Consideration of soil types for the calibration of molecular proxies for soil pH and temperature using global soil datasets and Vietnamese soil profiles

Distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils depends on environmental parameters such as mean annual air temperature (MAAT) and soil pH. MBT′/MBT′_5ME (methylation index of branched tetraethers) and CBT/CBT′ (cyclization ratio of branched tetraethers) are ratios based on the relative abundances of brGDGTs. Using these ratios, global and regional/local calibrations have been established using surface soils, but generally without any preliminary study of soil types. In this study, we reconsider global MAAT–pH/brGDGT calibrations by assigning soil types to the 358 components of a composite soil dataset. Additionally, we investigate brGDGT-derived proxies in five well-described soil profiles along an altitudinal transect in the Mount Fan Si Pan National Park, northwestern Vietnam. Our results show that at the global scale, traditional MAAT–pH/brGDGT relationships per soil type are significantly different and that MAAT (and pH) residuals per soil type may differ as well, for example between Alisols and Podzols. This effect persists when 5- and 6-methyl brGDGTs are quantified separately. Moreover, MAAT (and pH) residuals per soil type are generally not clearly reduced, including in Leptosols, in which 6-methyl brGDGTs are present. In the Fan Si Pan transect, MBT′/CBT–MAAT estimates showed no significant deviation from expected MAATs. We find, however, that soil type effect – related to vegetation changes and contrasting soil organic carbon properties – may bias MAAT–pH estimates in the Vietnamese soil transect. Furthermore, soil depth plays a role which differs between the different Fan Si Pan soil profiles, likely determined by soil type and history.