TRACKING DIAGENETIC ALTERATION OF MAGNETIC SUSCEPTIBILITY IN METHANE HYDRATE-BEARING MARINE SEDIMENTS OF THE CASCADIA MARGIN (ODP LEG 204 AND IODP EXPEDITION 311)

Magnetic susceptibility (κ) is a mixed signal in marine sediments, representing primary depositional and secondary diagenetic processes. Production of hydrogen sulfide via anaerobic oxidation of methane at the sulfate-methane transition (SMT) and organoclastic sulfate reduction above the SMT can result in the dissolution of iron oxides and precipitation of iron sulfides, altering κ in marine sediments. We investigated records of κ from gas hydrate-bearing sites on the Cascadia margin (ODP Sites 1249 and 1252; IODP Site 1325) using a heavy mineral proxy from X-ray fluorescence core scanning to identify intervals of primary detrital κ and predict intervals affected by dissolution of magnetite. We interpret these intervals of lower-than-expected κ in the context of measured total sulfur (TS) content, grain size distributions, total organic carbon (TOC) content, and rock magnetic properties.

At Site 1249, a seep site at the crest of Hydrate Ridge, κ is lower than predicted for almost the entire upper 90 m below seafloor (mbsf), mostly corresponding to high TS, and some intervals with higher κ correspond to magnetic iron sulfides. At Site 1252, in a slope basin adjacent to Hydrate Ridge, the upper 100 msbf contains repeated intervals of low κ over the majority of the cored record, correlated with elevated TS and TOC content. At Site 1325, κ is reduced relative to predicted detrital κ between 24-51 mbsf correlating to elevated TS. Across all sites, κ negatively correlates with total sulfur content.

Integrating heavy mineral proxies with rock magnetic and geochemical records allows for a prediction of primary κ and the amount of κ loss at each site when compared to actual κ measurements, and these predictions are supported by total sulfur results. These measurements highlight that decreases in κ due to magnetite dissolution and pyrite precipitation are likely widespread along the Cascadia Margin in methane-bearing sediments. This draw down in κ is more intense at sites with gas seeps and high saturations of methane hydrate. At non-seep sites with more disseminated, low concentration hydrate, alterations of κ are more episodic and correlated with TOC content. The results from these sites suggest that integrating magnetic and geochemical data with κ records can provide insight into the geologic evolution of hydrate systems.