Relative role of productivity vs. stagnation in dark laminated mud formation during the last 25,000 years in the Ulleung Basin, East/Japan Sea

摘要

Records of redox-sensitive elements (Mo, Cd, Cu, V and U), δ18O of planktonic foraminifera (δ18Opf), and organic carbon and biogenic opal contents in dark laminated mud (DLM) layers of the East/Japan Sea reveal relative role of primary productivity versus bottom-water stagnation for the formation of these layers during the last 25,000 years. Enrichments of redox-sensitive elements in these layers indicate that bottom waters were reduced during the formation of the lower DLM (22,500-15,00014C yr B.P.) and the upper DLM (10,400-10,20014C yr B.P.), probably to a greater extent in the lower layer. Anomalous low δ18Opf values and minimal contents of both organic carbon and biogenic opal suggest that bottom-water stagnation due to density-stratified water column could be more important than primary productivity for the lower DLM formation. In contrast, the relatively high contents of the organic carbon in the upper DLM invoke a primary role of enhanced productivity in the formation. This inference is, however, questioned by the absence of comparable increase in biogenic opal contents which is expected from generally coupled production of organic carbon and opal in surface water. The discrepancy between the organic carbon and opal records suggests that a bottom-water stagnation, which might have caused the preferential preservation of organic carbon in the reducing bottom-water condition, could be also more important than primary productivity for the upper DLM formation.