Wednesday May 19, 2021, 4pm GMT+2
Coralie ZORZI1,2* and Anne DE VERNAL1
Paleoceanographical changes of the Plio-Pleistocene based on marine palynological approach at ODP Sites 882 and 887, western and eastern North Pacific
1 GEOTOP, Université du Québec à Montréal, Montréal, Canada
2 EPHE-EPOC, Université de Bordeaux, France
The growth of glaciers in Alaska during the early Pliocene is in disagreement with the hypothesis suggesting that development of permanent continental ice in the circum-North Pacific is associated to onset of the modern halocline at 2.7 Ma (cf. Haug et al. 2005). However, the rarity of paleoceanography data documenting salinity in the eastern North Pacific prevents full understanding of the relationship between the halocline development and glaciers growth at regional scale. Numerical simulations have led to suggest that a strong contrast over the subarctic gyre during the Pliocene, with warm sea surface temperature (SST) in the east and cold SST in the west, might explain early ice extend in North America. Here again, the lack of paleoceanographic data is critical. In the view to fill this gap and to better constrain the scheme of the Plio-Pleisctocene climate-ocean-ice transition in the northern North Pacific region, we investigated the dinocyst assemblages at Ocean Drilling Program (ODP) Sites 882 and 887, located in the west and east respectively. Until 4.2 Ma, species associated with subpolar waters, Pyxidinopsis braboi, Filisphaera filifera and Impagidinium pallidum, are reported in the western Pacific. The presence of these species contrasts with the occurrence of Ataxiodinium zevenboomii, Impagidinium velorum and Impagidinium patulum in the Gulf of Alaska, which are associated with warm and temperate sea surface conditions. Between 4.2 and 2.7 Ma, the species assemblages at both sites are characterized by the dominance of the cold tolerant taxon Habibacysta tectata, which alternates with Impagidinium detroitense, and suggests homogenous cooling of the Pacific subarctic gyre until 2.7 Ma. After 2.7 Ma, dinocyst assemblages suggest temperate conditions and seasonal warming seat the surface due enhanced stratification, with the onset of the halocline. Concomitant extinction of Impagidinium detroitense, which marks a major biostratigraphical limit, is reported ~ 2.7 Ma at both ODP Sites 882 and 882, supports simultaneous onset of the modern halocline in the North Pacific. Dinocysts assemblages suggest the presence of warmer sea surface conditions in the Gulf of Alaska during the Pliocene and tend to support strong zonal contrast in the subarctic gyre prior to 4.2 Ma leading to early glaciers growth fostered by high evaporation and moisture supply in the east. However, our results also indicate a reduction of east-west gradient at 4.2 Ma, well before the development of the halocline at 2.7 Ma, which is recorded here for the first time in the Gulf of Alaska.