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

Abstract

Abstract. The combined application of the MBT (degree of methylation) and CBT (degree of cyclization) indices, based on the distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils, has been proposed as a paleoproxy to estimate mean annual temperature (MAT). CBT quantifies the degree of cyclization of brGDGTs and relates to soil pH. MBT and the simplified version MBT' quantify the degree of methylation of brGDGTs and relate to MAT and soil pH. However, other factors such as soil water availability have also been suggested to influence MBT' and possibly restrict the combined application of the MBT' and CBT indices as a paleotemperature proxy. To assess the effect of hydrological conditions on MBT' and CBT, a set of 23 Iberian Peninsula soil samples, covering a MAT range from 10 to 18 • C and a mean annual precipitation (MAP) range of 405 mm to 1455 mm, was analyzed. We found that the CBT was indeed significantly correlated with soil pH in our sample set. In contrast, MBT' was not correlated with MAT but had a significant correlation with the aridity index (AI), a parameter related to water availability in soils. The AI can explain 50 % of the variation of the MBT', and 70 % of the residuals of MAT estimated with the MBT/CBT proxy as compared to instrumentally measured MAT. We propose that, 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 be applied with caution in paleotemperature reconstructions in soils from dry subhumid to hyperarid environments.

Warm season bias of branched GDGT temperature estimates causes underestimation of altitudinal lapse rate

“…The decrease of MBT or MBT 0 with increasing elevation is, therefore, very likely associated with decreases of temperature and soil pH, which suggests that the MBT-or MBT 0 -CBT index may be valid for temperature estimation. The validity of MBT or MBT 0 as a temperature proxy in this arid area is different from previous findings (Peterse et al, 2012;Dirghangi et al, 2013;Menges et al, 2014), which might be associated with a strong monsoon climate. Summer monsoon accounts for 80% annual precipitation in the area, leading to relatively wet conditions in summer.…”

“…A number of studies have noted that the MBT or MBT 0 proxy showed no correlation with MAAT but significant correlation with MAP or aridity index in arid environments, e.g. in areas with MAP < 800 mm (Dirghangi et al, 2013;Menges et al, 2014). All the study sites here are in a relatively dry climate, with MAP generally < 500 mm as measured at nearby weather stations.…”

Transport of branched tetraether lipids from the Tagus River basin to the coastal ocean of the Portuguese margin: consequences for the interpretation of the MBT'/CBT paleothermometer

“…The MAP at the sampling sites is indeed low at 450-910 mm yr −1 (Table 1). A recent study of 23 Iberian soils (Menges et al, 2014) has revealed similar variable MBT' and DC (or CBT) values to those we report here. In that study it is suggested that the MBT' is primarily related with the MAP and soil aridity.…”

“…Indeed the pH of all but one of the soils studied were above 5.5. Recently, reduced values of the BIT index in soils have also been attributed to a dry climate Dirghangi et al, 2013;Menges et al, 2014). The reason for the low BIT could be that in dry soils, oxygen can penetrate further into the soil (Cleveland et al, 2010).…”

Sources and distributions of branched tetraether lipids and crenarchaeol along the Portuguese continental margin: Implications for the BIT index

“…We use the MBT' and DC to compare the brGDGT distribution in the different environments. The values of CL MBT' and DC of soils in the Tagus River basin (Zell et al, 2014a) and also in those of the whole Iberian Peninsula (Menges et al, 2014) vary widely. Compared to the average of the Tagus basin soils the CL MBT' and DC in the Tagus River were higher, which most likely means that brGDGTs are mainly produced in the Tagus River itself and are not derived from soil erosion (Zell et al, 2014a) (Fig.…”