NUTRIENT DECLINE, RAINFALL PATTERNS, AND THE END-PLIOCENE REGIONAL MASS EXTINCTION IN FLORIDA

Geochemical analyses of Pliocene Siderastrea spp. corals reveal that the hydrodynamic regime and surface nutrient levels in the sub-tropical western Atlantic were markedly different from today. Light stable isotopes (δ¹⁸O) and trace elements (Sr/Ca and P/Ca) were analyzed from samples taken along a continuous sample track, at a bi-monthly resolution (~6 samples/year). Utilizing modern calibrations for Siderastrea spp. corals from the Dry Tortugas, we can approximate temperature variations (Sr/Ca), salinity variations (δ¹⁸O), and phosphate levels (P/Ca) for the Pliocene nearshore marine environment.

Variations in the δ¹⁸O_­ values (-2.75 to -0.90‰) reveal a consistent record of high precipitation (low salinity) during the winter months, and low precipitation (high salinity) during the summer months. Excessive winter/spring precipitation on the southeastern North American continent is characteristic of conditions during a modern El Niño event, which influences the region through global climate teleconnections. A consistent El Niño-like precipitation pattern in the Pliocene, as indicated by our coral records from Florida, is in agreement with reports that the Pliocene equatorial Pacific Ocean lacked an east-west temperature gradient and was in a “permanent” El Niño-like (El Padre) state. Pliocene phosphate levels, as calculated from the P/Ca record, indicate nutrient pulses coincided with periods of high winter precipitation and point to a terrestrial runoff rather than an upwelling nutrient source as has been suggested in the past.

Pliocene phosphate levels averaged 3.6 umol/kg, which is an order of magnitude higher than modern observed values in surface western Atlantic water. These data provide the first direct evidence of higher nutrient levels prior to the end-Pliocene extinctions and support the view that the dense Florida shell beds were formed under conditions of high overall productivity.