This suggests that while phylogeny has a strong influence on GDGT distribution, the environmental niche that a thaumarchaeote inhabits also shapes its GDGT composition.

Sources of organic matter in two contrasting tropical coastal environments: The Caribbean Sea and the eastern Pacific

In this study, we evaluate sources of organic matter (OM) in two contrasting tropical coastal environments in Colombia, using a combination of glycerol dialkyl glycerol tetraethers (GDGTs) and n-alkane lipid biomarkers. A clear difference between the less productive Caribbean Sea and the more productive eastern Pacific is observed, with higher terrestrial contributions into the eastern Pacific strongly associated with riverine inputs. Although higher land plant-derived n-alkanes can be transported as eolian dust, our results suggest mainly riverine sources for our samples. When looking at the branched GDGT compositions, in situ production could be identified in most Caribbean stations and some of the more coastal sites in the Pacific with #ringstetra index values of above 0.7. The remaining stations in the eastern Pacific display soil-eroded GDGTs inputs associated with river discharges and deposited in the proximity of the coast. Marine production as measured by short chain n-alkanes and crenarchaeol are also higher in the eastern Pacific. It appears that the terrestrial inputs would bring nutrients as well as OM fueling both auto- and heterotrophs making the eastern Pacific overall more productive. We also observe OM of petrogenic origin, albeit in a low amount, especially in coastal Caribbean sites located near main harbors. This study helps improve the understanding of the complex mixture of OM inputs, origins and transport mechanisms into Colombian coastal areas. The present work emphasizes the need to integrate climate, geological setting and hydrology to fully understand the carbon cycle at a regional scale.

Assessing the carbon assimilation and production of benthic archaeal lipid biomarkers using lipid-RIP

Abstract Distributional patterns of intact polar lipids (IPLs) suggest that archaea are abundant in marine subsurface sediments. However, the production of archaeal lipids remains largely unresolved. We performed a lipid radioisotope probing (lipid-RIP) experiment using 14C-bicarbonate and 2-14C-acetate with surface and methanogenic marine sediments from the Rhone delta (Mediterranean Sea) to investigate mechanisms of carbon assimilation and to assess biomass production of benthic archaea. The direct determination of carbon assimilation rates into diagnostic IPLs in the experiments with 14C-bicarbonate were up to 9 times higher than in the incubations with 2-14C-acetate, implying that autotrophy is an important carbon pathway for the benthic archaea in the Rhone delta sediments. Production rates of polyglycosidic archaeols (AR) were one to two orders of magnitude higher than those of mono- and polyglycosidic glycerol dialkyl glycerol tetraethers (GDGTs), suggesting that the former IPLs are good biomarkers for active benthic archaea in marine sediments. In contrast, the low production rates of the monoglycosidic AR and GDGTs, indicate that a large fraction of these IPLs may represent fossil sedimentary remains from the water column. Unexpectedly, the lipid production rates of AR and GDGT core lipids (CLs) were similar to those of polyglycosidic IPLs. Considering the relatively short period of incubation (21 days), this suggests that CLs may be actively synthesized by benthic archaea in marine sediments and are not exclusively formed from the degradation of IPL precursors.

Biomarker-based Seawater Temperatures of Winter Sinking Particles and Core-top Sediment in the Ulleung Basin of the East Sea

This study evaluates the application of biomarkerbased temperature proxy data (alkenone with its resultant $$U_{37}^{K'}$$ index and glycerol dialkyl glycerol tetraether (GDGT) with its resultant TEX86and TEX index) in the Ulleung Basin of the East Sea. Based $$U_{37}^{K'}$$ on the and TEX86 indices from particles trapped for 10 days in December, 2015 and core-top sediment as well as the comparison of these indices to in situ seawater temperature data (from 2012 to 2016) from the Korea Oceanographic Data Center, this study confirms that the $$U_{37}^{K'}$$ index of core-top sediment indicates the annual mean SST of the surface water. This study also suggests that TEX86 data of core-top sediment may represent the annual mean temperature of the subsurface water. However, a reliable calibration equation for GDGT will be required in this regional environment for the proper application of GDGT-based paleotemperature reconstruction.

The coupled evolution of mid- to late Holocene temperature and moisture in the southeast Qaidam Basin

The southeast Qaidam Basin (SQB) lies near the boundary between the modern-day Asian summer monsoon (ASM) and mid-latitude westerly jet, and the paleoclimate variation in this region can be affected by these two atmospheric circulation systems. Reconstructions of paleotemperature and palaeohydrology are therefore critical to constraining the driving forces of climate in this region, where the ecological environment is fragile. Here, we analyzed glycerol dialkyl glycerol tetraethers (GDGTs), microbial membrane lipids occurring ubiquitously in aquatic and terrestrial environments across the globe, in an aeolian sediment profile over the last 7000 years from Xiangride Town (XRD) of southeast Qaidam Basin (SQB), China. The temperature record was generated using the global calibration of MATmr based on branched GDGTs (brGDGTs), whereas the palaeohydrological condition was reconstructed from the relative abundance of isoprenoid GDGTs vs. brGDGTs (Ri/b), the cyclisation index of brGDGTs (CBT), and the brGDGT-based pH indices. The results show that the paleoclimate during the mid-Holocene was relatively warmer and wetter in the SQB. Afterwards it was a trend to a cold and dry climate. Temperature variation was highly coupled with the moisture change during the mid- to late Holocene, as opposed to some previous studies showing a warm-dry and cold-wet climate pattern in the northern Qaidam basin. The palaeohydrological evolution agrees with the precipitation pattern of the Chinese loess plateau but opposes to that in Northeastern China, the middle reaches of Yangtze river and Arid Central Asia (ACA), implying that the ASM strength is the driving force of precipitation evolution during the mid- to late Holocene in the SQB and the spatial heterogeneity of the mid- to late Holocene precipitation pattern across China is remarkable. A rapid cold and drought event at around 4 ka before present (BP) was identified, which might be a key factor that caused the decline in the agricultural civilization during the late Neolithic Age in northwestern China.

Glycerol dialkyl glycerol tetraethers (GDGT) distributions from soil to cave: Refining the speleothem paleothermometer

The glycerol dialkyl glycerol tetraether (GDGT) paleothermometer has potential application as a speleothem paleoclimate proxy. However, the sources of GDGTs found in speleothems are poorly understood, with multiple potential sources of GDGTs from the soil to the speleothem surface. Here we analysed GDGTs in soils, soil leachates, in-cave surfaces and cave drip waters at two Australian montane caves. We observed significantly different GDGT distributions between soils, soil leachates, in-cave surfaces and drip waters and significant spatial differences in in-cave GDGT distributions. Comparison with published modern in-cave and karst groundwater GDGT datasets from Australia, Europe and China shows that speleothem GDGT distributions are different from those of all potential sources. We hypothesise that speleothem surfaces have a calcifying, alkaline, oxic, wet, carbon-available, environment that supports a microbial community that is different from other possible karst GDGT sources. We propose that the presence of GDGTs related to anoxic or methanogenic conditions, and observed in cave drip water and on in-cave surfaces, can be used to identify GDGTs from these sources. We confirm that TEX86 based paleothermometers are robust speleothem GDGT paleothermometers, whose calibration can be further refined through improved understanding and measurement of cave temperatures.

Glycerol dialkyl glycerol tetraethers (GDGTs) in high latitude Siberian permafrost: Diversity, environmental controls, and implications for proxy applications

Archaeal and bacterial glycerol dialkyl glycerol tetraethers (GDGTs) are globally abundant in soils under various climatic conditions, but little is known about their sources, relative distribution, and environmental controls on their diversity in high latitude permafrost deposits. Thus, constraints on GDGT-based proxies, such as methylation of branched GDGTs (MBT) or cyclization of branched GDGTs (CBT) used to infer mean annual temperature or soil pH, are also sparse. We investigated the GDGT diversity in typical North Siberian permafrost deposits including the active layer of polygonal tundra soils (seasonally frozen ground), fluvial terrace/floodplain sediments, Holocene and Pleistocene thermokarst sediments, and late Pleistocene Ice Complex (Yedoma). Our data show that isoprenoid GDGTs are produced by both methanotrophic and methanogenic Euryarchaeota, as well as Thaumarchaeota, but their abundance does not seem to be controlled by the investigated physicochemical parameters including %TOC, %TN, and soil pH. Branched GDGTs (brGDGTs) show similar distributional changes to those observed in other high latitude soil samples, i.e., a dominance of pentamethylated and hexamethylated brGDGTs, likely reflecting the adaptation to low temperatures and a positive correlation of the degree of cyclization with soil pH. Specifically, brGDGT-IIIa correlates positively with %TOC and %TN and negatively with soil pH, while brGDGT-Ib and brGDGT-IIb correlate negatively with %TOC and %TN and positively with pH. Moreover, we observe a negative correlation between 5-methyl and 6-methyl brGDGTs without cyclopentane moieties (except brGDGT-IIIa), but this anticorrelation does not seem to be related to the investigated physicochemical parameters. The observed brGDGT distribution yields a permafrost-specific soil pH calibration, pH'PF=1.8451×CBT'PF+8.5396 (r2 = 0.60, RMSE = 0.074; n = 109).The different investigated deposit types show some distinct GDGT distributional changes and appear to be distinguishable based on the relative abundance of crenarchaeol, GDGT-0/(crenarchaeol + GDGT-0) ratios, and CBT’PF values, although we also observe strong heterogeneity for each deposit type. In particular, Yedoma and the active layer of polygonal tundra soils represent distinct endmembers, which differ from each other, as well as from fluvial terrace/floodplain sediments and thermokarst sediments, while the latter two deposit types have similar GDGT fingerprints that are not easily distinguishable. Yet, the observed GDGT distributional differences have implications for GDGT proxies analyzed in aquatic suspended matter and sediments. Quantitative estimates of permafrost erosion, as well as soil pH inferred using BIT indices or CBT’PF, respectively, may be biased by changing relative contributions of different deposit types (carrying their respective GDGT signals) to the exported permafrost OC, particularly from Yedoma and the active layer of polygonal tundra soils.

Bacterial GMGTs in East African lake sediments: Their potential as palaeotemperature indicators

Glycerol monoalkyl glycerol tetraethers (GMGTs) are a group of membrane spanning lipids produced by some species of archaea and bacteria. They differ from the more commonly studied glycerol dialkyl glycerol tetraethers (GDGTs) in having an additional covalent carbon-carbon bond connecting the two alkyl chain. The relative abundance and distribution of bacterial branched GMGTs (brGMGTs) in surface sediments from a set of East African lakes were studied. The abundance of brGMGTs relative to the brGDGTs is positively correlated to measured mean annual air temperature (MAAT), although with a significant amount of scatter. BrGMGT abundance was not correlated to lake water pH. Seven major brGMGTs that vary in degree of methylation were identified, with m/z 1020, 1034 and 1048. Further, the mass chromatograms of the m/z 1020 and 1034 brGMGTs show an interesting distribution of peaks, which likely relates to the occurrence of distinct brGMGT isomers. This structural complexity is higher than previously observed in peats and marine sediments. Principal component analysis of the fractional abundance of bacterial tetraether lipids revealed the brGMGTs behave similarly to one another but differently from both the 5- or 6-methyl brGDGTs. This suggests the brGMGTs are produced by a common source organism and are methylated at a different position. The distribution of the seven brGMGTs showed considerable correlation with MAAT. This variability was captured in a new proxy index (the brGMGTI), which showed a strong positive linear relationship with MAAT. Lacustrine brGMGTs show potential to be applied to ancient settings to provide information about paleoclimate. © 2019 The Authors

Seasonality of archaeal lipid flux and GDGT-based thermometry in sinking particles of high-latitude oceans: Fram Strait (79° N) and Antarctic Polar Front (50° S)

Abstract. The relative abundance of individual archaeal membrane lipids, namely of glycerol dialkyl glycerol tetraethers (GDGTs) with different numbers of cyclopentane rings, varies with temperature, which enables their use as a paleotemperature proxy index. The first GDGT-based index in marine sediments called TEX86 is believed to reflect mean annual sea surface temperature (maSST). The TEX86L is an alternative temperature proxy for “low-temperature” regions (&lt;15 ∘C), where the original TEX86 proxy calibration shows a larger scatter. However, TEX86L-derived temperatures still display anomalous estimates in polar regions. In order to elucidate the potential cause of the disagreement between the TEX86L estimate and SST, we analyzed GDGT fluxes and TEX86L-derived temperatures in sinking particles collected with time-series sediment traps in high-northern- and high-southern-latitude regions. At 1296 m depth in the eastern Fram Strait (79∘ N), a combination of various transporting mechanisms for GDGTs might result in seasonally different sinking velocities for particles carrying these lipids, resulting in strong variability in the TEX86L signal. The similarity of flux-weighted TEX86L temperatures from sinking particles and surface sediments implies an export of GDGTs without alteration in the Fram Strait. The estimated temperatures correspond to temperatures in water depths of 30–80 m, where nitrification might occur, indicating the favorable depth habitat of Thaumarchaeota. In the Antarctic Polar Front of the Atlantic sector (50∘ S), TEX86L-derived temperatures displayed warm and cold biases compared to satellite-derived SSTs at 614 m depth, and its flux-weighted mean signal differs from the deep signal at 3196 m. TEX86L-derived temperatures at 3196 m depth and the surface sediment showed up to 7 ∘C warmer temperatures relative to satellite-derived SST. Such a warm anomaly might be caused by GDGT contributions from Euryarchaeota, which are known to dominate archaeal communities in the circumpolar deep water of the Antarctic Polar Front. The other reason might be that a linear calibration is not appropriate for this frontal region. Of the newly suggested SST proxies based on hydroxylated GDGTs (OH-GDGTs), only those with OH-GDGT–0 and crenarchaeol or the ring index (RI) of OH-GDGTs yield realistic temperature estimates in our study regions, suggesting that OH-GDGTs could be applied as a potential temperature proxy in high-latitude oceans.

Distribution of microbial lipids at an acid mine drainage site in China: Insights into microbial adaptation to extremely low pH conditions

Studies of acid mine drainage (AMD), a typically extremely acidic environment, can provide opportunities to determine how microorganisms adapt to low pH environments by regulating membrane lipid compositions. In this study, to gain an understanding of microbial pH adaptation mechanisms, we investigated the distribution of lipid biomarkers, primarily of microbial origin, in an AMD site at Dexing copper mine, Jiangxi, China. Microbial lipid biomarkers, including glycerol dialkyl glycerol tetraethers (GDGTs), monoalkyl glycerol ethers, fatty acids (FAs), 3-hydroxy fatty acids (3-OH-FAs), sterols, and hopanoids, were detected in surface and 10-cm-deep core sediments. Branched FAs and 3-OH-FAs were found to occur at low levels in the extremely low pH conditions. Similarly, there were fewer methyl moieties in branched GDGTs detected in the AMD, as reflected by the lower methylation degree of branched tetraethers, indicating that a reduction in membrane lipid methyl moieties may be a common strategy whereby bacteria adapt to low pH environments. The presence of only small amounts of crenarchaeol in the AMD suggests that the extremely acidic conditions are inimical to the growth of Thaumarchaeota. In contrast, abundant Euryarchaeota inhabit the AMD and produce an isoprenoid GDGT (isoGDGT) profile dominated by GDGT-2, along with GDGT-5 and GDGT-6, which are rarely detected in non-extreme environments. Furthermore, the GDGT-2/(GDGT-1 + GDGT-3) ratio in the AMD was found to be significantly higher than that in soils and was negatively correlated with pH. These findings indicate that increases in the cyclopentyl rings of isoGDGTs may be favorable for maintaining an osmotic balance across archaeal cell membranes. All the modifications adopted by bacteria and archaea result in more condensed cell membranes.

Sources and burial fluxes of sedimentary organic carbon in the northern Bering Sea and the northern Chukchi Sea in response to global warming

The Arctic and subarctic seas are the major CO2 sink areas on earth. In this study, the vertical variation characteristics of organic carbon, total nitrogen and their ratio (Corg/Nt), stable isotopes δ13C and δ15N, and BIT (branched and isoprenoid tetraether) index of GDGTs (glycerol dialkyl glycerol tetraethers) in combination with 210Pb-dating were used to analyze the changes in the marine and terrestrial sources of organic carbon in the northern Bering Sea (site 1), western Beaufort Sea slope (site 2) and northern Chukchi Sea (site 3). Organic carbon burial fluxes (OCBF) in the context of global warming were also explored at sites 1 and 3. The results showed that organic matter in these sediments were a mixed input of marine and terrestrial sources, and the BIT index and δ13C of site 2 suggested that the terrestrial soil organic matter was dominant. Based on a combination of 210Pb dating and Corg, the sedimentary OCBF at site 1 was 2.29–3.65 mg cm−2 y−1, and at site 3 was 0.00–0.41 mg cm−2 y−1. The temperature anomalies and sea ice changes in the Arctic in the past 100 years were compared with the burial fluxes of the terrestrial organic carbon. At site 1, the results indicated that fast melting of seasonal sea ice led to earlier arrival of ice algae bloom, enhanced zooplankton feeding and reduced carbon burial from 1947 to 2010, and the sudden increase in carbon burial after 2010 was attributed to an increase in primary productivity and terrestrial organic matter input due to an accelerated melting of sea ice. There was a smaller change in marine organic carbon content in site 3, but OCBF increased after a pre-1965 decrease, mainly controlled by terrestrial organic matter input associated with temperature rising and sea ice melting during recent decades.

Glycerol dialkyl glycerol tetraethers signature in sediments of the East China Sea and its implication on marine and continental climate and environment records

Abstract Glycerol dialkyl glycerol tetraethers (GDGTs) were determined in surface sediments from the East China Sea (ECS) in order to evaluate their potential for palaeo-climate and environmental reconstructions. Isoprenoid GDGTs (isoGDGTs) ranged from 516.75 ng/g to 1365 ng/g and crenarchaeol (cren) and caldarchaeol (GDGT-0) were the main components. IsoGDGTs were mainly derived from marine planktonic Thaumarchaeota. Branched GDGTs (brGDGTs) ranged from 12.3 ng/g to 123.5 ng/g and were dominated by GDGT-I and GDGT-II. The fractions and distributions, combined with branched/isoprenoid tetraether index (BIT) and methylation and cyclization indexes of brGDGT (MBT-CBT) suggested that brGDGTs in the ECS were mainly derived from terrestrial soils and they could be used to reconstruct climates. The TEX86-derived sea surface temperatures (SST) were close to the satellite-derived SST in autumn. The MBT/CBT-derived mean annual air temperatures (MAATs) and CBT-derived pH values in inner shelf by local and global calibrations matched well with the measured MAATs and soil pH in the lower Changjiang River basin, which seem to be appropriate for palaeo-climate and environment reconstructions of the continent around the ECS. Besides, cren and methane index showed a sensitive response to dissolved oxygen variation, suggesting their potential as palaeo-redox proxy.

New Insights Into the Polar Lipid Composition of Extremely Halo(alkali)philic Euryarchaea From Hypersaline Lakes.

We analyzed the polar membrane lipids of 13 strains of halo(alkali)philic euryarchaea from hypersaline lakes. Nine belong to the class Halobacteria, representing two functional groups: aerobic polysaccharide utilizers and sulfur-respiring anaerobes. The other four strains represent halo(alkali)philic methanogens from the class Methanomicrobia and a recently discovered class Methanonatronarchaeia. A wide range of polar lipids were detected across the 13 strains including dialkyl glycerol diethers (archaeols), membrane-spanning glycerol tetraethers and diether-based cardiolipins. The archaeols contained a range of core lipid structures, including combinations of C20 and C25 isoprenoidal alkyl chains, unsaturations, and hydroxy moieties. Several diether lipids were novel, including: (a) a phosphatidylglycerolhexose (PG-Gly) headgroup, (b) a N,N,N-trimethyl aminopentanetetrol (APT)-like lipid with a methoxy group in place of a hydroxy group on the pentanetetrol, (c) a series of polar lipids with a headgroup with elemental composition of either C12H25NO13S or C12H25NO16S2, and (d) novel cardiolipins containing a putative phosphatidylglycerolphosphate glycerophosphate (PGPGP) polar moiety. We found that the lipid distribution of the 13 strains could be generally separated into two groups, the methanogens (group) and the Halobacteria (class) based on the presence of specific core lipids. Within the methanogens, adaption to a high or more moderate salt concentration resulted in different ratios of glycerol dialkyl glycerol tetraethers (GDGTs) to archaeol. The methanogen Methanosalsum natronophilum AME2T had the most complex diether lipid composition of any of the 13 strains, including hydroxy archaeol and macrocyclic archaeol which we surmise is an order-specific membrane adaption. The zwitterionic headgroups APT and APT-Me were detected only in the Methanomicrobiales member Methanocalculus alkaliphilus AMF2T which also contained the highest level of unsaturated lipids. Only alkaliphilic members of the Natrialbales order contained PGPGP cardiolipins and the PG-Gly headgroup. The four analyzed neutrophilic members of the Halobacteria were characterized by the presence of sulfur-containing headgroups and glycolipids. The presence of cardiolipins with one or more i-C25 alkyl chains, generally termed extended archaeol (EXT-AR), in one of the Methanonatronarchaeia strains was unexpected as only one other order of methanogenic archaea has been reported to produce EXT-AR. We examined this further by looking into the genomic potential of various archaea to produce EXT-AR.