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

Abstract

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;' complexes significantly reduces the concentration of the free Mg+' ion. This decrease in the Mg+* activity results in desorption of magnesium from the solid surfaces due to the re-equilibration of the adsorbed Mg+' 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+* l&and. Therefore, significant changes in free Mg+2 concentration and thus in the magnesium desorption from solid surfaces by ligand competition for Mgt2 are more pronounced in sulfatedepleted 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+' between dissolved ligands and exchange sites, and Mg*'/NH: exchange reactions. The relative proportion of desorbed and displaced Mg+' from the solid surface depends on the characteristics of the sediment and on the ZCOz:NH: regenerative ratio in the pore waters. In sediments from Bransfield Strait, the Gulf of California, and the Peru margin, both release mechanisms for Mg"-l&and 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+' 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 sediments. By including the observed CEC changes in the multi-component model we show that the negative anomaly in the dissolved Mg+' profiles is a consequence of changes in the CEC of sediments during the very early stages of anoxic diagenesis.

Vivianite is a key sink for phosphorus in sediments of the Landsort Deep, an intermittently anoxic deep basin in the Baltic Sea

“…The twofold increase in Mg/Al during the MCA2 HI thus likely resulted from the incorporation of Mg in the Mn-carbonate structure (proposed Mg/Mnratio of 17; Suess, 1979) and other cation exchange processes (e.g. Drever, 1971;von Breymann et al, 1990).…”

Silicate weathering in anoxic marine sediments

“…Thus, Mg enrichments in anoxic pore fluids were generally attributed to ion exchange with dissolved ammonium (Cook, 1974;Gieskes et al, 1982;Suess et al, 1987;von Breymann et al, 1990). The cation exchange capacity (CEC) of rapidly accumulating diatomaceous hemipelagic mud determined at various sites falls into the range of 150-250 meq/kg (von Breymann et al, 1990). Experimental data (Sayles and Mangelsdorf, 1977;Sayles and Mangelsdorf, 1979;von Breymann et al, 1990) and numerical modeling (Charlet and Tornassat, 2005) show that the surface sites of clay minerals, river particles and sediments suspended in seawa- ter are occupied by Na, Mg, Ca, and K with relative proportions of 30-50% Na, 30-50% Mg, 10-20% Ca, and 2-10% K on equivalent base.…”

“…It may, however, also be argued that the cation release is caused by ion exchange processes. Thus, Mg enrichments in anoxic pore fluids were generally attributed to ion exchange with dissolved ammonium (Cook, 1974;Gieskes et al, 1982;Suess et al, 1987;von Breymann et al, 1990). The cation exchange capacity (CEC) of rapidly accumulating diatomaceous hemipelagic mud determined at various sites falls into the range of 150-250 meq/kg (von Breymann et al, 1990).…”

“…The cation exchange capacity (CEC) of rapidly accumulating diatomaceous hemipelagic mud determined at various sites falls into the range of 150-250 meq/kg (von Breymann et al, 1990). Experimental data (Sayles and Mangelsdorf, 1977;Sayles and Mangelsdorf, 1979;von Breymann et al, 1990) and numerical modeling (Charlet and Tornassat, 2005) show that the surface sites of clay minerals, river particles and sediments suspended in seawa- ter are occupied by Na, Mg, Ca, and K with relative proportions of 30-50% Na, 30-50% Mg, 10-20% Ca, and 2-10% K on equivalent base. These proportions translate into exchangeable cation concentrations of 60-100 mmol Na kg À1 , 30-50 mmol Mg kg À1 , 10-20 mmol Ca kg À1 , and 4-20 mmol K kg À1 assuming a CEC value of 200 meq/kg for Sakhalin slope sediments.…”

“…Detrital silicates derived from the physical denudation of the continents and products of continental weathering are major components of marine sediments. They affect the composition of seawater and pore fluids through ion exchange processes (Sayles and Mangelsdorf, 1977;Sayles and Mangelsdorf, 1979;von Breymann et al, 1990), chemical weathering (Maher et al, 2004), and reverse weathering (Michalopoulos and Aller, 1995;Michalopoulos et al, 2001). Maher et al (2004) recently proposed that finegrained clastic sediments deposited at the deep-sea floor off Iceland may weather as rapidly as terrestrial soils of similar age.…”