MIDDLE MIOCENE QUALITATIVE RECONSTRUCTION OF THE OLIGOTROPHIC SOUTH ATLANTIC GYRE, IODP EXPEDITIONS 390/393

The International Ocean Discovery Program South Atlantic Transect (SAT) expeditions 390/393 recovered Cenozoic successions and ~150 – 350 m of the underlying oceanic crust from six sites along a slow/intermediate spreading Mid-Atlantic Ridge crustal flow line at ~31°S. Recovered records span key intervals of climate shifts including the Miocene Climate Optimum (MCO; 17 – 15.1 Ma) and the mid-Miocene Climate transition (MMCT, ~ 15 – 13 Ma). The MCO was a time of warming and elevated atmospheric CO₂ concentrations, whereas the MMCT was a time of cooling and expansion of Antarctic ice sheets. Qualitative geochemical studies from the South Pacific across the MCO and MMCT have revealed poleward shifts of the westerlies in response to warming, and subsequent shifts of the winds back towards the equator during cooling. This study investigates the Hadley cell response to Miocene warming and cooling from the perspective of the oligotrophic South Atlantic gyre.

We carried out analysis of shipboard Natural Gamma Radiation (NGR) and X-ray fluorescence (XRF, collected at 2cm intervals at 10, 30, 50 kV) data from sediments of holes U1583C (30°43′S, 20°26′W, water depth 4214.6 m, 30Ma crust) and U1560A (30°24′S, 16°55′W, water depth 3724.4 mbsl, 15Ma crust). In Hole U1583C across the Middle Miocene, the K, Th and U signals become elevated near the onset of the MCO and remain so throughout the study interval. All three elements strongly correlate with one another in Hole U1583C. These findings suggest increased windiness and deposition of terrigenous sediments over the site in response to increased warming. However, this is not the case with NGR data from Hole U1560A. Here, K, Th and U values are lower through the Middle Miocene, and elements do not strongly correlate with one another. XRF data will help resolve differences between NGR signals at Sites U1583 and U1560. Future work will include conducting additional sediment and geochemical analyses to further characterize surface ocean changes in an oligotrophic gyre in response to the MCO and MMCT.