Tracking Sulfur Diagenesis in Methane Rich Marine Sediments on the Cascadia Margin: Comparing Sulfur Isotopes of Bulk Sediment and Chromium Reducible Sulfur

摘要

Methane gas is produced in anoxic marine sediments by methanogenic bacteria and can be ephemerally stored in gas hydrate deposits, escape to the seafloor at methane seeps, and/or be consumed at depth by the anaerobic oxidation of methane (AOM). One way to examine changes in methane flux in cold seep environments through time is to identify past positions of the sulfate methane transition zone (SMTZ) where AOM results in sulfate and methane consumption and bicarbonate and hydrogen sulfide production, often resulting in the precipitation of authigenic carbonates and iron sulfides. One method to identify paleo-positons of the SMTZ is through the sulfur isotopic composition of sulfides produced by AOM, which are typically enriched in 34S relative to sediments not influenced by AOM. Traditionally, a chemical extraction technique called chromium reduction has been used to extract reduced forms of sulfur, mainly pyrite and other iron sulfides, from the sediment. This separates only reduced sulfur from sediments leaving behind sulfur bound to organic matter and from oxidized sulfur species such as barite. In this study, bulk sediment delta 34S values are compared to measured delta34S from chromium reducible sulfur (CRS) to assess the utility of using bulk sediment delta 34S measured alone to investigate paleo-diagenetic conditions in methane rich marine sediments. The upper ~25 meters of sediment at three ODP core sites (1252A, 1247B, and 1244C) from Hydrate Ridge on the central Cascadia margin are examined. In addition to bulk sediment and CRS delta34 S, total sulfur and total organic carbon were also measured. The results reveal the bulk sediment is generally more enriched in 34S, compared to the CRS, except for a few samples at Site 1247B and some larger intervals at Site 1244C. At Site 1247B the greatest weight percent of total sulfur occurred at the modern SMTZ, and at Sites 1252A and 1244C the highest weight percent of sulfur occurred above the modern SMTZ, coincident with the most enriched (heaviest) bulk sediment sulfur isotope values at these two sites. Peaks in the delta34S value of the bulk sediment and total sulfur weight percent could be due to the presence of barite, making the bulk sediment in these locations more enriched in 34S than the CRS. In the majority of the sedimentary records examined here, the intervals where the chromium reducible sulfur has a heavier delta34S composition than the bulk sediment could indicate that the sediment has experienced intense AOM, leaving the iron sulfides to form from the heaviest hydrogen sulfide. This would make the delta34S value of the bulk sediment lighter than that of the highly enriched iron sulfides. There is a positive, linear relationship between the delta34S values of the CRS and the bulk sediment, which shows that the delta34S value of the bulk sediment is strongly influenced by the sulfur isotope composition of the CRS portion of the sediment. While the bulk sediment did not have the same delta34S values as the CRS it often showed the same trends and may be helpful in assessing the extent of AOM in methane rich marine sediments. Further comparisons of the sulfur isotope composition of sedimentary iron sulfides and bulk sediment, as well as other sulfur containing species, at other locations could indicate if the relationship observed at Hydrate Ridge exists in other methane rich seafloor environments.