Biostratigraphy, climate and ocean changes in the Gulf of Alaska during the Plio- Pleistocene

English Abstract:

The Pliocene (5.33 and 2.59 Ma) was a relatively warm epoch characterized by high atmospheric CO₂ concentration (about 400 ppmv) and the absence of continental ice sheet in the Northern Hemisphere. It contrasted with the ensuing Pleistocene Epoch, characterized by lower atmospheric CO₂ concentrations and the onset of glaciations in North America and Eurasia. The present study focuses on the Gulf of Alaska with the aim to document climatic and circulation changes in the subarctic Pacific Ocean in relation with the development of glaciers in northwest North America. More specifically, the aims are to provide clues on the vegetation history of the adjacent land from pollen assemblages, and to reconstruct sea-surface conditions with special attention to productivity and salinity linked with the onset of upwelling from dinocyst assemblages. Marine sediments from sites ODP 887 (54°21.9’N ; 148°26.8’W ; 3645 m) and IODP U1417 (56°95.9’N ; 147°10.9W ; 4187 m) are currently being analyzed for their palynomorph content to reconstruct environmental . Preliminary data indicate very large amplitude change in concentrations of both pollen (from 20 to 1780 grains/cc) and dinocyst (from 0 to 280 cysts/cc) suggesting very large change in biogenic fluxes from terrestrial origin and marine production. In general, Pliocene samples contain more pollen than dinocysts, which suggest large fluxes from the terrestrial vegetation, but low marine productivity. Many Pleistocene samples reveal barren, but some samples record high diversity contrasting with Pliocene intervals marked by the dominance of Brigantedinium spp. and moderately abundant dinocysts, which suggest episodes characterized by high marine productivity. The Plio-Pleistocene being marked by important biostratigraphic changes, the interpretation of palynological data is challenging. Our study is part of a multi-proxy collaborative project involving sedimentology, diatom and biomarker analyses. Hence, in addition to document environmental impact of both local and regional drivers on local vegetation and climate, the multi-proxy study should contribute to develop a comprehensive picture of regional changes during the Plio-Pleistocene climate transition.