The effect of orthophosphate on carbonate mineral dissolution rates in seawater

Abstract

The dissolution kinetics of a variety of synthetic and biogenic carbonates (low-Mg calcite, Mg-calcite and aragonite) were investigated in Gulf Stream seawater (salinity, S = 35‰) spiked with orthophosphate to a concentration of 50 μm. Phosphate is known to inhibit calcite dissolution and has been reported to catalyze the dissolution of aragonite. The main goal of the study was to assess whether the degree of inhibition depends on the mineralogy of the dissolving carbonate phase. Dissolution experiments were conducted at 25°C and a P CO 2 of 10 −2·5 atm. using the pH-stat technique. Dissolution rates were measured at saturation states ranging from 0.1 to 0.8 times saturation with respect to the dissolving carbonate mineral and were compared to rates measured in low-phosphate seawater. The degree of inhibition caused by phosphate addition increased as saturation state with respect to the dissolving carbonate mineral increased. At a saturation state of 0.2, addition of phosphate reduced dissolution rates by about a factor of 2. Closer to equilibrium, at a saturation state of 0.8, dissolution rates were reduced by about a factor of 4. More importantly, all carbonate samples whether of synthetic or biogenic origin were inhibited to similar degrees at equivalent saturation states. The “catalysis” reported by previous workers for synthetic aragonite appears to be an artifact of its anomalous non-linear dissolution rate over time. Because phosphate inhibits the dissolution of different carbonate minerals to similar degrees, selective dissolution of aragonite from metastable carbonate sediments cannot be promoted by the presence of orthophosphate.