Significant Size Change of Coccoliths at 2.7 Ma: Paleoenvironmental and/or Evolutionary Controls?

Coccolithophores have been the major carbonate-producing marine microplankton group since the late Jurassic. Today coccolithophores are still essential in regulating marine carbon cycling and ocean- atmosphere CO2 exchange with the calcification of their calcareous skeleton and the subsequent settling of calcium carbonate to the ocean floor. Consequently size variations in the remains of marine planktic organisms through time may well have affected global biogeochemical cycles and might do so again in the near future. In this context we study the size variability of oval to circular coccolithophore platelets (placoliths) in 49 globally distributed Holocene sediments and compare these results with 100 assemblages from a Pliocene section from ODP Site 999A. The site, located in the southern part of the Caribbean Sea, was chosen because of its well-preserved nannofossil assemblages and the broad variety of paleoproxies available. Our newly discovered size increase at 2.7 Ma is correlated to changing abundances of taxa with small versus large coccoliths. Some paleoproxies at Site 999A (δ18Osw,δ11B) and abundances of dominant coccolith taxa show significant changes in the same time interval. However, our analyses of global size variability of Holocene coccolith assemblages show different environmental dependencies. We infer that we either have non-analog size controls in the Holocene and the Pliocene data sets or that other proxies not examined so far may control size distributions of coccoliths.