Deep-Water Formation in the North Pacific During the Late Miocene Global Cooling

A multiproxy study including organic carbon and bulk nitrogen isotopes along with major and trace element concentrations in sediments from Integrated Ocean Drilling Program (IODP) Sites U1425 and U1430 in the Japan Sea have been conducted in order to trace deep-water evolution in the Japan Sea and the North Pacific since the late Miocene. The high total organic carbon (TOC) flux, as well as other published geochemical and sedimentary evidence, indicates the occurrence of anoxic deep-water in the Japan Sea before similar to 7.4 Ma. The low-nitrogen isotope values probably suggest nearly complete denitrification. In contrast, the sharply enhanced biological production but decreased burial of organic matter during similar to 7.4-4 Ma, as shown by high enrichment factor of Ba (Ba-EF) values, together with low TOC flux, highlights enhanced deep-water oxygenation in the Japan Sea during that time. We suggest that deep-water formation in the North Pacific ventilated the deep Japan Sea via northern deep seaways before the sea became semi-closed in the early Pliocene. The synchronously increased equator-to-pole temperature gradients driven by late Miocene global cooling may have caused southward shift of mid-latitude storm tracks, coupled with the weakened East Asian summer monsoon and moisture transport, leading to decreased precipitation in mid-latitude regions. The potential increases in surface salinity in the North Pacific may have broken the ocean stratification and favored deep-water formation, and further caused deep-water ventilation in the Japan Sea.