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Glycerol Dialkyl Glycerol Tetraethers (GDGTs) serve as important tools for the quantitative reconstruction of paleoclimate and paleoecology in both continental and marine environments. Previous studies of GDGTs in the terrestrial environments focused primarily on the soils from the relatively warm-humid or cold-dry regions. However, it is still unclear how GDGTs respond to environmental variables in the cold-humid regions. Here, we collected soils along an altitudinal transect of Mountain (Mt.) Changbai, which has a typical cold-humid climate, to investigate the distribution of GDGTs and the response of GDGT-based proxies to changes in climate along the transect. The shift in the distribution of archaeal isoprenoidal GDGTs (isoGDGTs) revealed that the archaeal community varied significantly along the transect, which can affect the relationship between TEX86 and mean annual air temperature (MAT). In addition, the increased temperature seasonality at higher altitudes exerted a significant impact on TEX86. We proposed a global calibration of TEX86 for the growing season temperature reconstruction in the soil environments: T=85.19×TEX86−46.30 (R2=0.84, p<0.001). The methylation indices for 5-methyl branched GDGTs (brGDGTs) including MBT′5me and MBT5/6, showed correlation with soil water content but no relationship with MAT, indicating that MBT′5meand MBT5/6 from cold-humid environments may be not suitable for temperature and altitude reconstruction. In contrast, the recently developed pH proxies, including MBT′6me (the methylation index for 6-methyl brGDGTs), CBT (Cyclisation index of Branched Tetraethers), IRIIa’ (Isomer ratio of IIa′) and IRIIIa′ (Isomer ratio of IIIa′) exhibited significant correlations with soil pH, suggesting these proxies can still be used for soil pH reconstruction in the coldhumid regions. The combination of MBT′5me and MBT′6me was strongly related to different types of climate (cold-dry, warmhumid, cold-humid, and warm-dry). For example, MBT′5me<0.65 and MBT′6me>0.55 are diagnostic for the cold-humid climate. Thus, the combination of MBT′5me and MBT′6me has the potential as a tool for the identification of different types of paleoclimate. [ABSTRACT FROM AUTHOR]

During the last decade, the distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in lacustrine sediments has been widely used to reconstruct past variations in lake temperature. A prerequisite for the application of brGDGTs to lacustrine paleoclimate reconstructions is to understand the sources of brGDGTs in lake systems and the processes that influence their distribution. In this study, we investigated the distribution of brGDGTs in core-top sediments from 35 lakes across China, with a broad mean annual air temperature (MAAT) range, but a constrained pH range, to explore the effect of temperature. The results reveal a contrasting response of MBT′ 5ME and MBT′ 6ME to temperature in lake environments compared to that in soils. The sedimentary distributions of 5- and 6-methyl brGDGTs exhibit different relationships with temperature, with most of the latter being correlated to MAAT while the former responding to temperature by only hexamethylated compounds. In both global and Chinese soils, most 6-methyl brGDGTs have no relationship with MAAT, but the distribution of 5-methyl brGDGTs is correlated with MAAT. The different behaviors suggest that communities producing 5- or 6-methyl brGDGTs might be different in lakes and soils. In addition, in lakes from cold regions (MAAT < 5 °C), the brGDGT distributions correlate only with warm season temperatures (April to October) but exhibit no correlation with cold seasons, suggesting a seasonal bias in brGDGT production in these lakes. This bias towards the warm season is not found in lakes from warmer regions (MAAT > 5 °C). Based on these results we propose new temperature calibrations for paleotemperature reconstructions in Chinese alkaline lakes. [ABSTRACT FROM AUTHOR]

Loess-paleosol sequences from the Chinese Loess Plateau (CLP) provide important archives for paleoclimate reconstruction in the Quaternary. Lipid biomarkers from the CLP have been shown to be useful as molecular paleoclimate proxies. Here, we report a Holocene record of n -alkane ratios and compound specific carbon isotope compositions from the Weinan section on the southern margin of the CLP. Throughout the latest deglaciation (L 1-1 ) and Holocene (S 0 ) layers, the average chain length, carbon preference index (CPI), relative abundance of low molecular weight to high molecular weight n -alkanes, and the δ 13 C values of n -C 29 and n -C 31 alkanes varied closely with the changes in paleotemperature and aridity as recorded by tetraether lipids from the same soil samples. Our results are consistent with the expectation that the CLP C 4 /C 3 plant ratio is sensitive to monsoon intensity. Furthermore, the carbon isotope offset between n -C 29 and n -C 31 alkanes (Δδ 13 C 29–31 ) correlated closely with the paleoclimate record. Existing data for C 4 and C 3 herb plants do not support the idea that the close relation between Δδ 13 C 29–31 and paleoclimate results from the isotopic difference between C 4 and C 3 plants. Instead, it is likely that it arises from climate effects on vegetation dynamics. Although more work is required to confirm this deduction, it appears that the Δδ 13 C 29–31 has potential to be considered a novel paleoclimate proxy in loess-paleosol sequences and similar settings. [ABSTRACT FROM AUTHOR]

The distribution of bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs) is influenced by growth temperature and pH. This results in the widespread application of the brGDGT-based MBT(′)/CBT proxy (MBT – methylation of branched tetraethers, CBT – cyclization of branched tetraethers) in terrestrial paleo-environmental reconstructions. Recently, it was shown that the amount of precipitation could also have an impact on CBT, as well as the abundance of brGDGTs relative to that of archaeal isoprenoidal (iso)GDGTs ( R i/b ) and the absolute abundance of brGDGTs, potentially complicating the use of MBT/CBT as paleothermometer. However, the full influence of hydrology, and in particular soil water content (SWC), on GDGT distributions remains unclear. Here we investigated variations in the GDGT distribution across a SWC gradient (0–61%) around Qinghai Lake in the Tibetan Plateau, an arid to semiarid region in China. Our results demonstrate that SWC affects the brGDGT distribution. In particular, we show that SWC has a clear impact on the degree of methylation of C6-methylated brGDGTs, whereas C5-methylated brGDGTs are more impacted by temperature. This results in a combined SWC and temperature control on MBT′. In this context we propose a diagnostic parameter, the IR 6ME (relative abundance of C6-methylated GDGTs) index, to evaluate the applicability of brGDGT-based paleotemperature reconstructions. Using the global dataset, expanded with our own data, MBT′ has a significant correlation with mean annual air temperature when IR 6ME < 0.5, allowing for the use of MBT′/CBT as temperature proxy. However, MBT′ has a significant correlation with mean annual precipitation (i.e., a substantial reflection of SWC impact) when IR 6ME > 0.5, implying that MBT′ may respond to hydrological change in these regions and can be used as a proxy for MAP. [ABSTRACT FROM AUTHOR]

Hydroxylated glycerol dialkyl glycerol tetraethers (OH-GDGTs) have been recently proposed as novel proxies for sea surface temperature (SST) estimation. However, it is still unclear whether these proxies are applicable to the tropical South China Sea (SCS). We assessed the possible impact of extraction methods on the OH-GDGT distribution in marine sediments. A batch of marine sediments from the SCS was subjected to lipid extraction using a modified Bligh/Dyer method (BDM) and an ultrasonic extraction (UE) method, respectively. A systematic difference in the concentration and distribution of OH-GDGTs was observed between the two methods. Sample-by-sample comparison also demonstrated that the BDM had a higher extraction efficiency for OH-GDGTs than UE. The concentration of OH-GDGTs for the BDM dataset was generally higher than that for the UE dataset. The fractional abundance of OH-GDGTs with one cyclopentyl ring was higher for the BDM, resulting in a different OH-GDGT distribution between the BDM and UE datasets. The BDM dataset was thus further used to explore the potential of OH-GDGTs as a proxy for SST in the SCS. OH-2/OHs, the ratio of OH-GDGTs with two cyclopentyl moieties to total OH-GDGTs, exhibited a negative correlation with annual SST when SST was > 25 °C, as opposed to the positive correlation between them when SST was < 25 °C. This suggests that OH-2/OHs holds promise for the reconstruction of SST in the SCS where the SST is close to the warm threshold and proxies, like U K' 37 and TEX 86 H , do not work well. [ABSTRACT FROM AUTHOR]

It is unclear whether or not the paleotemperature proxies based on the branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soil exhibits any seasonal bias in temperature reconstruction. To test this, we examined the brGDGT distributions and compared the mean annual air temperature (MAAT) estimates inferred from the brGDGT-based paleothermometers for surface soils from Nanyang (NY) and Kunming (KM), two sites in China with similar MAAT, but with contrasting climate seasonality (i.e. different summer and winter temperatures). If a significant bias towards summer or winter temperature were to occur, the soils from the locality with stronger seasonality would be expected to yield different temperature estimates than those from the other locality. The results show that, at a regional isothermal scale, the most important environmental control on the soil brGDGT distribution is pH at NY but soil water content (SWC) at KM, suggesting a regional difference in the key environmental control on the brGDGT distributions. MBT′ and CBT are both controlled primarily by pH at NY but by SWC at KM. In contrast, the MBT′ 5ME index, known to be dependent on MAAT alone, displays no relationship with pH or SWC at both sites, allowing us to evaluate the impact of temperature seasonality. MBT′ 5ME is statistically higher at the NY soils than at the KM soils; a MBT′ 5ME difference of 0.04 units is equivalent to a difference of ca. 1.3 °C in MAAT. Likewise, the MAAT mr estimates produced from multiple linear regression calibration for the two soil sites differ by only 0.8 °C. This suggests that there is no significant bias towards summer temperature in the brGDGT-based paleothermometer as the mean summer temperature differs by 6.4 °C at the two soil sites. By extension, temperature reconstructed from the brGDGT-based proxies for the loess-paleosol sequences in the southern Chinese Loess Plateau is unlikely to be biased towards summer. [ABSTRACT FROM AUTHOR]

Abstract: The bacterial membrane lipid-based continental paleothermometer, the MBT/CBT or MBT′–CBT proxy (methylation index of branched tetraethers/cyclization of branched tetraethers), results in a large temperature deviation when applied in semiarid and arid regions. Here we propose new calibration models based on the investigation of >100 surface soils across a large climatic gradient, with a particular focus on semiarid and arid regions of China, and apply them to a loess–paleosol sequence. As reported elsewhere, MBT values exhibit a much higher correlation with MAAT than with summer temperature, suggesting a minimal seasonality bias; however, MBT is apparently insensitive to temperature <5°C or >20°C. Additional complexities are apparent in alkaline and arid soils, which are characterized by different relationships to climatic parameters than those in the complete Chinese (or global) dataset. For example, MBT and CBT indices exhibit a negative correlation in alkaline and arid soils, in contrast to their positive correlation in acid soils. Moreover, the cyclization ratio of bGDGTs (CBT), previously defined as a proxy for soil pH, is apparently primarily controlled by MAAT in these alkaline soils. Thus, we propose (1) a local Chinese calibration of the MBT–CBT proxy and (2) an alternative temperature proxy for use in semiarid and arid regions based on the fractional abundances of bGDGTs; the latter has a markedly higher determination factor and lower root mean square error in alkaline soils than the Chinese local calibration and is suggested to be preferred for paleotemperature reconstruction in Chinese loess/paleosol sequences. These new bGDGT proxies have been applied to the Weinan Holocene paleosol section of the Chinese Loess Plateau (CLP). The fractional abundance calibration, when applied in the Weinan Holocene paleosol, produces a total Holocene temperature variation of 5.2°C and a temperature for the topmost sample that is consistent with the modern temperature. Previously, we showed that the ratio of archaeal isoprenoid GDGTs to bGDGTs (Ri / b ) increases at MAP<600mm, and elevated Ri / b values (>0.5) in the CLP suggest the presence of enhanced aridity in the late Holocene in North China. In combination, the high Ri / b ratios (>0.5) and the associated low MBT values (<0.4) reveal the co-occurrence of dry and cold events, especially in the latest Holocene, in the loess–paleosol sequences in CLP, and probably also in cold and arid regions outside of CLP. [Copyright &y& Elsevier]

Highlights • Archaeal isoGDGTs in peat and soil depth profiles investigated. • %Cren and Methane Index (MI) progressively decreased with depth in peats and soils. • Ring Index (RI) of OH-GDGTs correlated with the GDGT-0/crenarchaeol ratio in peats. • Anaerobic archaea-derived isoGDGTs increase with depth in peat and soil profiles. Abstract The distribution of archaeal isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs) has been increasingly used to characterize the archaeal community and to reconstruct palaeoenvironmental change. However, except for temperature, other environmental factors controlling the distribution of isoGDGTs remain to be explored. Environmental factors generally vary with depth along peat and soil profiles (PP and SP), which can provide unique opportunities to determine the impact of factors other than temperature on the distribution of archaeal ether lipids (AEL) in these settings. Here, we investigated the changes in AEL in two PPs and two red (ferralsol) SPs from southern China and determined the relationship of their distributions with peat and soil depth. The isoGDGT distribution in the two PPs and two SPs all show a significant dependence on depth. Crenarchaeol, which is likely to be produced by Thaumarchaeota, dominates in the upper parts of PPs and SPs, whereas GDGT-0 or isoGDGTs containing cyclopentyl moieties are the most abundant compounds in the deeper parts of these profiles. The change in the AEL distribution can be best characterized by %Cren (the proportion of crenarchaeol in total isoGDGTs) and MI (Methane Index), which are closely related to the water content in PPs and SPs. This suggests that %Cren and MI have potential for tracing the water content and, in turn, the redox conditions in paleosols and peats. The most remarkable differences in the AEL between PPs and SPs are the increased abundance of AEL, e.g., GDGT-5 (isoGDGTs with five cyclopentyl moieties), butanetriol dialkyl glycerol tetraethers (BDGTs), H-shaped isoGDGTs, methylated H-shaped isoGDGTs (Me-H-GDGTs) and the decreased abundance of hydroxylated isoGDGTs (OH-GDGTs) with depth in the two PPs, whereas these lipids are absent from the two SPs. This implies that the former group of lipids is likely derived from anaerobic archaea whereas OH-GDGTs and crenarchaeol are primarily of Thaumarchaeotal origin. The response of AEL to in situ environmental variables in the PPs and SPs reveals that the living archaea reside in the deep SPs and PPs and thus caution should be taken when applying these lipids to the high-resolution palaeoenvironmental reconstructions. [ABSTRACT FROM AUTHOR]

Highlights • Seasonal variation in core lipid (CL) GDGTs found in lake surface sediments. • Significant amount of GDGTs produced in situ in the lake sediments. • CL GDGT-0/Crenarchaeol in the surface sediments correlated with dissolved oxygen. • MBT′ 6ME in the surface sediments correlated with seasonal temperature. • Seasonal variation of isoGDGTs and brGDGTs in SPM was affected by precipitation. Abstract Bacterial branched and archaeal isoprenoid glycerol dialkyl glycerol tetraethers (brGDGTs and isoGDGTs, respectively) have become important tools for the paleoclimate reconstruction of lacustrine systems. However, it is still unclear how geochemical proxies based on GDGTs, particularly in lake surface sediments, respond to seasonal environmental changes in shallow water lakes, whose sediment cores are increasingly being used for paleoenvironmental reconstruction. In this study, we collected samples from the top 0–2 cm of sediments and water column suspended particulate matter (SPM) at a monthly interval from June 2015 to May 2016 in Lake Donghu, a shallow lake in central China. An updated liquid chromatography method was used to separate the 5- and 6-methyl brGDGTs. A remarkable difference between the lake sediments, SPM, and catchment soils suggested that microorganisms inhabiting the surface sediments contributed a significant proportion of isoGDGTs and brGDGTs to the GDGT pool in the lake sediments. We found a clear seasonality in the concentration and distribution of core lipid (CL) isoGDGTs and brGDGTs in the lake surface sediments, in contrast to the absence of seasonality in brGDGTs in mid-latitude soils reported by previous studies. The intact polar lipid/CL ratios for both isoGDGTs and brGDGTs in the lake surface sediments were significantly higher than those in SPM and catchment soils and were higher in the warm seasons than in winter. This collectively suggested that the turnover of brGDGTs in the lake sediments was faster than that in soils. Furthermore, the GDGT-0/Cren ratio for the lake sediments exhibited a negative correlation with dissolved oxygen in the water column. The cyclisation degree of C5-methylated brGDGTs (CBT 5ME) was strongly related to water pH. Seasonal temperature variation clearly impacted the methylation degree of C6-methylated brGDGTs (MBT′ 6ME) but had no influence on C5-methylated brGDGTs (MBT′ 5ME), as opposed to the dependence of MBT′ 5ME on temperature in global soils, peatlands and African lakes. Due to the fast turnover rate and the sensitive response to seasonality, we suggest that the seasonal variation of fossil GDGTs in lake sediments should be considered when developing a calibration using fossil GDGTs in shallow-water lake surface sediments. [ABSTRACT FROM AUTHOR]

Archaeal isoprenoidal glycerol dibiphytanyl glycerol tetraether lipids (iGDGTs) and their non-isoprenoidal branched bacterial analogues (brGDGTs) have widespread applications in biogeochemistry and paleothermometry. Analysis of GDGTs usually involves separation using high performance liquid chromatography, typically coupled via atmospheric pressure chemical ionisation to mass spectrometric detection in selected ion-monitoring mode (HPLC–APCI-MS). However, reliable determination of ratios and, in particular, quantification by this technique, can be challenging due to differences in ionisation efficiencies of the various compounds. Quantification of GDGTs also relies on external calibration of the relative response to an internal standard with authenticated GDGTs, which are often not readily accessible. Here, we tested the suitability of high temperature gas chromatography with flame ionisation detection (HTGC-FID) for the determination of concentrations and tetraether lipid-based ratios in marine and terrestrial samples. For this, we identified GDGTs in environmental samples using HTGC coupled to time-of-flight mass spectrometry (HTGC–MS). Using a purified GDGT standard, we show we can quantify GDGT-0 in environmental samples by GC-FID. Some GDGT-based ratios measured by HTGC-FID exhibited a linear correlation (1:1) with ratios derived from HPLC–MS and weight-based ratios of mixtures of purified standards. However, ratios relying on minor isomers, such as TEX 86 and MBT/CBT have many unresolved challenges for determination by HTGC. Detection limits were higher than for HPLC–MS. However, the advantages of employing HTGC-based methods include: (1) the independence from MS tuning-related differences in ionisation energies; (2) the potential for direct comparison with other, non-GDGT based biomarkers; and (3) a more complete insight into biomarker distributions in environmental samples by the extension of the temperature range. Quantitative elution of GDGTs from a HTGC column as demonstrated herein, will also enable their analysis by compound-specific isotope ratio mass spectrometry. [ABSTRACT FROM AUTHOR]

Understanding the origin and evolutionary history of drought events is of great significance, providing critical insight into future hydrological conditions under the changing climate. Due to the scarcity of drought proxies from northern China, the occurrence and underlying mechanisms of the drought events remains enigmatic on longer timescales. Here we utilize microbial lipid proxies to reconstruct significant drought events over the last four ice age terminations in the southernmost section (Weinan section) of the Chinese Loess Plateau. The abundance of archaeal isoprenoid GDGTs (glycerol dialkyl glycerol tetraethers) relative to bacterial branched GDGTs, measured by R i/b and BIT indices, is diagnostic of enhanced drought conditions. The R i/b (and BIT) indices are stable and low (high) throughout most of the loess section spanning the last 350 thousand years, but they do exhibit sharp transient peaks (valleys) during the intervals associated with the four ice age terminations, and especially Terminations II and IV. These enhanced drought events are, non-intuitively, associated with a significant decrease in the relative abundance of C 4 plants, inferred by a decrease in the carbon isotope composition of bulk organic matter. Although the microbial records show some consistency with the Weinan grain size profiles, indicative of Eastern Asian winter monsoon variability, they also show some apparent difference. In fact, some features of the microbial records exhibit strong similarities with marine sediment planktonic foraminiferal δ 13 C records from the western Pacific warm pool, which reflect ENSO-like changes during glacial terminations. Therefore, enhanced droughts immediately before the interglacial warming in northern China could be explained, at least in part, by teleconnections in tropical ocean–atmosphere circulation via shifts in the Intertropical Convergence Zone (ITCZ) and associated Jet Stream over the Asian continent. According to our microbial biomarker data, these enhanced megadroughts are apparently different, both in terms of severity and causal mechanism, from the more commonly discussed dry conditions observed during glacial periods. [ABSTRACT FROM AUTHOR]

The mineral protection of microbial necromass is critical for soil carbon stabilization. The water impact on the interactions between minerals and microbial necromass, however, remains unclear due to a lack of proper methods to quantify the mineral-protected microbial necromass. Here, we used offline hydrous pyrolysis to release the archaeal necromass—isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) bound to clay minerals by heating samples at increasing temperatures (150, 200, 250, 300, and 350 °C) under argon atmosphere. The content of extractable isoGDGTs in soils reached the maximum at 200–250 °C, indicating the release of bound isoGDGTs. By investigating the free and bound isoGDGTs along a soil transect with increasing soil water content (SWC), we found that the portion of bound isoGDGTs reached up to 97% of total isoGDGTs in dry soils, and decreased with increased SWC, suggesting that SWC controls isoGDGT stabilization on clay minerals. This relationship was confirmed in the soils from the Northeast China Transect, where the proportion of bound isoGDGTs decreased with increased mean annual precipitation from 160 to 814 mm. Such a connection was further supported by the water-saturated samples of the lake and marine sediments, which have a much lower proportion of bound lipids than the soils. The decrease of bound lipid portion with increasing water content is related to the lower attachment of microbial cells to minerals in wetter soils. Microbial cells are more strongly attached to the mineral phase under drier conditions due to the thin water film on the mineral surfaces. In addition, microorganisms secrete more extracellular polymeric substances under drier conditions, which reinforces the interactions between microbial-derived organic matter and minerals. The increased microbe-mineral interactions under drier conditions offer more stable microbial legacies and stabilize organic matter in the (semi)arid regions. • Mineral-bound archaeal lipids were released using offline pyrolysis. • Bound archaeal lipids could account for 97% of total archaeal lipids in soils. • The proportion of bound archaeal lipids negatively correlated with soil water content. • Aridity favors the preservation of archaeal necromass in soils. [ABSTRACT FROM AUTHOR]

The methylation index (MBT or MBT′) of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils along altitudinal transects of some mountains, e.g. Mount (Mt.) Shennongjia and Mt. Gongga, exhibited no correlation with altitude or mean annual air temperature (MAT) alone. However, a significant correlation between MBT′, cyclization ratio (CBT) of brGDGTs and MAT for each transect could still be found. To investigate the reason, we reanalyzed the brGDGTs in soils from Mt. Shennongjia using an improved liquid chromatography method and examined whether the newly described 6-methyl brGDGTs were the main cause of the large scatter in the correlation between MBT′ and MAT. In contrast to MBT′, the MBT′ 5ME , based on 5-methyl brGDGTs alone, correlated significantly with MAT ( R 2 0.89, p < 0.001) but had no relationship with soil pH. However, MBT′ 6ME , based only on 6-methyl brGDGTs, was strongly dependent on soil pH. This suggests that pH dependent 6-methyl brGDGTs introduce large scatter in the correlation between MBT′ and MAT, which is likely to mechanistically explain the poor relationship between MBT′ and MAT, yet good correlation between MBT′, pH and MAT. A series of indices derived from 6-methyl brGDGTs, including IR IIIa′ and IR IIa′ (isomer ratio of structures IIIa′ and IIa′, respectively), correlated with soil pH and can be used as new paleo pH indicators to supplement the classic CBT proxy. The tetramethylated brGDGTs with 0–1 cyclopentyl rings (Ia and Ib) were found to comprise unknown isomers, whose abundances also appear to increase with soil pH. A further optimized liquid chromatography method is needed to quantify these new isomers more accurately and to determine the environmental controls on them. [ABSTRACT FROM AUTHOR]

Highlights: [•] GDDs and GDGTs in surface soil and a loess–paleosol sequence investigated. [•] iGDDs and bGDDs followed same distribution pattern as corresponding GDGTs, suggesting common source. [•] GDGTs/(GDGTs+GDDs) ratio decreased exponentially with depth. [•] GDDs in soils may be degradation products of GDGTs. [Copyright &y& Elsevier]

• Distinct distributions of 3-OH-FAs are found in soils, marine- and lake sediments. • First systematic investigation of 3-OH-FAs in lake environments. • We propose a novel temperature proxy based on 3-OH-FAs for lake environments. Bacterial 3-hydroxy fatty acids (3-OH-FAs) show potential for terrestrial paleoclimate reconstruction; however, the possibility of using 3-OH-FAs as proxies in lacustrine environments remains to be investigated. We analysed the composition and distribution of 3-OH-FAs in surface sediments from 20 Chinese alkaline lakes and investigated the development of climate proxies based on 3-OH-FAs in these lacustrine environments. The results show that lake sediments contain higher relative abundances of anteiso 3-OH-FAs compared to soils. Analysis of the gene community composition from Lake Liangzi reveals that Gram-negative bacterial communities in that lake environment are distinct from marine sediments from the South China Sea but share many characteristics with soils from Mt. Yujia at the phylum level. At the class level, the Lake Liangzi samples yield distinctly higher proportions of Deltaproteobacteria (23%), Betaproteobacteria (13%), Gammaproteobacteria (14%), and Bacterioidia (10%). pH proxies based on 3-OH-FAs (Branching Ratio, Branched index, RIN and RIAN) yield weak linear relationships with lake water pH, while RAN 13 (previously proposed as a sea surface temperature proxy) shows a linear relationship with mean annual air temperatures (MAAT; ranging from 5 °C to 17 °C). The soil temperature proxies RAN 15 (previously applied to a stalagmite) and RAN 17 do not appear to be suitable for temperature calibration in lacustrine environments. We propose a novel temperature proxy (RIN 17 , ratio of iso to normal C 17 3-OH-FA) for lake environments. These field-based correlations demonstrate that the physiological response of Gram-negative bacterial cell membranes to external environmental changes are recorded in lake sediments on a large regional scale. This further highlights the potential use of bacterial 3-OH-FAs as proxies for paleoclimate reconstruction in both aquatic and terrestrial environments. [ABSTRACT FROM AUTHOR]