Orbital-scale competition of biogenic carbonate and opal production at the Del Cao Rise in the Indian sector of the Southern Ocean during the late Pleistocene

The Southern Ocean is characterized by various ocean fronts that separate surface waters with different hydrographic properties. Ocean fronts position changed over glacial-interglacial cycles, which, consequently, affected surface water properties and productivity through time at any given location. The present study reconstructs changes in biogenic carbonate and opal burial at Del Cao Rise (DCR), Indian Ocean sector of the Southern Ocean, over the last three climate cycles. Based on Accelerator Mass Spectrometry radiocarbon dating of planktonic foraminifera as well as comparison of oxygen isotopes of planktonic foraminifera with LR04 stack, the studied core MD19-3575CQ, located today in the Subantarctic Zone, covers the last ~250,000 years. Biogenic carbonate content shows consistent low values during all glacial periods (average 32.4%) and high values during all interglacial periods (average 61.5%). In contrast, biogenic opal content increased during all glacial periods (average 12.8%), compared to interglacial periods (average 8.5%). TOC exhibited a similar pattern to the biogenic opal content, with higher values during glacial periods (average 0.38%) and lower values during interglacial periods (average 0.17%). The increased biogenic opal content during glacial periods is attributed to enhanced silica supply to surface waters due to northward migration of the Subantarctic Front, possibly shifting north of DCR during these periods. However, the relatively low biogenic opal content in the study area compared to other regions of the Polar Frontal Zone (average 55.6%) suggests insufficient silica supply to support high diatom production. The reduction in biogenic carbonate content during glacial periods is related to the decrease in coccolithophore productivity due to low ocean temperatures (~2C) and the competition for nutrients and light with diatoms. Despite the shallow water depth (~2400 m) at the core location, the weakened Atlantic Meridional Overturning Circulation and thickening of the corrosive Southern Ocean bottom waters during glacial times may have increased carbonate dissolution. Our study emphasizes that changes in surface water properties by ocean fronts migration impact on regional productivity, which may influence global biogeochemical cycles.