Orbital-scale competition of biogenic carbonate and opal production and its implication on carbon cycle at Del Caño Rise in the Indian sector of the Southern Ocean

Geochemical properties measured in core MD19-3575CQ, collected from the Del Caño Rise within the Subantarctic Zone in the Indian sector of the Southern Ocean, provide evidence of orbital-scale changes over eccentricity, obliquity, and precession cycles in phytoplankton productivity and its influence on the organic matter burial over the last 235 kyr. CaCO₃ and biogenic opal contents varied oppositely with high CaCO₃ and low biogenic opal contents during the interglacial periods and vice versa during the glacial periods. During the glacial periods, the biogenic opal production by diatoms increased as a result of more silica supply when the Subantarctic Front shifted northward across the Del Caño Rise. In addition, Fe-bearing dust and remobilized sediments likely enhanced diatom productivity. In contrast, biogenic carbonate production by coccolithophores and foraminifera was reduced during the glacial periods because of cold ocean temperature and competition for nutrients between coccolithophores and diatoms. More organic carbon was buried at the core site during the glacial periods than during the interglacial periods, probably in relation to a greater export efficiency by diatoms relative to coccolithophores as well as a better preservation of the organic matter when the Circumpolar Deep Water was less ventilated during the glacial periods.