RECONSTRUCTING NUTRIENT CONSUMPTION IN THE EXPANDED SOUTHERN OCEAN OF PLEISTOCENE GLACIAL PERIODS
The emergence of the longer, high amplitude glacial cycles of the last ~1 million years are attributed, in part, to variations in the global overturning circulation and iron stimulation of biological carbon drawdown. Previously, data resolving the relative importance of these processes within the Southern Ocean did not exist. Here, I present high resolution records of foraminifera-bound nitrogen isotope values from ODP Site 1090, within the Subantarctic Zone (SAZ), and IODP Site U1475, beneath the modern Subtropical Frontal Zone (STFZ), spanning both the last glacial period (0 – 160,000 years ago) and the Mid-Pleistocene Transition (MPT, 1,200,000 – 600,000 years ago). New biogenic opal deposition, bulk sedimentary nitrogen isotope values, and total nitrogen content records spanning the MPT are also presented from IODP Site U1475. These records, when considered alongside existing data, reveal that the Southern Ocean expanded across the MPT and in each glacial stage after, with the largest expansions observed ~900,000 years ago and in the last glacial period. I find that, during the last glacial period, expansion of the Southern Ocean was accompanied by enhanced STFZ nutrient consumption, likely due to iron fertilization as expansion not only increases the geographic area for iron fertilization to occur, but also brings nutrient rich water in closer proximity to aeolian iron sources. Conversely, during the MPT, our data imply that the expansion of the Southern Ocean impacted physical carbon storage ~900,000 years ago. This increased deep ocean carbon storage may have been a precondition to the establishment of 100,000 year glacial cyclicity. The dramatic expansion of the SAZ ~900,000 years ago, and during subsequent glacial periods, likely enhanced the regions ability to drawdown atmospheric CO2 through physical and, later, biological feedbacks.