21-9 The Miocene of the Western Ross Sea, Antarctica: The MCO and MMCT of DSDP Site 273

Session: Insights from Microfossils and Their Modern Analogs: From Traditional to Emerging Approaches

Presenting Author:

Samantha Bombard

Authors:

Bombard, Samantha¹, Leckie, Robert Mark², Harwood, David M.³, Browne, Imogen⁴, Shevenell, Amelia E.⁵, Stanchak, Felipe⁶

(1) Department of Earth, Geographic, and Climate Sciences at UMass Amherst, Amherst, MA, USA, (2) Univ Massachusetts - Amherst, Geosciences, Amherst, MA, USA, (3) Univ Nebraska - Lincoln, Dept of Earth & Atmospheric Sciences, Lincoln, NE, USA, (4) Binghamton University, Binghamton, NY, USA, (5) University of South Florida, Saint Petersburg, FL, USA, (6) University of South Florida, College of Marine Science, Saint Petersburg, FL, USA,

Abstract:

Foraminifera provide invaluable insight into paleoceanographic, depositional, and climatic conditions within the Ross Sea embayment from the Early to Middle Miocene. This study investigates the foraminifera from Deep Sea Drilling Project (DSDP) Site 273 during the Miocene Climatic Optimum (MCO; 16.9 to 14.7 Ma), Middle Miocene Climate Transition (MMCT; 14.7 to 13.8 Ma), and the Mi3 glaciation (~13.8 Ma). The MCO-MMCT interval at Site 273 spans ~237.5 m of section (Cores 273-6R to 273A-22R), with a diatom-based age model indicating an average sedimentation rate of ~110.3 m/myr. The benthic foraminiferal assemblages suggest the Ross Ice Shelf expanded northward between the MCO and Mi3. During the MCO, the benthic biofacies are dominated by Uvigerina and Globocassidulina subglobosa, suggesting active bottom currents on the continental shelf. However, the shipboard observations of only sparse laminations and faint stratification in the diatom-bearing, silty clay indicate that any bottom current activity was not vigorous. The sporadic occurrence of thin intervals of diatom-rich muds in Cores 273A-6R and 273A-17R suggests less turbid waters and open water conditions at times during the MCO, including the end-MCO at ~14.7 Ma and an interval correlative with peak warmth at ~15.6 Ma, respectively, according to our age model. Preliminary benthic foram oxygen isotopes at Site 273 track d¹⁸Osw values at ODP Site 1171 south of Tasmania. While the open water conditions existed during the MCO, the presence of IRD throughout the Site 273 sequence demonstrates the presence of a marine based ice sheet within the Ross Embayment and/or abundant runoff. The benthic biofacies prior to the MCO (~18.6 to 18 Ma) and during the MMCT, are dominated by G. subglobosa and Nonionella spp., suggesting a sub-ice shelf environment during cooler conditions in the Ross Sea. In addition to the benthic biofacies, we note five occurrences of warm planktic species (Globigerinita glutinata, Paragloborotalita continuosa), before and during the MCO, and one during the MMCT. We propose the increase in abundance of G. subglobosa, a proxy of Circumpolar Deep Water (CDW) incursion, and sporadic warm water planktic foraminifera indicates the import of warm water, providing ideal conditions to facilitate ice sheet growth prior to the MCO, and during the MMCT and Mi3.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9795

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