Magnetic Mineral Assemblages of Diagenetically Reduced Sediments and Their Contributions to Paleomagnetic Signals

Abstract

During early diagenesis of marine sediments, iron-bearing minerals undergo a series of redox reactions until they reach equilibrium with reactive chemical components. Paleomagnetic records in the sediments subjected to severe diagenesis can be distorted or lost due to iron mineral dissolution, and hence these sediments were often excluded from paleomagnetic studies without detailed examination. Silicate-hosted magnetic inclusions and hematite are likely more resistive to reductive diagenesis compared with unprotected magnetite. Thus, these minerals have potential for preserving paleomagnetic records in reduced sediments. To better understand this issue, we conducted a paleo- and rock magnetic study of a sediment core taken from the Ontong Java Plateau in the western equatorial Pacific Ocean using various techniques including isothermal remanent magnetization (IRM) component analyses, first-order reversal curve diagrams, low-temperature magnetic measurements, thermal demagnetization of three-component IRM, chemical separation, and electron microscopy. The magnetite dissolution front occurs at 5.7 m in depth in the studied core. Below this horizon, silicate-hosted magnetic inclusions and hematite carry 46%–63% and 21%–34% of saturation IRM, respectively. Depositional remanent magnetization acquisition efficiency of silicate-hosted magnetic inclusions is estimated to be relatively low in the studied core based on the grain sizes of silicate hosts and concentration of magnetic inclusions. It was revealed that magnetostratigraphy and relative paleointensity correlative to the global stacks could still be recovered from the sediments below the dissolution front. Relict hematite may be an important carrier of the paleomagnetic records.