Identifying chemical remanent magnetization in marine sediments via anhysteretic magnetization: Case study from the Bermuda Rise, North Atlantic Ocean

Paleomagnetic studies of marine sediments often assume that the remanence acquisition process occurs via torque from the Earth's magnetic field called a depositional remanent magnetization (DRM), which implies that paleomagnetic recording is time-transgressive. However, magnetic minerals can also grow authigenically in the sediment, leading to a chemical remanent magnetization (CRM), which is not necessarily time-transgressive. Not only are the physics of the DRM and CRM acquisition processes different, but their remanence lock-in times are likely non-contemporaneous, potentially leading to misinterpretations of the magnetic signal. The question arises how to distinguish the two? The hallmark of a detrital remanence can be expressed by the anisotropy of remanence. Since single domain prolate grains should be distributed statistically parallel to the magnetic field direction, the maximum anisotropy axis should also be in the magnetic field direction, as suggested by our theoretical models and confirmed by our experimental data. But what about CRM? To test this, we applied a wide range of experiments using anhysteretic methods with the automated SushiBar system on Bruhnes-Matuyama-aged marine sediments from the Bermuda Rise (ODP Site 1063). Previous work assumed the remanence in these sediments was carried solely by magnetite; however, we found evidence for a gyroremanent magnetization (GRM), which is commonly attributed to greigite. Of interest is that these samples, as well as neighboring samples or other intervals that do not show evidence for GRM, yield distinctly different characteristics either in their remanence anisotropy parameters and/or in their partial anisotropy spectra. We present new methodology that helps identify potential CRM-bearing horizons that was overlooked by existing techniques.