Mantle buffering of the early oceans

Archean calcites and dolomites, if compared to their Phanerozoic counterparts, are enriched in Sr²⁺, Ba²⁺, Mn²⁺, Fe²⁺, depleted in ¹⁸O, (Na⁺), and contain mantle-like ⁸⁷Sr/⁸⁶Sr and, in associated S phases, mantle-like ³⁴S/³²S. This may be a consequence of massive seawater pumping through, and equilibration with, the coeval basaltic oceanic crust. The exponential decline of oceanic geothermal gradient in the course of terrestrial evolution led to a waning of this “mantle” flux and to the enhancement of the continental river discharge as the controlling factor of seawater composition; the major transition occurring probably during the late Archean — early Proterozoic time interval. Such evolution is consistent with the observed tectonic, sedimentological, geochemical and metallogenic secular patterns and may also provide an alternative, or complementary, inorganic explanation for the development of the post-Archean oxygenic atmosphere.