Comparison with existing data
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

The sulfur isotopic composition of a few total reducible inorganic sulfur (TRIS; n = 5) samples at the ODP 1123 site has been measured on board the research vessel and reported (71). As these data have not been released in tabulated form, to allow comparison with our measurements, we digitized figure 5 of (71) using DigitizeIt (http://digitizeit.de/). To account for the uncertainties associated with such an exercise, we performed three independent digitizations and report the mean and SD of the three replications (table S3).

As shown in fig. S3, there is overall good agreement between our data (in blue) and the sparser TRIS data (in purple), although we do observe a few per mil differences between the two datasets at depths where they overlap. Differences in the reference frame between the datasets and inaccuracies because of the data digitization are possible but minor compared to the differences in the reported δ34Spyr values. We suggest that the observed mismatch may reflect the freshness of the on-board samples relative to our “aged” samples. The fresh samples probably contained porewater H2S and/or AVS (mostly as iron monosulfides), both of which are sensitive to oxidation and would not survive long-term exposure to oxygen during storage. We performed a few exploratory acidifications of samples from both cores, which yielded immeasurable amounts of AVS. Consequently, in the rest of the samples, we did not attempt to extract AVS, and all reduced sulfur was extracted as CRS. We suggest that our CRS extractions were composed mostly of pyrite, whereas the on-board TRIS results also contained H2S and/or AVS. Isotopic differences between these sulfur species are the likely cause of differences in reported δ34Spyr values. Despite the effect of years of exposure to oxygen on the pyrite abundance, the negligible sulfur isotopic fractionation associated with aerobic oxidation of pyrite (72) allows interpretation of the sulfur isotopic variability preserved in the remaining pyrite as negligibly altered [e.g., (73)].

One main point of disagreement seems to be preserved in the data point immediately overlying the Marshall Paraconformity, which is the only one that has a δ34Spyr value that is more negative than our nearest sample (by ~10‰). Without access to complementary analyses from the on-board study (e.g., sediment description–TOC–δ13Corg–Fe speciation), we cannot confidently discuss the possible causes of this mismatch. Adding to the uncertainty, this single sample appears to be deposited at the onset of drift deposition (i.e., immediately after the Marshall Paraconformity). This is a time when one may expect more energetic conditions, more frequent and more intense reworking, and also greater uncertainty in the age model.

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