Variations in the radiolarian assemblages in the Bering Sea since Pliocene and their implications for paleoceanography

Radiolarian assemblages were analyzed using samples from the IODP Site U1340 to reconstruct the paleoceanographic conditions in the Bering Sea. Based on the characteristic faunal changes, the radiolarian evolution at Site U1340 was divided into four major intervals from Stages I to IV, with Stage IV divided into Substages IVa and IVb. The radiolarians in each stage recorded significant paleoenvironmental conditions. In general, the Bering Sea was governed by an ocean ecological environment with stable warm and saline surface water during Stage I (4.15 Myr to 3.91 Myr). The environment in the Bering Sea fluctuated strongly during Stage II (3.91 Myr to 2.75 Myr) and was controlled by the cold-water masses and sea ices during Stage III (2.75 Myr to 1.07 Myr) with the gradual development of cold and well-ventilated intermediate water. Stage IVa (1.07 Myr to 0.47 Myr) was a transitional period characterized by the enhanced formation of cold subsurface and intermediate water as well as of the oxygen-rich deep water. During Stage IVb (after 0.47 Myr), the Bering Sea was mainly characterized by enhanced warmth during interglacial episodes and well-developed water layers that were generally comparable to those of the modern Bering Sea. These conditions indicated that the vertical water-mass structure of the modern Bering Sea began to form since 0.47 Ma. Every Stage boundary in the studied core was marked by notable changes in the radiolarian assemblages. These changes corresponded to the climatic cooling event at ~ 3.91 Ma, the intensification of Northern Hemisphere glaciation at ~ 2.75 Ma, the beginning of the middle Pleistocene transition at ~ 1.07 Ma, and the low-latitude radiolarian ecology event at ~ 0.47 Ma. In addition, the relative abundance pattern of Cycladophora davisiana indicates that the Bering Sea was the main source of the past North Pacific Intermediate Water at ca. 0.85 Ma (MIS22), ca. 0.63 Ma (MIS16), and ca. 0.18 Ma (MIS6), just as it was during the last glacial maximum.