Natural remanent magnetization acquisition in bioturbated sediment: General theory and implications for relative paleointensity reconstructions

Abstract

We present a general theory for the acquisition of natural remanent magnetizations (NRM) in sediment under the influence of (a) magnetic torques, (b) randomizing torques, and (c) torques resulting from interaction forces. Dynamic equilibrium between (a) and (b) in the water column and at the sedimentwater interface generates a detrital remanent magnetization (DRM), while much stronger randomizing torques may be provided by bioturbation inside the mixed layer. These generate a so-called mixed remanent magnetization (MRM), which is stabilized by mechanical interaction forces. During the time required to cross the surface mixed layer, DRM is lost and MRM is acquired at a rate that depends on bioturbation intensity. Both processes are governed by a MRM lock-in function. The final NRM intensity is controlled mainly by a single parameter c that is defined as the product of rotational diffusion and mixed-layer thickness, divided by sedimentation rate. This parameter defines three regimes: (1) slow mixing (c < 0.2) leading to DRM preservation and insignificant MRM acquisition, (2) fast mixing (c > 10) with MRM acquisition and full DRM randomization, and (3) intermediate mixing. Because the acquisition efficiency of DRM is larger than that of MRM, NRM intensity is particularly sensitive to c in case of mixed regimes, generating variable NRM acquisition efficiencies. This model explains (1) lock-in delays that can be matched with empirical reconstructions from paleomagnetic records, (2) the existence of small lock-in depths that lead to DRM preservation, (3) specific NRM acquisition efficiencies of magnetofossil-rich sediments, and (4) some relative paleointensity artifacts.

Short-Term Magnetic Field Variations From the Post-depositional Remanence of Lake Sediments

“…Although we have made no attempt to explain the underlying physical process, we propose that using this or other similar functions is better than ignoring lock-in altogether when considering paleomagnetic data for geomagnetic field modeling or for dating. Our new method provides a platform that could be used to experiment with other, more physically motivated (e.g., Egli and Zhao, 2015), variations of filter functions, as long as they can be defined with a limited number of parameters.…”

“…Other efforts have questioned this classical pDRM acquisition concept arguing that flocculation of sediments prevents substantial postdepositional movement of grains within pore spaces (Katari et al, 2000). Alternative sediment mixing models have instead demonstrated the potential role of bioturbation in pDRM acquisition (Mao et al, 2014;Egli and Zhao, 2015). In addition, the presence of magnetotactic bacteria either living at the sediment/water interface or in the sediments, giving rise to either bio-depositional or bio-geochemical magnetizations, is becoming more recognized (e.g., Tarduno et al, 1998;Heslop et al, 2013;Roberts et al, 2013b;Larrasoaña et al, 2014;Mao et al, 2014).…”

Refining Holocene geochronologies using palaeomagnetic records

“…In contrast to palaeomagnetic data from archaeological materials and volcanic rocks, the process by which sediments acquire a magnetization is not fully understood (see, e.g., Roberts et al, 2013;Tauxe and Yamazaki, 2015), despite considerable work to understand the issues that influence sediment remanence acquisition (e.g. Egli and Zhao, 2015;Zhao et al, 2016;Valet et al, 2017;Chen et al, 2017). The overriding principle is that some fraction of magnetic particles aligns with the geomagnetic field after their last disruption event after deposition as sediment.…”

Global and Regional Assessments of Paleosecular Variation Activity Over the Past 100 ka

“…Sediments record the paleomagnetic field through the acquisition of a detrital (depositional or post depositional) remanent magnetization (e.g., Griffiths, 1955;Irving & Major, 1964;Johnson et al, 1948;King, 1955;Tauxe, 2002). Estimates of the depth over which magnetization gets locked in sediments vary, ranging from 1 to 25 cm (Boudreau, 1994;Channell & Guyodo, 2004;Egli & Zhao, 2015;Hyodo, 1984;Lund & Keigwin, 1994;Roberts & Winklhofer, 2004;Stockhausen, 1998;Suganuma et al, 2011;Yamazaki, 1984). Tauxe et al (2006) argued that the evidence for significant (deep) lock-in depth in marine sediments is weak based on natural sediments as well as laboratory experiments.…”