Elevation controls bacterial branched GDGT-based temperature proxies: A regional to global perspective

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial lipids used as proxies for past temperature and pH reconstruction in terrestrial environments. Nevertheless, global calibrations often show discrepancies between observed and proxy-based predictions across soils spanning both latitudinal and elevational gradients. As orographic barriers in mountain regions create distinct elevation-dependent environmental conditions, soils across elevational transects may result in distinct growth conditions of brGDGT producers than expected from their latitude. To assess the impact of elevation on brGDGT proxies, we investigated a Western Himalayas (300–5500 m) transect spanning natural gradients in soil properties, precipitation, temperature and seasonality. BrGDGT-estimated pH agrees with the observed soil pH, while brGDGT-estimated temperatures show deviations of −10 to +10 °C from observed mean annual temperatures across our transect as well as other Himalayan transects. We find that these deviations are dependent on cumulative heat, quantified as growing degree days above 0 °C (GDD<inf>0</inf>). An analysis of globally distributed soils and peats (n = 1795) shows that GDD<inf>0</inf> is positively correlated with the degree of methylation of brGDGTs, suggesting that microbial lipid production is seasonally biased and favors colder-season signals in warm climates (high GDD<inf>0</inf>) and warm-season signals in cool climates (low GDD<inf>0</inf>) across elevational and latitudinal gradients. We further suggest that scatter in brGDGT temperature estimates increases in regions where elevation-driven orographic effects create localized variability in bacterial growth conditions. Such non-uniform growth conditions induced by regional (orographic) factors can locally modulate broader climate trends, which results in biases and scatter within global calibrations and paleotemperature reconstructions.