Deciphering biotic responses to rapid climate change at the Pliocene-Pleistocene boundary

The Pliocene-Pleistocene is a dynamic period for taxonomic loss and origination resulting from the closing of the Panama Isthmus. Evidence from the Atlantic and the Pacific indicates that this tectonic change altered ocean circulation patterns and redistributed nutrients and sea surface temperature (SST). It is unknown how Pacific coast marine taxa responded to those oceanic changes. This study aims to document changes in marine community diversity and abundance, in response to a hypothesized decrease in SST and increase in nutrient availability at the Plio-Pleistocene boundary. Samples were collected from 9 fossil beds within the San Diego Formation, Tourmaline Surf Park, San Diego County, CA. Foraminifera biostratigraphy confirmed the location of the Pliocene-Pleistocene at Tourmaline Surf Park. The collected samples were identified and diversity and abundance were calculated. While diversity declined into the Pleistocene, abundance levels varied sporadically. To determine the variables controlling changes in diversity and abundance, SST was tracked via stable isotope analysis and nutrient availability was recorded via Ba/Ca ratios calculated through Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). SST decreased during this interval, while nutrient availability increased. NMDS, Clustering Analysis, and ANOSIM were used to statistically determine differences between the San Diego Formation Plio-Pleistocene fossil beds and known Pleistocene fossil sites in Orange and Los Angeles Counties. The statistical results confirm that there are major differences between the Pliocene and Pleistocene horizons. The Plio-Pleistocene boundary provides a unique laboratory to examine how environmental change affects life on Earth and can provide direct insights into our present-day biodiversity crisis for two reasons: (1) many Plio-Pleistocene taxa are extant thus providing a direct prediction of how modern taxa might respond to rapid climate change; and (2) the environmental analyses provides more rigorous and detailed analysis then previous studies, thus providing a more direct comparison with modern climate patterns.