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The combined application of the MBT (degree of methylation) and CBT (degreeof cyclization) indices, based on the distribution of branched glyceroldialkyl glycerol tetraethers (brGDGTs) in soils, has been proposed as apaleoproxy to estimate mean annual temperature (MAT). CBT quantifies thedegree of cyclization of brGDGTs and relates to soil pH. MBT and thesimplified version MBT' quantify the degree of methylation of brGDGTs andrelate to MAT and soil pH. However, other factors such as soil wateravailability have also been suggested to influence MBT' and possibly restrictthe combined application of the MBT' and CBT indices as a paleotemperatureproxy. To assess the effect of hydrological conditions on MBT' and CBT, a setof 23 Iberian Peninsula soil samples, covering a MAT range from10 to 18 °C and a mean annual precipitation (MAP) range of 405 mm to1455 mm, was analyzed. We found that the CBT was indeed significantlycorrelated with soil pH in our sample set. In contrast, MBT' was not correlatedwith MAT but had a significant correlation with the aridity index (AI), aparameter related to water availability in soils. The AI can explain 50%of the variation of the MBT', and 70% of the residuals of MAT estimatedwith the MBT/CBT proxy as compared to instrumentally measured MAT. We proposethat, in arid settings, where water may be an ecologically limiting factor,MBT' is influenced by hydrological conditions rather than temperature. Thus,our results suggest that the combination of MBT' and CBT indices should beapplied with caution in paleotemperature reconstructions in soils from drysubhumid to hyperarid environments

Influence of water availability in the distributions of branched glycerol dialkyl glycerol tetraether in soils of the Iberian Peninsula