The anatomy of the Indian summer monsoon variability during the penultimate deglaciation (Termination II)

Abstract

Precisely dated East Asian summer monsoon (EASM) speleothem δ¹⁸O records frequently mirror Greenland ice-core variability during deglaciation and stadial–interstadial transitions, however, pronounced regional heterogeneity is evident, particularly during the penultimate deglaciation (Termination II; TII). Not all records align, and mismatches are often ascribed to chronological uncertainty despite high dating precision, yet they persist even in annually band-counted, confocal-imaged speleothem δ¹⁸O records, implying complexity beyond dating artifacts. This ambiguity sustains debate over whether EASM cave δ¹⁸O primarily encodes Asian monsoon intensity via moisture-source shifts or reflects upstream rainout, progressive isotopic distillation and is potentially modulated by precipitation seasonality.

In contrast, speleothem δ¹⁸O records from the Indian subcontinent provide a complementary perspective, with δ¹⁸O variability more directly reflecting Indian Summer Monsoon (ISM)  circulation strength. To better constrain first-order Asian monsoon variability, we present a high-temporal-resolution, precisely dated speleothem δ¹⁸O record from Mawmluh Cave in northeastern India (hereafter ML11 δ¹⁸O). The ML11 δ¹⁸O record spans Termination II (TII) and is derived from a ~70-cm-long stalagmite, with a mean temporal resolution of ~5 years and average ²³⁰Th age uncertainties of ±600 years. Our ML11δ¹⁸O  record resolves the evolution of the Indian Summer Monsoon (ISM) across TII, enabling robust assessment of monsoon structure and variability under changing boundary conditions. We examine what constitutes a “penultimate deglaciation” in a monsoon-dominated system, considering not only its precise timing but also its sensitivity to external forcing. Leveraging the high chronological precision of ML11 δ¹⁸O record, we evaluate similarities and potential differences between the ISM and EASM speleothem δ¹⁸O variability. Our new ISM δ¹⁸O record further tests whether the EASM speleothem δ¹⁸O reflects pan-Asian monsoon dynamics or is dominated by regional-to-local hydroclimate processes. These results highlight the need to integrate chronologically robust archives with regionally diagnostic proxy interpretations to better resolve monsoon behavior and improve constraints on monsoon sensitivity under future climate change.