Geochemical and sedimentological control of the magnetic properties of hemipelagic sediments

<jats:p>Owing to their high sedimentation rates and wide areal extent, hemipelagic sediments along continental borderlands are potentially important high‐resolution recorders of geomagnetic secular variation. To assess this possibility, we studied suboxic hemipelagic muds from the Oregon continental slope and anoxic laminated diatomaceous oozes from the Gulf of California. These sediments were rapidly deposited, with average sedimentation rates of 121 and 135 cm/kyr, respectively. Bulk sediment sedimentological and geochemical analyses indicate that the two areas represent contrasting depositional regimes. The remanence in both environments resides in fine‐grained magnetite particles, although minor amounts of hematite were observed in the topmost Gulf of California sediments. Despite first‐order differences, the sediments from both areas show similar downcore patterns of systematic increases in solid sulfur (mainly as pyrite), dramatic decreases in natural (NRM), anhysteretic, and isothermal remanent magnetization, intensities, and accompanying shifts in the magnetic stabilities. These changes are consistent with reduction and dissolution of the ferrimagnetic iron oxides with depth due to early diagenesis of organic matter. The magnetic grain size distribution first appears to rapidly coarsen downcore as the smallest and most abundant grains are removed, then slowly grows finer as the remaining particles dissolve. No evidence was observed for authigenic formation of magnetite. Provided sufficient concentration and stability of the detrital magnetic particles, paleomagnetic directions can survive this dissolution diagenesis. However, relative paleointensity determinations based on normalization of NRM by concentration in related parameters will give erroneous estimates of paleofield behavior in such diagenetically altered sediments.</jats:p>