Influence of salinity on glycerol dialkyl glycerol tetraether-based indicators in Tibetan Plateau lakes: Implications for paleotemperature and paleosalinity reconstructions

Glycerol dialkyl glycerol tetraethers (GDGTs) are powerful molecular tools that are suitable for paleoenvironmental reconstruction. The widely distributed lakes on the Tibetan Plateau (TP) are important terrestrial paleoenvironmental archives. However, studies on the distributions and environmental responses of branched GDGTs (brGDGTs; analyzed by a new method) are scarce, which makes their environmental response uncertain, further hindering the development of paleoclimate research and the reliability of reconstruction results for the TP region. Herein, we report the distribution of brGDGTs and archaeols in surface sediments from 83 lakes and 22 surrounding soils across the TP. The potential source, environmental response of sedimentary brGDGTs, and the implications for the application of GDGT-based indicators are further discussed. Our results indicate that the sedimentary brGDGTs in the TP lakes were mainly produced in-situ. Lake salinity had a noticeable effect on the brGDGT-based temperature indicators. Coupled with the global lake dataset, we found that the methylation index of 5-methyl brGDGTs (MBT'5ME) responded to temperature in freshwater lakes, while that of 6-methyl brGDGTs (MBT'6ME) showed a better correlation with temperature in saline lakes. However, both indices showed a poor correlation in brackish lakes. These varied responses implied that biological communities producing 5- and 6-methyl brGDGTs were likely to be different, and that 6-methyl brGDGTs might be predominantly synthesized in high-salinity environments. Furthermore, these responses will affect the reliability of paleotemperature records reconstructed by MBT'5ME or MBT'6ME paleothermometers. Therefore, it is essential to evaluate the applicability of these two proxies before using them for paleotemperature reconstruction. In addition, the applicability of GDGT-based salinity indicators on the TP was assessed. Both the archaeol and caldarchaeol ecometric (ACE) and its alternative approach ACE' showed good correlation with lake salinity on the TP, and their high response thresholds to salinity were also noted. It is therefore that these salinity proxies may only be applicable to large paleo-hydrological/salinity fluctuations. In contrast, the isomer ratio of 6- and 7-methyl brGDGTs (IR6 + 7ME and its revised form IR'6 + 7ME) was sensitive to salinity and covered different salinity ranges, which may represent novel salinity proxies. The integration of ACE and IR'6 + 7ME indicators c