HR: 0800h
AN: PP51B-1338    [Abstracts]
TI: Marine Plankton Revolution and the Closure of the Circum-Tropical Seaways
AU: * Schmidt, D N
EM: d.schmidt@gl.rhul.ac.uk
AF: Royal Holloway University of London, Department of Geology, Egham, TW20 0EX United Kingdom
AU: Kucera, M
EM: m.kucera@gl.rhul.ac.uk
AF: Royal Holloway University of London, Department of Geology, Egham, TW20 0EX United Kingdom
AB: Climatic and tectonic events in the Neogene had a profound effect on marine pelagic environments. With the cooling in the high latitudes, latitudinal and vertical temperature gradients in the ocean increased. Additionally, the throughflow through the major tropical seaways became increasingly restricted, limiting the exchange of planktic organisms. We have analysed the size record of Neogene foraminifers and compared it with existing data on size in calcareous nannoplankton. Our data show that dramatic size increase in planktic foraminifers was not unidirectional, but showed size reductions during the Late Miocene carbonate crash, the biogenic bloom event and the Pliocene climate optimum. From 4.1 Ma onwards, unprecedented large sizes are recorded in the oligotrophic tropical settings, which do not occur in the temperate or upwelling settings. We relate the size trend to a closure of the circum-tropical seaways. By 4.2 Ma, an effective closure of the Isthmus appears to be followed by the evolution of foraminifers of an unprecedented large size. The changes in size do not reflect evolution of new, larger species, but they affect existing lineages, e.g. {\it G. conoidea - G. inflata} lineage or the {\it G. plesiotumida - tumida} lineage. The closure of the tropical seaways is a novel tectonic constellation in the evolutionary history of planktic foraminifera. The closure has strengthened the meridional heat transport, enhanced stratification, changed surface water density, and thermocline depth. This will have increased the number of available niches in the upper water column. Enhanced adaptation to the new niches in the tropical oceans might have allowed for larger sizes in assemblages, but have not lead to an increased number of species, as shown by the decoupling of size and diversity in the Plio-Pleistocene. Increased stratification restricts the transport of nutrients to the surface and growth to large size in coccoliths, but promoting foraminiferal growth. This decoupling of size and diversity and the divergence of mid- and low latitude size evolution in foraminifers, in contrast to the reduction in size in coccolithophorids , point to a revolutionary change in the niches in the upper water column during the Neogene caused by the unique closure of the circum-tropical seaways.
DE: 4855 Plankton
DE: 4203 Analytical modeling
DE: 4267 Paleoceanography
DE: 3030 Micropaleontology
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting