Evaluating middle Miocene eastern equatorial Pacific organic carbon export using marine pelagic barite and extraterrestrial 3He

A key tipping point in Earth's climate occurred during the middle Miocene, with a shift from the greenhouse-like warmth of the middle Miocene Climate Optimum (mMCO; 16.9 - 14.7 Ma) to the more stable icehouse conditions since the middle Miocene Climate Transition (mMCT, ~13.9 Ma). A decrease in pCO2 over the 3 m.y. period of more positive benthic δ13C (the Monterey excursion) suggests Earth's long-term carbon cycle (i.e., organic carbon burial) was responsible for at least part of this shift. However, the role of eight shorter-term carbon isotope maxima (CM) events occurring at 400 k.y. long eccentricity minima is less well understood. While recent studies have proposed increases in primary productivity and the marine organic carbon pump - which sequesters organic carbon for shorter timescales -- various paleoproductivity proxies provide inconsistent results for such changes.

Within the eastern equatorial Pacific (EEP), a key upwelling region for ocean-atmosphere carbon exchange, increases in opal productivity and organic carbon export occurred during the last CM event or CM6 (Carter et al., 2016, Geology). However, changes in organic carbon export in prior CM events have not been examined in this region. We report a new marine pelagic barite record of organic carbon export to the deep ocean from Integrated Ocean Drilling Program (IODP) Site U1337. Interpreting this barite record, in combination with new extraterrestrial 3He concentrations and previously published XRF records by other authors from the same site, we find no evidence to support an increase in organic carbon export in the EEP during CM events prior to CM6. We instead argue that high latitude cooling at the mMCT was responsible for tipping the EEP into a regime of expanded organic carbon export, potentially in response to reduced remineralization rates and/or increases in nutrient supplies (i.e., silicic acid, iron, or macronutrients) through upwelling.