Magnetic diagenesis, involving a sequence of iron reduction steps resulting in initial authigenesis of magnetite and progressing through reduction to greigite and on to pyrite, can be traced in Leg 164 sites by downhole changes in susceptibility, the hysteresis parameter D (sub JH) , and coercivity spectra. An undisturbed reference site on the Blake Ridge (Site 995) shows an initial increase in susceptibility between 0 and 4 mbsf, consistent with authigenesis of fine-grained magnetite and greigite, followed by almost complete reduction to pyrite below 4 mbsf. Site 991 shows a similar pattern restricted to Unit I; Unit II, although it commences at only 2 mbsf, is thoroughly reduced, reflecting the removal of material in a slide prior to the deposition of Unit I. At Site 992 the authigenesis of magnetite in Unit I has a different character; it is dominated by single-domain magnetite, suggesting that the multiple remobilization and redeposition of Unit I sediments at this site may have encouraged the growth of slightly larger magnetite grains, perhaps by incorporation of additional sulfate in the redeposited sediments. Unit I sediments at Site 992 have never been exhaustively reduced, suggesting that they were not buried by more than about 4 m prior to remobilization. In contrast, the entire sequence at both Sites 993 and 996 are thoroughly reduced. At Site 993 this reflects the exhumation of the site, which was previously estimated by physical properties (shear strength) measurements to be 50-80 m. Site 996 has not been exhumed significantly; instead reduction here has proceeded extensively even near the seafloor because of the supply of methane along a fault.