NEOGENE DIVERSITY OF DEEP-SEA CARIBBEAN BENTHIC FORAMINIFERA RELATIVE TO THE CLOSURE OF THE CENTRAL AMERICAN SEAWAY

The rise of the Isthmus of Panama closed the Central American Seaway by ~4-3 Ma, profoundly affecting the evolution and biogeography of marine biotas on either side of the isthmus. We test the hypothesis that the diversity of deep-sea benthic foraminifera prior to seaway closure was similar in the Caribbean and equatorial Eastern Pacific, and showed divergent values by the time of early shoaling ~12 Ma. The diversity measures Shannon’s H, Fisher’s alpha, Dominance and Evenness, determined for assemblages from the Late Oligocene (~26 Ma) to Late Pliocene/Early Pleistocene (~2.5 Ma), were calculated for Caribbean and Eastern Equatorial Pacific assemblages at five sites. Species’ occurrences in the Eastern Equatorial Pacific (DSDP Sites 503, 568, and 569) and Caribbean (DSDP Site 502, ODP Site 999) were taxonomically standardized with new data (ODP Site 999).

The effects of early shoaling on this deep-sea community were apparently greater than the effects of complete seaway closure. Shannon’s H proved to be the most effective index, as it took into account the relative abundances of species and gave greater weight to common species, thereby addressing the issue of comparative sites with low abundance counts. The greatest changes in diversity for both the Caribbean and Eastern Equatorial Pacific occurred 15.5-8 Ma. This period encompasses a known faunal turnover in deep-sea benthic foraminifera that is attributed to an increase in oxygenated, cold, bottom waters. This interval also contained the largest increases in dominance, where fewer species dominated the assemblages. Caribbean diversity decreased at 8 Ma when there was an effective barrier to deep-water exchange, and was followed with slight increases at 6 Ma when surface water interchange was renewed. Finally, at complete closure of the Central American Seaway ~4-3 Ma, there was a slight decrease in diversity at Caribbean sites, followed by an increase, at which time the Eastern Equatorial Pacific sites began to decrease in diversity. Changes in bottom-water source at 15.5 Ma, together with the mid-Miocene turnover, and a barrier to deep circulation at 12 Ma, are correlated with the greatest effects on diversity, with the most variation in diversity levels occurring prior to ~8 Ma.