Multiple sulfur isotope record from the Precambrian of South America shows an unusual trend.
Abstract
Constraining the evolution of the Archean atmosphere is primarily undertaken using multiple S isotope. For example, a negative slope, of -1, between Δ33S and Δ36S is common through most of the Archean Eon. In contrast, here we report a set of samples exhibiting both negative Δ33S and Δ36S with a slope around +1.
Multiple S isotope compositions were analyzed from 68 samples collected in 3 drill cores from the Carajás mineral province (CMP), in the state of Pará, northern Brazil. One core intercepts the Águas Claras Group and 2 cores intercept the Igarapé Bahia Group. In addition δ 13C org and datation are currently investigated. The Igarapé Bahia Group comprises massive to finely laminated siltstone and fine sandstones with frequent ripples marks interpreted to represent offshore marine deposits and is dated around 2.6Gy. The Águas Claras Formation overlies the Igarapé Bahia Formation and comprises massive to finely laminated siltstones to sandstones, sometimes exhibiting wave and current ripples attributed to shallow marine depositional environments. Though its age is still debated, a best estimate lies around 2.1Gy. In the case of the Igarapé Bahia Fm., there are black shales deposited underlying potential diamictites, while in Águas Claras Fm. there is a good preservation of organic matter (0.4 to 1% of total organic carbon). The oldest samples show δ34S between -3‰ and 3‰, with restricted Δ33S<0.2, unexpected for this age. Yet, drill core in the Aguas Claras shows δ34S falling between -7‰ and +2‰, Δ33S between -1.2‰ and -0.1‰ and Δ36S between -1.2‰ and -0.2‰. Most of the samples define a trend with a slope 1. Guy et al. (2013) reported samples with both negative Δ33S and Δ36S values in the Witwatersrand but there were few within samples having values common in the Archean while all our samples are in this range. Our data compare favorably with the experiments by Whitehill et al. (2013). In their study organosulfur molecules generated by the laser-induced photolysis of SO2 show positive Δ33S and Δ36S values, and our analyzed sulfides could be the complementing reservoir. Variations along the -1 slope documented among Australian and South African cratons have been interpreted as changes in atmospheric chemical composition and accordingly our first results seem to underline much more significant changes.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V31B..04B
- Keywords:
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- 9820 Techniques applicable in three or more fields;
- GENERAL OR MISCELLANEOUSDE: 1030 Geochemical cycles;
- GEOCHEMISTRYDE: 1041 Stable isotope geochemistry;
- GEOCHEMISTRYDE: 1094 Instruments and techniques;
- GEOCHEMISTRY