Orbital-scale biotic and paleoceanographic changes in Japan Sea during the late Miocene global cooling (LMGC)

The late Miocene global cooling (LMGC; ca. 7.9–5.8 Ma) was accompanied by global environmental changes including monsoon intensity, ocean circulation, and biotic turnover. Recent studies have revealed drastic environmental and biotic changes in the Japan Sea across the LMGC; the radiolarian fauna in the Japan Sea suggested decreased inflow of Pacific Central Water (PCW) species Tricolocapsa papillosa, increased subarctic species Cycladophora sphaeris and extinction of a dominant endemic radiolarian Cycladophora nakasekoi. These radiolarian fluxes showed ∼100-kyr eccentricity signals; however, the dominant paleoclimatic/paleoceanographic signal during the LMGC appeared to be ∼40-kyr obliquity cycle, which was not detected in the radiolarian fluxes possibly due to lower sampling resolution. Thus, orbital-scale variations of these radiolarian fluxes and their potential forcing mechanisms remain unclear.

In this study, we establish changes in the abundance of selected radiolarian species with a resolution that exceeds the 10-kyrs and suggest obliquity-paced changes in the paleoceanography in the Japan Sea during the LMGC. We detected ∼40-kyr cycles in abundance of T. papillosa, suggesting that PCW inflow into the Japan Sea was controlled by a 40-kyr obliquity-paced glacial cycle. We also detected a ∼ 40-kyr signal in C. sphaeris abundance, implying that the obliquity-paced winter monsoon probably promoted the inflow of subarctic water into the Japan Sea. Contrary, variation in C. nakasekoi abundance lacks a ∼ 40-kyr signal. A 100-kyr cycle is also observed in a summer monsoon proxy from lacustrine sediment in China. Hence, it is possible that C. nakasekoi decreased with weakened summer monsoon across the LMGC.