Magnetic Mineral Diagenesis in a High Temperature and Deep Methanic Zone in Izu Rear Arc Marine Sediments, Northwest Pacific Ocean

Abstract

Magnetic mineral diagenesis is an important process in sediments that is responsible for the partial or total destruction of records of Earth's magnetic field variations and also plays an important part in iron and sulfur cycling. A rock magnetic study has been carried out at International Ocean Discovery Program Expedition 350 Site U1437 in the Izu Bonin rear arc to investigate magnetic mineral diagenesis in deeply buried sediments from ~775 to ~1,002 m below sea floor (mbsf) with burial temperatures ranging from ~67 to 85 °C. Nonsteady state geochemical environments occur within an unusual deep methanic zone (below 850 mbsf) because of a release of sulfate below the shallow sulfate reduction zone. A drastic decline in magnetic susceptibility and remanence is observed at ~850 mbsf due to a decrease in ferrimagnetic iron oxide contents. Reduction of (titano)‐magnetite and pyritization occur at this inferred deep sulfate‐methane transition zone. Below ~850 mbsf, lower magnetic mineral contents coincide with further methane release. Geochemical analyses support a change in redox conditions and secondary precipitation of iron sulfides and carbonates in the methane‐rich zone. Magnetic mineral alteration is enhanced in zones where methane accumulates underneath low porosity intervals that act as seals. Although geochemical processes due to methane occurrence are likely dominant, more than one mechanism, possibly involving microbial activity, is probably responsible for the observed magnetic mineral assemblage changes.

Progressive and Punctuated Magnetic Mineral Diagenesis: The Rock Magnetic Record of Multiple Fluid Inputs and Progressive Pyritization in a Volcano‐Bounded Basin, IODP Site U1437, Izu Rear Arc

“…Sulfidic alteration in response to AOM has been reported to generate monoclinic pyrrhotite (Kars & Kodama, ; Larrasoaña et al, ). No unambiguous rock‐magnetic evidence for pyrrhotite is provided in any samples from this study, but Kars et al () in their study of Site U1437 recognized a candidate for the Besnus transition of pyrrhotite (Dekkers et al, ; Rochette et al, ) at ~32 K in ZFC LTSIRM warming curves for samples in intervals with lower methane contents within the overall methane‐rich zone between fluid anomalies 6 and 8. Diagenetic pyrrhotite which forms in methanic sediments has been reported to lack a Besnus signature (Horng & Roberts, ), so the presence of diagenetic pyrrhotite in the methane‐rich zones cannot be ruled out.…”

“…This enhanced pyritization in the gas hydrate setting has been suggested to be the result of an accumulation of H 2 S in company with methane (“sour gas”) below an impermeable seal formed by gas hydrate filling porosity (Housen & Musgrave, ). We consider likewise that H 2 S‐rich horizons are responsible for the D JH notches at Site U1437, with the seals in this case provided by intervals of less porous clay (Kars et al, ).…”

“…Higher concentrations of hydrocarbons in the intervals below anomalies 7 and 9 appear to be responsible for SIRM/κ values above the trend line, and may be acting to renew greigite growth. Kars et al () suggested the formation of relatively fine, SP greigite in the approximately 100‐m interval below anomaly 7, but at least some of this must be in the SD range to explain the elevated SIRM/κ values.…”

“…The stratigraphic age through most of the section is well constrained by a combination of magnetostratigraphy and biostratigraphy, and a detailed age model combining biostratigraphic, magnetostratigraphic, and oxygen isotope data for the last 1 Myr has been produced (Mleneck‐Vautravers, ). Climate controls on nonsteady state magnetic diagenesis from the uppermost 120 m of the sedimentary record at Site U1437 have already been reported (Kars et al, ), as has a detailed description of the late magnetic mineral diagenesis associated with a deep methane‐rich zone between 850 and 1,000 mbsf (Kars et al, ), while the challenges presented in the recognition of magnetostratigraphy at Site U1437 have been presented by Musgrave and Kars ().…”

“…Headspace gas profiles provide additional evidence of fluid transport below the depth at which recovery of interstitial water became impracticable (700 mbsf). Methane, which is present in markedly higher concentrations in the interval from 750 to 1,035 mbsf, may have been locally sourced, but the correlation between total organic carbon contents and gas concentrations is poor (Kars et al, ), favoring gas migration. Ethane appeared above detection limits for the first time in the first core below an interpreted fault at the break between the bottom of Hole U1437D and the beginning of coring at the same subbottom depth in Hole U1437E (Musgrave & Kars, ; Tamura et al, ).…”