Trends in Ocean S-Isotopes May Be Influenced by Major LIP Events

Abstract

Mega volcanic eruptions associated with the formation of Large Igneous Provinces (LIPS) pump vast amounts of carbon dioxide and sulfur-rich gases into the atmosphere and stratosphere with the potential to totally change the chemistry of the global ocean. Here we investigate the sedimentary pyrite sulfur isotope record of black shales through time and demonstrate two coherent populations termed P1 and P2. Population P1 dominates the Archean pyrites, has a mean of δ34S = +3.7‰, standard deviation of 5.3‰, and is considered to represent S of mantle origin. Population P2 appears toward the start of the Proterozoic, dominates the Phanerozoic, has a mean around +25‰ and standard deviation of 13.5‰, and is considered to represent S of seawater sulfate origin. Population P1 can be identified in sedimentary pyrite at certain times in the Proterozoic and Phanerozoic, which correspond, within error, with the timing of 25 major LIP events. Our data suggest that at regular times through the Proterozoic, coinciding with major LIP events, the oceans contained a mixture of seawater sulfate and dissolved mantle sulfide derived from the LIPs. LA-ICP-MS analyses of the sedimentary pyrite indicate that metals, particularly gold, nickel, cobalt, and PGE were also enriched in the oceans at these times. The long periods between major LIP eruptions enabled the oceans to return to a seawater sulfate background equilibrium with a decrease in the mantle-derived metals.