Distinguishing detrital versus chemical remanent magnetization in sediments: Toward a better understanding of relative paleointensity records

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Sediments provide detailed and continuous records of geomagnetic field behavior. Amongst other things, these time series provide estimates for relative field intensity variations in the past, which are used and to calibrate climate proxies stemming from nuclide production in the upper atmosphere. An overriding assumption when extracting and comparing geomagnetic field information from sediments is that the recording mechanism remained constant. The common hypothesis is that magnetic remanence is acquired via torque from the external field acting on a magnetic particle, which is termed a depositional (detrital) or post-depositional remanent magnetization. However, diagenetic processes can precipitate ferrimagnetic iron oxides and/or sulphides during or after deposition, thereby producing a secondary, chemical remanent magnetization. The physics behind the recording mechanisms between depositional and chemical remanences are fundamentally different, yet the identification of the latter remains challenging with existing methods, and thus it's potential influence is often ignored--especially concerning relative paleointensity records. With preliminary results from redeposition experiments we show that anisotropy of anhysteretic remanent magnetization parameters vary with relative paleointensity, which means we may have found a means to reduce the scatter in paleointensity estimates based on magnetic anisotropy. On the other hand, a chemical remanent magnetization has a much different anisotropy signature than a depositional remanence. We show an example of sediments from the Bermuda Rise (ODP site 1063) that contains horizons of the chemical precipitate greigite. Previous studies from different working groups on this site conclude detrital magnetite is the sole remanence carrier. The identification of greigite was made possible through experiments using anhysteretic remanent magnetization techniques. We would like to use anhysteretic remanent magnetization-based experiments to better understand how the paleointensity recording process differs between chemical and depositional processes, as well as when they act simultaneously, with the ultimate goal of producing more robust relative paleointensity records.