Tectonic control of chemical and isotopic composition of ancient oceans: The impact of continental growth

A numerical model that couples carbon-sulfur-strontium and atmospheric oxyg en cycles is used here to explore the impact of continental growth on the l ong term (greater than or equal to 10(8) yrs) evolution of the isotopic com position of seawater. Three growth scenarios are tested: "big bang" generat ion of continents shortly after the accretion of the Earth and two more gra dual scenarios, with a major growth episode around the Archean-Proterozoic boundary. The corresponding Sr-87/Sr-86, delta(34)S, and delta(13)C Of seaw ater and the sizes of the respective crustal sedimentary reservoirs are cal culated for each scenario and compared to the available data. The gradual c ontinental growth scenarios yield a better fit to the existing Sr-87/Sr-86 and delta(34)S isotope data for ancient seawater than does the "big bang" m odel and can be in agreement also with the measured seawater 6 degrees C, p roviding C-org participates in carbon subduction flux over the Earth histor y. These scenarios also generate a progressive oxygenation of the ocean/atm osphere system, with a large pO(2) rise coincident with (and due to) the ma jor continental growth event around the Archean-Proterozoic transition, in accord with the geologic record that indicates a major oxidation event in t he early Proterozoic. The advancing oxygenation of the planetary exogenic s ystem may therefore be a consequence of tectonic evolution rather than of b iological innovations such as the photosystem 2. The latter may have predat ed considerably the impact of oxygenation visible in the geologic record. I n 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 discrepa ncy 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.