Experimental dissolution of diatom silica in concentrated salt solutions and implications for paleoenvironmental reconstruction

Two diatom frustule dissolution experiments (DE1 and DE2) were performed on material from Lakes Geneva and Nantua, SE France. In DEl, the dissolution of diatom silica from sediments was measured over a 92-d period at room temperature in six 3 M salt solutions (NaCl, Na,CO,, KN09, CaCl,, LiN03, MgCl,), in 0.6 M NaCl, and in distilled water. As expected, dissolution was greatest in Na,CO, where high pH leads to ionic dissociation. A similar process was also apparent in solutions of KNO, and LiNO, which were only slightly alkaline. In contrast, weakly acid solutions of MgCl, and CaCl, produced less dissolution than distilled water. Between 0.6 and 3 M NaCI, silica solubility appeared to reach an undetermined optimum which could correspond to minimum values of ion activity coefficients. In a second experiment (DE2), the dissolution of living diatoms from Lake Geneva was monitored over 46 d in 3 M solutions of Na,CO, and NaCl and in distilled water. The results were not in accordance with the expected theoretical levels predicted by Pitzer’s equations when applied to silica solubility in conccntrated solutions. Clear interspecific differences in diatom dissolution rates occurred in both experiments. The gross valve surface area : volume ratio was found to bc a useful first-order approximation of a particular diatom species’ propensity to dissolve. Only minor differences in diatom assemblage composition were observed in NaCl solutions over the experimental period whereas major changes were caused by Na,CO, solutions.