Exploring Archean seawater sulfate via triple S isotopes in carbonate associated sulfate.

Abstract:

Multiple sulfur isotope ratios in Archean sedimentary rocks provide powerful insights into the behavior of the ancient sulfur cycle, the redox state of fluid Earth, and the timing of the rise of atmospheric oxygen [1]. The Archean sulfur isotope record is marked by pronounced mass-independent fractionation (Δ³³S≠0)—signatures widely interpreted as the result of SO₂ photolysis from “short–wavelength” UV light resulting in a reduced phase carrying positive Δ³³S values (ultimately recorded in pyrite) and an oxidized phase carrying negative Δ³³S values carried by sulfate [2]. Support for this hypothesis rests on early laboratory experiments and observations of negative Δ³³S from barite occurrences in mixed volcanic sedimentary strata in Mesoarchean greenstone terrains. Despite forming the framework for understanding Archean sulfur cycle processes, this hypothesis is still largely untested, notably due to the lack of sulfate minerals in Archean strata.

Using a new MC-ICP-MS approach combined with petrography and X-ray spectroscopy we have generated a growing S isotope dataset from CAS extracted from Archean carbonates from a range of sedimentary successions, including: the 2.6 to 2.521 Ga Campbellrand-Malmani carbonate platform (Transvaal Supergroup, South Africa), 2.7 Ga Cheshire Formation (Zimbabwe), and 2.9 Ga Steep Rock Formation (Canada). Importantly, we observe positive δ³⁴S and Δ³³S values across a range of different lithologies and depositional environments. These results demonstrate that dissolved sulfate in seawater was characterized by positive Δ³³S values—a result that receives additional support from recent laboratory and theoretical experiments [e.g. 4, 5].

[1] Farquhar et al., 2000, Science

[2] Farquhar et al., 2001, Journal of Geophysical Research: Planets

[3] Paris et al., 2014, Science.

[4] Whitehill et al., 2013, Proceedings of the National Academy of Sciences.

[5] Claire et al., 2014 Geochimica et Cosmochimica Acta