Superheavy pyrite ( 34Spyr >  34SCAS) in the terminal Proterozoic Nama Group, southern Namibia: A consequence of low seawater sulfate at the dawn of animal life

Abstract

Sulfur isotope analysis (δ 34 S) of well-preserved carbonates spanning an ~10 Ma interval of the terminal Proterozoic Nama Group reveals that disseminated pyrite is consistently enriched in 34 S relative to coeval seawater sulfate as preserved in carbonate-associated sulfate (CAS). This observation is not consistent with the current paradigm for interpreting the geologic record of sulfur isotopes, which assumes that pyrite δ 34 S (δ 34 S pyr ) will be equal to or less than co-occurring CAS δ 34 S (δ 34 S CAS ) due to the kinetic isotope effect of bacterial sulfate reduction (BSR) that favors the lighter isotope of sulfur ( 32 S) during sulfur-oxygen bond breakage. Although the precise mechanism of pyrite sulfur isotope enrichment is debatable, our combined observations of extremely 34 S-enriched pyrite, low bulk-rock concentrations of sulfur from CAS and pyrite, and high-frequency fl uctuations in δ 34 S CAS and δ 34 S pyr throughout the Nama Group carbonates point to very low concentrations of sulfate in portions of the terminal Proterozoic ocean. The additional occurrence of 34 S-enriched pyrite in contemporaneous terminal Proterozoic sections from Poland and Canada reveal that low seawater sulfate may have been widespread in the oceans at this time. However, the absence of such extremely 34

Mechanism for Burgess Shale-type preservation

“…A growing body of evidence also points to low-sulfate concentrations in Cambrian oceans relative to the modern (24,39). Low-sulfate conditions are indicated by prominent global excursions in the isotopic composition of seawater sulfate (39) and by the presence of "superheavy" δ 34 S of sedimentary pyrite in some settings (40). Low-sulfate conditions may have been promoted by the extensive burial of pyrite under widespread anoxic or dysoxic bottom waters (37) and by the lack of extensive bioturbation (41).…”

Ocean redox conditions between the snowballs – Geochemical constraints from Arena Formation, East Greenland

“…Although extremely heavy pyrite δ 34 S values are not a worldwide signature in the ocean (e.g. Oman), it is clear that an extremely sulfate-depleted ocean is required to achieve these values (Ries et al, 2009 If secondary alteration induced the negative δ 13 C CARB trend observed in the lower Arena Fm, then the positive δ 13 C ORG trend (from -34‰ to -30‰) could be interpreted in two ways. Firstly, δ 13 C ORG could record a positive carbon isotope trend in the global exogenic carbon cycle.…”

“…The sulfur isotope composition of marine pyrites, δ 34 S PY , rarely exceeds average Phanerozoic oceanic input (<8‰) (Halevy et al, 2012) simply because pyrite is the major sink for reduced sulfur that preferentially removes the lighter sulfur isotopes from the marine sulfate reservoir (Canfield, 2004 (Feng et al, 2010;Li et al, 2012;Ries et al, 2009). Although extremely heavy pyrite δ 34 S values are not a worldwide signature in the ocean (e.g.…”

Correlations of some Neoproterozoic carbonate-dominated successions in South America based on high-resolution chemostratigraphy

“…Reported δ 34 S pyrite values (24 -32.5 ‰ CDT) in the Loma Negra Formation are heavier than δ 34 S CAS for the same unit (24 -29 ‰, Bagnoud-Velásquez et al 2013). This "superheavy pyrites" have been interpreted for the coeval Nama Group (southern Africa) as the result of enhanced bacterial sulphate-reduction activity in a low sulphate environment, hinting at low atmospheric O 2 contents in the latest Ediacaran (Ries et al 2009). …”