The first application of RI-OH lipid biomarkers to reconstruct the upper water column thermal structure. An example from the tropical Indian Ocean (Bay of Bengal) during the past 24 kyr

Millennial-scale upper-ocean thermal structure variations of the northern tropical Indian Ocean are a robust indicator of past southwest monsoon (SWM). Lipid biomarker isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs) and hydroxylated isoprenoid GDGTs (OH-GDGTs) are promising proxies for reconstructing past upper water thermal conditions. Yet, their application in the tropical Indian Ocean remains limited. Here, we applied the TEX 86 H and RI-OH methods to quantitatively reconstruct mixed-layer and thermocline temperatures in the Bay of Bengal (BoB) over the last 24 kyr. The thermal gradient (ΔT = mixed-layer temperature – thermocline temperature) exhibits pronounced variations in response to millennial-scale abrupt climate events due to changes in the SWM. Our data revealed a narrow ΔT during relatively arid periods, specifically the Last Glacial Maximum, late Heinrich Stadial 1 (HS1), and the Younger Dryas. In contrast to arid periods, large ΔT occurred during humid intervals, namely early HS1, the Bølling-Allerød, and the early mid-Holocene (7.95–7.0 cal ka BP). The upper water structure was intensely stratified during strong SWM phases, but increased vertical mixing during weak SWM periods. These weak-strong patterns align with the Ganga-Brahmaputra-Meghna runoff inferred from the branched GDGT indices (ΣⅢa/ΣⅡa and Branched and Isoprenoid Tetraether) and the northeast Indian speleothem records, indicating that freshwater barrier layers suppressed vertical convection. The southward contraction of the Intertropical Convergence Zone likely contributed to the humid conditions in the BoB during early HS1. Moreover, the BoB upper ocean structure responded to the variability of the Atlantic Meridional Overturning Circulation (AMOC). The weakening of the two-phase AMOC resulted in contrasting stratification (early HS1) and mixing (late HS1) regimes. Reduced interbasinal heat exchange between the North Atlantic and the Indian Ocean, due to the slowdown of the AMOC, drove synchronous upper ocean warming in the BoB during the Younger Dryas. The strongest SWM strength and enhanced Ganga-Brahmaputra-Meghna freshwater discharge led to pronounced water column stratification between 7.95 and 7.0 cal ka BP. • First biomarker-based reconstruction of upper water thermal structure in the Bay of Bengal. • TEX 86 H and RI-OH (GDGTs) used for mixed layer and thermocline temperatures, respectively. • Mixed layer and thermocline varied inversely over the past 24,000 years. • BoB's upper water thermal gradient varied between arid and humid climate intervals. • Upper water thermal gradient significantly stronger during early HS1 than in later HS1.