Early Neogene biochemostratigraphy of Pohang Basin: a paleoceanographic response to the early opening of the Sea of Japan (East Sea)

The location of the Pohang Basin near the Korea Strait in the southwest of the Sea of Japan (East Sea) makes this area appropriate for providing a rec ord of paleoenvironmental/biotic changes associated with the early Neogene (similar to 16.5 Ma) opening of this gateway to the Pacific Ocean. Stable i sotopic and planktonic foraminiferal records are presented that assist with the understanding of paleoenvironmental changes in the Sea of Japan result ing from the opening of the Korea Strait. The oldest sediments of early Neo gene age of the Pohang Basin overlie Cretaceous basement and are Early Mioc ene (>similar to 16.5 Ma) shallow, estuarine facies containing a benthic fo raminiferal assemblage dominated by Ammonia beccarii (L.). The oldest early Neogene planktonic foraminiferal assemblages in the basin are similar to 1 6.5 Ma in age (latest Early Miocene foraminiferal zone N8), The migration o f these planktonic assemblages to the Sea of Japan at that time appears to have resulted from the initial opening of the Korea Strait, The overlying e arly Neogene marine sequence of the Pohang Basin extends from 16.5 Ma (Zone N8) through 14 Ma (Zone N10). Change in the oxygen isotopic record of the Pohang Basin sequence suggests strong local paleoenvironmental control rela ted to the early opening of the Korea Strait. Early Middle Miocene isotopic temperatures of planktonic foraminifera are relatively cool at similar to 15 Ma at a time when global temperatures were high in the middle/low latitu de regions. It was not until 14.8 Ma that isotopic temperatures of plankton ic foraminifera increased markedly. This distinct warming is inferred to re flect the major intrusion of the Kuroshio Current into the Sea of Japan pro bably as a result of further critical opening and deepening of the Korea St rait. At this time planktonic foraminifera increased in abundance reflectin g expanding oceanic influence. A cooling that followed at similar to 14.5 M a, when the strait was well open is unlikely to reflect local tectonic cont rol on the paleoceanography, but global cooling during the early Middle Mio cene associated with the expansion of the East Antarctic ice sheet. (C) 199 9 Elsevier Science B.V. All rights reserved.