EXPERIMENTAL DISSOLUTION OF DIATOM SILICA IN CONCENTRATED SALT-SOLUTIONS AND IMPLICATIONS FOR PALEOENVIRONMENTAL RECONSTRUCTION

Two diatom frustule dissolution experiments (DE1 and DE2) were perform ed on material from Lakes Geneva and Nantua, SE France. In DE1, the di ssolution of diatom silica from sediments was measured over a 92-d per iod at room temperature in six 3 M salt solutions (NaCl, Na2CO3, KNO3, CaCl2, LiNO3, MgCl2), in 0.6 M NaCl, and in distilled water. As expec ted, dissolution was greatest in Na2CO3 where high pH leads to ionic d issociation. A similar process was also apparent in solutions of KNO3 and LiNO3 which were only slightly alkaline. In contrast, weakly acid solutions of MgCl2 and CaCl2 produced less dissolution than distilled water. Between 0.6 and 3 M NaCl, silica solubility appeared to reach a n 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 solu tions of Na2CO3 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 concentrated solutio ns. Clear interspecific differences in diatom dissolution rates occurr ed in both experiments. The gross valve surface area: volume ratio was found to be a useful first-order approximation of a particular diatom species' propensity to dissolve. Only minor differences in diatom ass emblage composition were observed in NaCl solutions over the experimen tal period whereas major changes were caused by Na2CO3 solutions.