Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging: Extracting local paleo‐environmental information from modern and ancient sediments

Abstract

Rationale Sulfur isotope ratio measurements of bulk sulfide from marine sediments have often been used to reconstruct environmental conditions associated with their formation. In situ microscale spot analyses by secondary ion mass spectrometry (SIMS) and laser ablation multiple‐collector inductively coupled plasma mass spectrometry (LA‐MC‐ICP‐MS) have been utilized for the same purpose. However, these techniques are often not suitable for studying small (≤10 μm) grains or for detecting intra‐grain variability. Methods Here, we present a method for the physical extraction (using lithium polytungstate heavy liquid) and subsequent sulfur isotope analysis (using SIMS; CAMECA IMS 7f‐GEO) of microcrystalline iron sulfides. SIMS sulfur isotope ratio measurements were made via Cs+ bombardment of raster squares with sides of 20–130 μm, using an electron multiplier (EM) detector to collect counts of 32S− and 34S− for each pixel (128 × 128 pixel grids) for between 20 and 960 cycles. Results The extraction procedure did not discernibly alter pyrite grain‐size distributions. The apparent inter‐grain variability in 34S/32S in 1–4 μm‐sized pyrite and marcasite fragments from isotopically homogeneous hydrothermal crystals was ~ ±2‰ (1σ), comparable with the standard error of the mean for individual measurements (≤ ±2‰, 1σ). In contrast, grain‐specific 34S/32S ratios in modern and ancient sedimentary pyrites and marcasites can have inter‐ and intra‐grain variability >60‰. The distributions of intra‐sample isotopic variability are consistent with bulk 34S/32S values. Conclusions SIMS analyses of isolated iron sulfide grains yielded distributions that are isotopically representative of bulk 34S/32S values. Populations of iron sulfide grains from sedimentary samples record the evolution of the S‐isotopic composition of pore water sulfide in their S‐isotopic compositions. These data allow past local environmental conditions to be inferred.

Shifting modes of iron sulfidization at the onset of OAE-2 drive regional shifts in pyrite δ34S records

“…The contents of these tubes were mixed for 1 min using a vortex mixer, then placed in an ultrasonic bath (35 kHz) for 15 min to de-flocculate the powders. Following Bryant et al (2019), the tubes were spun in a centrifuge for 38 min at 3000 rpm to allow Fe sulfide grains of diameters ≥0.5 µm to settle out, thus maintaining the original size distribution of iron sulfide grains. After centrifugation, heavy fractions were removed from the tubes using a plastic micropipette, placed in new 50-mL centrifuge tubes, rinsed, and centrifuged (5 min at 2000 rpm) five times in deionized water, and dried overnight at 60°C in a vacuum oven.…”

“…δ SCRS values can reflect variable contributions by multiple generations of mineral growth (Cui et al, 2018) and can be further complicated by the presence of multiple mineralogies (e.g., pyrite and marcasite; Bryant et al, 2019). Sulfide minerals record an integrated value representing the  34 S of aqueous sulfide during their formation.…”

“…Recorded δ 34 SCRS values can thus reflect local environmental factors (e.g., the 'openness' of the environment of mineral formation with respect to sulfate, or the abundance and reactivity of iron minerals; Bryant et al, 2019;Fike et al, 2015) and ecological factors [e.g., the magnitude of the biological fractionation associated with microbial sulfate reduction, εmic (Leavitt et al, 2013;Sim et al, 2011bSim et al, , 2011aWing and Halevy, 2014)]. Additionally, late-stage fluids can produce highly 34 S-enriched pyrites, often with irregular textures such as overgrowths and cements (Berelson et al, 2018;Bryant et al, 2019;Cui et al, 2018;Raiswell, 1982). Any single or a combination of these factors could have contributed to the observed δ 34 SCRS decrease toward the onset of OAE-2 at Demerara Rise and cannot be ruled out based on bulk δ 34 SCRS data alone.…”

“…Here, we use spatially resolved S isotope analyses to assess regional C, S, and Fe cycling. A scanning ion imaging method of secondary ion mass spectrometry (SIMS) was recently developed for microcrystalline Fe sulfides, making it possible to investigate both inter-and intra-grain variability within the CRS pool (Bryant et al, 2019). By applying this method in a comparative study of Fe sulfides from pre-and syn-OAE-2 sediments at Demerara Rise, we can establish the various contributions to and mechanisms responsible for the observed secular decrease in δ 34 SCRS values.…”

An improved femtosecond laser‐ablation fluorination method for measurements of sulfur isotopic anomalies (∆33S and ∆36S) in sulfides with high precision

“…Several methods and techniques have been developed to determine the multiple isotope ratios of sulfur. These are conventional methods by fluorination of sulfide minerals in nickel reaction vessels to produce SF 6 for sulfur isotopic analyses by isotope ratio mass spectrometry (IRMS), fluorination methods with laser heating of samples, a laser‐ablation (LA) technique for direct solid sampling in combination with multicollector inductively coupled plasma mass spectrometry (LA‐MC‐ICP‐MS), and in situ spot analyses by secondary ion mass spectrometry (SIMS) . These methods are currently used to examine S‐MIF signals in the geological record.…”