354-8 Greenhouse Warming Without Bottom-Water Deoxygenation: Multiproxy Evidence from the Southern Ocean During the Middle Eocene Climatic Optimum (MECO)

Abstract:

The Middle Eocene Climatic Optimum (MECO) represents one of the most significant transient global warming events of the Cenozoic and provides an important natural analogue for understanding marine biogeochemical responses to sustained greenhouse climates. Although previous studies have documented changes in ocean circulation, carbonate chemistry, and productivity during the MECO, the response of high-latitude Southern Ocean redox conditions remains poorly constrained. We generated a high-resolution multiproxy geochemical record from International Ocean Discovery Program (IODP) Hole U1514 in the Mentelle Basin, southeast Indian Ocean, integrating sedimentary trace metal geochemistry, carbonate content (CaCO₃), total organic carbon (TOC), and bulk organic carbon and nitrogen isotopes (δ¹³C_org and δ¹⁵N_org) across the pre-MECO, MECO, and post-MECO intervals. Strong positive correlations between Al and lithogenic elements, together with inverse relationships between CaCO₃ and lithogenic element concentrations, indicate that variations in bulk sediment geochemistry are primarily controlled by dilution of the lithogenic fraction by biogenic carbonate. Redox-sensitive trace metals show no significant authigenic enrichment, TOC contents remain uniformly low, and δ¹⁵N_org exhibits no systematic changes across the MECO. Together, these independent proxy records consistently support predominantly oxic depositional conditions throughout the studied interval, with no evidence of persistent oxygen depletion. Despite persistently oxic depositional conditions, CaCO₃ concentrations record substantial perturbations to the marine carbonate system, including transient carbonate dissolution and prolonged reductions in carbonate saturation. A post-MECO negative δ¹³C_org excursion (CIE) likewise occurred without corresponding changes in redox-sensitive trace metals, TOC, or δ¹⁵N_org, suggesting that the CIE was not accompanied by measurable changes in sedimentary redox conditions. Overall, the Hole U1514A record indicates that major perturbations to the marine carbonate system during MECO were not accompanied by persistent bottom–water deoxygenation.

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