Why are geomagnetic excursions not always recorded in sediments? Constraints from post-depositional remanent magnetization lock-in modelling

Abstract

Many geomagnetic excursions have been documented during the Brunhes Chron. However, high-resolution paleomagnetic studies of sediments often provide evidence of few (if any) excursions. We have investigated the reasons for this observation by modelling the post-depositional remanent magnetization (PDRM) lock-in process. A cubic lock-in function produces more rapid lock-in compared to linear and exponential functions proposed in the literature. We therefore used a cubic function to model “best-case” scenarios for the quality of the paleomagnetic record when a high-frequency geomagnetic input signal is convolved with the sediment lock-in function for a wide range of sedimentation rates. Even for a lock-in depth of 10 cm, where 95% of the PDRM is locked in 5 cm below the surface mixed layer, an input signal containing abundant excursions (with 1-kyr duration) is smoothed so drastically at low sedimentation rates (1–3 cm/kyr) that no excursions are recorded. Excursions are further attenuated by increasing the lock-in depth, as well as by sampling at discrete stratigraphic intervals. The PDRM process acts as a low-pass filter for high-frequency secular variation signals even at moderate sedimentation rates (>10 cm/kyr). We have also modelled the effects of modulation of sedimentation rate by a climatic forcing function (e.g., insolation). Even though the average sedimentation rate might be substantial, PDRM recording becomes much less reliable in intervals where sedimentation rates are lowest. In these cases, some excursions are reliably recorded in detail, while others may not be recorded at all. Our modelling suggests that in order to consistently detect the presence of geomagnetic excursions, it is ideal to work with sediments that maintain minimum sedimentation rates above 10 cm/kyr. If the normal spectrum of geomagnetic field behaviour contains abundant excursions, failure to document excursions in many apparently high-resolution analyses probably results from PDRM lock-in within relatively slowly deposited sediments, or to unrecognized intervals of slow sedimentation in environments with higher average sedimentation rates.