Changes in the upper water-column structure of the eastern equatorial Indian Ocean during 12.8–4.7 ka

The eastern equatorial Indian Ocean (EEIO), a pivotal region of the Indo-Pacific Warm Pool, plays a crucial role in modulating both the regional and global climate. Its complex hydrography is mainly influenced by an interplay of multiple ocean and atmospheric processes, i.e., inter-oceanic heat exchange (Indonesian throughflow; ITF), ocean-atmosphere interactions (Indian Ocean Dipole; IOD and El Niño-Southern Oscillation; ENSO) and seasonal monsoon circulations, at varying time-scales. However, the associated feedbacks of these climatic components and the impact on EEIO hydrography over millennial time-scales remain unclear, thus highlighting the need for further investigation. Here, we present well-dated records of the planktic foraminiferal proxies (% Globigerina bulloides, % symbiont-barren and symbiont-bearing species) and stable oxygen isotope (δ¹⁸O of Globigerinoides ruber (r) and Pulleniatina obliquiloculata (o), and Δδ¹⁸O_r-o) at multi-centennial to millennial scale resolutions from a spade core SC-04B, retrieved from a specific location of the EEIO (8.40°S, 98.00°E). The core spans the late deglacial to middle Holocene period (12.8–4.7 ka). Using faunal and isotope proxies, we reconstructed changes in surface hydrography, productivity, and the upper water-column structure during 12.8–4.7 ka, and explored their potential links with the ITF, IOD-like mean state, and South Asian Monsoon (SAM) variability. We document the increased influence of surface ITF during 12.8 to ∼9.8 ka that switched to thermocline ITF during ∼9.8 to 4.7 ka. When compared with existing oceanic and land-based paleoclimate records from the Indian Ocean and adjacent continents, our faunal proxies suggest an intensification of the southeast (SE) monsoon winds and upwelling-induced surface productivity in the EEIO during the periods of 12–9.2, 8.0–7.3, 6.4–5.6, and 5.2–4.7 ka. These intervals correspond to conditions similar to a positive IOD (pIOD)-like mean state in the tropical Indian Ocean, when SAM was also intensified. In contrast, during 12.8–12, 9.2–8.0, 7.3–6.4, and 5.6–5.2 ka, weakened SE monsoon winds led to increased precipitation-induced stratification, creating surface conditions similar to a negative IOD (nIOD)-like mean state, which suppressed surface productivity in the EEIO. Further, we record four distinct intervals (∼6, ∼8.2, ∼10.5, and ∼12 ka) of low % G. bulloides abundance and corresponding low values of Δδ¹⁸O_r-o, which appear to coincide with the cold phases in the North Atlantic [Bond Events (BEs) 4, 5, 7 and Younger Dryas (YD)]. We think that these changes were possibly linked with the intensified northwest (NW) monsoon winds and could have been modulated by the North Atlantic cold events potentially through atmospheric teleconnections.