Emergence of a Southern Ocean oxygen minimum zone during glacial periods

Oxygen Minimum Zones (OMZ) in the upper ocean are commonly associated with poor ventilation and storage of respired carbon, potentially linked to atmospheric CO2 level. Iodine to calcium ratios (I/Ca) in recent planktonic foraminifera suggest that values less than ~2.5 mmol/mol indicate the presence of an OMZ. We apply this proxy to estimate past dissolved oxygen concentrations in the currently well oxygenated Southern Ocean, which played a critical role in carbon sequestration during glacial times. A down-core planktonic I/Ca record from South of the Antarctic Polar Front (APF) shows that minimum O2 concentrations in the upper ocean fell below 1.5 ml/l during the last two glacial periods, indicating the development of a persistent glacial OMZ at the heart of the carbon engine of our climate system. New estimates of past ocean oxygenation variability may assist in resolving mechanisms responsible for the much-debated ice age atmospheric CO2 decline.