Evaluating Spatial Heterogeneity and Environmental Variability Inferred from Branched Glycerol Dialkyl Glycerol Tetraethers (GDGTs) Distribution in Soils from Valles Caldera, New Mexic
Tuesday, 16 December 2014
Branched glycerol dialkyl glycerol tetraethers (GDGTs) are recently discovered bacterial membrane lipids, ubiquitously present in peat bogs and soils, as well as in rivers, lakes and lake sediments. Their distribution appears to be controlled mainly by soil pH and annual mean air temperature (MAT) and they have been increasingly used as paleoclimate proxies in sedimentary records. In order to validate their application as paleoclimate proxies, it is essential evaluate the influence of small scale environmental variability on their distribution. Initial application of the original soil-based branched GDGT distribution proxy to lacustrine sediments from Valles Caldera, New Mexico (NM) was promising, producing a viable temperature record spanning two glacial/interglacial cycles. In this study, we assess the influence of analytical and spatial soil heterogeneity on the concentration and distribution of 9 branched GDGTs in soils from Valles Caldera, and show how this variability is propagated to MAT and pH estimates using multiple soil-based branched GDGT transfer functions. Our results show that significant differences in the abundance and distribution of branched GDGTs in soil can be observed even within a small area such as Valles Caldera. Although the original MBT-CBT calibration appears to give robust MAT estimates and the newest calibration provides pH estimates in better agreement with modern local soils in Valles Caldera, the environmental heterogeneity (e.g. vegetation type and soil moisture) appears to affect the precision of MAT and pH estimates. Furthermore, the heterogeneity of soils leads to significant variability among samples taken even from within a square meter. While such soil heterogeneity is not unknown (and is typically controlled for by combining multiple samples), this study quantifies heterogeneity relative to branched GDGT-based proxies for the first time, indicating that care must be taken with samples from heterogeneous soils in MAT and pH reconstructions.