Magnetic effects of maghemitization of Plio‐Pleistocene marine sediments, northern California

Abstract

A magnetic study conducted on reversed polarity, Plio‐Pleistocene marine sediments from the Rio Dell formation suggests that maghemitization occurring in the source area may have enhanced their paleomagnetic reliability. In a section characterized by variable amounts of magnetic overprinting, some of the reliable horizons had natural remanent magnetization (NRM) intensities nearly 1 order of magnitude greater than other reliable and all the unreliable horizons from the Rio Dell. These high‐NRM horizons had high anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM) intensities, suggesting a greater concentration of magnetic minerals. Thermal demagnetization data indicate that maghemite is the magnetic carrier in the high‐NRM horizons. ARM versus χ and Lowrie‐Fuller tests show that these horizons have magnetic grains dramatically smaller than magnetite‐bearing horizons. Contraction cracking during maghemitization may have reduced the effective grain size of the maghemite‐bearing horizons. The small magnetic grain size of the maghemite increases these horizons' coercivity and viscous acquisition coefficient but not their JVRB. Sediment grain size analyses show that the magnemite‐bearing horizons have the smallest sediment grains and unreliable magnetite‐bearing horizons have the largest grains. Assuming hydrodynamic sorting of the magnetic oxides, these data suggest that the finest‐grained magnetic minerals are the most susceptible to oxidation. Maghemite occurs in horizons 1.5–0.9 m.y. old. Reduced chemical and increased mechanical weathering during the Pleistocene glaciations could have increased the concentration of magnetic iron oxides in these sediments and caused their maghemitization in the provenance area.