Distinguishing detrital versus chemical remanent magnetization in sediments: Toward a better understanding of relative paleointensity records
Geophysics
Final Report Abstract
Relative paleointensity data from sedimentary rocks play an important role to decipher the workings of the geodynamo and to correct for atmospheric cosmogenic radionuclide production, so it is important to understand how sediments acquire remanent magnetizations and to better assess the quality of relative paleointensity data. We published experimental results from sediments deposited in controlled magnetic fields to observe the changes in magnetic anisotropy as a function of applied field strength going from near Earth-like values to almost full saturation. Relative paleointensity values followed a very well defined power law through the entire range of applied field intensities. Magnetic remanence fabrics evolved from oblate with maximum anisotropy axes in the sedimentary plane at low field strengths, to prolate with maximum anisotropy axes parallel to the applied field direction at high fields. Anisotropy of magnetic susceptibility also evolved with field strength, but in a much less coherent manner than anisotropy of magnetic remanence. Our experiments used well-characterized, natural sediment containing single domain magnetite, which made it possible to numerically model the data. The model matched the field dependency of both relative paleointensity and magnetic fabric development using a simple assumption that a large proportion (~80%) of the remanence carriers in the sediments were unable to align with the magnetic field while a small fraction was free to align. Our results demonstrated that anisotropy of magnetic remanence can improve and assess relative paleointensity estimates and can help improve theoretical treatment of magnetic recording in sediments. We also developed a new method to synthesize greigite (Fe3S4), a common authigenic magnetic mineral. We documented how sediments become magnetic as greigite crystallites grow over time following a synthesis pathway of γFeOOH to FeS to Fe3S4, analogous to early diagenetic redox reactions. We then directly compared, for the first time, a chemical remanent magnetization (CRM) with a depositional remanent magnetization (DRM) acquired by the same samples as a function of magnetic field intensity. All remanences were proportional and parallel to the applied magnetic fields ranging from 25 to 100 µT, yet CRMs recorded the magnetic field ca. 5 times more efficiently than DRMs, demonstrating that undetected CRM components can produce significant biases in paleointensity estimates of the geomagnetic field. Based on the observation that CRM was significantly more resistant to alternating field demagnetization than DRM, a comparison of the demagnetization spectra of the natural versus laboratory-induced magnetizations may provide a useful tool to discriminate CRM and DRM in natural sediments. Many of our new results were made possible thanks to further technological development of the so-called SushiBar. A virtually non-magnetic sample holder was designed that can rotate standard paleomagnetic cores about two axes with extremely high precision. Two new workstations and three new instruments were added to the SushiBar that, in addition to existing infrastructure, enable the SushiBar to measure virtually any parameter needed to understand the magnetic properties of sediments.
Publications
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(2019), Relative paleointensity estimates from magnetic anisotropy: Proof of concept, Earth and Planetary Science Letters, 519, 83-91
Gilder, S., He, K. Wack, M. & Ježek, J.
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(2019). Chemical vs. Detrital Remanence: DGG Jahrestagung 2019, Braunschweig
Roud, S. & Gilder, S.
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(2019). Greigite synthesis and its magnetic recording in sediments. AGU Fall Meeting Abstracts, GP31B-0713
Roud, S. & Gilder, S.
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(2019). Relative paleointensity estimates from magnetic anisotropy. AGU Fall Meeting Abstracts 2019, GP14A-04
Gilder, S., He, K. Wack, M. & Ježek, J.
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(2021). A Multi-Proxy Approach to Unravel Late Pleistocene Sediment Flux and Bottom Water Conditions in the Western South Atlantic Ocean. Paleoceanography and Paleoclimatology, 36(4), e2020PA004058
Mathias, G., Roud, S., Chiessi, C., Campos, M., Dias, B., Santos, T., Albuquerque, A., Toledo, F. Costa, K. & Maher, B.
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(2021). Miocene to Early Pleistocene Depositional History and Tectonic Evolution of the Issyk-Kul Basin, Central Tian Shan. Geochemistry, Geophysics, Geosystems, 22(4), e2020GC009556
Roud, S., Wack, M., Gilder, S., Kudriavtseva, A., & Sobel, E.
