T105 Oceans and Climates through Earth History: Modelling, Proxy, and Proxy Development Studies (Posters)

Monday, 16 October 2023
8:00 AM - 5:30 PM
Hall B

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

Paper number: T105-14
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

The International Ocean Discovery Program (IODP) South Atlantic Transect Expeditions 390/393 recovered sedimentary records that span the Miocene Climate Optimum (MCO; 16.9 – 14.7 Ma) and the mid-Miocene Climate Transition (MMCT, 14.7 – 13.8 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 (AIS). Previous studies indicate that during the Middle Miocene, the AIS became very sensitive to obliquity-paced mid-latitude atmospheric circulation processes. 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, which may be the mechanism for AIS sensitivity to obliquity. This study investigates the Hadley Cell response to the MCO and MMCT from the perspective of the oligotrophic South Atlantic gyre to test if East Antarctic ice was also affected by latitudinal migrations of the westerlies.

We conducted X-ray fluorescence (XRF) from Holes U1583C (30°43′S, 20°26′W, water depth 4214.6 mbsl, 30Ma crust) and U1560A (30°24′S, 16°55′W, water depth 3724.4 mbsl, 15Ma crust) and combined this data with shipboard natural gamma radiation (NGR) measurements. We observe two dramatic increases in the NGR data from site U1583, one that coincides with the MCO at ~16 Ma, and one that corresponds with the MMCT at ~14.5 Ma., suggesting increased windiness and deposition of terrigenous sediments. This is further confirmed by XRF data, which show an increase in the wind intensity (Ti/Al), a change in sediment provenance (Fe/Zr) and increased amounts of lithogenic sediments (Ca/Ti) across these intervals. However, in Hole U1560A, NGR and XRF data do not strongly correlate with each other and show faster but lower-amplitude changes suggesting a possible contraction of the Hadley cell over the sites. Additional sedimentological and geochemical analyses, along with Antarctic sediment records and spectral analyses of the XRF and NGR data, will further characterize the atmospheric and surface ocean changes in the South Atlantic Gyre and response of AIS across the MCO and MMCT.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-391232

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