Late Pliocene to early Pleistocene (2.4–1.25 Ma) paleoproductivity changes in the Bering Sea: IODP expedition 323 Hole U1343E

Late Pliocene to early Pleistocene paleoproductivity changes in the Bering Sea were reconstructed using geochemical concentrations and mass accumulation rates (MARs) of CaCO3, biogenic opal, and total organic carbon (TOC), and sedimentary nitrogen isotope ratios (δ15N) at IODP Expedition 323 Hole U1343E, drilled in the northern slope area (1956 m deep) of the Bering Sea. CaCO3 concentration is generally low, but prominent CaCO3 peaks occur intermittently due to subseafloor authigenic carbonate formation rather than biogenic accumulation, regardless of glacial–interglacial variations. Biogenic opal concentrations reflect orbital-scale glacial–interglacial variations. However, TOC concentration did not show clear glacial–interglacial variation, probably due to poor preservation. The sedimentary δ15N values vary synchronously with biogenic opal concentration on orbital timescales. The co-varying pattern of opal productivity and δ15N values at Hole U1343E is a result of nutrient utilization controlled by diatom productivity in the Bering slope area where Fe is not a limiting factor. Biogenic opal and TOC MARs showed a temporal shift at around 1.9 Ma from a high productivity period under nutrient-enriched conditions to a low productivity period under relatively nutrient-depleted conditions. High diatom productivity with low δ15N values before 1.9 Ma is associated with abundant nutrient supply by upwelling in relation to strong surface current system. This productivity decrease at about 1.9 Ma was also found in the southern Bering Sea (Site U1341) and may be related to global opal reorganization.