Interhemispheric Temperature Gradient Was the Primary Driver of Indian Monsoon Intensity Since the Last Glacial Maximum

Knowledge of glacial-interglacial changes in the Indian summer monsoon (ISM) can provide insights into future hydrological changes in the monsoon region. We analyzed the grain size, branched glycerol dialkyl glycerol tetraethers (brGDGTs), and leaf wax hydrogen isotopes (δD_wax) of sediments from Lake Tianchi in the ISM region, to reconstruct the temperature and monsoon history for the past 23 kyr. The results reveal a notable negative correlation between the abundance of brGDGT-IIIa and grain size, indicating the non-negligible influence of soil-derived brGDGTs on temperature. Using the grain size-brGDGT-IIIa relationship, we assessed the contribution of soil-derived brGDGTs and corrected the temperature accordingly. The results reveal a ∼6°C warming from the Last Glacial Maximum (LGM; 23–20 kyr) to the mid-Holocene (∼8–6 kyr), punctuated by several millennial-scale events, including Heinrich event 1 (H1; 16.5–15 kyr), the Bølling-Allerød (B/A; 15–12.8 kyr), and the Younger Dryas (YD; ∼12.8–11 kyr), with H1 being the coldest interval. The δD_wax records show that H1 was the driest interval, with a drier LGM and YD and a wet B/A and early- to mid-Holocene. Furthermore, the δD_wax records display a pattern of variation similar to the interhemispheric temperature gradient, indicating that the intensity of this gradient is the primary factor controlling the ISM precipitation. Given the significant influence of Antarctic ice on the interhemispheric temperature gradient, the anticipated loss of half of the Antarctic sea ice by 2100 would lead to the southward shift of the Intertropical Convergence Zone and associated drought in the ISM region.