Geochemical cycles of heavy metals on the early Earth

Abstract

Availabilities of various heavy metals for organisms in near surface waters have played critical roles in the origin and evolution of life on Earth. Geochemical cycles of heavy metals through the Earth’s surface environments can be divided into two groups based on the solubility changes of metal-bearing minerals as a function of pO2. The solubility of metal-bearing minerals decreases with increasing pO2 for the first group of elements (e.g., Fe, Mn, Ni, Cu). As a result, in oxic environments these elements basically remain in soils during the weathering of rocks (except in organic acid-rich environments), resulting in the oceans poor in these elements. In contrast, the solubility of metal-bearing minerals increases with increasing pO2 for the second group of elements (e.g., Cr, Mo, W, U). As a result, in oxic environments these elements are leached out of soils, resulting in the oceans rich in these elements. Examinations of the geochemistry of paleosols, sedimentary rocks, and alteration zones around submarine hydrothermal deposits of Archean ages suggest that the geochemical cycles of both groups of metal elements have been basically the same as today since the Archean. However, the concentrations of Group-II elements in the Archean oceans could have been higher than today.