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 ∼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 ∼7.4–4 Ma, as shown by high enrichment factor of Ba (BaEF) 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. Plain Language Summary Deep‐water formation plays a significant role in deep ocean ventilation in the modern North Atlantic and South Ocean, while absent in the North Pacific. Whether deep‐water formation occurred in the North Pacific in the late Miocene global cooling remains unknown. The Japan Sea is an ideal place to test this phenomenon since it had deep‐water exchange with the North Pacific during the late Miocene‐early Pliocene. We analyze the organic carbon and bulk nitrogen isotopes and major and trace element concentrations of sediments since 11 Ma from IODP Sites U1425 and U1430 in the Japan Sea. During ∼7.4–4 Ma, the enhanced biological production along with decreased organic carbon burial indicates enhanced deep‐water oxygenation in the Japan Sea. We attribute this to potential deep‐water formation in the North Pacific, which was probably caused by weakened precipitation in mid‐latitude regions of the Northern Hemisph