Varied terrestrial sources of branched tetraether lipids in sediments of the Lower Bengal Fan affect the temperature reconstruction proxy

Sedimentary branched glycerol dialkyl glycerol tetraether (brGDGT) lipids are commonly used to reconstruct continental paleotemperatures. However, the influence of varied terrestrial sources of brGDGTs either from highland or floodplain in a river basin on paleotemperature reconstruction has not been addressed. In this study, brGDGT lipids were analyzed in a gravity core taken from the active channel of the Lower Bengal Fan to evaluate the source effect on paleotemperature reconstruction during 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 5-methyl branched tetraethers (MBT’ 5ME ) (stage I: R 2 = 0.64, p < 0.01; stages II and III: R 2 = 0.69, p < 0.01) indicated that the reconstructed paleotemperatures were intimately associated with shifts in sediment sources. At stage I, enriched highland sediment sources with low smectite/ (illite + chlorite) ratio at the low sea level during the Bølling-Allerød event led to an underestimation of the paleotemperature (15 °C) compared to the Younger Dryas (17 °C) event. During late stage II (6-2 ka), gradually waning highland sediments indicated by the increasing smectite/ (illite + chlorite) ratio along with rising sea level and deltaic subsidence, resulted in a progressive overestimation of the reconstructed temperatures (19 to 20 °C) than the early stage II (18 °C at 10-6 ka). Stage III (2-0 ka) marked an incrementally increasing contribution from highland sediments due to progressive river relocation, leading to a cooling paleotemperature reconstruction from 20 °C to 18 °C. All pointed to the importance of distinguishing varied terrestrial sources of branched tetraether lipids in large-elevation river basins to reconstruct paleotemperature by MBT’ 5ME proxy. 1. Terrestrial-derived brGDGTs can survive thousands of kilometers of transport to be deposited in the Lower Bengal Fan. 2. The decreasing sediment supply from cool Himalaya highland resulted in elevated MBT’ 5ME values and their reconstructed temperatures. 3. The reconstructed MAT proxy is affected by terrestrial sources of branched tetraether lipids. Plain language summaryTerrestrial branched tetraether lipids preserved in coastal sediments have the potential to reconstruct the land temperature. However, when rivers carry these molecules from different landscapes — such as the Himalaya highland versus floodplains — they can distort temperature estimates. By analyzing a 15 ka sediment core from the Lower Bengal Fan, we discovered that highland-derived sediments make temperatures appear colder than reality, while floodplain sediments make them seem warmer. During the actual warm period of the Bølling-Allerød event, increased erosion from the Himalaya highland flooded the Bengal Fan with "cold-signature" sediments, causing reconstructed temperatures to 15°C — colder than the subsequent actual cold period of the Younger Dryas event, which registered as 17°C due to reduced highland inputs. Later, between 10-2 ka, rising sea level and deltaic subsidence amplified floodplain sediments, creating a warming trend as sea levels rose, increasing floodplain sediments created a false warming trend (19°C to 20°C). While the river draining Himalaya highland relocation during 2-0 ka imposed a cooling trend (20°C to 18°C). Crucially, these distortions show that where sediments originate — highland versus floodplains — can override actual climate signals in major river basins. Untangling these geological interference signals from true climate melodies is necessary to accurately decode past land temperatures.