Changes in intermediate circulation waters along the tropical eastern Indian Ocean during quaternary climatic oscillations

The Indo-Pacific Warm Pool (IPWP) is the warmest and most dynamic ocean-atmosphere-climate system on Earth and has undergone significant climatic changes during the Pleistocene glacial periods (De Deckker et al., 2012; Lea et al., 2000; Russell et al., 2014). During the Last Glacial Maximum, the latitudinal position of the Southern Ocean fronts, both south of Africa and Australia, was shown to be critical in controlling the outflow of warm water of the Agulhas Current from the Indian Ocean and the IPWP area. Yet, there is no direct evidence for such oceanic change on the scale of the Late Pleistocene glacial-interglacial transitions.

Here, we combine sea surface temperature proxies (d¹⁸O and Mg/Ca) with the neodymium (Nd) isotopic signature to reconstruct changes in climate and oceanic circulation in the eastern tropical Indian Ocean over the last 500 ka. The most striking feature of our dataset is the oscillating Nd signal that mimics the glacial-interglacial cycles. While interglacial periods are characterized by a more significant contribution from the less radiogenic Antarctic intermediate water mass (AAIW, ~ -7 ε_Nd), glacial periods are marked by more radiogenic water mass of Pacific origin (~ -5 ε_Nd). We argue that under global cooling, the northward penetration of the AAIW has weakened due to the general slowdown of the global thermohaline circulation. Furthermore, the oscillating pattern is also recorded in the sea surface temperature and salinity, indicating the settlement of cooler and more saline surface water masses probably linked to a less expanded IPWP and weaker Leeuwin Current during glacial intervals.

We suggest that under low AAIW a less intense advective mixing occurred, allowing a deepening of both halocline and thermocline in the tropical eastern Indian Ocean. Our new proxy-derived dataset confirms results from models (DiNiezo et al., 2018), suggesting that these ocean conditions could amplify the externally forced climate changes resulting from drier atmospheric conditions and weaken the monsoon during glacial periods in the Indonesian region.