Constraining magnesium cycling in marine sediments using magnesium isotopes

Magnesium concentrations in deep-sea sediment pore-fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust. To better characterize and differentiate these processes, we have measured magnesium isotopes in pore-fluids and sediment samples from Ocean Drilling Program sites (1082, 1086, 1012, 984, 1219, and 925) that span a range of oceanographic settings. At all sites, magnesium concentrations decrease with depth. At sites where diagenetic reactions are dominated by the respiration of organic carbon, pore-fluid delta Mg-26 values increase with depth by as much as 2 parts per thousand. Because carbonates preferentially incorporate (24)mg (low delta(26)mg), the increase in pore-fluid delta Mg-26 values at these sites is consistent with the removal of magnesium in Mg-carbonate (dolomite). In contrast, at sites where the respiration of organic carbon is not important and/or weatherable minerals are abundant, pore-fluid delta Mg-26 values decrease with depth by up to 2 parts per thousand. The decline in pore-fluid delta Mg-26 at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore-fluid. The identity of this enriched magnesium sink is likely clay minerals. Using a simple ID diffusion advection reaction model of pore-fluid magnesium, we estimate rates of net magnesium uptake/removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites. Independent estimates of magnesium isotope fractionation during dolomite precipitation from measured delta Mg-26 values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites, suggesting that local exchange of magnesium between sediment and pore-fluid at these sites can be neglected. Our results indicate that the magnesium incorporated in dolomite is 2.0-2.7 parts per thousand depleted in delta Mg-26 relative to the precipitating fluid. Assuming local exchange of magnesium is minor at the rest of the studied sites, our results suggest that magnesium incorporated into clay minerals is enriched in delta Mg-26 by 0 parts per thousand. to +1.25% relative to the precipitating fluid. This work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources/sinks in low-temperature aqueous systems. (C) 2010 Elsevier Ltd. All rights reserved.