EVIDENCE FOR ENHANCED EXPORT PRODUCTIVITY DUE TO VOLCANIC ASH FERTILIZATION AROUND THE MIDDLE MIOCENE CLIMATE TRANSITION (MMCT) IN THE CARIBBEAN (ODP SITE 999)

The Middle Miocene (~16 to ~11.6 Ma) included a period of climatic cooling and Antarctic ice sheet expansion known as the Middle Miocene Climate Transition (MMCT, ~14 Ma). One possible contributor to cooling may have been atmospheric carbon dioxide (CO₂) drawdown caused by increased oceanic organic carbon (C) burial. Volcanic ash fall in marine waters has been shown to provide limiting nutrients (e.g., phosphorus [P] or iron [Fe]) to phytoplankton, causing increased primary productivity. We hypothesize that nutrients supplied by middle Miocene Caribbean volcanism could have stimulated an increase in export productivity and organic C burial. To test this hypothesis, we measured total reactive P (the sum of oxide-bound P, authigenic P, and organic P) concentration, bio-available trace metal concentrations (Fe, Mn, Ni, Cu, & Zn), and total organic C (wt %) above, within, and below two Miocene ash layers from ODP Site 999 (Core 28X, ~254 mbsf, ~16.5 Ma, and Core 46X, ~423 mbsf, ~9.5 Ma) in the Caribbean. We compared these to the planktic to benthic foraminiferal δ13C gradients in the same samples (when available), as an indicator of the strength of the oceanic biological pump. Across the ash layers, bio-available trace metal concentrations start relatively high and then decline, while reactive P concentrations increase throughout. Both possibly indicate enhanced nutrient supply. The planktic to benthic foraminiferal δ13C gradient also increases across the Core 28X event, indicating an increase in productivity response. We further speculate that the open Panamanian gateway between the Caribbean Sea and the Pacific Ocean during the Miocene may have allowed transport of nutrients from Caribbean volcanoes around the Pacific, stimulating primary productivity, organic C burial, and atmospheric CO2 drawdown during the mid- to late Miocene.