Nitrate utilization and stratification changes during the early and late Pleistocene in the Bering Sea

The subarctic North Pacific and its marginal seas have been known to be characterized by high primary productivity related to the excessive nitrate concentrations in surface water. However, studies on relationship between nitrate utilization and biogenic opal productivity in these regions are restricted either during the last ~180 ka or at the Northern Hemisphere Glaciation (~2.73 Ma). In this study we document bulk nitrogen δ¹⁵N and diatom-bound δ¹⁵N (δ¹⁵N_db) during the late (0-600 ka) and the early (1.25-2.4 Ma) Pleistocene at IODP Exp. 323 Site U1343 in the Bering slope area to understand how the relationship between nitrate utilization and biogenic opal productivity changed in terms of the biogenic opal contents during the Pleistocene. Variation between bulk δ¹⁵N and δ¹⁵N_db at Site U1343 during the early and late Pleistocene is generally parallel, which implies that δ¹⁵N_db can be used for a nitrate utilization parameter in the Bering Sea. Nitrate utilization is positively related with biogenic opal productivity before 400 ka; high during interglacial and low during glacial periods, which contrasts to the previous results in the subarctic North Pacific. This may be ascribed primarily to regional strong sea-ice effect at Site U1343, otherwise stratification in the Bering slope area might be not as strong as the subarctic North Pacific. Before Mid-Pleistocene Transition (MPT), coupling between large amplitude of biogenic opal productivity and small change of nitrate utilization is distinct, whereas it became reversed after MPT. It indicates that the water column in the Bering Sea is relatively more stratified after MPT. Thus, strong stratification after 400 ka in the Bering slope area may be related to the enhanced stratification in the subarctic North Pacific.