Post-depositional remanent magnetization lock-in and the location of the Matuyama-Brunhes geomagnetic reversal boundary in marine and Chinese loess sequences

摘要

Bioturbation disturbs detrital magnetic particles after deposition, so accurate recording of the ancient geomagnetic field in bioturbated sediments is widely attributed to acquisition of a post-depositional remanent magnetization (PDRM) whereby the geomagnetic field exerts a torque on a magnetic particle and aligns it with the held after the final mixing event experienced by the particle. The relationship between the Matuyama-Brunhes boundary (MBB) and oxygen isotope age tie points in marine sediments has been widely used to determine the depth at which the paleomagnetic signal is locked-in. However, such analyses can be badly affected by age discrepancies among different paleoclimatic proxies and by varying isotopic compositions of seawater in different locations and from the presence of different water masses at different depths at the same location. It is therefore necessary to separately compare paleomagnetic data with respect to either benthic or planktonic foraminiferal oxygen isotope records for sites from the same water mass to avoid inadvertently introducing age differences to the analysis. When the global data set is subjected to such a rigorous analysis, few reliable data remain for the MBB. Using two complementary approaches, we estimate that the MBB is, on average, shifted <= 20 cm below its true position in marine sediments. This offset is the sum of the thickness of the bioturbated surface mixed layer, which is possibly dominant, and the PDRM lock-in depth. There is also controversy concerning observed differences in the position of the MBB relative to paleoclimatic proxies in marine sediments and Chinese loess deposits. For the Chinese loess, quartz grain size is insensitive to pedogenic alteration and is a useful parameter for determining the true position of the MBB with respect to paleoclimatic boundaries. We conclude that the MBB occurs late in marine oxygen isotope stage 19, and in the upper part of Chinese paleosol unit 58, rather than at the mid or lower part of loess unit L8. Our results require adjustment of the generally accepted positions for the MBB, and resolve a longstanding chronological conundrum for marine and Chinese loess sequences.