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 lxansformation into iron sulfides are a common feature in these suboxic to anoxic lutites. These processes are evidenced in rapid decreases in natural reinanent 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 concenlxation 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 !xm. 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 concenlxation decrease exponentially, and the concenlxation, 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 concenlxation and surface area. Despite extensive magnetite deslxuction, magnetic directions in such sediments appear to reliably record long-wavelength lxends of the geomagnetic field. Consequently, the potential for using rock magnetism as a sedimentological tool has not been fully developed. In recent years, a new generation of sediment paleomagnetic studies has begun to explore the possibilities of using rock magnetic properties as proxy indicators of climatic change to explore the expression of Milankovitch cyclicities in the sediment record as well as to delineate patterns of paleocean and paleowind circulation [Mead et al., 1986, Ledbetter and Ciesielski, 1986; Doh et al., 1988]. Since magnetic techniques are rapid, nondestructive, and very sensitive, rock magnetism is becoming increasingly important in examining long sedimentary sections such as recovered by the Ocean Drilling Program. Rock magnetic variations in sediments reflect changes in the composition, concentration, and grain size of magnetic minerals, especially magnetite. In a given section, downcore trends arise from changes in provenance of the magnetic minerals, dilution effects from other sources (particularly biogenic), depositional perturbations, and any postdepositional modifications of the original remanence due to authigenesis or diagenesis. Before the potential of rock magnetism as a paleoclimatic tool can be fully realized, the interaction of magnetic phases with physical and chemical processes in different depositional environments must be understood. Very few studies have addressed the causes of magnetic fluctuations in sediments or the biogeochemical and physical processes responsible for acquisition, fixation, and preservation of remanence. However, there is a growing realization that the amb...

A biogenic origin for anomalous fine‐grained magnetic material at the Paleocene‐Eocene boundary at Wilson Lake, New Jersey

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Paleoclimatic and diagenetic history of the late quaternary sediments in a core from the Southeastern Arabian Sea: Geochemical and magnetic signals

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