804 / 2024-09-19 14:50:47

Provenance of branched tetraether lipids affects the temperature reconstruction proxy in the Lower Bengal Fan

Lower Bengal Fan, BrGDGT, Paleotemperature reconstruction, Indian Monsoon, Sea level change

Sedimentary branched glycerol dialkyl glycerol tetraether (brGDGT) lipids are commonly used to reconstruct continental paleotemperature. However, it remains unclear if provenance shifts of branched tetraether lipids have affected the applicability of the brGDGT paleotemperature proxies. Therefore, the brGDGT lipids were analyzed in a gravity core taken from the active channel of the Lower Bengal Fan to evaluate the provenance effect on paleotemperature reconstruction. The sediment record indicated three distinct stages over the past 15 ka: stage I (15‒10 ka), stage II (10‒2 ka), and stage III (2‒0 ka). The positive correlation between smectite/(illite + chlorite) and methylation index of branched tetraethers (MBT'_5ME) (R² of 0.64 for stage I, and 0.69 for stages II and III) indicated the reconstructed paleotemperatures were intimately associated with shifts in sediment provenance. At stage I, the increasing smectite/(illite + chlorite) ratio, indicative of waning highland sediments due to sea level rise, corresponded with a nominal increase in paleotemperature that the Younger Drays (17 ℃) was warmer than the preceding Bølling-Allerød event (15 ℃). Stage II (10‒2 ka) saw a continued increase in the smectite/(illite + chlorite) due to ongoing sea level rise (10‒6 ka), deltaic subsidence, and river migration (6‒2 ka), resulting in a further surge in reconstructed temperatures from 17.5 to 20 ℃. Conversely, stage III marked a resurgence of highland sediment input due to river relocation, leading to a cooling trend, with reconstructed paleotemperature decreasing from 20 to 18 ℃. This study underscores the necessity of being cautious when interpreting paleotemperatures from the MBT'_5ME proxy, particularly in river basins characterized by significant variations in altitude.