Enhanced dissolution of basaltic glass in brackish waters: Impact on biogeochemical cycles

Abstract

In order to better constrain the geochemical budget of Si in the ocean, and potentially other elements released by the dissolution of silicates, the alteration of riverine particulate material in estuaries and seawater needs to be estimated. For this, a series of alteration experiments of basaltic glass were performed at various degrees of salinity (from 0 to 3.5 g L−1) in far-from-equilibrium conditions. The solution used is a filtered natural seawater standard from the Atlantic Ocean. The forward dissolution rates increase from 2.1⋅10−7 molSim−2s−1 (S=0 gL−1) to 7.7⋅10−7 molSim−2s−1 (S=3.5 gL−1) at 90 °C and were extrapolated at 16 °C (from 2.9⋅10−10 molSim−2s−1 at S=0 gL−1 to 1.1⋅10−9 molSim−2s−1 at S=3.5 gL−1). This positive relationship between glass dissolution rate and salinity degree is consistent with published investigations concerning the role of specific cations and ligands present in seawater, which can promote dissolution at the glass surface. These results illustrate the potential of river basaltic glass particles to dissolve quickly in the water column after entering into the brackish waters of estuaries, and before sinking on continental margins. Based on these dissolution rates and on assumptions on the particulate solid flux of fresh basaltic glass exported by rivers towards the ocean, the corresponding flux of dissolved Si is estimated to range between 2 and 8⋅1012 molSiyr−1. This is of the same order of magnitude as the estimated river dissolved Si flux, which represents therefore a significant input of Si into the ocean. Additionally, if the glass dissolution process remains congruent during the residence time of suspended particles into the water column, the K flux to the ocean could also be significantly affected.