Iron speciation analyses
This protocol is extracted from research article:
Strong local, not global, controls on marine pyrite sulfur isotopes
Sci Adv, Feb 26, 2021; DOI: 10.1126/sciadv.abb7403

Iron speciation measurements were made at the Weizmann Institute of Science using the calibrated extraction of (63), as modified for application to recent sediments [e.g., (64, 65)]. The extracted fractions are defined operationally, after (66, 67). The first step consists of a 0.5 N HCl extraction for 1 hour, targeting ferrous iron (FeII) phases, including AVS, surface-bound FeII, and FeII carbonate/phosphate phases. As the sulfur extractions indicated that the AVS component is negligible in our samples, we assume that no FeII is associated with iron monosulfides and assign the FeIIHCl pool to nonsulfidized FeII minerals (i.e., mostly carbonates). The 0.5 N HCl also extracts the most reactive FeIII minerals in the sediment (termed FeIIIHCl), in particular, ferrihydrite. Because of its rapid kinetics of sulfidization (68) and transformation to more crystalline FeIII oxides, it is highly unlikely that the ferrihydrite extracted by 0.5 N HCl was a part of the original sediment. Instead, we consider the FeIIIHCl fraction in this core, which was not stored under anoxic conditions, to most likely represent the oxidation product of pyrite. Accordingly, the FeIIIHCl fraction was added to the chromium-reducible fraction to correct the pyrite abundance for postsampling oxidation. A second sequential extraction step uses a buffered Na-dithionite solution to extract more crystalline FeIII (oxyhydr)oxide minerals, such as goethite and hematite (termed Fedi-ct). A third extraction step targets magnetite, using Na-oxalate (termed Feoxa). A fourth extraction step targets pyrite by reduction with chromium (termed FeCRS). The extraction is similar to that described above for CRS (62). As mentioned above, we added FeIIIHCl to FeCRS to estimate the original amount of pyrite present in the cores (termed Corr.Fepyr).

The FeIIHCl, Fedi-ct and Feoxa and FeCRS fractions were determined by spectrophotometry using a ferrozine assay (69), immediately after completion of the extraction. Determination of the FeIIIHCl abundance requires reduction of all FeIII to FeII, which is achieved by reaction with ascorbic acid. The total FeHCl is then measured by spectrophotometry, and the FeIIIHCl abundance is determined by subtraction of FeIIHCl from the total FeHCl (69).

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