The Neoarchaean surficial sulphur cycle: An alternative hypothesis based on analogies with 20th-century atmospheric...

We revisit the S-isotope systematics of sedimentary pyrite in a shaly limestone from the ca. 2.52 Ga Gamohaan Formation, Upper Campbellrand Subgroup, Transvaal, South Africa. The analysed rock is interpreted to have been deposited in a water depth of ca. 50-100 m, in a restricted sub-basin on a drowning platform. A previous study discovered that the pyrites define a nonzero intercept δ³⁴ S_V-CDT -Δ³³ S data array. The present study carried out further quadruple S-isotope analyses of pyrite, confirming and expanding the linear δ³⁴ S_V-CDT -Δ³³ S array with an δ³⁴ S zero intercept at ∆³³ S ca. +5. This was previously interpreted to indicate mixing of unrelated S-sources in the sediment environment, involving a combination of recycled sulphur from sulphides that had originally formed by sulphate-reducing bacteria, along with elemental sulphur. Here, we advance an alternative explanation based on the recognition that the Archaean seawater sulphate concentration was likely very low, implying that the Archaean ocean could have been poorly mixed with respect to sulphur. Thus, modern oceanic sulphur systematics provide limited insight into the Archaean sulphur cycle. Instead, we propose that the 20th-century atmospheric lead event may be a useful analogue. Similar to industrial lead, the main oceanic input of Archaean sulphur was through atmospheric raindown, with individual giant point sources capable of temporally dominating atmospheric input. Local atmospheric S-isotope signals, of no global significance, could thus have been transmitted into the localised sediment record. Thus, the nonzero intercept δ³⁴ S_V-CDT -Δ³³ S data array may alternatively represent a very localised S-isotope signature in the Neoarchaean surface environment. Fallout from local volcanic eruptions could imprint recycled MIF-S signals into pyrite of restricted depositional environments, thereby avoiding attenuation of the signal in the subdued, averaged global open ocean sulphur pool. Thus, the superposition of extreme local S-isotope signals offers an alternative explanation for the large Neoarchaean MIF-S excursions and asymmetry of the Δ³³ S rock record.