Ocean Drilling Program, United States

The middle/late Miocene "carbonate crash", a sharp decrease in carbonate mass accumulation rates in the eastern and central equatorial Pacific, as well as the Caribbean region, has previously been considered only a dissolution event associated with changes in global ocean chemistry, which is in turn believed to be tied to the production of the North Atlantic Bottom Water and/or ventilation via the Panama Seaway. delta (super 13) C data in the bulk-isotope record from Ocean Drilling Program Site 1256 show a close parallel with CaCO (sub 3) mass accumulation rates (MARs) in the 5- to 14-Ma interval (correlation coefficient = 0.87), suggesting a relationship likely coupled to surface water productivity by calcite-secreting organisms. The decoupling between delta (super 13) C and MARs after 5 Ma probably indicates a dominance of dissolution over carbonate production. Therefore, the coincidence in delta (super 13) C excursions with the stages of sharp reduction in CaCO3 MARs during the carbonate crash points to a causative mechanism induced by surface circulation-induced low productivity. We speculate that the major middle/late Miocene sea level drop may have caused the complete closure of the Indonesian Seaway. We propose a model wherein the blockage of Indonesian Throughflow would have resulted in a piling-up of surface warm water in the west Pacific, thereby strengthening the Equatorial Undercurrent system. The eastward spread of this nutrient-poor water then warmed sea-surface temperature and reduced upwelling in the central and eastern Pacific, reducing in turn biological productivity of phytoplankton. A coincident reduction in Central America and circum-Caribbean volcanism plus the deflection of the delivery of volcanic ash as a result of the then prevailing southeastern trade winds across the equator further deprived these regions of trace element nutrients, which added to lowered surface water carbonate production. Surface water warming and reduced upwelling is documented by negative excursions in delta (super 18) O values. The reduction in carbonate supply to the deep waters caused a rapid shoaling of the carbonate compensation depth and triggered the carbonate crash. The close correlation between CaCO (sub 3) mass accumulation rates and biological productivity suggests that the carbonate crash is best characterized as a low-productivity event.