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Abstract: Suspended particulate matter (SPM) was collected above and below the Three Gorges Dam (TGD) on the Yangtze River in May, July and December 2009, respectively, for a study of the spatial and temporal variation in glycerol dialkyl glycerol tetraethers (GDGTs) in the riverine system. The branched GDGTs (bGDGTs) were > 0.6ng/l and isoprenoid GDGTs (iGDGTs) > 0.3ng/l in most SPM samples in May and July, whereas, iGDGTs were on average about 10× more abundant than bGDGTs in December. The results suggest that the abundance of bGDGTs and iGDGTs in the Yangtze River at the TGD varied seasonally, which might be affected by soil runoff and in situ production. Despite these effects, the TEX86-inferred temperature derived from SPM reflected the measured river surface water temperature in the sampling months, suggesting that planktonic Thaumarchaeota might actively make iGDGTs in the river water near the TGD throughout the year. The bGDGT-derived CBT/MBT temperature from the SPM was not significantly different among the three seasons and the values together averaged 15.2±0.7°C, close to the annual mean air temperature (16.9°C) in the region near the TGD. However, the bGDGT-derived pHCBT values were close to the water pH and slightly higher than the pH of the local soil. These results suggest that the sources of bGDGTs in the river water are complicated and that bGDGTs may not solely derive from the local soil runoff. Nevertheless, our study highlights the potential of using TEX86 and MBT/CBT proxies for a better understanding of the source and mechanism of GDGT production in riverine systems increasingly affected by the construction of dams. [Copyright &y& Elsevier]

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]

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 incursion of euxinic waters into the ocean-surface layer is hypothesized to have been an important killing agent during the end-Permian mass extinction. However, both the causes and extent of oceanic euxinia during this crisis remain poorly known, making assessment of its role in the mass extinction difficult. Here, we document the distribution of aryl isoprenoids (AIs), which are biomarkers of obligate anaerobic green sulfur bacteria (Chlorobiaceae) that are indicative of photic-zone euxinia, in 12 Permian-Triassic Boundary (PTB) sections with a wide distribution globally. Profiles of AI abundance for the 12 study sections show significant spatio-temporal variation. No AIs were identified in the shallowest sections, but AIs are present both prior to and following the mass extinction in intermediate-depth sections and following the mass extinction in deep-water sections. This pattern suggests a combination of upward and oceanward expansion of photic-zone euxinia during the PTB crisis, possibly fueled by elevated riverine nutrient fluxes as a consequence of climatic warming, terrestrial ecosystem destruction, and enhanced erosion. This hypothesis is supported by the close association of AIs with elevated abundances of moretanes and dibenzofuran (DBF), which are biomarkers for terrestrial erosion. Expansion of oceanic euxinia at intermediate water depths may have established a long-term reservoir that fed episodic incursions of H 2 S-bearing waters into shallow-marine environments, delaying the recovery of marine ecosystems during the Early Triassic. The biomarker records of the present study thus provide significant evidence of terrestrial-marine linkage during the PTB crisis. [ABSTRACT FROM AUTHOR]

Isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) of archaeal origin and branched (br)GDGTs of bacterial origin occur in a diverse range of lacustrine sedimentary environments. They have attracted attention as potential temperature proxies, providing high resolution (palaeo)environmental reconstruction from continental interiors. For this study, the distribution of GDGTs in, and application of GDGT-based proxies to, surface samples from Chaka Salt Lake (China), as well as soils and in-flow river sediments, were investigated to assess whether GDGT-based proxies were applicable to this hypersaline lake system. We show that iso- and brGDGTs were present in all sediments and soils from the Chaka Salt Lake system. GDGT-0 and crenarchaeol were generally the two most abundant isoGDGTs, suggesting Thaumarchaeota as a major biological source of isoGDGTs. The low ratio of crenarchaeol/crenarchaeol regioisomer suggests that Thaumarchaeota of the lake sediments is likely Thaumarchaeota group I.1b derived from the surrounding alkaline soils, arguing against the use of the TEX 86 proxy in this system. Because alkaline soils generally have high isoGDGT concentrations, it is likely that a large allochthonous input of isoGDGTs would be a pervasive challenge to palaeoclimate applications in such settings. On the other hand, the brGDGT distributions in the lake and river sediments differed markedly from those in the surrounding soils, suggesting that, instead of deriving from the surrounding soils, at least part of the brGDGTs is synthesized in situ or delivered from more distal upland soils. Taken together, our results indicate that the mixed sources of GDGTs in Chaka Salt Lake complicate the application of GDGT-based proxies, and it will be challenging to use such proxies in this system. [ABSTRACT FROM AUTHOR]

Hydroxylated isoprenoid glycerol dialkyl glycerol tetraethers (OH-GDGTs), with one or two hydroxyl groups in one of the biphytanyl moieties, are major compounds in planktonic Thaumarchaeota , and occur widely in marine and lacustrine sediments. In order to examine their potential as an indicator of local variation in sea surface temperature (SST), we collected 70 surface sediments from the Chinese coastal seas (CCSs), including 16 from the Yangtze Estuary, to determine their tetraether lipid composition. The proportion of OH-GDGTs relative to total isoprenoid GDGTs increased with latitude from 1.8% to 12.8% and correlated strongly with SST, as indicated from redundancy analysis. The variation in OH-GDGT composition was best captured by the weighted average number of cyclopentane moieties in OH-GDGT-1 and -2 (RI-OH), which correlated significantly with both summer and autumn SST and was associated with minimum residual SST relative to remote sensing data. The large input of terrigenous organic matter to the Yangtze Estuary appears to have no significant influence on the OH-GDGT based proxy. Our study suggests that RI-OH could serve as a useful proxy for recording warm seasonal SST in the CCSs. A global surface sediment compilation further indicates that RI-OH and its derivative, RI-OH′, which additionally contains OH-GDGT-0, may be used to estimate SST in warm (ca. > 15 °C annual mean SST) and cold (ca. < 15 °C) environments, respectively. [ABSTRACT FROM AUTHOR]

To shed more light on the applicability of glycerol dialkyl glycerol tetraether (GDGT) based proxies for reconstructing environmental change in shallow marginal seas, we analyzed the GDGT composition of surface sediments from the east coastal sea of China (ECSC). The GDGT distribution in 58 surface sediment samples from the ECSC shows that the isoprenoid GDGTs ( i GDGTs) are mainly from autochthonous planktonic Thaumarchaeota and that the relative abundance of the acyclic GDGT (GDGT-0) in the ECSC increases with latitude, whereas that of cyclic GDGTs decreases. The fractional abundances of i GDGTs correlated significantly with annual and seasonal sea surface temperature (SST), especially with summer SST. Based on this, we propose a new calibration of the TEX 86 H proxy to reconstruct past SST in the ECSC. The new calibration shows a higher calibration coefficient ( R 2 0.84, p < 0.001), leading to a more robust reconstruction of temperature in China marginal seas. The 90 yr SST record derived with the new calibration for a sediment core from the ECSC is consistent with climate data from NOAA, demonstrating the applicability of the new calibration for summer SST reconstruction in this shallow marginal sea [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]

Abstract: Lipid biomarkers in stalagmites have potential in palaeoenvironmental reconstruction, but glycerol dialkyl glycerol tetraethers (GDGTs) in stalagmites have rarely been investigated. We report the occurrence of archaeal isoprenoid GDGTs and bacterial branched GDGTs in Heshang Cave stalagmites in central China. The archaeal component, crenarchaeol, dominated bacterial branched GDGTs in the stalagmites and on the weathered parent rock surface collected inside the cave. In contrast, the distribution of GDGTs in the overlying soil shows that branched GDGTs dominated archaeal GDGTs. Hence, the majority of archaeal GDGTs in stalagmites may be produced by Crenarchaeota inhabiting the cave rather than being inherited from the overlying soil. Bacterial branched GDGT distributions, expressed as MBT and CBT indices, showed remarkable differences between stalagmite and soil samples, implying that most branched GDGTs may also be produced in situ in the cave. The TEX86 or values were found to increase with more depleted δ 18O values representing much stronger monsoon and therefore usually higher temperatures, in the study region. However, the MBT/CBT index in stalagmites may be influenced by a variety of factors besides palaeotemperature. [ABSTRACT FROM AUTHOR]

The distribution of archaeal ether lipids is thought to be governed by salinity in hypersaline and marine environments due to differences in their biological sources. However, little is known about their source and relationship with salinity in lacustrine environments. To explore this, we analyzed the distribution of specific archaeal lipids (isoprenoid GDGTs and diethers) of 11 ponds along a salinity gradient (0.8‰ to 279‰) in Inner Mongolia, NE China. Functional gene mcrA and compound-specific stable carbon isotope data were used to explore the biological source(s) of archaeal diether and tetraether lipids. Both lipid- and DNA-based methods showed methanogens to be abundant in ponds, which is consistent with the dominance of GDGT-0 relative to other isoprenoid GDGTs as well as the occurrence of archaeol (C 20 /C 20 DGD), hydroxyarchaeol and the mcrA gene. However, the occurrence of the extended archaeol (C 20 /C 25 DGD) also suggests the presence of Halobacteriales, non-halophilic methanogens and halophilic methanogens. This means that the biological sources of archaeal diethers and tetraethers are complicated. We subsequently evaluated the applicability of TEX 86 temperature and ACE salinity proxies in these settings; however, due to the mixed sources of archaeal diethers and tetraethers, these proxies are not applicable in these ponds and it appears that salinity is not the dominant control on archaeal ether lipid distributions in these saline ponds. [ABSTRACT FROM AUTHOR]