Multiple sulfur-isotope signatures in Archean sulfates and their implications for the chemistry and dynamics of the early atmosphere

Abstract

Sulfur isotopic anomalies (Δ 33 S and Δ 36 S) have been used to trace the redox evolution of the Precambrian atmosphere and to document the photochemistry and transport properties of the modern atmosphere. Recently, it was shown that modern sulfate aerosols formed in an oxidizing atmosphere can display important isotopic anomalies, thus questioning the significance of Archean sulfate deposits. Here, we performed in situ 4S-isotope measurements of 3.2-and 3.5-billion-year (Ga)-old sulfates. This in situ approach allows us to investigate the diversity of Archean sulfate texture and mineralogy with unprecedented resolution and from then on to deconvolute the ocean and atmosphere Archean sulfur cycle. A striking feature of our data is a bimodal distribution of δ 34 S values at ∼+5‰ and +9‰, which is matched by modern sulfate aerosols. The peak at +5‰ represents barite of different ages and host-rock lithology showing a wide range of Δ 33 S between −1.77‰ and +0.24‰. These barites are interpreted as primary volcanic emissions formed by SO 2 photochemical processes with variable contribution of carbonyl sulfide (OCS) shielding in an evolving volcanic plume. The δ 34 S peak at +9‰ is associated with non-33 S-anomalous barites displaying negative Δ 36 S values, which are best interpreted as volcanic sulfate aerosols formed from OCS photolysis. Our findings confirm the occurrence of a volcanic photochemical pathway specific to the early reduced atmosphere but identify variability within the Archean sulfate isotope record that suggests persistence throughout Earth history of photochemical reactions characteristic of the present-day stratosphere.Archean | sulfate | sulfur isotopes | atmosphere photochemistry T he amount of sulfate and its sulfur isotopic composition in the ocean through time is a function of the dynamic changes of sulfate sources (oxidative weathering on land, magmatic and hydrothermal input, and atmospheric photochemical reactions) and sulfate sinks (microbial and hydrothermal sulfate reduction and sulfate mineral precipitation). Although the Earth's sulfate budget can be reasonably well constrained after ∼2.3 billion years (Ga) ago, when free oxygen became a permanent component of the atmosphere, our understanding of the ocean sulfate budget before 2.3 Ga ago is subject to uncertainties. The occurrence of mass-independent sulfur-isotope anomalies (MIF-S, noted Δ 33 S and Δ 36 S) in sedimentary sulfur (sulfide and sulfate) of Archean age (1) and the photochemical models (2) for the production and preservation of these anomalies support the view that the Archean atmospheric O 2 concentration was lower than 10 −5 times the present atmospheric level. In this model, photochemical reactions involving volcanic SO 2 in the anoxic atmosphere yields both a reduced sulfur reservoir that can carry a highly positive Δ 33 S and an oxidized sulfur reservoir with modestly negative Δ 33 S, the Δ 36 S values being of opposite sign. The corollary to this model is that sulfate influx from oxidative weathering on land should...

Use of Isotope Effects To Understand the Present and Past of the Atmosphere and Climate and Track the Origin of Life

“…[117,118] Given the potential warming effect of COS,t his hypothesis is with significant implications for explaining the remarkable presence of liquid water (a necessity for life) on the early Earth despite a3 0% weaker sun than at present (also known as the faint young sun paradox). [118,119] Another relevant greenhouse gas in the Archean atmosphere to solve the faint young sun paradox is CH 4 .O rganic haze formed from CH 4 photochemistry might exist in the early Earthsa tmosphere and serve as am ajor source of organics to the early Earths ocean, which could have been the birthplace of life. [118,119] Another relevant greenhouse gas in the Archean atmosphere to solve the faint young sun paradox is CH 4 .O rganic haze formed from CH 4 photochemistry might exist in the early Earthsa tmosphere and serve as am ajor source of organics to the early Earths ocean, which could have been the birthplace of life.…”

“…[170] Given that biomass burning contributed to COS emissions,i tw as proposed that widespread wildfire during the 1997-1998 ENSO provided an additional in situ source of SO 2 in the higher stratosphere to form stratospheric sulfates. [117] This indirect pathway for producing sulfur isotopic anomalies is not considered in modern and Archean models.A sC OS was proposed as am ajor greenhouse gas in the Archean, [116][117][118] its role in the early Earth should be further investigated. [117] This indirect pathway for producing sulfur isotopic anomalies is not considered in modern and Archean models.A sC OS was proposed as am ajor greenhouse gas in the Archean, [116][117][118] its role in the early Earth should be further investigated.…”

“…Thep ilot photochemistry experiments by Farquhar et al, [76,97] which revealed wavelength-dependent sulfur isotope effects,allow the reconstruction of the past atmospheric composition (apart from oxygen) and solar actinic flux spectrum based on sulfur isotopic anomalous signatures in Archean rocks.Itwas suggested that carbonyl sulfide (COS or OCS) from volcanic activities might be an important component in the Archean atmosphere to reproduce the observed Archean d 34 S-D 33 Sr elationship because it prevents SO 2 photolysis at l > 202 nm by shielding UV, [116] and particularly since it also converts to SO 2 through photolytic reactions. [117,118] Given the potential warming effect of COS,t his hypothesis is with significant implications for explaining the remarkable presence of liquid water (a necessity for life) on the early Earth despite a3 0% weaker sun than at present (also known as the faint young sun paradox). Unique d 34 S-D 33 Sp atterns in some 3.5-3.2 Ga samples (different from other Archean samples) was also interpreted as deriving from complex changes in atmospheric composition as ac onsequence of intense burst of subaerial volcanic activity during that period, which might be linked to significant crustal growth and mantle plume events.…”

“…Unique d 34 S-D 33 Sp atterns in some 3.5-3.2 Ga samples (different from other Archean samples) was also interpreted as deriving from complex changes in atmospheric composition as ac onsequence of intense burst of subaerial volcanic activity during that period, which might be linked to significant crustal growth and mantle plume events. [118,119] Another relevant greenhouse gas in the Archean atmosphere to solve the faint young sun paradox is CH 4 .O rganic haze formed from CH 4 photochemistry might exist in the early Earthsa tmosphere and serve as am ajor source of organics to the early Earths ocean, which could have been the birthplace of life. [120] This hypothesis has been tested by carbon and sulfur isotope measurements and relevant photochemistry models,w hich showed that observed changes in Archean D 36 S/D 33 Ss lopes (from À0.9 to À1.5) could be explained, in part, by aC H 4derived organic haze,a nd the CH 4 production is likely of abiological origin.…”

GGR Biennial Critical Review: Analytical Developments Since 2014

“…Roerdink et al (2016) measured the multi S isotopes of pyrites associated with barite and explained the large isotopic variations using a model that combined biological and abiotic processes, including hydrothermal leaching, juvenile sulfur mixing and photochemical processes. Muller et al (2016) measured the in situ multi S isotope composition of barites from the Dresser (3.5 Ga, Australia) and the Mapepe (3.2 Ga, South Africa) deposits, with a precision of 0.2‰ for d 34 S and D 33 S and 0.5‰ for D 36 S, and proposed that this sulfate represents an exceptional period of volcanic and non-volcanic sulfate aerosol production and preservation. The same group also investigated the sulfur and the sulfate present in the Sargur Formation (3.2 Ga, India), which revealed similar pristine sulfur isotope composition and trends observed in Mapepe and Dresser at a micrometre scale, despite intense metamorphic re-equilibration (Muller et al 2017).…”