[MIS06-P09] Sulfur isotope geochemistry of the Japan Sea sediments (IODP Exp. 346) 30 ~ 220 kyr ago: Implications for the evolution of Asian Monsoon climate system
The light layers have lower Spy (0.03~0.25 wt.%) contents when compared to the dark layers (0.26~1.49 wt.%). The Corg contents have similar distribution (0.34~1.10 wt.% for light layers and 1.16~3.38 wt.% for dark layers). However, the SSO4 contents (0.02~0.64 wt.%), the d34Spy values (-34 to -38‰) and the d34SSO4 values (0 to -10‰) did not show such light-dark distinction. Elevated Spy/Corg ratios (0.03~1.00) in the dark layers are interpreted to represent sulfide formation in the anoxic water column by bacterial sulfate reduction. During deposition of light layers, oxidation of sulfide minerals could have released sulfate without significant isotope fractionation. Regardless of the type of the sediments (dark vs. light), sulfate was not limiting during bacterial sulfate reduction, as reflected in the sulfur isotope compositions.
We speculate that, during deposition of dark layers, enhanced summer monsoon activity caused heavy rainfall and increased source-rock weathering, runoff of the Yangtze River, and nutrient input into the East China Sea and the Tsushima Warm Current. Inflow of nutrient-rich and less salty water into the Japan Sea triggered enhanced biological activity, water-column density stratification, transport of organic matter into deeper ocean and consumption of dissolved oxygen, and ultimately the creation of anoxic water body to allow bacterial sulfate reduction (syngenetic sulfide formation) to occurr. On the other hand, during deposition of light layers, enhanced winter monsoon activity caused cooling of surface water of the Japan Sea, often creating down-going oxygen-rich water mass to ventilate the deep water (and break up density stratification). Oxic condition in the deep water did not allow accumulation of massive organic matter and formed light-color sediments. Biogenic sulfide formed, if any, during diagenesis (no syngenetic sulfide formation).