Potential degradation effect on paleo-moisture proxies based on the relative abundance of archaeal vs. bacterial tetraethers in loess-paleosol sequences on the Chinese Loess Plateau

Two indices based on the relative abundance of isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) to branched GDGTs (brGDGTs), i.e., the branched and isoprenoid tetraether (BIT) index and the ratio of isoGDGTs to brGDGTs (Ri/b) index, have recently been proposed as promising proxies for tracing humidity variations in terrestrial soil deposits. However, the applicability of these proxies in loess-paleosol sequences (LPSs) remains unclear on the Chinese Loess Plateau (CLP). In this study, we analyzed BIT, Ri/b and concentrations of related GDGTs in 37 surface soil samples on the CLP. The results showed that the concentrations of brGDGTs correlate positively (strongly or moderately) with soil water content (SWC; r = 0.74) and mean annual precipitation (MAP; r = 0.58), while the concentrations of isoGDGTs correlate negatively (but generally weakly) with SWC (r = −0.31) and MAP (r = −0.45), and consequently, there is a positive relationship between BIT and moisture, and a negative relationship between Ri/b and moisture. Despite the sensitivity of BIT and Ri/b to moisture variations on the modern CLP, the published records in 3 LPSs consistently show an increasing trend in BIT and a decreasing trend in Ri/b with age, which are insensitive to past changes in Asian summer monsoon intensity. This fact suggests that the two indices might not be sensitive recorders of past hydrological changes in LPSs on the CLP. Further analysis of GDGT concentrations in the Lantian LPS showed strong and negative exponential relationships between isoGDGT concentrations and age but weak relationships between brGDGT concentrations and age, possibly indicating faster degradation of isoGDGTs vs. brGDGTs in the LPSs. The preferential degradation of isoGDGTs vs. brGDGTs may potentially bias the BIT and Ri/b indices towards higher and lower values, respectively, and thus might be at least partly responsible for the insensitivity of the two proxies for paleohydrologicial reconstructions in LPSs on the CLP. However, it should be noted that the preferential degradation of isoGDGTs vs. brGDGTs cannot affect other paleoproxies which are based only on isoGDGTs or brGDGTs, such as the TEX86 and MBT′/CBT indices.