STABLE ISOTOPE EVIDENCE FOR DECLINING FRESHWATER INPUT TIED TO PLIO-PLEISTOCENE FAUNAL OVERTURN IN THE CARIBBEAN

Faunal overturn in the Southwest Caribbean (SWC) during the Plio-Pleistocene has been primarily attributed to either decreased upwelling and productivity collapse associated with uplift of the Central American Isthmus (CAI; ~3.5 Ma), or decline in marine temperatures associated with Northern Hemisphere glaciation (NHG) beginning ~3.3 Ma. Extinction preferentially selected against taxa adapted to nutrient-rich environments uncommon in the SWC today, but the turnover acme occurred 1-2 myr after the final closure of the CAI, which is coincident with the onset of severe NHG. We use ~3000 new stable isotope measurements of ~60 serially-sampled gastropods (Conus and Strombus) from Costa Rica and Panama to examine nutrient delivery by upwelling and freshwater input during and after the final closure of the Central American Isthmus ~3.5 Ma. Gastropod δ¹⁸O profiles were referenced to an open-ocean baseline using δ¹⁸O values for planktonic foraminifera that lived near the same depth. Foraminiferal δ¹⁸O data for the Plio-Pleistocene were derived from sediment cores ODP 999A, DSDP 502B, and MD03-2628 in the Caribbean, and ODP 1242 in the Pacific. Deviation of δ¹⁸O profiles from baseline values allows quantification of seasonal upwelling and freshening in the nearshore environments.

δ¹⁸O profiles of fossil shells do not show the strong upwelling and freshening signals found in modern Pacific Conus profiles. However, SWC specimens older than ~2.5 Ma show δ¹⁸O ranges greater than those of modern specimens, suggesting potential refugia where seasonal upwelling and/or freshening enhanced productivity. The baseline approach points to significantly greater freshening from 2.5-3.6 Ma and no upwelling after 3.5 Ma. The freshening signal in SWC shells declines after 2.5 Ma, signaling a decline in nutrient delivery and providing a causal mechanism for delayed extinction in the SWC.