Magnesium adsorption and ion exchange in marine sediments: A multi-component model

The observed distribution of dissolved magnesium in the pore water of rapidly accumulating sediments shows significant deviations from the seawater value. We have shown that deviations during early diagenesis can be explained by reactions occurring at the surface of sediment particles. In anoxic pore water environments with high levels of dissolved total carbon dioxide the formation of Mg +2·CO 3 −2 complexes significantly reduces the concentration of the free Mg +2 ion. This decrease in the Mg +2 activity results in desorption of magnesium from the solid surfaces due to the re-equilibration of the adsorbed Mg +2 with the dissolved species. The effect of increasing carbonate complexation of Mg +2 in anoxic environments is initially compensated by the loss of sulfate, which is also a strong Mg +2 ligand. Therefore, significant changes in free Mg +2 concentration and thus in the magnesium desorption from solid surfaces by ligand competition for Mg +2 are more pronounced in sulfate-depleted systems undergoing methanogenesis. Such conditions are characteristic of most continental margin sediments. Another consequence of the decomposition of organic matter in hemipelagic sediments is the accumulation of high levels of ammonium ions which also displace Mg +2 from sediment-particle surfaces by ion exchange. These equilibria in the pore water-sediment systems can be described by empirical parameters, which were experimentally obtained. A computer model was used to determine the equilibrium conditions for solid-solution reactions as a function of changes in the pore-water composition in organicrich hemipelagic environments. This model includes complex formation, competition for Mg +2 between dissolved ligands and exchange sites and Mg +2 NH + 4 exchange reactions. The relative proportion of desorbed and displaced Mg +2 from the solid surface depends on the characteristics of the sediment and on the ΣCO 2:NH + 4 regenerative ratio in the pore waters. In sediments from Bransfield Strait, the Gulf of California, and the Peru margin, both release mechanisms for Mg +2—ligand competition and ion exchange with ammonium—were evaluated as part of the complex reaction system in order to explain the observed maxima in the dissolved magnesium profiles. Overlying the Mg +2 maxima, the Bransfield Strait and Gulf of California pore waters show minima in the dissolved magnesium concentration, concurrent with a measured increase in the cation exchange capacity (CEC) of the sediment