The relationship between ocean circulation and monsoon systems over orbital to sub-millennial timescales is a crucial but poorly-constrained component of the climate system. Here, using foraminiferal and detrital neodymium (Nd) isotope records from the intermediate-depth northern Indian Ocean, we provide new evidence revealing that both monsoon-driven weathering inputs and water mass advection from the Southern Ocean influenced past seawater Nd isotope changes in this region. Our results suggest that Indian Summer Monsoon weakening coincided with enhanced northward Antarctic Intermediate Water (AAIW) advection during the last deglaciation, reflecting a strong interhemispheric coupling. In contrast, the Early Holocene was characterised by enhanced monsoon strength but persistently strong AAIW inflow, indicating a relationship in the opposite sense. These differing interhemispheric relationships indicate asynchronous changes in the global atmosphere – ocean – climate system, and may represent a previously unrecognised component of the ocean-atmosphere reorganization during the deglacial to Holocene transition.

Key Points

We present a 17 kyr record of foraminiferal Nd isotopes from the intermediate-depth northern Indian Ocean with a resolution of ∼200 years

Both regional Himalayan weathering inputs and intermediate water advection from the Southern Ocean influenced the record

Different interhemispheric relationships are revealed between AAIW and the ISM during the last deglaciation compared to the Early Holocene

Plain Language Summary

Deciphering the interactions between processes in the Northern and Southern hemispheres is crucial for understanding the mechanisms of paleoclimate change, and therefore for predicting future climate evolution. Resolving the nature of such interactions requires high-resolution datasets from key components of the system. To this end, we present a new intermediate-depth foraminiferal neodymium (Nd) isotope record from the Indian Ocean covering the last 17,000 years at an unprecedented resolution of ∼200 years. Variability in this Nd isotope record reflects both changes in regional Nd inputs related to continental weathering driven by the Indian Summer Monsoon (ISM), and changes in the Nd signal carried northwards from the Southern Ocean by Antarctic Intermediate Water (AAIW). By disentangling these continental weathering and ocean circulation signals, we can compare Northern and Southern Hemisphere climate processes on sub-millennial timescales. We found that ISM weakening coincided with enhanced northward AAIW advection during the last deglaciation, whereas enhanced monsoonal activity was accompanied by persistently strong intermediate water inflow during the Early Holocene. This variable interhemispheric connection between the summer monsoon and intermediate water advection may have played a crucial role in the last deglacial climate transition.