Constraints on the late Ediacaran sulfur cycle from carbonate associated sulfate

Abstract

We report new sulfur isotope compositions (δ 34 S) in carbonate associated sulfate (CAS) and pyrite from the lower Nama Group, Namibia (~550 to <547 Ma), and use these data to interrogate terminal Ediacaran sulfur cycle dynamics. Our extraction method utilizes an improved pre-leaching procedure that reduces the likelihood of contamination from matrix-bound sulfur. Data generated with the improved extraction method show CAS δ 34 S as much as 12‰ higher (34 S-enriched) than previously reported which suggests a reevaluation of the phenomenon of 'superheavy' pyrite. Seawater sulfate δ 34 S increases from 30 to 38‰ in the lower Nama Group, and we correlate this rise in δ 34 S among contemporaneous marine basins. Global seawater sulfate δ 34 S >35‰ is highly unusual in Earth history, and in the terminal Ediacaran is best explained by a high pyrite burial flux. Pyrite in the Nama Group is close in isotopic composition to coeval sulfate, but the sulfur isotope fractionation between sulfate and pyrite varies widely among different studied basins, suggesting highly heterogeneous redox and depositional conditions.

Sedimentology and chemostratigraphy of the terminal Ediacaran Dengying Formation at the Gaojiashan section, South China

“…Viewed at a global scale, the finding of a δ 13 C carb excursion of up to +6 ‰ in the Dengying Formation is also consistent with published chemostratigraphic profiles of the roughly correlative terminal Ediacaran strata in Namibia (Saylor et al 1998;Wood et al 2015;Tostevin et al 2017) and Arctic Siberia (Knoll et al 1995;Pelechaty et al 1996b;Cui et al 2016a;Vishnevskaya et al 2017) where δ 13 C carb positive excursions with similar magnitude (up to +6 ‰) have also been reported.…”

“…Sulfur isotope chemostratigraphy of the DYF@GJS shows overall positive δ 34 S pyrite values ranging between +20 ‰ and +30 ‰ through most of the section, except for two episodes of anomalous negative excursions down to c. -30 ‰ in the lower and middle Gaojiashan Member (Figs 12e, 13f). The occurrence of highly positive δ 34 S pyrite values in the terminal Ediacaran strata has also been reported from the roughly equivalent strata in Oman (Fike & Grotzinger, 2008), Namibia (Ries et al 2009;Tostevin et al 2017), Arctic Siberia (Cui et al 2016a) and Newfoundland (Canfield et al 2007;Hantsoo et al 2018), suggesting a global phenomenon. The origin of the low δ 34 S pyrite signals in the Gaojiashan Member has been fully discussed in our earlier paper (Cui et al 2016b); we will mainly focus on the overall high δ 34 S pyrite signals below.…”

Integrated records of environmental change and evolution challenge the Cambrian Explosion

“…Mo and U isotopes show that an increase in oxygenation was punctuated by intervals of expanded anoxic seafloor (41-43); low Th/U ratios show a positive correlation with  13 C values at multiple sites across the Ediacaran-Cambrian boundary, suggesting that the BACE is a response to the widespread development of shallow marine anoxia (44)(45). A shift in the marine sulphur cycle, as recorded by δ 34 S, is possibly consistent with increasingly widespread sulphate reduction under anoxic conditions around ~550 Ma (22,46). Diverse proxies suggest that the global ocean became progressively more oxygenated through the early Cambrian until ~520 Ma, after which time there was a return to more widespread anoxia (49,50).…”

Uranium isotope evidence for an expansion of anoxia in terminal Ediacaran oceans

“…The integration of local and global redox proxies, in a section that directly preserves changes in biota, allows for co-interpretation without a need to correlate between possibly contemporaneous sections. These new results constrain global oceanic redox conditions spanning the interval from ~550-~547 Ma, and suggest anoxic bottom waters expanded to cover at least a third of the sea floor, following the emergence of the first skeletal animals and coincident with major perturbations in the sulfur and carbon cycles (Cui et al, 2016;Fike et al, 2006;Tostevin et al, 2017;Wood et al, 2015).…”

“…The carbonates were probably originally deposited dominantly as aragonite, but have since neomorphosed to calcite. Dolomite-rich samples were excluded due to uncertainty surrounding the impact of dolomitisation on δ 238 U. δ C ( 13 C/ C; reformulated as  C), δ O ( 18 O/ 16 O; reformulated as  18 O), δ 34 S ( S/ 32 S; reformulated as  34 S), major element, Fe-speciation and rare earth element data for the same samples, and the associated methods are published in Tostevin et al, (2017Tostevin et al, ( , 2016 and Wood et al, (2015).…”