MIDDLE PROTEROZOIC OCEAN CHEMISTRY

Banded iron formations (BIFs) ceased depositing around 1.8 billion years ago (Ga). While BIFs deposited, ocean bottom waters contained appreciable dissolve Fe. However, the nature of Proterozoic ocean chemistry after 1.8 Ga is highly uncertain and both oxic and sulfidic ocean models have been proposed. In this contribution we present stable sulfur isotope and major element data from the MacArthur Basin, northern Australia. These results provide compelling evidence for euxinic conditions, much like the modern Black Sea, for sediment deposition in the MacArthur Basin. Furthermore, stable sulfur isotope results, along with a new model describing the intensity of sulfide accumulation in euxinic basins, constrain ocean sulfate concentrations at 1.6 Ga to < 5 mM.

We also consider the evolution of the stable sulfur isotope systematics of sulfate and sulfides through the Proterozoic and into the early Phanerozoic. From this analysis we conclude that sulfide was the major pathway of sulfur removal from the oceans through the middle and late Proterozoic. This result is completely consistent with a middle and late Proterozoic sulfide-rich ocean with suppressed concentrations of seawater sulfate.