Foraminiferal records of bottom-water oxygenation and surface-water productivity in the southern Japan Sea during 160–15 ka: Associations with insolation changes

Analyses of the benthic and planktonic foraminiferal record were conducted to understand temporal variations in sea-floor oxygenation and surface productivity during periods of dark-layer deposition between 160 and 15 ka, based on data from sediment core IMAGES MD01-2407 (water depth, 932 m) from the southern Japan Sea. The dark layers in the core were classified into six types on the basis of benthic foraminiferal assemblages, which reflect the nature and amount of organic material on the sea floor, as well as the degree of oxygenation of bottom waters. Three of the dark layer types were deposited in conditions strongly associated with increased surface productivity, which correlates with positive or negative peaks of summer insolation in the Northern Hemisphere. During periods of insolation maxima, summertime surface productivity was enhanced because of the inflow of nutrient-rich surface waters associated with high precipitation in eastern Asia, caused by intensification of the East Asian summer monsoon. In contrast, relatively sustained organic matter fluxes to the sea floor occurred during insolation minima, possibly affected by active vertical mixing. When the East Asian summer and winter monsoons were both intensified in a given period well oxygenated bottom-water environments combined with high surface productivity may have resulted in dark-layer deposition, because of a combination of strong wintertime water convection and increased summertime surface productivity. It is likely that the organic carbon content of sediments underestimates surface productivity during depositional conditions favoring the sedimentary decomposition of organic matter.