Transfer patterns of Mn, Cd and Mo in sediments during early diagenesis: Evidences from sediment cores at ODP Site 1148

Authigenic Mn, Cd, and Mo contents of the top 600 mcd (meter of composed depth) sediments from the sediments cores at ODP Site 1148, Northern South China Sea were reported in this paper. Based on these data, as well as some other shipboard chemical data, the redox condition changes within the sediments were discussed. Also, the transfers of the Mn, Cd and Mo between sediments and interstitial water were discussed. These may help to understand the changes of the deposit environments in the past. The results indicate that authigenic Mn contents in the sediments above 387 mcd are high, indicating an oxic environment. Sediments between 387 and 485 mcd show high Cd authigenic concentration with significantly decreasing authigenic Mn contents, indicating a suboxic environment. For the sediment bellow 485 mcd, the authigenic Mn and Cd contents are negligible and authigenic Mo contents are significantly high, indicating an anoxic environment. The variation patterns of the contents of these elements can also indicate that Mo may downward transfer in interstitial water, and deposit into sediments when reaching the anoxic boundary, where an enriched Mo layer could be found. On the other hand, it is at the suboxic/anoxic boundary other than the oxic/suboxic boundary that authigenic Cd peak values could be found. This may indicate that Cd transfers upward in interstitial water. As solid phase Cd in anoxic environment might be unstable and the dissolved Cd moves upward and deposit into sediments when across the suboxic/anoxic boundary. Moreover, the changes of the redox conditions within sediments as revealed by these elemental records are the response to the changes of the deposit environment at the areas around ODP Site 1148. In details, during the early periods as represented by the sediment bellow 485 mcd, the fluxes from continents were high, as a result the productivity was high. As the South China Sea enlarged, the fluxes from continents to this area decreased, and the productivity decreased, consequently.