Geochemical response of aragonite on pressure and oxygen depletion in seawater: an experimental study
Aragonite precipitation experiments were conducted in high pressure vessels to study partitioning of S, Mn, Li, B, Mg, Sr, Ba and fractionation of 13C/12C between aragonite and artificial seawater. Crystallization was promoted by the one-time addition of an aliquot of 0.1M Na2CO3 solution into stirring seawater in the amount insufficient to cause immediate precipitation. Visual examination confirmed the absence of precipitates prior transferring of this carbonate ion – enriched seawater to pressure vessel, where aragonite crystallized without stirring in a few days. Fluid samples were collected during some experiments and pH measurements suggested that crystallization started within a few hours after the beginning of experiments. Nitrogen gas was used to vary experimental pressure from 90 to 345 bars. In two experiments aragonite precipitated under 5 bars using a gas mixture of 0.1%CH4-N2. A few experiments were performed at 1 atm. Iron powder was added to some experiments to sequester oxygen from the fluid. The proportion of Na2CO3 solution to seawater was the same in all experiments causing saturation state of initial fluids to be 59 at atmospheric pressure (1.01 bar) and room temperature. Precipitated aragonite and fluids were analyzed with ICP-MS, electron microprobe, and isotope ratio mass spectrometer.
Electron microprobe analysis yielded high heterogeneity of aragonite spherulites especially in Mg and S. We intend to present the data of δ13C and element to calcium ratios in aragonite grown at 1-345 bars, at 7.8 and 22°C, with presence and absence of methane, in normal and low oxygen environments.