Correlations between microbial tetraether lipids and environmental variables in Chinese soils: Optimizing the paleo-reconstructions in semi-arid and arid regions

摘要

The bacterial membrane lipid-based continental paleothermometer, the MBT/CBT or MBT0–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 (R_(i/b)) increases at MAP < 600 mm, and elevated R_(i/b) values (>0.5) in the CLP suggest the presence of enhanced aridity in the late Holocene in North China. In combination, the high R_(i/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.