Responses of Japanese Cenozoic molluscs to Pacific gateway events
The Cenozoic history of Pacific Ocean gateways can be divided into seven stages, extending from the Eocene onset of the Tasmanian seaway to the Pliocene closure of the Central American seaway. The first stage was the interval before 43 Ma, tentatively named the proto-Tasmanian stage. Development of the Tasmanian seaway (43-29 Ma) occupied the second stage. The third stage (29-23.5 Ma) was the oceanographically and paleoceanographically most open system, because the Pacific was not isolated from other oceans. The 4th stage consists of the closing of the Bering Strait prior to closure of the Indonesian seaway (23.5-17 Ma). The 5th stage was the time during which the Indonesian and Bering Seaways were both closed (17-5 Ma). The 6th stage consisted of a closed Indonesian seaway, and an open Bering Strait and Central American seaway (5 -2 Ma). The 7th and final stage is the modern situation, with a closed Central American seaway.
The responses of Japanese Cenozoic molluscan faunas to these Pacific gateway events reflect Pacific-wide patterns. The remarkable faunal changes from the late Eocene to the early Oligocene record the transition from warm-water environments to temperate or cool-water environments. The Oligocene Ashiya and Asagai molluscan faunas evidently developed as a response to oceanic cooling, which may have been related to the onset of both the Drake and Tasmanian Seaways (32-29 Ma to 23.5 Ma). A drastic change in bio-siliceous sedimentation in the early Miocene, recently recognized in Sakhalin may have been related to diatom floral turnover. As with the early Miocene Kunugidaira-Akeyo molluscan fauna that lived from 20 to 16.4 Ma, the diatom turnover may have been related to the stepwise closure of the Indonesian seaway. The Kadonosawa fauna lived during the first Neogene climatic optimum, at about 16 Ma. Formation of the modern north-flowing Kuroshio Current may have resulted from closure of the Indonesian seaway at that time. The wid espread occurrence of the bivalve Chlamys cosibensis during the late Miocene and early Pliocene in the North Pacific implies that surface waters were within the temperate realm.