CONCLUSIONS

The late/middle Miocene carbonate crash has previously been considered only a dissolution event associated with production of North Atlantic Bottom Water and ventilation via the Panama Seaway. A coupling between ¹³C and CaCO₃ MARs data in the 5- to 14-Ma interval (R = 0.87) observed at ODP Site 1256 suggests a dominant role for carbonate production by calcite-secreting organisms in surface waters, whereas the decoupling thereafter probably indicates enhanced dissolution caused by the effective closure of the Panama Seaway. Therefore, the coeval occurrences of negative ¹³C excursions and stages of sharp reduction in CaCO₃ MARs during the middle/late Miocene carbonate crash point to a causative mechanism related to surface circulation-induced low fertility.

We speculate that the major middle/late Miocene sea level drop caused complete closure of the Indonesian Seaway. Blockage of the ITF resulted in a piling-up of surface warm water in the west Pacific, strengthening the EUC system. We further speculate that the eastward spread of this nutrient-poor water warmed the SST and reduced upwelling in the central and eastern Pacific, primarily triggering a reduction in the standing stock in calcareous phytoplankton. The synchronous reduction in Central America and circum-Caribbean volcanism and deflected delivery of volcanic ash would have further deprived these regions of trace elements, which added to lowered surface water carbonate production. Surface water warming and reduced upwelling is documented by a negative excursion in ¹⁸O values. The reduction in carbonate supply to the deep waters caused a rapid shoaling of the CCD and triggered the carbonate crash. The close coupling between CaCO₃ MARs and biological productivity suggests that the carbonate crash is not a dissolution event but one caused by a marked drop in productivity.

Further work to test the mechanism proposed here is needed to compare the variations at the sea surface, thermocline, and, if possible, seafloor. This should be possible by comparing paleothermal records (e.g., ¹⁸O, Mg/Ca ratios, Sr/Ca ratios, U^k´₃₇ index, etc.) spanning the carbonate crash from shallower localities in the east and west Pacific, respectively, where foraminifers are well preserved. To simplify the interpretation of the isotopic data, it would be ideal to analyze a single foraminiferal species from niches at different water depths.