SIMULTANEOUS INCREASE IN PLANKTONIC-BENTHIC CARBON ISOTOPE GRADIENTS AND REACTIVE PHOSPHORUS DEPOSITION IN SEDIMENTS FROM THE CARIBBEAN (ODP SITE 999) PROVIDE EVIDENCE FOR INCREASED EXPORT PRODUCTIVITY AND ORGANIC CARBON BURIAL DURING A LATE MIOCENE VOLCANIC ASH EVENT

The late Miocene experienced a cooling trend and simultaneous positive carbon (C) isotope excursion. A possible mechanism for the cooling is increased global volcanism leading to increased nutrient supply, increased export productivity, increased organic C burial, and atmospheric carbon dioxide drawdown. We test the hypothesis that intense volcanism in the Caribbean during the late Miocene could have served as an external source for an increase of nutrients, thereby locally increasing export productivity and organic C deposition. We measured biologically reactive phosphorous (P, the sum of oxide bound, authigenic, and organic P) as a tracer for organic C burial in volcanic ash layers deposited in the Caribbean basin (ODP Site 999, Core 28X) of approximate age 9.6 million years. The highest concentrations of reactive P were 17.08 µmol g^-1 and 19.68 µmol g^-1 at 254.75 and 254.55 meters below sea floor (mbsf), respectively, and were located in and just above the ash layer (~254.75-254.65 mbsf). The first peak in reactive P (~254.75 mbsf) occurs coincident with a peak in the planktonic to benthic C isotope gradient, whereas the second reactive P peak occurs subsequently. We interpret the overlapping sequence of events of the occurrence of the ash layer, an increase in the planktonic to benthic C isotope gradient, and an increase in reactive P burial to indicate that Caribbean volcanism provided increased nutrients causing increased export productivity and increased organic C burial at this time. Furthermore, the effect of a volcanism event would have had a global effect on climate of the Miocene; the Panamanian isthmus was not yet closed, so phosphorous would have flowed to the Pacific Ocean as well. This is the first evidence that we know of to support a late Miocene volcanic event causing productivity increases. Moreover, this increased productivity, both locally in the Caribbean and wider spread to the Pacific, is a plausible mechanism for carbon drawdown and a general cooling trend.