Constraints on the acquisition of remanent magnetization in fine-grained sediments imposed by redeposition experiments

The magnetization of sediments is acquired through complex processes involving a large number of physical, mineralogical and magnetic parameters. Despite many attempts, the degree to which these processes distort the record of the geomagnetic field as it is archived as a natural remanent magnetization (NRM) remains poorly documented. Among many other parameters, it is important to evaluate the amount of smoothing inherent to the signal, its relation with the field intensity and its variability in the sediment column. In order to address these problems, we performed new redeposition experiments using carbonate-rich, Ocean Drilling Program (ODP) Site 851, and clay-rich, ODP Site 854, sediments. We used a dilute solution of gelatin, which gels below 20 °C, thereby allowing mechanical blocking of the magnetic grains. We observed two critical results: (1) The efficiency of detrital remanent magnetization (DRM) decreases with increasing sediment concentration for a given slurry. Sediment concentration is defined as: c = m s / ( m s + m H2O), where m s and m H2O are the sediment and water mass, respectively. Higher c would then reflect greater compaction, lower water content and, presumably, greater depth in the sediment column. This effect reduces DRM efficiency nearly to zero for c > ∼ 50%. (2) Post-depositional remanent magnetization (pDRM) is important for c < ∼ 50%. pDRM is carried by grains covering the entire coercivity spectrum. By comparing the mean value of NRM divided by anhysteretic remanent magnetization from the previous magnetostratigraphic study at Site 851 with the relevant ratio derived from our redeposition experiments, we were able to estimate that pDRM was significant within the depth interval where ∼ 44% < c < ∼ 56%. If the sediment concentration profile for the uppermost sediment was known at Site 851, we could define the transfer function for the deconvolution of the field variations. Finally, the dependence of DRM efficiency on c suggests that changes in the thickness of the surface mixed layer would change DRM efficiency. Thus, fluctuations in maximum bioturbation depth could possibly cause DRM intensity changes, regardless of changes in earth's field.