Distribution of branched tetraether lipids in geothermally heated soils: Implications for the MBT/CBT temperature proxy

Branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids occur in soils and peat bogs and are assumed to be produced by anaerobic bacteria. Two indices based on the distribution of these lipids in soils, the Cyclisation of Branched Tetraethers (CBT) and the Methylation of Branched Tetraethers (MBT) index have been shown to linearly relate to pH, and to mean annual air temperature (MAT) and pH, respectively. To directly evaluate the impact of changes in soil temperature on the MBT/CBT proxy, we determined these indices in soils sampled from a transect away from two hot springs in California, which provided a set of soils similar in composition but with different temperatures (12–41 °C). The CBT values of these geothermally heated soils show a good relation with pH (R2 0.76), similar to that of a global MBT/CBT calibration set. Also, the relationship between MBT, soil pH and temperature for the geothermally heated soils is similar to that of a global soil calibration set, although the intercept for the geothermally heated soils is significantly lower, likely because our data set is based on in situ soil temperatures rather than MAT. The results confirm the dependence of the MBT index on soil temperature and pH and support the applicability of the MBT/CBT indices as a proxy for continental palaeotemperatures and past soil pH.