South Asian summer and winter monsoon evolution during the last deglaciation

The South Asian monsoon (SAM) system significantly influences the hydroclimate of the Indian subcontinent, affecting nearly two billion people. However, much of our paleoclimate knowledge is centered on the summer monsoon (SASM), while the winter monsoon (SAWM) remains poorly understood. This study investigates seasonal monsoon variability during the last deglaciation, focusing on abrupt climate transitions that provide natural experiments for understanding past monsoon dynamics. We analyzed sediment core SO130-289KL from the Northeastern Arabian Sea, a region sensitive to both the SASM and the SAWM. Laminated sediments deposited during the Bølling–Allerød Interstadial (~14,690–12,890 years BP) offer a rare high-resolution archive for reconstructing past climate variability at ~decadal timescales.

To overcome the limitations of traditional analytical techniques, we employed mass spectrometry imaging and hyperspectral imaging, achieving micrometer-scale spatial resolution. SST reconstructions rely on two independent biomarkers: the alkenone-based U^K’₃₇ index and the GDGT-based Crenarchaeol-Caldarchaeol Tetraether (CCaT) index. Hyperspectral imaging quantified chloropigments-a as a proxy for primary production, while leaf wax hydrogen (δD C₃₁) and carbon (δ¹³C₃₁) isotopes provide insights into atmospheric moisture and terrestrial vegetation dynamics in lower resolution.

Our results reveal distinct seasonal responses of the SAM system to deglacial climate changes. Alkenone-based SSTs, which are more sensitive to change in SAWM winds, show a progressive weakening of the northeastern boreal winter winds during the Allerød, aligning with a progressive cooling trend in the Southern Hemisphere. This weakening likely reflects a boreal winter (austral summer) northward shift of the Intertropical Convergence Zone (ITCZ) towards the equator driven by decreasing Southern Hemisphere austral summer temperatures. In contrast, CCaT-derived SSTs, linked to SASM wind strength, closely correlate with Northern Hemisphere temperature proxies, demonstrating that SASM variability was primarily controlled by boreal summer conditions.

Seasonal precipitation patterns reconstructed from leaf wax isotopes highlight hydroclimatic changes during the Bølling-Allerød. Lower δD C₃₁ values during the Bølling indicate increased summer precipitation, while the early Allerød more positive δD C₃₁ suggest decrease in precipitation. Following concurrent decreases in δ¹³C₃₁ and δD C₃₁ values during the mid to late Allerød suggest reduced seasonality with enhanced precipitation in both summer and winter.

The reconstructed seasonal evolution of SASM and SAWM has significant implications for other paleoclimate archives, such as speleothem δ¹⁸O values, traditionally interpreted as summer monsoon proxies. Our findings suggest that speleothem δ¹⁸O values reflect a combined signal of summer and winter precipitation. During the Bølling-Allerød, depleted δ¹⁸O values may indicate an increased contribution from isotopically lighter winter precipitation associated with subtropical westerly jets, rather than solely stronger summer monsoon rainfall. The observed decrease in δ¹⁸O values during the late Allerød likely reflects enhanced winter precipitation from isotopically depleted far-distance moisture sources.

Our findings underscore the dual hemispheric influence on the SAM. SASM strength was directly linked to Northern Hemisphere forcing, particularly shifts in Atlantic Meridional Overturning Circulation and associated ITCZ migrations, while SAWM variability was modulated by both Northern and Southern Hemisphere climate changes.