Characterising paleomagnetic field recording efficiency of sedimentary magnetic mineral inclusions: implications for relative paleointensity determinations

Relative paleointensity (RPI) signals obtained from sedimentary records were carried by complex magnetic mineralogies, including detrital, biogenic magnetic minerals, and others. Recent studies demonstrate that magnetic minerals inclusions of detrital origin are potentially widespread in marine sediments that make significant contributions to sedimentary palaeomagnetic records. But little is known about how detrital magnetic mineral inclusions record the palaeomagnetic field. To assess the role of magnetic mineral inclusions in sedimentary paleomagnetic records, we analyze paleomagnetic and rock magnetic data of a marine sedimentary core from the North Pacific Ocean offshore of central Japan that is known to contain abundant magnetic mineral inclusions and other form of detrital magnetic minerals. Sedimentary intervals that contain abundant magnetic mineral inclusions as the dominant magnetic mineralogy were isolated from the sedimentary sequence based on their distinct rock magnetic and geochemical properties compared to those of other sedimentary intervals. Such separation enables a quantitative assessment of the contribution of magnetic mineral inclusions to sedimentary RPI signal. NRM and remanence after AF-demagnetization from different sedimentary intervals were analyzed and compared. RPI record from the studied core does not show a clear correlation to global RPI stacks, which we attribute to different palaeomagnetic recording efficiency of magnetic mineral inclusions compared to other form of detrital magnetic minerals. For example, normalized NRM by susceptibility, ARM and IRM for intervals containing abundant magnetic mineral inclusions have significantly different slopes compared to those from other sedimentary intervals. In addition, we carried out the first laboratory re-depositional experiment of magnetic inclusions using the core materials under controlled earth-like magnetic field. Our data show a lower magnetic recording efficiency for samples containing abundant magnetic mineral inclusions compared materials from other sedimentary intervals, consistent with RPI data from the core. Our demonstration of the role of sedimentary magnetic inclusions in palaeomagnetic records are widely useful for understanding the complex processes how marine sediments get magnetized.