Distributions of isoprenoid and branched glycerol dialkanol diethers in Chinese surface soils and a loess-paleosol sequence: Implications for the degradation of tetraether lipids

Isoprenoid and branched glycerol dialkanol diethers (iGDDs and bGDDs) have recently been found in marine and peat deposits, whereas their distributions and sources in soils are undetermined. We present the distributional characteristics of GDDs, as well as their corresponding glycerol dialkyl glycerol tetraethers (GDGTs), in Chinese surface soils and a loess-paleosol sequence (LPS) in northwest China to study the source of GDDs and their relationship with GDGTs. The distributions of iGDDs and bGDDs were comparable with those of the corresponding GDGTs, with a dominance of iGDGTs over bGDGTs in alkaline soil and the opposite in acid soil. By extension, the GDD- and GDGT-based BIT indices exhibited the same trends in both surface soils and the LPS. The fractional abundances of individual iGDDs and bGDDs were also similar to those of the corresponding GDGTs, resulting in similar cyclization patterns for iGDGTs and iGDDs, and similar methylation indices for bGDGTs and bGDDs. These similarities suggest that bGDDs and iGDDs may share a common biological source with the corresponding GDGTs. In the LPS, the GDGT/(GDGT. +. GDD) ratio decreased exponentially with depth, fitting a first order kinetic degradation (or more specifically, transformation) model that has commonly been applied to other lipid classes; this is strong evidence for a diagenetic origin for GDDs. Although our results do not exclude production of GDDs directly by microorganisms, they do suggest that the GDDs may be the degradation products of GDGTs.