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Micro-scale δ34S variation of sulfide species

Project description

Probing sulfur isotope variability in organic sedimentary rocks

The delta 34 S (δ34S) value is a standardised method for reporting measurements of the ratio of two stable isotopes of sulfur, 34S:32S, in a sample against the equivalent ratio in a known reference standard. The method has been widely used to reconstruct the paleo-redox conditions of early Earth environments. Existing δ34S compilations of sulfur isotopes on pyrite rocks possibly do not reflect the accurate isotope composition deposited on the rock. The EU-funded MicroS project will use high-resolution in situ analytical methods – secondary-ion mass spectrometry and X-ray spectroscopy – to investigate micro-scale δ34S variability in organic-rich sedimentary rocks. The project's work will help increase our understanding of the processes that govern sulfur isotope fractionations, δ34S transfer to the rock record and its preservation over the geologic time scale.

Objective

Sedimentary sulfur (S) isotope systematics (δ34S) has been widely used in paleoredox reconstruction of early Earth’s environments. Large-scale variations in the δ34S record are typically interpreted to reflect fluctuations in the global S cycle. However, the expression of S isotope fractionation is sensitive to open- vs. closed-system conditions and heterogeneous micro-scale δ34S patterns can form in S-bearing phases that precipitate over time in evolving pore-water conditions. Such intra-sample δ34S variability is undetectable in traditional bulk analyses, which has been the conventional approach in time-series δ34S compilations. Moreover, early- and late-stage interactions between organic matter and inorganic sulfide (thermochemical sulfate reduction) can impact δ34S signatures at different stages of sulfide formation leading to sulfide species with distinct δ34S compositions. Therefore, the existing stratigraphic bulk-rock pyrite δ34S compilations possibly integrate a whole spectrum of δ34S variability that may not reflect the depositional isotope composition. MicroS project will take advantage of high-resolution in situ analytical methods (SIMS, synchrotron-based X-ray spectroscopy) to investigate the micro-scale variability in organic-rich sedimentary rocks and its applicability to key intervals of Earth’s history. The project focuses on three organic-rich sedimentary basins: ~2.0 Ga Onega, ~2.1 Ga Francevillian and the modern Guaymas Basins, that share several characteristics, such as a semi-restricted rift environment, syndepositional magmatism and hydrocarbon seepage. By examining the δ34S composition of the co-existing sulfide species (e.g. pyrite, pyrrhotite, organic sulfur) and placing them into the geologic context of the individual basins, the project aims to decipher the current gap in understanding processes that govern S isotope fractionations, the transfer of δ34S to the rock record and its preservation over geologic time.

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Coordinator

TARTU ULIKOOL
Net EU contribution
€ 254 362,56
Address
Ulikooli 18
51005 Tartu
Estonia

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Region
Eesti Eesti Lõuna-Eesti
Activity type
Higher or Secondary Education Establishments
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Other funding
€ 0,00

Partners (1)