Diagenesis of magnetic minerals in Recent haemipelagic sediments

Abstract

Rapidly deposited sediments from marine and lake environments are being used increasingly to study decadal to millennial fluctuations in the Earth's magnetic field. The objectives are to gain more fundamental understanding of the geodynamo and to establish a new dating technique for sediments. While lacustrine sections are generally restricted to temperate latitude glacial lakes (≤ 20,000 yr old), rapidly deposited marine sediments along continental margins potentially offer continuous high resolution, yet long-term records of geomagnetic secular variation. However, to interpret the sedimentary magnetic record accurately, geochemical processes that affect the reliability of the magnetic signal must be understood. We now report downcore magnetic profiles from undisturbed Kasten cores taken in rapidly deposited laminated sediments from the Gulf of California and in bioturbated haemipelagic muds on the Oregon continental slope which give apparently reliable directions, but show dramatic decreases in the intensities of natural (NRM) and artificial (ARM, IRM) remanences with depth. Downcore porewater and solid sulphur analyses show concave-down decreases in porewater sulphate and systematic increases in pyrite and metastable monosulphides. The maximum curvature of the sulphide profile occurs directly below the high magnetization zone. Combined with other compositional and mineralogical analyses, these data suggest that due to oxidative decomposition of organic matter, magnetites and other iron oxides become progressively reduced and subsequently sulphidized and pyritized with depth. Iron reduction seems to occur before sulphide formation. Changes in magnetic stability parameters are consistent with selective dissolution of the finer-sized grains causing downcore coarsening of the magnetic fraction.