A deep-sea piston core containing more than 3 m of sediment of freshwater origin was recovered from the northeast flank of the Yamato Bank in the central part of the Sea of Japan. We identify this layer on the basis of abundant frustules of fresh-water diatoms with no admixture of marine forms. All data indicate that the layer is in place. Micropaleontological evidence from the overlying marine sediment dates this layer as older than the early Pliocene Denticula kamtschatica zone of Koizumi. These considerations and the pollen data suggest a late Miocene age for the sediment. Possible explanations for the occurrence of this fresh-water deposit are (1) the sediment originated in a turbidity current from the Japanese mainland, (2) it represents a wind-blown deposit, (3) the Yamato Bank originated as an island (with lakes) and subsequently subsided to its present depth, (4) the Yamato Bank was originally a part of the Japanese mainland and subsequently broke off and spread westward, carrying with it continental sediment, and (5) the sediment is an in situ deposit, formed when the Sea of Japan was a fresh-water lake. Restraints imposed by topography and time tend to favor explanation 5. We realize that this fresh-water sediment may occupy the same time frame as the latest Miocene (Messinian) Mediterranean dessication.

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