Southern Ocean Water Column Hydrography during the mid-Brunhes Event

The mid-Brunhes climate event (MBE; Marine Isotope Stages 11-12) is considered to be a major shift towards warmer interglacial conditions, with an increase in interglacial atmospheric CO₂ levels relative to those during the mid-Pleistocene. We hypothesize that the CO₂ increase during this climatic shift is associated with a concurrent decrease in Southern Ocean stratification, allowing upwelling of nutrient- and CO₂-rich deep waters. To test this hypothesis, we measure percent biogenic silica (opal) content in bulk sediments as a proxy for upwelling at Ocean Drilling Project Site 745B (59°37'S, 85°52'E, water depth of 4,082m). The site lies in the Antarctic Zone of the Indian sector of the Southern Ocean, an important region in ocean-atmosphere carbon transfer. Preliminary results indicate that there is a 40% increase in biogenic opal during the deglaciation from MIS 12 to MIS 11 (Termination V), with glacial opal averaging about 20% and the interglacial values averaging about 60%. This range is larger than the 20% average increase in opal content from glacial to interglacial intervals during the mid-point of the mid Pleistocene transition (Billups et al., 2018). It is also larger than the 30% increase in opal during the MIS 18 to 17 transition. These observations suggest that there is a relatively large increase in opal content at Site 745B associated with the more extreme warmth of interglacial MIS 11. Next steps involve generating a nitrogen isotope record to assess potential changes in nutrient utilization in relation to changes in opal content.