Influence of cooling event at 2.75 Ma on the molluscan fauna of the Japan Sea borderland:

Abstract

We examined Plio-Pleistocene mollusks from the Taihei and Minehama areas in Akita Prefecture with the aim of elucidating the change in faunal composition at Datum A ( . Ma), when glaciers expanded in the Northern Hemisphere. In both areas, the deposits below the Datum A consist of finer sediments than those above. In the Taihei area, species of mollusks were recovered from localities. On the other hand, species were found from seven localities in the Minehama area. In terms of paleobathymetry, the sediments below the Datum A in both areas were deposited in the lower sublittoral zone, while those above the datum were deposited in the upper sublittoral zone. In both areas, the assemblages above the Datum A contain a higher proportion of individuals of cold-water species, despite the fact that they lived in shallower water depths. Moreover, some species now living in Hokkaido and northwards are confined to the upper horizons than the Datum A. In both areas, two Miocene relict species were collected from the deposits below the Datum A. These species might suffer from extinction by the cooling event at the Datum A. The results show that the cooling event at . Ma affected the molluscan fauna in Akita Prefecture.

The effects of meteoric diagenesis on the geochemical composition and microstructure of Pliocene fossil Terebratalia coreanica and Laqueus rubellus brachiopod shells from northeastern Japan

“…Assuming that the brachiopods occupied similar habitats during the Pliocene to those occupied by their modern counterparts, this indicates the presence of cooler and/or more saline seawater in the Minehama region during the Pliocene compared with that of the modern Otsuchi and Sagami bays. This interpretation is supported by the occurrence of coolwater molluscan species within the outcrop from which the fossil brachiopod shells of this study were collected (MH1; Amano et al 2011). This suggests that AFR/AF-PCR and MCLI are good indices for quantitatively estimating the extent of diagenetic alteration within brachiopod shells.…”

“…1b-f ) and L. rubellus (MNLR1-MNLR5; Fig. 1h-l) were collected from sandy conglomerates of the upper part of the Pliocene Tentokuji Formation exposed at Minehama (MH1, Amano et al 2011), Happo-cho, Akita Prefecture, northeastern Japan ( Fig. 2; 40°17.18′ N, 140°4.52′ E).…”

“…The Sasaoka Formation conformably overlies the Tentokuji Formation and consists of poorly sorted, very coarse to coarsegrained sandstone. Bivalves that co-occur with the brachiopods indicate a lower sublittoral zone depositional environment (Higo et al 1999;Amano et al 2011).…”