NOAA/WDS Paleoclimatology - Japan Sea Organic Carbon and Bulk Nitrogen Isotopes and Major and Trace Elements over the Past 11 Ma

A multi-proxy 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 has been conducted in order to trace deep-water evolution in the Japan Sea and the North Pacific since the late Miocene. The moderate Ba enrichment factors (BaEF) and high total organic carbon (TOC) flux in sediments, as well as other published geochemical and sedimentary evidence, indicate the occurrence of anoxic deep-water in the Japan Sea before ~7.4 Ma. The lower nitrogen isotope values probably suggest (nearly) complete denitrification in redox-stratified ocean. In contrast, the sharply enhanced primary production but decreased burial of organic matter during ~7.4−4 Ma, as shown by high BaEF values, together with low TOC flux, highlights deep-water oxygenation in the Japan Sea since that time. We argue 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 ocean oxygenation occurred synchronously with increased pole-to-equator temperature gradients driven by late Miocene global cooling, which could have intensified Hadley circulation and caused southward shift of mid-latitude storm tracks, leading to significantly decreased precipitation in mid-latitude regions; this is supported by rapid expansion of C4 vegetation and aridification in the mid-latitudes of the Northern Hemisphere. The resultant 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.