At Site 991, the break between the undisturbed Unit I and the strongly disturbed Unit II, which includes soft-sediment deformation features and clay conglomerates, is taken to represent the sole of the last slide to affect this site. Sediments below this slide have overconsolidated physical properties; in rock magnetism, the time-break between the deposition of Units I and II appears as a small discontinuity in the reduction pattern expressed by the susceptibility record, so that Unit II sediments have completed the magnetite-greigite-pyrite reduction sequence at a depth 2 m shallower than at the reference Site 995.
Disturbance is different at Site 992; here, Unit I is itself composed of clay conglomerates, involving soft-sediment deformation, displacement and redeposition. Rock magnetism suggests that authigenesis in Unit I at Site 992 results in an SD-dominated ferrimagnet population (DJH around 0.5). This contrasts with Unit I at Site 991 and the interval above 2 mbsf at Site 995, where DJH values do not indicate authigenic addition of SD-sized ferrimagnets. The magnetic diagenesis in Unit I at Site 992 may represent a more extreme form of the situation that appears to apply at around 3 mbsf at Site 995, where the second peak in susceptibility seems to involve generation of some SD- or near-SD-sized greigite. It is not clear why growth of large proportions of SD-sized grains should characterize Unit I at Site 992. It may be that the multiple displacements and assemblies of the sequence in Unit I at Site 992 have refreshed the supply of sulfate through to about 6 mbsf, making more sulfate available than was supplied at other sites from initial deposition and later diffusion. This may have encouraged a different style of bacterial activity (perhaps involving a different balance of species or metabolic pathways), resulting in greater proportions of larger magnetite and/or greigite grains, in the SD-size range. What is clear is that the redeposited sediments in Unit I at Site 992 had not already been fully reduced prior to redeposition; hence, these sediments have not been buried more than 4 mbsf prior to remobilization. This appears to conflict with physical properties evidence (Shipboard Scientific Party, 1996c), suggesting about 4 m of overburden have been removed at Site 992. It may be that the increased strength of the Site 992 sequence results from transport and redeposition mechanisms, rather than significant prior burial.
In contrast, the entire sequence at Site 993 has been completely reduced prior to its later exhumation, consistent with physical properties evidence for the stripping of 50-80 m of overburden.
Near-surface reduction is also unusually complete at Site 996, but this does not reflect removal of an overburden. Instead, the supply of methane via the fault at this site, greatly exceeding local microbial methane production or supply by normal diffusion, has pushed the magnetite-greigite-pyrite reduction series close to complete reduction to pyrite even at very shallow depths below the seafloor.