Evaluating the potential of soil bacterial tetraether proxies in westerlies dominating western Pamirs, Tajikistan and implications for paleoenvironmental reconstructions

Abstract

The widely distributed paleoclimate archives in Tajikistan, such as loess-paleosol sequences and the fluvial and lacustrine sedimentary rocks, are excellent materials for studying the aridification histories and mechanisms of the westerlies and the uplift of the Pamirs. The branched glycerol dialkyl glycerol tetraethers (brGDGTs)-based proxies have been increasingly used to reconstruct the paleoenvironmental parameters, e.g., paleo-pH, paleotemperature and paleoaltitude, from sedimentary records and represent potential tools to address the above issues. However, a knowledge of the applicability and factors affecting brGDGT-based proxies remains incomplete, especially in westerlies-dominating areas. Here, we present results from a study with 53 surface soils that are collected along altitudinal transects in the windward slope of the western Pamirs, Tajikistan with large environmental parameter gradients (mean annual air temperature (MAT) varying from −5.1 to 12.6 °C; mean annual precipitation (MAP) spanning a range from 135 to 882 mm; pH ranging from 5.9 to 8.9; soil water content (SWC) changing from 0.102 to 0.183 m3/m3). The proxies (CBT5ME, IR6ME, CBT′ and IBT) show significant correlations with measured pH values (r2 = 0.51, 0.52, 0.51 and 0.53, respectively), implying the great potential of these proxies for pH reconstructions in Tajikistan. For the alkaline soils, the reconstructed temperature lapse rates (TLR) are constrained to be −1.0 °C/km (actual values ranging from −4.9 to −5.4 °C /km) using the MATmr, a brGDGT-based paleothermometer which is thought to be independent of pH. Similar to the traditional brGDGT-based paleothermometer MBT′/CBT, the anomalies in alkaline soils are related to the fact that pH-dependent 6-methyl brGDGTs are not completely excluded from the calculations in MATmr. The paleothermometer which excludes 6-methyl brGDGTs (MBT′5ME) is no longer influenced by pH, and records MAT variations along the altitudinal transects for soils lower than ~3.3 km. For soils higher than ~3.4 km, the correlations between the brGDGT-derived MAT and altitude are statistically significant only in samples with high SWC. However, the calculated MAT and TLR are both overestimated in high SWC samples. Our results suggest that brGDGT-based proxies have the potential to reconstruct palaeoenvironmental parameters in arid and semi-arid Tajikistan with the discretion of the appropriate proxies and calibrations.