Chemistry of sediment profile GeoB4906

Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O2, pH, pCO2 and Ca2+ (in situ), pore-water concentration profiles of NO3-, NH4+, Fe2+, and Mn2+ and SO42- (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O2, [NO3]-, Fe(OH)3 and [SO4]2-, reoxidation reactions involving Fe2+ and Mn2+, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 µmol C cm**-2 yr**-1, of which 77% were due to O2, 17% to [NO3]- and 3% to Fe(OH)3 and 3% to [SO4]2-. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d-1 and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO3 flux to the sediment was estimated to occur at a rate of 42 g m**-2 yr**-1 in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m**-2 yr**-1, thus indicating that ~90% of the calcite flux to the sediment is redissolved.本研究针对加蓬近海的上陆坡表层沉积物，开展了有机质矿化与后续方解石溶解过程的模拟研究。实验采用微传感器（microsensors）原位（in situ）测定了O₂、pH、pCO₂及Ca²⁺浓度，异位（ex situ）测定了NO₃⁻、NH₄⁺、Fe²⁺、Mn²⁺与SO₄²⁻的孔隙水（pore-water）浓度剖面，并通过培养实验得到硫酸盐还原速率（sulfate reduction rates）。本研究采用输运-反应模型（transport and reaction model）CoTReM，模拟了O₂、NO₃⁻、Fe(OH)₃与SO₄²⁻介导的有机质降解过程、涉及Fe²⁺与Mn²⁺的再氧化反应，以及FeS的沉淀过程。模型应用结果显示，有机质矿化总速率可达50 μmol C·cm⁻²·yr⁻¹，其中77%由O₂介导，17%由NO₃⁻介导，3%由Fe(OH)₃介导，剩余3%由SO₄²⁻介导。通过调整0.01%~0.5% d⁻¹范围内的3个不同方解石溶解速率常数，并考虑沉积物中不同方解石物相的影响，最终实现了pH剖面的最优拟合。该动力学速率定律（kinetic rate law）中假设反应级数为4.5。在赤道上升流（equatorial upwelling）区域，碳酸钙（CaCO₃）输入至沉积物的通量估算为42 g·m⁻²·yr⁻¹。模型预测方解石再溶解通量可达36 g·m⁻²·yr⁻¹，表明输入至沉积物的方解石中约90%发生了再溶解。