Paleoproductivity and paleoceanography of the last 4.3 Myrs at IODP Exp. 323 Site U1341 in the Bering Sea based on biogenic opal content

The Bering Sea is known as a North Pacific marginal sea with a high biological productivity with its key location between the Pacific Ocean and the Arctic Ocean for the surface water circulation. Because that biogenic opal productivity is influenced by surface water structure, water circulation and sea ice cover, such conditions can be reconstructed by measuring biogenic opal content. Previous knowledge from the region including the vicinity indicates that biogenic opal productivity is generally high during interglacials and low during glacials (Narita et al., 2002; Okazaki et al., 2005; Iwasaki et al., in press). During the IODP Expedition 323, seven sites were drilled in the Bering Sea. Among them, Site U1341 was drilled to approximately 600 mbsf with a more or less complete record of the last 4.3 Myrs. The objectives of our study are to: (1) reconstruct the paleoproductivity change; and (2) compare the productivity change with that of other regions. Based on the analysis of over 1,400 samples, several notable trends are observed. First, the overall change is decreasing in biogenic opal content (from ~60% to 10-35%) during the last 4.3 Myrs. During the warm Pliocene without sea-ice diatoms the convection was sufficient to keep up high nutrient concentrations in the surface layers, also seen in the ship board data of "heavily silicified diatoms" (Takahashi et al., 2011). On the other hand, during the late Pleistocene, for example, overall biogenic opal values are low while substantial fluctuations (10-65%) due to change in climate are seen. Sufficient sea-ice cover, with sea-ice diatoms and dinoflagellates in ship board data (Takahashi et al., 2011), during the cold intervals of the Milakovitch cycles is responsible for the notably low values. The changes from warm to cold intervals are matched up generally well with other available ship board data: natural ganmma ray radiation, bulk density, & color reflectance (b*). Furthermore, the amplitudes of biogenic opal change increase as the age goes younger. These trends appear to be analogous to those of global benthic foraminiferal δ18O record (LR04: Lisiecki and Raymo., 2005) whereas the details are sufficiently different. A major deviation from the LR04 is that our opal records contain a substantially higher frequency than that of the 100 ky eccentricity signals of the LR04 during the Milakovitch cycles. Moreover, the change in biogenic opal content around the onset of Northern Hemisphere Glaciation (NHG ~2.65 Ma) shows a rapid decrease after an over all gradual increase, reflecting the specific paleoceanographic changes in the Bering Sea. It is notable and significant that the change seen at OPD Site 882, the end of opal dump, (Maslin et al., 1995) did not occur at Site U1341. In stead, the values kept increasing with time with a clear cyclicity of ~ca. 40 ky in our data set.