SATURATION STATE OF SEAWATER WITH RESPECT TO THE OTAVITE-CALCITE SOLID SOLUTION

The concentrat ion of c a dmium ( C d ) in seawater exhibits a nutrient-type profile (Chester, 2003), i . e . its ver­ tical distribution in the oceans resembles that of the nutrients phosphate and nitrate : it is greatly depleted in the surface waters, with its concentration increasing with depth, until it reaches a maximum value deeper into the water column, at depths around 1 000 m. As suggested by this similarity of distributions, the view that currently prevails, in trying to account for the characteristic verti­ cal pa ttern of Cd, is that its concentration must be regulated by marine biogeochemical processes (Abe, 200 1 ) . These processes include the uptake of Cd by phytoplankton in the euphot ic z o n e and, as a consequence of the decay of organic matter, the later re­ mineralization of Cd and the other nutrients at greater depths, with the ensuing increase in their concentrations (Wright, 1995) . However, despite this seeming connection to marine organisms, the true biogeochemical role of Cd is still uncertain, as the mechanisms of its uptake by cells are yet to be determined (Kremling and Strew, 2001 ) . On the other hand, it has also been suggested that the interact ions b e tween trace meta l s in s o lut ion and particulate matter could be a major process in regulating the concentrations of these metals in seawater (Sherrell and Boyle, 1 992) . At near-surface waters, Cd is sorbed onto biogenic particles, including calcitic and aragonitic particles secreted by organisms; these particles later sink towards the bottom, thus depleting the surface waters in Cd. As a result of the change in seawater chemistry with depth (Prothero and Schwab, 1996) characterized by a decrease in temperature and an increment in pressure and in the t o t a l d i s s o l v e d inorganic c a r b o n the solubility of calcite and aragonite will increase (Wright, 1 9 9 5 ) . This increment leads to further dissolution of these carbonates, with the consequent release of Cd2+ ion s into solution. Experiments on the sorption of Cd2+ to calcite and/or to aragonite surfaces have been carried out by several authors (Tesoriero and Pankow, 1996; Prieto et al., 2003; Cubillas et al., 2005) . Tesoriero and Pankow ( 1996) have measured the distribution coefticients for Cd (DCd) in freshwater and determined a range of 1 000-4500. These high values imply that, if a calcitic and/or aragonitic phase is present, partition of Cd2+ to the solid phase will a lways occur, even if Cd2+ is only present at t race concentrations in solut ion . AIso , the extremely low solubility of otavite, compared to that of calcite, further corrobora tes this strong partition tendency. When calculating the saturation state of seawater with respect to the possible precipitating phases, solid solutions are not usually considered. Our purpose is, then, to further investigate the mechanisms of Cd sorption onto cal cite and aragonite, considering the formation of (Cd,Ca)C03 solid solutions in seawater. We expect this study might lead us to a better understanding of the extent, efficiency and importan ce of this inorganic removal process as a sink for Cd in the oceans, and how it may superimpose on the processes of organic removal, thence influencing the observed distributions of this metal in the ocean. In this wo rk, pr ior to any experiments, we have calculated the saturation indices for pure calcite, pure otavite, and (Cd,Ca)C03 solid solutions, using existing data of sea water composition.