Resolving micron-scale heterogeneity in porewater δ³⁴S_(H₂S) by combining films for in-situ sulfide capture and secondary ion mass spectrometry

Abstract

Sulfur cycling is ubiquitous in marine sedimentary environments and is influenced by microbial and abiotic processes that alter both the abundance and isotopic composition of sulfur species that can ultimately be captured as sedimentary minerals. Microbial metabolisms that generate sulfur isotopic (δ³⁴S) signatures in hydrogen sulfide have a spatial distribution that varies on the micron scale, yet porewater hydrogen sulfide is most often measured in bulk samples representing much larger volumes. This mismatch of scales can lead to erroneous or non-unique interpretations of biogeochemical processes and environmental conditions. Recently, an in-situ film-based technique was described that captures dissolved sulfide (H₂S) in porewaters and which can be subsectioned to reconstruct the δ³⁴S_(H₂S) profiles on the sub-cm scale within sediments. Here, we investigate the use of a Cameca 7f-GEO secondary ion mass spectrometer (SIMS) to analyze the δ³⁴S_(H₂S) captured from porewaters on these films on even smaller spatial scales and particularly in films with low sulfide abundance that could not otherwise be processed with bulk extraction techniques. We present a best-practice method for film analysis that minimizes analytical artifacts from varying sulfide abundance and interactions with silver halide nanocrystals imbedded in the organic-based film amalgam. This method was tested on several films from field deployments, including examples with heterogeneities on small (~100 μm) scales, steep isotopic gradients, and very low sulfide abundance across the sediment-water interface. The results demonstrate that analysis using SIMS can accurately measure δ³⁴S of in-situ sulfide captured by film with high precision (1σ ~ 0.3‰) in both spot and image modes and that the film itself can accurately record δ³⁴S variability down to 25 μm spatial resolution, below which physical limitations of the film can create artifacts.