Complex interplay of forcings drives Indian vegetation and summer monsoon variability during MIS 11

Abstract. Marine Isotope Stage (MIS) 11 has long been considered a unique Quaternary interglacial due to its orbital similarities with the Holocene, persistence of high atmospheric CO₂ concentrations and extended duration triggering unusual polar ice-sheet loss. Despite its importance, Indian summer monsoon (ISM) variability within the core monsoon zone (CMZ), as well as its impacts on vulnerable tropical forests, remain unexplored. Here, we document, for the first time, MIS 11 ISM-driven vegetation changes and their underlying forcings by combining pollen analysis from IODP Site U1446, strategically retrieved from the Bay of Bengal to represent the CMZ, with model simulations. Our results reveal the distinct roles of insolation, CO₂, ice volume, and millennial-scale variability in driving coupled ISM-vegetation changes, depending on the changing boundary conditions through MIS 11. Orbital- and millennial-scale tropical forest changes mirror southern European vegetation and atmospheric methane variability, ultimately reflecting shifts in the Intertropical Convergence Zone (ITCZ) that impact the tropical regions, a primary source of CH₄ emissions.

Our proxy and model reconstructions show that ISM-vegetation changes during MIS 11c closely followed boreal summer insolation, revealing its dominant role under warm background conditions with high CO₂ and reduced ice volume. Conversely, during MIS 11b-a, ISM-vegetation decreased while insolation remained high, indicating that its influence was overshadowed by expanding ice sheets, lower CO₂, and the interaction of orbital and millennial-scale variations. Millennial-scale climate variability during the younger MIS 11b-a substages is expressed by prominent forest contractions tied to southward ITCZ shifts, Atlantic meridional overturning circulation (AMOC) reductions and high-latitude ice sheet dynamics, which were rapidly followed by abrupt forest expansions associated with northward ITCZ shifts, AMOC strengthening and CH₄ overshoots. Conspicuously, the first and most severe forest setback interrupted MIS 11 full interglacial conditions, suggesting that extreme ISM weakening could also occur under similarly warm future conditions. Our findings provide new insights into ISM behavior during MIS 11, highlighting its high sensitivity to climate changes in the context of projected ISM intensification and its effect on the extent and composition of the tropical forest, which is key component of both global carbon and methane cycles.