Evolution of East-West contrast in the subarctic Pacific gyre during the Plio-Pleistocene based on new palynological evidences at ODP Site 882

Evolution of the temperature contrast in the subarctic gyre is fundamental to better constrain the scheme of the Plio-Pleistocene climate-ocean-ice transition in the northern North Pacific region. 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. Hence, development of warm surface conditions in eastern Pacific during the early Pliocene has been proposed as key mechanism fostering regional evaporation. These assumptions are sustained by model experiments simulating a strong W-E contrast in subarctic Pacific during the Pliocene (Burls et al., 2017) and a reduction of the surface gradient at 2.7 Ma (Haug et al., 2005). However, evolution of the zonal subarctic temperature gradient remains unclear in geological records. In the view to fill this gap, we investigated the dinocyst assemblages at Ocean Drilling Program (ODP) Site 882 located in western subarctic gyre, which were be compared with the Plio-Pleistocene palynostratigraphy previously performed in the Gulf of Alaska.

Prior 4.2 Ma, dinocyst assemblages at ODP Site 882 are characterized by cold and cool-tolerant species such as Pyxidinopsis braboi and Filisphaera filifera with occasional occurrence of thermophilic species reflecting cold sea-surface water conditions. This observation is in line with U^K₃₇ SST analysed at the study site but contrasts with the highest percentages of thermophilic dinocysts, including Ataxiodinium zevenboomii, Impagidinium velorum and Impagidinium patulum, recorded in the Gulf of Alaska. Hence, our results documented for the first time a strong temperature contrast in the subarctic Pacific gyre with warmer conditions in the East than in the West as previously simulated by the models (Bursl et al., 2017). However, reduction of the zonal temperature contrast was reported at 4.2 Ma, well before the development of the halocline at 2.7 Ma, by the increase of Impagidinium detroitense and Habibacysta tectata over the subarctic gyre. Despite uncertainties regarding the ecology of these two extinct taxa, we tentatively interpreted this change as the development of cool but saline conditions in the subarctic Pacific. Our study acknowledges that a better understanding of the mechanisms driving major climate changes is reinforced by integrating approach combining proxy data and climate models.