Multiple sulfur isotopes (δ34S, Δ33S) and trace elements (Mo, U, V) reveal changing palaeoenvironments in the mid-Carboniferous Chokier Formation, Belgium

Abstract

In this study, multiple sulfur isotopes are combined with widely-used redox-sensitive trace element concentrations (Mo, V, and U) in order to further validate the use of δ34S and Δ33S as a paleoredox proxy. The approach is applied to the mid-Carboniferous Chokier Formation, for which a detailed microfacies study identified a marked change in the environmental conditions during sediment deposition.Sediments from the upper part of the Chokier Formation exhibit negative δ34S (mean −24.6‰) and positive Δ33S (mean +0.040‰) values that are attributed to microbial sulfate reduction (MSR) and/or microbial sulfur disproportionation (MSD). Samples from the lower part of the Chokier Formation display less negative δ34S (mean −9.7‰) and negative Δ33S (mean −0.029‰) values. Paired δ34S-Δ33S values are located on a mixing curve between two endmembers: (1) sulfur generated by MSR and/or MSD, and (2) seawater sulfate. This latter observation is attributed to a restricted seawater exchange that limited the sulfate supply, thus, promoting closed-system conditions. Consequently, sulfur isotope values approached the isotopic composition of contemporaneous seawater.Trace element abundances exhibit a stronger enrichment in the upper Chokier Formation (EFMo=9–77, EFV=1–4; EF=Enrichment Factor) than in the lower Chokier Formation (EFMo=16–21, EFV ≈1). However, a hydrographic rather than a redox control is inferred for both sections based on Mo/TOC≤15 and Mo/U ratios well below seawater values. For the upper Chokier Formation, Mo/TOC≈15 indicates a moderately restricted basin, whereas the lower part is more strongly restricted (Mo/TOC≪15).Multiple sulfur isotopes and trace element abundances/ratios reveal a consistent picture of restricted basinal conditions during deposition of the lower Chokier Formation and more open conditions for the upper Chokier Formation. Hence, it can be concluded that results support the application of multiple sulfur isotope analyses for revealing paleoenvironmental conditions.