Methylation Index Of Branched Tetraethers Research Articles

A nonlinear model for resolving the temperature bias of branched glycerol dialkyl glycerol tetraether (brGDGT) temperature proxies

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are a class of ubiquitous, bacteria-derived lipid biomarkers in terrestrial and aquatic environments. Many studies have demonstrated the potential for brGDGTs as paelotemperature proxies, but the observed seasonal bias in brGDGT-inferred temperatures still remains poorly understood, particularly for Arctic or seasonally dry climates. Here we introduce a new physical framework for understanding variations in the methylation index of branched tetraethers (MBT) by explicitly modeling the production and preservation of brGDGTs for a bacterial population with an exponential rate dependence on temperature. The dependence of MBT ratios on temperature is predicted to be nonlinear, and thus has a different form from other empirically defined MBT models. We apply the model to understanding how the methylation index depends on not only the mean annual air temperature (MAAT) but also how it depends on the amplitude of the seasonal temperature cycle and the phase and amplitude of the soil moisture content. We performed mesocosm growth experiments using natural lake waters that confirm that the model correctly accounts for lower brGDGT production rates during cold seasonal temperatures. Comparing the new model predictions with global compilations of MBT’5Me and MBT’ data, we determine a new calibration of best fitting set of model coefficients that more accurately account for the expected physical constraints on MBT-type data. Our new results account naturally for the expected saturation of MBT’5Me and MBT’ at low and high temperatures, a bias of up to 5–7 °C in MAAT for high seasonal temperature fluctuations, and an additional bias of up to 1–3 °C in MAAT for strongly seasonal soil moisture fluctuations. Taken together, the 3 effects explain differences of up to 15 °C in inferred MAAT compared with traditional empirical models for MBT’5Me and MBT’ and results in improved mean squared errors for both. At MAAT above 18 °C, MBT’ temperature errors are lower than those of MBT’5Me, suggesting MBT’ may still be a useful proxy in some situations despite its added complexity. While we apply the framework only to global MBT’5Me and MBT’ proxy data, the physical framework can be adapted to other types of paleoproxies and may therefore be more widely applicable. Preliminary application of our model to the Pliocene North Sea Hank Core yields ∼1 °C cooler temperatures than a previous calibration and application to 130 kyr-long Chinese Loess Plateau records yields ∼4 °C warmer glacial and glacial stadial temperatures compared to previous calibration.

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 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.

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.

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.

Impact of seasonal hydrological variation on the distributions of tetraether lipids along the Amazon River in the central Amazon basin: implications for the MBT/CBT paleothermometer and the BIT index

Suspended particulate matter (SPM) was collected along the Amazon River in the central Amazon basin and in three tributaries during the rising water (RW), high water (HW), falling water (FW) and low water (LW) season. Changes in the concentration and the distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs), i.e., the methylation index of branched tetraethers (MBT) and the cyclization of brGDGTs (CBT), were seen in the Amazon main stem. The highest concentration of core lipid (CL) brGDGTs normalized to particulate organic carbon (POC) was found during the HW season. During the HW season the MBT and CBT in the Amazon main stem was also most similar to that of lowland Amazon (terra firme) soils, indicating that the highest input of soil-derived brGDGTs occurred due to increased water runoff. During the other seasons the MBT and CBT indicated an increased influence of in situ production of brGDGTs even though soils remained the main source of brGDGTs. Our results reveal that the influence of seasonal variation is relatively small, but can be clearly detected. Crenarchaeol was mostly produced in the river. Its concentration was lower during the HW season compared to that of the other seasons. Hence, our study shows the complexity of processes that influence the GDGT distribution during the transport from land to ocean. It emphasizes the importance of a detailed study of a river basin to interpret the MBT/CBT and BIT records for paleo reconstructions in adjacent marine setting.

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.