Seawater residence times of some elements of geochemical interest and the salinity of the oceans

Residence times of several dissolved chemical elements in seawater are revisited on the basis of updated river and groundwater chemical fluxes. A classification of the considered elements of geochemical interest (B, C, N, Na, Mg, Si, P, S, Cl, Ca, K, Li and Sr) is proposed accordingly to the solubility products of their main host minerals as well as their biological reactivity in the seawater column. The origin of seawater salinity is also discussed through both processes of early mantle degassing and continental growth. Finally, changes in seawater chemistry as well as in the total concentration of dissolved elements are investigated in the light of geological and biological processes such as continental erosion, hydrothermal activity, closure and opening of marine basins, climate forcing, marine productivity, and the Devonian rising of vascular plants. Taking into account groundwater fluxes and chemistry, and a revised lower mass of the oceans, residence times of chemical elements in the ocean have been calculated lower than commonly accepted in the literature for several decades. Both changes in salinity and chemistry of seawater took place throughout the Earth’s history at the m.y. timescale. Continents did not emerge before 3 Gy ago, thus excluding the weathering of their surfaces and the transport of alkali and calc-alkali elements by the rivers to the oceans. The only available sources, albeit to a limited extent, were possibly volcanic islands. As sodium is the main cation responsible for seawater salinity, calcium could have contributed to the charge balance of seawater to preclude the development of highly acidic waters. Indeed, the high-temperature hydrothermal alteration of oceanic crust is an important source of calcium that could have accumulated in seawater in the absence of its removal by calcifying organisms. It means that the earliest oceans could have been ‘CaCl₂ dominated oceans’ instead of the ‘NaCl dominated oceans’ that prevailed during the Proterozoic and Phanerozoic.