ROLE OF THE CLOSURE OF THE CENTRAL AMERICAN SEAWAY IN FORAM GIGANTISM
Size is fundamentally important in individual planktic foraminifers as it determines the number of offspring and hence reproductive success. Over evolutionary timescales, individuals tend to increase in size, altering the average size of assemblages; in the last million years, individuals were larger than at any other time in the geological record. This pattern is specifically driven by size increases in tropical and subtropical taxa in the Plio-Pleistocene. Results from “Planktic foraminiferal test size and weight response to the late Pliocene environment” instigated a global investigation of the drivers of size change in planktic foraminifera across the Pliocene.
A taxon-free approach is used to assess what facilitated novelty size changes and quantify the planktic foraminifers' response to long- and short-term-scale environmental changes. The research focuses on the Pliocene, a time interval with carbon dioxide levels as high as today and temperatures 2-3ºC warmer. It is also characterised by the closure of the Central American Seaway (CAS) and the short glaciation at marine isotope stage (MIS) M2. Size in foraminiferal assemblages was measured using automated microscopy across 24 globally distributed ocean drilling sites, covering both early (ePRISM) & late Pliocene (PRISM) (2.6 - 5.4 million years ago (Ma)) and subpolar to tropical environments. The 95th percentile was calculated on the maximum diameter measurements of 1.73 million specimens. Although there is a slight decrease in the average size of the assemblage in the high latitudes from the Early to Late Pliocene, with minimal changes in the tropics, results indicate little to no effect on foraminiferal size across MIS M2. The results show unexpected stability and resilience against the reorganisation of the tropical oceans associated with the closure of the CAS and call for further research to untangle what is causing the shift in the size-temperature relationship across the Pliocene.
