Microbial and environmental controls on the S-isotopic (δ34SPYR-Δ33SPYR) composition of pyrite in modern sediments and sedimentary rocks: The joined force of bulk and in-situ technics

Séminaire de Virgil Pasquier (Université de Lausanne)

Reconstructions of past environmental conditions and biological activity are often based on stable isotope proxies whose interpretations are inherently non-unique. This is particularly true of the sulfur (δ³⁴S_PYR-Δ³³S_PYR) isotopic composition of sedimentary pyrite, which is used to reconstruct ocean-atmosphere oxidation state and track the evolution of several microbial metabolic pathways.

My use and approach of sulfur, i.e., bulk and in-situ, present a novel microanalytical approach to deconvolve the multiple signals that influence pyrite isotopes, yielding both the unambiguous determination of biological fractionations and novel information about depositional conditions.

Over the last few years, I apply this approach to recent glacial-interglacial sediments, which feature among the largest observed S isotopic variability across these environmental transitions. Despite profound environmental changes, the biological fractionation (ε_mic) remained essentially invariant throughout this interval and the observed isotopic range in δ³⁴S_PYR-Δ³³S_PYR were instead driven by climate-induced variations in the depositional environment.