Contrasting Scenarios for Upper Temperature Ocean Changes at the Last Glacial Maximum Inferred From Inorganic and Organic Paleothermometers: A Case Study From the Eastern Tropical Indian Ocean

Reconstructing paleo seawater temperatures is essential for understanding past climate changes, yet different paleotemperature proxies can yield divergent signals with opposing climatic interpretations. This study reconstructs the upper ocean thermal structure in the eastern tropical Indian Ocean offshore southwest Sumatra across two 2‐kyr periods during the Last Glacial Maximum (LGM, 19–21 ka) and the Late Holocene (LH, 0–2 ka). Our multiproxy approach combines updated, published results from organic paleothermometers (, TEX 86 ), published and newly generated Mg/Ca data from four species of surface and thermocline dwelling planktic foraminifera, alongside new data from clumped isotope analysis (Δ 47 ) on the same foraminiferal species. The new Δ 47 data suggest surface dwellers experienced more pronounced cooling (6.1°C; 95% confidence interval 4.7–7.5°C) than the thermocline dwellers (1.6°C; 1.1–2.1°C) during the LGM compared to the LH, resulting in a weaker LGM vertical temperature gradient in the upper ocean and a deeper thermocline. While this weaker thermal gradient is generally consistent with previous Mg/Ca reconstructions, our new Δ 47 data for the LH suggest a higher thermocline temperature than the Mg/Ca‐based data, which may have been affected by dissolution effects as indicated by the correlation between Mg/Ca values and shell weights of thermocline dwellers. In contrast, paired and TEX 86 suggest a stronger or similar upper ocean thermal gradient during the LGM. Based on our analysis of modern data, we propose that the apparent discrepancy between the foraminiferal and biomarker approaches might have stemmed from proxies reflecting different depths with different sensitivities to upwelling dynamics.