Magnetic mineral diagenesis in suboxic sediments at Bettis Site W‐N, NE Pacific Ocean

Abstract

The effects of iron diagenesis on paleomagnetism and rock magnetism were examined in suboxic sediments from Bettis Site W‐N in the NE Pacific Ocean. As part of the oxidative decomposition of organic matter, ultrafine‐grained authigenic magnetites are produced immediately above the iron reduction zone. This zone is commonly observed in deep‐sea sediments as a prominent color change from brown to greenish grey. The magnetite authigenesis is evidenced by systematic increases in natural magnetic intensities and stabilities and by comparable shifts in other rock magnetic properties. The magnetites are composed of relatively pure Fe 3 O 4 and fall within a restricted size range of 0.06–0.12 μm, which is nominally single domain. Some of these finest‐grained magnetites are rapidly destroyed during burial upon entering the zone of iron reduction, leaving a relatively coarser population. Magnetite dissolution appears to continue with depth according to a first‐order surface area reaction process when at steady state. Iron reduction is independent of sulfide formation in these sediments, because solid Sulfur profiles show no increase at depth and most of the sulfur occurs as barite. Despite a well‐constrained carbonate stratigraphy, comparisons of paleomagnetic directions between nearby cores show little agreement, suggesting that the directions are composed of a complex mixture of remanences acquired at different depths. Subsurface magnetic peaks may represent paleoredox markers preserved by non‐steady state redox conditions caused by climatically induced variations in organic matter flux and sedimentation rates.