Late Quaternary productivity and hydrographic variability in the upper water column of the Agulhas Return Current region as inferred from planktic foraminifera

Abstract

The Southern Ocean plays a pivotal role in the Earth's climate system by interconnecting the Atlantic, Pacific, and Indian Oceans, facilitating the exchange of climate signals across regions. The western Indian sector of the Southern Ocean, characterized by dynamic interactions among water masses, wind patterns, and biogeochemical processes, is particularly influenced by the Subtropical Front (STF)—a critical boundary separating colder, less saline southern waters from warmer, saltier northern waters. In this study, we examined a marine sedimentary core from the southern Agulhas Plateau located in the Agulhas Return Current (ARC) region, to reconstruct oceanographic conditions over the past ∼350 ka using planktic foraminiferal relative abundance and stable oxygen isotope (δ¹⁸O) data. Our findings reveal that glacial periods (MIS 4 and 2) were marked by increased productivity, likely driven by intensified Southern Hemisphere westerlies and a northward-shifted STF, facilitating nutrient-rich Sub-Antarctic Surface Water (SASW) influx. In contrast, glacial periods MIS 8 and MIS 6 showed reduced productivity, linked to an increase in the transport of warm, nutrient-poor Subtropical Surface Waters (STSW) through the Agulhas Return Current to the region. The MIS 6/5 transition (Termination II) marked a significant shift, characterized by the replacement of warmer, stratified, nutrient-poor waters with colder, well-mixed, nutrient-rich conditions. Evidence of thermocline deepening during warm periods as well as during MIS 8 and MIS 6 underscores enhanced stratification. Furthermore, variability in Agulhas Return Current strength across glacial and interglacial phases reflects the sensitivity of tropical-subtropical assemblage transport to STF positioning. These results underscore the intricate interactions between ocean circulation, frontal dynamics, and regional hydrography in shaping past marine environments, and offer critical insights into the palaeoceanographic evolution of the southwest Indian Ocean and its broader climatic implications.