Branched glycerol dialkyl glycerol tetraethers in Pleistocene loess-paleosol sequences: Three case studies

Branched glycerol dialkyl glycerol tetraethers (branched GDGTs) are membrane lipids presumably derived from anaerobic, heterotrophic soil bacteria. Empirical studies have shown that the composition of branched GDGTs in topsoils varies in degree of cyclization and methylation (expressed as CBT and MBT, i.e. cyclization and methylation index of branched tetraethers) depending on soil pH and mean annual air temperature (MAT). Here we present results from three case studies, namely three well-studied loess-paleosol sequences (LPSs), in which we measured the branched GDGTs in order to test the applicability of these biomarkers for paleoclimate reconstruction. The results show that major disagreements exist between expectations based on available stratigraphic, pedological and geochemical data on the one hand, and GDGT-derived reconstructions on the other hand. Caution is therefore advised when interpreting the respective indices in LPS studies. We discuss several potential directions for future research: (i) Some of the target peaks in the HPLC chromatograms consist of multiple, unidentified compounds. This needs to be addressed by way of improved component separation, identification and new calibration studies. (ii) Factors other than MAT and soil pH that could affect membrane composition should be further investigated, e. g. nutrient availability and redox conditions, as well as seasonality and changes in bacterial communities. (iii) GDGTs may be produced in non-negligible amounts at depth and thus overprint the original in situ signal, a concern that could be termed 'growth depth effect'. (iv) GDGTs should also be analyzed in dust source regions and contemporaneous eolian sediments in order to address a potential redeposition effect. (C) 2012 Elsevier Ltd. All rights reserved.