沉积物成岩蚀变过程中的Mn、Cd和Mo元素活动特征:以ODP 1148站钻孔沉积物记录为例

通过对南海北部的ODP 1148站岩芯600 mcd以上(约30 Ma以来)的沉积物中自生富集Mn、Cd和Mo等过渡金属元素的含量变化的研究,并结合相关的化学组成结果,探讨了岩芯内部氧化-还原条件的变化以及相关元素的活动特征,反演了相应沉积时期的环境演变.结果显示,岩芯387 mcd以上,自生Mn富集明显,代表氧化的环境;387～485 mcd之间,自生Cd含量明显富集,Mn含量显著降低,代表少氧的环境;485 mcd以下,Mn和Cd含量极低,自生Mo明显富集,代表缺氧的环境.随氧化-还原条件的变化,Mo存在明显的向下迁移并在缺氧界面的缺氧一方达到最大值的趋势,而Cd在少氧环境形成的固相态则可能在缺氧环境下不稳定,溶解态的Cd有向上迁移的趋势,并且在少氧/缺氧界面的少氧一方富集.这些过渡金属元素记录的氧化-还原条件的变化,反映出ODP 1148站所在海区的沉积环境变化:早期有较丰富的陆源输入,表层海水生产力较高,随着南海不断扩张以及全球海平面上升,该海区表层海水生产力逐渐降低.

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.