Foraminiferal I/Ca Reveals Regional Variation in Eastern Tropical Pacific Oxygen Deficient Zone Intensity Across the Miocene Climatic Optimum

Documenting the response of marine oxygen deficient zones (ODZs) to past intervals of climate change is key to anticipating the dynamics of these regions in the future. The foraminiferal iodine-to-calcium (I/Ca) proxy is a powerful geochemical tool for tracking ODZs in ancient oceans. I/Ca works because the oxidized iodine species, iodate, is exclusively incorporated into foraminiferal calcite, and it facilitates reconstruction of both benthic and pelagic redox states from the fossil tests of foraminifera. Here, we present work applying the I/Ca proxy to study paleo-ODZ dynamics in the Eastern Tropical Pacific (IODP 321-1338 and IODP 202-1237) over a geologically recent greenhouse climate interval, the Miocene Climatic Optimum (MCO, ~14.5-17ma). The MCO is characterized by a global average temperature increase of up to 3°C, elevated atmospheric CO2 levels, expansions in organic carbon burial, and evidence for intensification of regional ODZs. We also tested for an impact of diagenesis on I/Ca signal using samples from IODP 321-1337 and 321-1338, two adjacent localities which differ in planktonic foraminiferal preservation quality, using SEM imaging and trace metal analyses. Diagenetic considerations are especially relevant because recrystallization of foraminiferal tests in corrosive pore fluids could result in a false OMZ signal, as porewaterslike oxygen-poor waters of a severe ODZare depleted in iodate. Our results show a range of I/Ca values which encompass the levels indicative of hypoxic conditions at the two localities, with varying patterns throughout the interval. Further, our analyses do not indicate diagenetic recrystallization as the primary driver of low I/Ca values at these localities.