Rock-magnetic signature of gas hydrates in accretionary prism sediments

Sediments from two Ocean Drilling Program Leg 146 sites from the Cascadia margin of western North America have magnetic properties indicating diagenesis of magnetic minerals associated with the presence of gas hydrates. Two indices combining coercivity, remanence, and susceptibility parameters, DJH ( = {Jrs/Js}{Hcr/Hc”) and DS ( = {Jrs/k}Hcr), when combined with thermo-magnetic data, can be diagnostic of these changes. At Site 892, DS values are distinctly higher and more scattered above the bottom simulating seismic reflector (BSR), which marks the base of the hydrate stability zone. Within the hydrate stability zone at Site 892, DJH shows two trends: an increase from about 50 mbsf to the BSR at 73 mbsf, corresponding to an expected increase in hydrate concentration near the BSR; and a second increase upwards from 50 mbsf to peak values at less than 21 mbsf, associated with hydrate recovered in cores above 19 mbsf. At Site 889/890 DJH increases downhole to about 285 mbsf, substantially below the BSR at 225 mbsf. High DJH sediments within a low Cl− zone at this site have magnetic mineralogies which are dominated by fine-grained magnetic sulfides, whereas sediments from above and below this zone are characterised by magnetite-magnetic sulfide mixes. This trend at Site 889/890 is consistent with an interpretation based on pore-water geochemistry (low Cl−) and bottom-water temperature that a ‘fossil gas hydrate zone’ extended downwards to about 295 mbsf during the last glacial. served changes in magnetic properties can be attributed to steps in the reduction series from magnetite through SD greigite to pyrite (or to overgrowth of SD greigite to MD size). Diagenetic growth of magnetic iron sulfides (greigite and/or pyrrhotite) has been reported in other accretionary wedge sediments. Thermal demagnetization of multi-component isothermal remanent magnetization (mIRM) indicates the presence of a low-coercivity magnetic mineral with an unblocking temperature (Tub) between 310° and 350°C. High Jrs/k ratios suggest that the low-coercivity, low unblocking temperature mineral is predominantly greigite rather than pyrrhotite. A low- to medium-coercivity mineral with Tub ca. 580°C — magnetite — is also present in varying amounts. e apparently controls the presence of greigite by incorporating H2S, shown to be present as a hydrate phase together with methane in hydrate recovered at Site 892. Release of H2S below the base of the hydrate layer allows overgrowth of greigite grains to MD size or the conversion of some of the greigite to pyrite.