Cyclization Ratio Of Branched Tetraethers Research Articles

Global calibration of novel 3-hydroxy fatty acid based temperature and pH proxies

3-Hydroxy fatty acids (3-OH-FAs), derived from Gram-negative bacterial outer membranes, have received recent attention for their potential as new terrestrial pH and temperature proxies for palaeoclimate studies. Initial studies from altitudinal transects of contemporary soils – correlating bacterial 3-OH-FA compositions to air temperature and pH – have shown promising results. But the geographical extent of recent calibrations is limited. In this study, we analyse 3-OH-FA lipid distributions in 186 globally distributed soil samples to study the environmental factors controlling the relative distribution of the 3-OH-FA isomers. Our sample-set covers a wide range of temperatures (−0.4 to 27 °C) and pH (3.6–9.2). For the global compilation we find that the ratio of anteiso to normal 3-OH-FAs of the C15 or C17 homologues (RAN15 or RAN17) shows a strong linear relationship with mean annual air temperature (MAAT) (R2 = 0.48, p < 0.001 and R2 = 0.41, p < 0.001, respectively). Additionally, the negative logarithm of the ratio of the summed iso and anteiso to the total amount of normal 3-OH-FAs (RIAN) is also strongly anticorrelated with the soil pH (R2 = 0.66, p < 0.001). However, we find that for our 3-OH-FA based proxies there are significant differences in slope and intercept of the linear corrections at regional scales. Thus local or regional calibrations are likely preferable (at this stage of 3-OH-FA proxy development) for application to specific palaeoclimate archives. We also explore the relationship of 3-OH-FA isomer fractional abundances to environmental parameters using machine learning tool (a Gaussian Process (GP) emulator). This confirms the first order relationships to environmental parameters highlighted by the empirical equations and also derives several alternative GP emulator models for reconstructing MAAT and pH which give higher R2 values (0.66 for MAAT; 0.63 for pH) and lower RSME values (3.5 °C for MAAT; 0.76 for pH) compared to simple linear regressions at the global scale. We compare our 3-OH-FA based indices with bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs) based indices from the same soil samples. At a global scale RAN15 and RAN17 show negative correlations with the MBT′5ME-MAAT (MBT′5ME, methylation index of 5-methyl branched tetraethers) (r = −0.59, p < 0.001 and r = −0.42, p < 0.001, respectively), whilst RIAN shows strong linear correlations with the cyclisation ratio of branched tetraethers (CBT) (r = 0.77, p < 0.001). Similar to 3-OH-FA based temperature proxies, GDGT based temperature proxy MBT′5ME also showed different regional calibrations. Our new field-based correlations demonstrate the broad physiological response of Gram-negative bacterial cell membranes to external environmental changes on a global scale. We suggest that 3-OH-FA based proxies have widespread potential for palaeoenvironmental studies to estimate past MAAT and soil pH, but that regional/local and context specific calibrations may need to be applied.

Temperature changes reconstructed from branched GDGTs on the central Loess Plateau during the past 130–5 ka

This study investigated the glycerol dialkyl glycerol tetraethers (GDGTs) in a loess-paleosol sequence (LPS) to quantitatively reconstruct the mean annual air temperature (MAT) on the central Chinese Loess Plateau (CLP) during the past 130–5 ka. The results show that: (1) MAT variations reconstructed by global MBT (methylation index of branched tetraethers) and CBT (cyclisation ratio of branched tetraethers) transfer functions at Xifeng range from 7.3 °C to 28.4 °C. The MAT variation on the central CLP (Xifeng site) is between the previously reported MAT variations on the western (Yuanbao site) and eastern CLP (Mangshan site), in agreement with the pattern that temperature increases from the northwestern CLP to the southeastern CLP. (2) MAT variations estimated by transfer functions of global MBT-CBT, global MBT′-CBT and Chinese MBT-CBT for LPS are nearly all higher than the modern MAT at Xifeng, indicating that the reconstructed MAT values are probably overestimated. (3) The maximum value of magnetic susceptibility at Xifeng lagged that of the MAT derived by MBT-CBT proxy during the last interglacial (∼9 ka lag) and since the last deglacial (∼6 ka lag).

Distribution of branched glycerol dialkyl glycerol tetraethers in soils on the Northeastern Qinghai-Tibetan Plateau and possible production by nitrite-reducing bacteria

Abstract Branched glycerol dialkyl glycerol tetraethers (bGDGTs) are ubiquitous and abundant in soils, but their sources remain elusive. Recent studies demonstrate that the distributions of bGDGTs are sensitive to various environmental factors. In an effort to understand how and to what extent soil moisture (expressed as soil water content (SWC) or mean annual precipitation (MAP), pH and temperature may impact the distribution of bGDGTs, and to shed more light on the biological sources of bGDGTs in cold and arid regions, we investigated the distribution of bGDGTs as well as bacterial 16S rRNA gene and functional genes involved in the N cycle (including amoA , nirS and nirK ) in 41 surface soil samples from around Lake Qinghai and east of Qaidam Basin on the Northeastern Qinghai-Tibetan Plateau. We found that lower soil moisture reduced the fractional concentrations of cyclic bGDGTs and thus the cyclisation ratio of branched tetraethers (CBT) index correlated negatively with SWC and MAP, suggesting that soil moisture is an important factor controlling bGDGT distributions in soils in this arid and semi-arid region. Two subgroups of bGDGTs were assigned on the basis of cluster analysis, and bGDGT indices behaved differently in the two groups, hinting at different biosynthetic mechanisms for bGDGTs under different environmental conditions. Real time PCR results showed that nirS and nirK genes correlated significantly with the concentration of bGDGTs, suggesting that the nirS- or nirK -encoding bacteria involved in denitrification might potentially be an additional biological source for soil bGDGTs (besides Acidobacteria). Moreover, our results also support the application of new indices based on 5-methyl bGDGTs and 6-methyl bGDGTs in reconstructing past temperature and pH variations in this region.

Distribution of branched GDGTs in surface sediments from the Colville River, Alaska: Implications for the MBT′/CBT paleothermometer in Arctic marine sediments

AbstractSignificant climate fluctuations in the Arctic over the recent past, and additional predicted future temperature changes, highlight the need for high‐resolution Arctic paleoclimate records. Arctic coastal environments supplied with terrigenous sediment from Arctic rivers have the potential to provide annual to subdecadal resolution records of climate variability over the last few millennia. A potential tool for paleotemperature reconstructions in these marine sediments is the revised methylation index of branched tetraethers (MBT′)/cyclization ratio of branched tetraethers (CBT) proxy based on branched glycerol dialkyl glycerol tetraethers (brGDGTs). In this study, we examine the source of brGDGTs in the Colville River, Alaska, and the adjacent Simpson Lagoon and reconstruct temperatures from Simpson Lagoon sediments to evaluate the applicability of this proxy in Arctic estuarine environments. The Colville catchment soils, fluvial sediments, and estuarine sediments contain statistically similar brGDGT distributions, indicating that the brGDGTs throughout the system are soil derived with little alteration from in situ brGDGT production in the river or coastal waters. Temperatures reconstructed from the MBT′/CBT indices for surface samples show good agreement with regional summer (June through September) temperatures, suggesting a seasonal bias in Arctic temperature reconstructions from the Colville system. In addition, we reconstruct paleotemperatures from an estuarine sediment core that spans the last 75 years, revealing an overall warming trend in the twentieth century that is consistent with trends observed in regional instrumental records. These results support the application of this brGDGT‐based paleotemperature proxy for subdecadal‐scale summer temperature reconstructions in Arctic estuaries containing organic material derived from sediment‐laden, episodic rivers.

Distribution of tetraether lipids in agricultural soils – differentiation between paddy and upland management

Abstract. Rice paddies constitute almost a fifth of global cropland and provide more than half of the world's population with staple food. At the same time, they are a major source of methane and therewith significantly contribute to the current warming of Earth's atmosphere. Despite their apparent importance in the cycling of carbon and other elements, however, the microorganisms thriving in rice paddies are insufficiently characterized with respect to their biomolecules. Hardly any information exists on human-induced alteration of biomolecules from natural microbial communities in paddy soils through varying management types (affecting, e.g., soil or water redox conditions, cultivated plants). Here, we determined the influence of different land use types on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs), which serve as molecular indicators for microbial community structures, in rice paddy (periodically flooded) and adjacent upland (non-flooded) soils and, for further comparison, forest, bushland and marsh soils. To differentiate local effects on GDGT distribution patterns, we collected soil samples in locations from tropical (Indonesia, Vietnam and Philippines) and subtropical (China and Italy) sites. We found that differences in the distribution of isoprenoid GDGTs (iGDGTs) as well as of branched GDGTs (brGDGTs) are predominantly controlled by management type and only secondarily by climatic exposition. In general, upland soil had higher crenarchaeol contents than paddy soil, which by contrast was more enriched in GDGT-0. The GDGT-0 ∕ crenarchaeol ratio, indicating the enhanced presence of methanogenic archaea, was 3–27 times higher in paddy soils compared to other soils and increased with the number of rice cultivation cycles per year. The index of tetraethers consisting of 86 carbons (TEX86) values were 1.3 times higher in upland, bushland and forest soils than in paddy soils, potentially due to differences in soil temperature. In all soils brGDGT predominated over iGDGTs with the relative abundance of brGDGTs increasing from subtropical to tropical soils. Higher branched vs. isoprenoid tetraether (BIT) values in paddy soils compared to upland soils together with higher BIT values in soils from subtropical climates indicated effects on the amounts of brGDGT induced by differences in management as well as climate. In acidic soils cyclization ratio of branched tetraethers (CBT) values correlated well with soil pH. In neutral to alkaline soils, however, no correlation but an offset in CBT between paddy and upland managed soils was detected. This is interpreted as indicating soil moisture exerting an additional control on the CBT in these soils. Lower modified methylation index of branched tetraether (MBT′) values and temperatures calculated from this (TMC) in paddy soils compared to upland soils are attributed to a management-induced (e.g. enhanced soil moisture via flooding) effect on mean annual soil temperature (MST).

Impacts of pH and temperature on soil bacterial 3-hydroxy fatty acids: Development of novel terrestrial proxies

Gram-negative bacterial 3-hydroxy fatty acids (3-OH-FAs) biomarkers are widespread in a variety of environments including both marine and terrestrial sediments (including speleothems). In this study we analysed the hydroxylated membrane lipids of 26 soil samples from an altitudinal transect of Shennongjia Mountain in central China to study the environmental factors controlling the relative distribution of 3-OH-FAs. Our results show that both the ratio of the summed iso and anteiso to the total amount of normal 3-OH-FAs (RIAN), and the ratio of summed iso and anteiso to the total amount of all 3-OH-FAs (Branched Index) were primarily related to the pH of soil (R2=0.70 and 0.70, respectively). Additionally, the anteiso to normal 3-hydroxy fatty acids ratio of the C15 and C17 homologues (RAN15 and RAN17) shows a significant negative correlation with mean annual air temperature (MAAT) (R2=0.51 and 0.48, respectively). When comparing the 3-OH-FA based indices with established glycerol dialkyl glycerol tetraether (GDGT) based indices from the same soil samples, the RIAN and Branched Index show strong linear correlations with the cyclisation ratio of branched tetraethers (CBT) (R2=0.77 and 0.74, respectively), and the RAN15 and RAN17 show negative correlations with the MBT/CBT-MAAT (MBT, methylation index of branched tetraethers) (R2=0.61 and 0.36, respectively). Our new field-based correlations demonstrate the physiological response of Gram-negative bacterial cell membranes to the external environment and suggest that 3-hydroxy fatty acids can be applied in palaeoenvironmental studies to estimate past MAAT and soil pH.

Seasonal variability in concentrations and fluxes of glycerol dialkyl glycerol tetraethers in Huguangyan Maar Lake, SE China: Implications for the applicability of the MBT–CBT paleotemperature proxy in lacustrine settings

Glycerol dialkyl glycerol tetraethers (GDGTs) are increasingly used for paleoclimate studies of marine and lacustrine environments. Although GDGT-based proxies have been applied to various lake environments globally, little is known about the distribution of GDGTs in lakes in subtropical South China, which are mainly controlled by the East Asian monsoonal system. Therefore, we investigated the distribution of GDGTs in Huguangyan Maar Lake (HML) in order to examine their characteristics as paleoclimate proxies in subtropical South China. Suspended particulate matter (SPM) in the water column was sampled monthly over a one year period at three different depths. Sediment traps were deployed simultaneously at the same locations and settling particles were collected monthly over a one year period too. In order to identify the sources of GDGTs in HML, lake bottom sediments and soil samples from the catchment were collected. The distribution and abundance of GDGTs in all samples were determined and GDGT-based temperatures were calculated.The concentrations and fluxes of GDGTs of settling particles in HML were dominated by branched GDGTs, with a minor contribution of isoprenoid homologues. The maximum concentrations and fluxes of branched GDGTs occurred during the winter months, when the water column was well mixed due to the strong East Asian winter monsoon. Also, maximal concentrations and fluxes of branched GDGTs were observed at the deeper water mass. These observations suggest that in situ production of branched GDGTs takes place mainly in the deeper part of the water column and/or at the sediment–water interface, but may be influenced by seasonal variability in the primary production in the shallower water mass of HML. The temperatures calculated from the methylation of branched tetraethers (MBT) and the cyclization ratio of branched tetraethers (CBT) of SPM and settling particles were lower than local air temperature (AT) during spring and summer when the water column was in thermal stratification, and were higher than AT during fall and winter when the water column was isothermal. However, the MBT–CBT based temperatures were in good agreement with AT on an annual time scale. Therefore they can be used as a proxy of local air temperature. Furthermore, the variations in concentration and flux of GDGTs are synchronous with the East Asian monsoon annual cycle. The overall amplitude of the estimated monthly temperature is however smaller than that of the measured air temperature. The mean MBT–CBT based temperature estimates (23.4±0.6°C) of lake bottom sediments for that one year period agree with the local average mean air temperature (23.1±0.5°C) for the preceding 45years, providing further support to the reliability of this proxy and the interpretation of the results. Both long term stratification and mixing events in water column might present a challenge to the application of branched GDGT-based paleoenvironmental proxies in lakes. The present study reveals that GDGTs in lakes within the area influenced by the East Asian monsoon have a considerable potential for paleoclimatic studies but it is important to determine the location and nature of the sources of GDGTs sampled in lacustrine environments prior to their interpretation as paleoclimatic indicators. More generally, this study identifies factors which have to be considered in order to obtain reliable palaeotemperature data from GDGTs in lacustrine settings.

Distribution of branched glycerol dialkyl glycerol tetraethers in surface soils of the Qinghai–Tibetan Plateau: implications of brGDGTs-based proxies in cold and dry regions

Abstract. The methylation index of branched tetraethers (MBT) and cyclization ratio of branched tetraethers (CBT) based on the distribution of branched glycerol dialkyl glycerol tetraethers (brGDGT) are useful proxies for the reconstruction of mean annual air temperature (MAT) and soil pH. Recently, a series of 6-methyl brGDGTs were identified which were previously co-eluted with 5-methyl brGDGTs. However, little is known about 6-methyl brGDGTs in the Qinghai–Tibetan Plateau (QTP), a critical region of the global climate system. Here, we analyze 30 surface soils covering a large area of the QTP, among which 6-methyl brGDGTs were the most abundant components (average 53 ± 17% of total brGDGT). The fractional abundance of 6-methyl brGDGTs showed a good correlation with soil pH, while the global MBT'5ME calibration overestimates MAT in the QTP. We therefore proposed a MBT5/6 index including both 5- and 6-methyl brGDGTs, presenting a strong correlation with MAT in QTP: MAT = −20.14 + 39.51 × MBT5/6 (n = 27, r2 = 0.82; RMSE = 1.3 °C). Another index, namely IBT (isomerization of branched tetraether), based on carbon skeleton isomerization of the 5-methyl to 6-methyl brGDGTs, is dependent on soil pH: pH = 6.77 − 1.56 × IBT (n = 27; r2 = 0.74, RMSE = 0.32). Our study suggests that changing the position of methyl group of brGDGTs may be another mechanism for some soil bacteria to adapt to the ambient pH change in addition to the well-known cyclization.

Temporal variation in community structure and lipid composition of Thaumarchaeota from subtropical soil: Insight into proposing a new soil pH proxy

Crenarchaeol is a unique glycerol dialkyl glycerol tetraether (GDGT) lipid specific to Thaumarchaeota, which play an important role in the global C and N cycles. GDGTs from Thaumarchaeota have been used to develop proxies for paleoclimate or paleoenvironment studies in aquatic environments. However, our understanding of their response to environmental change in soil remains poor. We addressed this question by investigating the change in archaeal lipid composition and community structure in the context of other environmental variables over a period of 12months in a subtropical soil from Chongming Island, China. The results showed that Nitrososphaera spp. were the dominant archaeal population producing crenarchaeol in the soil. The relative abundance of GDGTs with one and three cyclopentane moieties, and crenarchaeol and its isomer in the core lipid (CL) fraction correlated with seasonal pH change in the soil. We therefore propose a molecular fossil proxy, the thaumarchaeota index (TI), which significantly related to pH in not only Chongming Island soil (R2 0.56, RMSE 0.14, P<0.05), but also global soils (R2 0.51, RMSE 1.39, P<0.001) and five thaumarchaeotal enrichments (R2 0.99, RMSE 0.04, P<0.001). This suggests that TI could be a useful proxy for soil pH, which may corroborate the use of the bacterial GDGT proxy for soil pH, the cyclisation ratio of branched tetraethers (CBT).

Glacial–interglacial contrast in MBT/CBT proxies in the South China Sea: Implications for marine production of branched GDGTs and continental teleconnection

Two proxies derived from branched glycerol dialkyl glycerol tetraethers (br GDGTs), the methylation index of branched tetraethers (MBT) and the cyclization ratio of branched tetraethers (CBT), are often used to reconstruct paleo mean annual air temperature (MAAT) and soil pH based on the premise that br GDGTs in the marine environment are mainly of terrigenous origin. However, mounting evidence indicates that br GDGTs can be produced in situ in oceanic settings, which may affect MAAT reconstruction and the use of other related paleoenvironmental proxies. We have determined br GDGT distributions in a sedimentary core (MD05-2896/7) from the southern South China Sea, which provided high resolution profiles of MBT and CBT indices as well as the branched and isoprenoid tetraether (BIT) index. BIT varied systematically with glacial–interglacial cycles, reaching much lower (<0.1) values during the interglacial periods (MIS 1 and MIS 5) than during the glacial periods (MIS 2, MIS 3, MIS 4 and MIS 6). MBT/CBT-derived temperature showed, on the other hand, lower values during the interglacial periods but higher values during glacial periods. We hypothesize that the lower MBT/CBT-derived temperature during interglacial periods reflects bottom water temperature registered via br GDGTs produced under marine conditions, whereas the higher MBT/CBT derived temperature during glacial periods reflects terrestrial MAAT because of the overwhelming input of br GDGTs from land when the sea level was low. Similarly, the CBT-derived soil pH appeared to have been overprinted by marine br GDGT production during interglacial periods but responded to precipitation on land during glacial periods, showing patterns similar to, or as a positive response to, the southern hemispheric climate oscillation due to teleconnection. Our results demonstrate an unprecedented pattern of MBT/CBT variation constrained by glacial–interglacial cycles in the South China Sea. Under this constraint, MBT/CBT revealed deep water production of br GDGTs during interglacial periods and recorded changes in paleohydrology on land during glacial periods, providing a new perspective for paleoclimate studies using organic proxies.

Distribution, source and transportation of glycerol dialkyl glycerol tetraethers in surface sediments from the western Arctic Ocean and the northern Bering Sea

Isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGTs) and grain size distribution were investigated in surface sediments from the western Arctic Ocean including the Chukchi Sea and the adjacent northern Bering Sea to understand their source and transportation in the Arctic region and test environmental proxies derived from the GDGT composition.Coarse sediments such as sand and silty sand are distributed in the Yukon and Mackenzie River estuaries, the northern Bering Sea near Bering Strait, and some areas of the outer shelf of the Bering Sea. In the Chukchi Sea, silt, grading from sandy to clayey silt, predominates and becomes finer northward towards the deep Arctic OceanIsoprenoid GDGTs are abundant on the outer shelf and slope of the Chukchi Sea and the upper slope of the Bering Sea. The higher abundances are attributed to a combination of higher production of marine Archaea (Thaumarchaeota) at the shelf edge, redeposition of GDGT-carrying fine particles, and better preservation of GDGTs at sites with higher sedimentation rates. The TEX86- and TEX86L-derived temperatures are not consistent with sea surface temperatures in the study area, with unrealistically high TEX86- and TEX86L-derived temperatures in samples north of 73 °N probably biased by factors other than temperature.Branched GDGTs are abundant on the Chukchi shelf and in the Yukon and Mackenzie River estuaries. At the shelf edge of the Chukchi Sea, both branched and isoprenoid GDGTs are abundant, indicating common concentration processes such as sediment redeposition and efficient preservation at sites with high sedimentation rates. Sediments from the western Arctic Ocean north of 75 °N, the Yukon and Mackenzie River estuaries, and the Yukon River have higher cyclization ratio of branched tetraethers (CBT) than sediments from the Chukchi and Bering Seas, suggesting two different sources of branched GDGTs inferred as soil and marine bacteria.

Potential of GDGTs as a temperature proxy along an altitudinal transect at Mount Rungwe (Tanzania)

Glycerol dialkyl glycerol tetraethers (GDGTs) are membrane lipids of high molecular weight and include the isoprenoid GDGTs (iGDGTs) produced by Archaea and the branched GDGTs (brGDGTs) produced by unknown bacteria. Several indices have been developed to describe the relationship between GDGT distribution and environmental parameters: the TEX86 (tetraether index of tetraethers consisting of 86 carbons), based on the relative abundances of iGDGTs, and the MBT (methylation index of branched tetraethers) and CBT (cyclisation ratio of branched tetraethers), based on the relative abundance of brGDGTs. For soils, most studies have focused only on the relationship between brGDGT distribution and environmental parameters, without examining the iGDGT distribution. We have analysed both brGDGT and iGDGT distributions and determined GDGT-derived proxies along an altitudinal transect at Mt. Rungwe, Tanzania. MBT/CBT-derived temperature values were found to correlate linearly with altitude (R2 0.75), as observed before for the other altitudinal transects investigated. The MBT/CBT-derived temperature lapse rate along Mt. Rungwe (0.7°C/100m) was similar to the in situ measured one and was consistent with results obtained previously for an altitudinal gradient in the same region at higher altitude. This confirms the robustness of the MBT/CBT as a paleoelevation proxy in East Africa. In addition, a linear correlation (R2 0.50) between TEX86 and altitude was observed, likely reflecting the effect of the adiabatic cooling of air with altitude on iGDGT distribution. This suggests that TEX86 might be used to track temperature change in terrestrial settings, in addition to the MBT/CBT indices.

Glycerol dialkyl glycerol tetraethers in surficial coastal and open marine sediments around China: Indicators of sea surface temperature and effects of their sources

Accurate reconstruction of past sea surface temperature (SST) is important for the understanding of past climate and its variations. Recently, the TEX86 indicator (TetraEther indeX of tetraethers consisting of 86 carbon atoms), based on glycerol dialkyl glycerol tetraethers (GDGTs) has been developed into a promising proxy for SST reconstruction. In this study, we investigated the distribution of GDGTs in surficial sea bottom sediments from coastal and open marine settings around China between 18°N and 38°N in water depth of either <100 or >1000m.As the distribution patterns of GDGTs reflect their biological sources and their respective habitats these can be identified from the sediment samples investigated. The distributions of branched GDGTs (brGDGTs) in the Yellow Sea (YS) was found to be similar to that of the lower Yellow River basin indicating that the brGDGTs in the YS originated from terrestrial soil. While the variability of the cyclisation ratio of branched tetraethers (CBT)-derived pH values in the Pearl River estuary (PRE) suggested that brGDGTs might also be produced in situ in coastal sediments or in the water column. In the shallow areas (water depth <100m, the YS, East China Sea and PRE) TEX86H-based SST (SST86H) was overestimated or underestimated when compared with mean annual SSTsatellite, most likely caused by seasonality or overprint by a cold-biased terrestrial signal. In the South China Sea (SCS), at water depth >1000m, the SST86H matched well the mean annual SST determined by satellite, indicating the applicability of the TEX86H temperatures for paleoclimate investigations in these study areas and, potentially, in similar settings of deep water. These findings provide the basis for application of branched/isoprenoid index (BIT) and TEX86 as indicators for temperature and paleotemperature in coastal and open marine environments around China.

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.

Distributions and temperature dependence of branched glycerol dialkyl glycerol tetraethers in recent lacustrine sediments from China and Nepal

[1] Glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in surface sediments from 24 lakes in China and Nepal were analyzed. The result shows that branched GDGT lipids occur widely in the studied lake sediments. The different distribution of branched GDGTs between the Chinese soils from Weijers et al. (2007b) and the Chinese lake sediments indicates that the branched GDGTs in the lacustrine sediment are the mixture of allochthonous and autochthonous branched GDGTs. Our result supports the suggestion that temperature is the principal factor affecting the distribution of GDGTs. Seasonality, sources, and precipitation increase the uncertainty of the correlation between the methylation index of branched tetraethers (MBT)/cyclization ratio of branched tetraethers (CBT) proxy and the temperature. The correlations of the MBT/CBT index with mean annual air temperature (T) for the global lakes are T = 6.803 − 7.062 × CBT + 37.090 × MBT (n = 139, p < 0.0001, R2 = 0.62, and RMSE = 5.24°C); for the global lakes with pH < 8.5, T = 3.949 − 5.593 × CBT + 38.213 × MBT (n = 100, p < 0.0001, R2 = 0.73, and RMSE = 4.27°C). The significant correlation suggests that the MBT/CBT proxy might be used as a potential paleotemperature proxy in the limnic environment. The MBT/CBT proxy provides an independent way to evaluate and verify the sensitivities of the different proxies for paleotemperature reconstruction from lacustrine sediments.

Environmental controls on bacterial tetraether membrane lipid distribution in soils

Over the last years a novel group of branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids has been discovered in peat bogs and soils. They consist of components with 4–6 methyl groups attached to the n-alkyl chains and 0 to 2 cyclopentyl moieties in the alkyl chain. These branched membrane lipids are produced by an as yet unknown group of anaerobic soil bacteria. In this study we analysed the branched membrane lipid content of 134 soil samples from 90 globally distributed locations to study the environmental factors controlling the relative distribution of the different branched GDGT isomers. Our results show that the relative amount of cyclopentyl moieties, expressed in the cyclisation ratio of branched tetraethers (CBT), is primarily related to the pH of the soil ( R 2 = 0.70) and not to temperature ( R 2 = 0.03). The relative amount of methyl branches, expressed in the methylation index of branched tetraethers (MBT), is positively correlated with the annual mean air temperature (MAT) ( R 2 = 0.62) and, to a lesser extent, negatively correlated with the pH of the soil ( R 2 = 0.37). If both parameters are combined, however, it appears that the variation in the MBT is largely explained by both MAT and pH ( R 2 = 0.82). These results suggest that the relative distribution of soil-derived GDGT membrane lipids can be used in palaeoenvironmental studies to estimate past annual MAT and soil pH.