4 GA OF SEAWATER EVOLUTION–EVIDENCE FROM SULFUR ISOTOPES

The history of seawater composition over the past ca. 4 Ga of Earth evolution is recorded in the sulfur isotopic composition of sulfate, such as marine evaporitic calcium sulfates and barite, or trace sulfate in marine carbonates, phosphates and cherts. The record is constrained by some 1350 measurements, largely following the traditional approach of studying marine evaporites as reflecting seawater composition. In addition, a growing body of data exists for structurally substituted sulfate (SSS) in whole-rock and biogenic carbonates, providing enhanced stratigraphic resolution and – during Phanerozoic time – biostratigraphic control of the sulfur isotope record.

The Precambrian sulfur isotope record is very fragmentary due to the fact that only few sulfate occurrences have been preserved. Thereby, the Archean record is based on the analyses of barite which is believed to reflect the seawater signature. For these, primary barite formation as well as pseudomorphic replacement after calcium sulfate are being discussed. Sulfur isotope values display a range between +2.7 and +8.7 ‰ around an average of +4.0 ± 1.1 ‰ (n=67). The temporal evolution throughout the Archean and Proterozoic displays a first-order increase towards an average value of +32.1 ± 3.7 ‰ (n=134) for the latest Neoproterozoic, i.e. between the Marinoan glaciation and the lower Cambrian. This rise is likely not linear, but temporal resolution for most of the time is rather poor.

A detailed sulfur isotope record is available for Phanerozoic seawater, based on the analysis of SSS in biogenic and whole-rock carbonates and with a temporal resolution of a stage or even a biozone. Substantial fluctuations of the sulfur isotopic composition of seawater sulfate are discernible on different time scales. Following an early Paleozoic maximum around +30 ‰, values decline to a minimum around +12 ‰ in the Permo-Carboniferous, followed by a rise towards the modern value of +21 ‰.

Modeling of the Phanerozoic sulfur isotope record reveals substantial fluctuations in sulfate concentration and pyrite burial rate. Controversial opinions exist in respect to a generally lower sulfate concentration in Precambrian oceans.