Miocene benthic foraminifera of the Japan Sea

Benthic foraminifers from ODP Sites 794 and 797 (Yamato Basin) and Site 795 (Japan Basin) were investigated to clarify the Miocene paleoceanography and paleobathymetry of the Sea of Japan. Miocene benthic foraminifers from the three sites are generally characterized by agglutinated deep-water species apparently living below calcium carbonate compensation depth (CCD), except for faunas indicative of environments above the CCD in the early middle Miocene (~14 to ~17 Ma) and redeposited faunas in upper Miocene sediments. Based on species occurrences, the following assemblage zones are recognized from lower to upper horizons: at Site 794, the Chilostomella oolina/Cyclammina sp. C and the Spirosigmoilinella compressa/Martinottiella communis Zones; at Site 795, the Martinottiella communis, the Oridorsalis umbonatus/Cibicidoides wuellerstorfi, the Martinottiella communis/Cribrostomoides cf. crassimargo, and the Cyclammina ezoensis Zones; at Site 797, the Cyclammina sp. A/Cyclamminal sp., the Gyroidina orbicularis/Gyroidinoides planulatus, the Cibicidoides wuellerstorfi and the Martinottiella communis/Globobulimina pupoides Zones and two barren intervals below the Cyclammina sp. A/Cyclammina? sp. Zone and between the Cibicidoides wuellerstorfi and the Martinottiella communis/Globobulimina pupoides Zones, respectively.
Benthic faunal changes are principally reflected by variations in the ratio between agglutinated and calcareous tests. Dominantly calcareous faunas developed in the early middle Miocene at Sites 794 and 797 and in the middle middle Miocene at Site 795. The first appearance of a dominantly calcareous fauna in the G. orbicularis/G. planulatus Zone at 17.1 Ma suggests a change from low-oxygen to high-oxygen bottom water. This paleoceanographic change was likely caused by widening of the Proto-Japan Sea in a tensional stress field. Thus, the significant opening of a basin along with the Pacific deep waters in the Sea of Japan is considered to have occurred at 17.1 Ma.
On the basis of an appearance of a barren interval and/or agglutinated fauna during the middle Miocene, two significant benthic foraminifer events are recognized in the Sea of Japan. The First Foram Sharp Line (1st FSL), representing an abrupt change from calcareous to agglutinated faunas, appears at 14.4 Ma at Site 794 and 14.3 Ma at Site 797. These ages are also close to the ages obtained for occurrence of the FSL in onshore Miocene sequences in the Honshu arc. The coeval appearance of the 1st FSL in the Yamato Basin and along the Sea of Japan coast indicates that the uplift of backbone ranges of the Honshu arc resulted in oxygen-deficient bottom water in the evolving back-arc basin. Uplift was likely associated with the clockwise rotation of the Japanese Islands. The Second Foram Sharp Line (2nd FSL) at Site 795 is estimated to occur at 11.2 Ma, which is about 3 m.y. later than the first FSL at Sites 794 and 797. These differences between the Yamato and Japan Basins suggest that sub-basins in the Sea of Japan did not respond simultaneously to changes in deep-water masses and currents. Paleotopographic features were changed dramatically through the back-arc opening process. The FSL at Site 795 may be related to global paleoceanographic changes and rather than caused by local silled basin effects. Thus, two types of the FSL are recognized in the Miocene Sea of Japan, which express both global and provincial paleoceanographic events.