In situ S‐isotope compositions of sulfate and sulfide from the 3.2 Ga Moodies Group, South Africa: A record of oxidative sulfur cycling

Abstract

Sulfate minerals are rare in the Archean rock record and largely restricted to the occurrence of barite 5 (BaSO 4). The origin of this barite remains controversially debated. The mass-independent fractionation 6 of sulfur isotopes in these and other Archean sedimentary rocks suggests that photolysis of volcanic 7 aerosols in an oxygen-poor atmosphere played an important role in their formation. Here we report 8 on the multiple sulfur-isotopic composition of sedimentary anhydrite in the ca. 3.22 Ga Moodies Group 9 of the Barberton Greenstone Belt, southern Africa. Anhydrite occurs, together with barite and pyrite, 10 in regionally traceable beds that formed in fluvial settings. Variable abundances of barite vs. anhydrite 11 reflect changes in sulfate enrichment by evaporitic concentration across orders of magnitude in an 12 arid, nearshore terrestrial environment, periodically replenished by influxes of seawater. The multiple 13 S-isotope compositions of anhydrite and pyrite are consistent with microbial sulfate reduction. S-14 isotope signatures in barite suggest an additional, oxidative sulfate source probably derived from 15 continental weathering of sulfide possibly enhanced by microbial sulfur oxidation. Although 16 depositional environments of Moodies sulfate minerals differ strongly from marine barite deposits, 17 their sulfur-isotopic composition is similar and most likely reflects a primary isotopic signature. The 18 data indicate that a constant input of small portions of oxidized sulfur from the continents into the 19 ocean may have contributed to the observed long-term increase of Δ 33 S sulfate values through the 20 Paleoarchean.

The emergence of subaerial crust and onset of weathering 3.7 billion years ago

“…Bladed and granular barite is characterized by δ 18 O and δ 34 S values that fall within the range reported for Paleoarchean seawater sulfate from sulfate minerals and carbonate-associated sulfate (Fig. 1b, 1c) 15,26,41,42 . Seawater-like δ 18 O and δ 34 Table S2).…”

“…Previous work has demonstrated that the magnitude of seawater sulfate D 33 S decreases throughout the Paleoarchean, as shown in Fig. 1d 26,27,42 . The observed correlation between Sr and S isotopes in bladed barite is therefore best explained by co-evolution of D 33 51,52 and hydrothermal uid (150°C) that lead to oversaturation with respect to barite 53 .…”

Anoxic photochemical weathering of pyrite on Archean continents

“…Under higher sulfate conditions, microbial sulfate reduction results in S isotope fractionations up to 45 per mil (‰) ( 44 ), and several Archean deposits exhibit large S isotope fractionations, suggesting locally high sulfate concentrations ( 45 , 46 ). For example, the pyrite in the Hemlo gold deposit in Wawa, Ontario, shows δ 34 S values ranging from −17.5 to +12.6‰ ( 45 ), and the Dresser Formation in North Pole ( 46 ) and the Moodies Group in Barberton Greenstone Belt, Africa, show fractionations up to 21.1 and 34‰, respectively ( 47 ). All three of these locations also contain sulfate minerals such as barite and anhydrite.…”

“…These types of environments could have had extensive Fe 3+ -mediated pyrite dissolution due to the shallow water depth that allowed light penetration and Fe 2+ photooxidation. Moreover, Archean pyrites are known to contain mass-independent fractionation S isotope signals ( 7 , 8 , 14 , 47 , 48 ), and the dissolution of these pyrites should release sulfate with Δ 33 S anomalies inherited from the pyritic sulfur. Currently, it is unknown whether UV-driven sulfate production from pyrite oxidation causes additional isotope fractionations that might be discernable in the Archean geologic record.…”

Paleomagnetismo, estratigrafia magnética e sedimentologia do Ediacarano na Formação Avellaneda (Argentina)

“…Features related to evaporite relics consist of lenticular biterminated crystal and isolated/coalesced nodular features with acicular-lath crystals (Figure 3.10). These features are associated with gypsum or anhydrite phase formed in the surface environment and require a sulfate enrichment process such as evaporative brine formation or oxidative sulfide weathering (Nabhan et al, 2020). Indeed, barite (as well as carbonate) appears as a substitute for likely calcium sulfate phases (Figure 3.10).…”