3-hydroxy fatty acids and branched GDGT-based proxy temperature calibrations for Southeast Asia

This study presents the first region-specific calibrations of 3-hydroxy fatty acids (3-OH-FAs),and branched glycerol dialkyl glycerol tetraethers (brGDGTs) for reconstructing temperature and hydroclimate across mainland Southeast Asia. Surface sediments and soils from forty-eight lakes and thirty catchments spanning tropical Thailand to temperate Yunnan, China, were analyzed to establish empirical relationships between biomarker indices and mean annual air temperature (MAAT). Among the 3-OH-FA indices, RAN13 and the newly proposed RIN15 exhibit the significant correlations with MAAT in lake sediments (slope = −0.026, R2 = 0.63, p < 0.001 and slope = −0.162, R2 = 0.63, p < 0.001, respectively), while RAN15 performs best in soils (slope = −0.06, R2, = 0.83, p < 0.001). For brGDGTs, the MBT′5ME index shows robust correlations with MAAT in lakes (slope = 0.026, R2 = 0.71, p < 0.001), whereas the relationship is notably weaker in soils (slope = 0.013, R2 = 0.28, p < 0.01), although it still outperforms MBT′6ME. pH-related indices (e.g., BR, RIN, CBT′ and IR6ME) display weaker relationships, reflecting limited pH variability across sites. Together, these results highlight distinct bacterial contributions to lipid assemblages and demonstrate that 3-OH-FA- and brGDGT-based temperature proxies respond systematically to regional thermal gradients. The new calibrations provide a robust framework for reconstructing, past temperature and hydroclimatic variability across Southeast Asia′s monsoon region.