The Effect of Bioturbation on Relative Paleointenstiy Records

Abstract:

Bioturbation is one of the key factors affecting the acquisition of a natural remanent magnetization (NRM) in sediments featuring a top mixed layer. In this case, a rotational diffusion process controls the acquisition timing, which is described in terms of a lock-in function, and NRM intensity. In general terms, NRM acquisition by rotational diffusion is described by a Smoluchowski-Debye differential equation, which yields analytical solutions describing how an initially acquired depositional remanent magnetization (DRM) is progressively replaced by a post-depositional remanent magnetization (PDRM) [Egli and Mao, Geochem. Geophys. Geosyst. 16, 995-1016, 2015]. These solutions in turn support the calculation of lock-in functions. Results are controlled by the following parameters: (1) a rotation diffusivity constant γ = 2D_rL/ω, where D_r is the rotational diffusion coefficient, L the thickness of the mixed layer, and ω the sedimentation rate, and (2) the ratio between magnetic aligning torques τ_m = mB and the torques τ_p associated with mechanical interactions between sediment particles and with the action of perturbing forces. The PDRM acquisition rate and the extent of DRM replacement is controlled by γ, while PDRM intensity is a Langevin function of τ_m/ τ_p. Associated lock-in functions range from a constant (NRM is acquired only at the sediment surface) to the classical lock-in function starting below the mixed layer, though intermediate situations where PDRM is partially acquired in the mixed layer. This model has been confirmed by redeposition experiments performed with fresh sediment containing living microorganisms. Redeposition experiments show that the intensity of bioturbation-driven PDRMs can reach ~50% of the originally acquired DRM. Our model has profound consequences for the evaluation of relative paleointensity records, where variations can be driven by changes of the depositional environment. While this knowledge is not new, we provide for the first time a key for understanding, in a quantitative manner, how the NRM acquisition efficiency is controlled by bioturbation. A combination of proxies leading to estimates of the bioturbation activity might provide a new path for improving the reliability of relative paleointensity records.