Bacterial sulfur disproportionation constrains timing of Neoproterozoic oxygenation

Abstract

Various geochemical records suggest that atmospheric O2 increased in the Ediacaran (635–541 Ma), broadly coincident with the emergence and diversification of large animals and increasing marine ecosystem complexity. Furthermore, geochemical proxies indicate that seawater sulfate levels rose at this time too, which has been hypothesized to reflect increased sulfide oxidation in marine sediments caused by sediment mixing of the newly evolved macrofauna. However, the exact timing of oxygenation is not yet understood, and there are claims for significant oxygenation prior to the Ediacaran. Furthermore, recent evidence suggests that physical mixing of sediments did not become important until the late Silurian. Here we report a multiple sulfur isotope record from a ca. 835–630 Ma succession from Svalbard, further supported by data from Proterozoic strata in Canada, Australia, Russia, and the United States, in order to investigate the timing of oxygenation. We present isotopic evidence for onset of globally significant bacterial sulfur disproportionation and reoxidative sulfur cycling following the 635 Ma Marinoan glaciation. Widespread sulfide oxidation helps to explain the observed first-order increase in seawater sulfate concentration from the earliest Ediacaran to the Precambrian-Cambrian boundary by reducing the amount of sulfur buried as pyrite. Expansion of reoxidative sulfur cycling to a global scale also indicates increasing environmental O2 levels. Thus, our data suggest that increasing atmospheric O2 levels may have played a role in the emergence of the Ediacaran macrofauna and increasing marine ecosystem complexity.

A productivity collapse to end Earth’s Great Oxidation

“…This is consistent with a similar degree of S cycling during both intervals and, by inference, no diminution of the syn-GOE Δ 17 O signal by enhanced O exchange with water (14,46). Although the distinctly more positive Δ 33 S values in syn-GOE samples could be interpreted to reflect greater S reoxidation at this time (35,37), another explanation is that the lack of any covariation with δ 34 S (cf. refs.…”

Dating the late Proterozoic stratigraphic record

“…720 Ma as currently defined; [44]) overall is poorly calibrated radiometrically. Furthermore, the Tonian sedimentary succession in Svalbard, perhaps the best preserved and most complete in the world for this time period [69,70] and a critical archive of important fossil [39,60,62,[71][72][73] and geochemical [69,[74][75][76][77][78] data, has not been directly dated. In the absence of direct radiometric ages, easily identified GTS boundaries, or biostratigraphic zonation (e.g.…”

“…In the absence of direct radiometric ages, easily identified GTS boundaries, or biostratigraphic zonation (e.g. [63,76]), other approaches are required to tell time in this and many other sedimentary successions of this time interval.…”

Evolution of cellular metabolism and the rise of a globally productive biosphere

“…The evolutionary process that produced such innovations in marine picocyanobacteria (Fig. 4) stretches out across the Neoproterozoic and Phanerozoic [63,64], which is relevant to discussion that ocean oxygenation played out over hundreds of millions of years [38,[93][94][95][96]. Further, the innovations seen in marine picocyanobacteria (Fig.…”

“…400 Mya) [92] or even the Triassic (ca. 200 Mya) [93] -but evidence for the initiation of this process reaches back to the Neoproterozoic [38,[92][93][94][95][96]. Several biogeochemical feedbacks have been proposed to explain why the deep open ocean remained anoxic during the Proterozoic.…”