Ž . Earth and Planetary Science Letters 139 1996 509–519 Rock-magnetic signature of gas hydrates in accretionary prism sediments Bernard A. Housen a,  , Robert J. Musgrave b a Institute for Rock Magnetism, UniÕersity of Minnesota, 293 Shepherd Laboratories, 100 Union Street SE, Minneapolis, MN 55455-0128, USA b School of Earth Sciences, La Trobe UniÕersity, Bundoora, VIC 3083, Australia Received 21 November 1995; accepted 25 November 1995 Abstract 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 Ž  4  4. Ž  4 . combining coercivity, remanence, and susceptibility parameters, D  J J  H H and D  J k H , when JH rs s cr c S rs cr combined with thermo-magnetic data, can be diagnostic of these changes. At Site 892, D values are distinctly higher and S Ž . 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, D shows two trends: an increase from about 50 mbsf to the BSR at 73 mbsf, JH 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 D JH increases downhole to about 285 mbsf, substantially below the BSR at 225 mbsf. High D sediments within a low Cl  JH 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 Ž  . 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. The observed changes in magnetic properties can be attributed to steps in the reduction series from magnetite through SD Ž . Ž 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 Ž . isothermal remanent magnetization mIRM indicates the presence of a low-coercivity magnetic mineral with an unblocking Ž . temperature T between 310° and 350°C. High J k ratios suggest that the low-coercivity, low unblocking temperature ub rs mineral is predominantly greigite rather than pyrrhotite. A low- to medium-coercivity mineral with T ca. 580°C— ub magnetite — is also present in varying amounts. Hydrate apparently controls the presence of greigite by incorporating H S, shown to be present as a hydrate phase 2 together with methane in hydrate recovered at Site 892. Release of H S below the base of the hydrate layer allows 2 overgrowth of greigite grains to MD size or the conversion of some of the greigite to pyrite. Keywords: ODP Site 892; accretionary wedges; gas hydrates; remanent magnetization; greigite  Corresponding author. E-mail: house009@gold.tc.umn.edu 0012-821X96$12.00  1996 Elsevier Science B.V. All rights reserved Ž . SSDI 0012-821X 95 00245-6