Magnetite diagenesis in marine sediments from the Oregon continental margin

Abstract

The magnetochemistry of sediments from the Oregon continental margin is examined to determine the effects of iron‐sulfur diagenesis on the paleomagnetic record. Magnetic mineral dissolution and transformation into iron sulfides are a common feature in these suboxic to anoxic lutites. These processes are evidenced in rapid decreases in natural remanent magnetization intensities and stabilities, systematic changes in other rock magnetic properties, and increases in solid phase sulfur concentrations with depth. Hysteresis measurements are used to evaluate downcore changes in magnetite concentration and grain size. Magnetite abundances decrease downcore from initial values of about 0.1%, and nominal grain diameters lie within a narrow pseudosingle domain range of 0.08 to 0.6 μm. A first‐order surface area reaction model, dA/dt = ‐kA, is proposed to explain the magnetite dissolution mechanism, where A is the total magnetite surface area and k is the rate constant. The solution of this equation predicts that the surface area and concentration decrease exponentially, and the concentration, in addition, depends on grain size. Application of this model in two cores where grain size varies with depth successfully explains the downcore profiles of both concentration and surface area. Despite extensive magnetite destruction, magnetic directions in such sediments appear to reliably record long‐wavelength trends of the geomagnetic field.