Tectonic control of chemical and isotopic composition of ancient oceans; the impact of continental growth

Abstract

A numerical model that couples carbon-sulfur-strontium and atmospheric oxygen cycles is used here to explore the impact of continental growth on the long term (> or =10 ⁸ yrs) evolution of the isotopic composition of seawater. Three growth scenarios are tested: "big bang" generation of continents shortly after the accretion of the Earth and two more gradual scenarios, with a major growth episode around the Archean-Proterozoic boundary. The corresponding ⁸⁷ Sr/ ⁸⁶ Sr, delta ³⁴ S, and delta ¹³ C of seawater and the sizes of the respective crustal sedimentary reservoirs are calculated for each scenario and compared to the available data. The gradual continental growth scenarios yield a better fit to the existing ⁸⁷ Sr/ ⁸⁶ Sr and delta ³⁴ S isotope data for ancient seawater than does the "big bang" model and can be in agreement also with the measured seawater delta ¹³ C, providing C _org participates in carbon subduction flux over the Earth history. These scenarios also generate a progressive oxygenation of the ocean/atmosphere system, with a large PO ₂ rise coincident with (and due to) the major continental growth event around the Archean-Proterozoic transition, in accord with the geologic record that indicates a major oxidation event in the early Proterozoic. The advancing oxygenation of the planetary exogenic system may therefore be a consequence of tectonic evolution rather than of biological innovations such as the photosystem 2. The latter may have predated considerably the impact of oxygenation visible in the geologic record. In contrast to the above isotope systematics, the model does not approximate well experimental observations of large delta ¹⁸ O variations at 10 ⁸ yrs time scales, at least during the Phanerozoic. The reason for this discrepancy may depend on the model structure that permits large variations in the oxygen isotopic composition of seawater only on time scales of 10 ⁹ yrs.