Inorganic Precipitation of Aragonite from Artificial Seawater at Low Oxygen Content and in the Presence of Methane.

Calcium carbonate minerals are important for our understanding of past marine conditions as well as tools for constructing paleoclimate. However, very little work has been done to determine the influence of oxygen depletion on the geochemistry of calcium carbonate minerals. At lower depths, conditions become increasingly anoxic. Methane oxidation, sulfate reduction, and respiration and fermentation of organic compounds are among the associated microbial metabolisms that may cause precipitation of carbonate or dolomite [1]. As a result the oxidation state of a number of elements change, which should affect their incorporation into carbonate phase.

To determine how oxygen depletion affects elemental incorporation and partitioning, aragonite was inorganically grown in the laboratory using artificial seawater. In order to induce precipitation, a small amount of 0.1 M Na₂CO₃was added into solution. Pumping at a slow rate and stirring was done in order to avoid immediate CaCO₃ precipitation. N₂ was bubbled through solution in order to minimize oxygen content in experiments at atmospheric pressure. Two experiments were conducted at 5 bars of 0.1%CH₄-N₂mixture. Further oxygen trapping was done by the addition of Fe powder to some experiments. The aragonite was precipitated over a period of five to seven days, during which fluid subsamples were taken daily to measure pH. A drop in pH indicated that precipitation had started. Precipitated aragonite samples as well as final fluid solutions were collected and analyzed with ICP-MS, and isotope ratio mass spectrometer. In addition, solids were measured with electron microprobe.

Our preliminary data showed aragonite grown with the presence of Fe powder is depleted in S/Ca and is enriched in Mn/Ca relative to aragonite precipitated without Fe powder. No significant effect of CH₄ on aragonite d¹³C was observed.

1. Boetius A. et al. (2000) Nature, 407, 623-626.