Difference in Relative Paleointensity Recording Efficiency of Magnetic Mineral Constituents in a Sediment Core Off Chile.

Progress of relative paleointensity (RPI) estimations using marine sediments has greatly contributed to better understanding of the behavior of the past geomagnetic field. To enhance further the reliability of RPI estimations, we must overcome the problem that climatically induced variations of magnetic‐mineral assemblages in sediments may influence RPI records. Two major constituents of magnetic‐mineral assemblages in marine sediments are magnetofossils and detrital magnetic minerals, and the latter consists of silicate‐hosted magnetic inclusions and unprotected magnetic minerals. It is necessary to understand different RPI recording efficiencies among those constituents to tackle the problem, but previous evaluations were inconclusive. We studied this issue using a sediment core taken from the southeast Pacific Ocean. Rock‐magnetic investigations revealed that the magnetic mineral assemblage of this core during the last ∼900 Kyr is a mixture of low‐coercivity magnetofossils and middle‐coercivity detrital unprotected partially oxidized magnetite. Natural remanent magnetization versus isothermal remanent magnetization demagnetization diagram showed strong convex curvature, and RPI signals carried by the two components could be separated by calculating RPI from the gradients of 20–40 and 70–160 mT segments. We confirmed that RPI recording efficiency of magnetofossils is lower than that of detrital unprotected magnetites/maghemites. Because of the marginal overlap between the coercivity ranges of the two components, changes in their relative abundance do not influence RPI estimations. This condition is ideal for RPI estimations, and the resulted RPI curve closely coincides with that of the PISO‐1500 stack despite changes in the relative abundance of the two components. Plain Language Summary: Marine sediments potentially preserve records of geomagnetic field intensity variations in the past (paleointensity), which may be important for life on the Earth to shield influx of high‐energy particles from the universe. A serious problem recognized by previous studies is that paleointensity records from sediments sometimes include artificial changes induced by sediment lithology changes reflecting varying depositional environments. Two major constituents of magnetic minerals in sediments, which records paleointensity variations, are magnetites produced by magnetotactic bacteria, called magnetofossils, and inorganic magnetic minerals originated from weathering of rocks on land or materials of volcanic eruptions. Our study using a sediment core taken from the southeast Pacific Ocean revealed that magnetofossils yield lower paleointensity values than inorganic magnetic minerals. This implies that temporal changes in the relative abundance of magnetofossils and inorganic magnetic minerals in sediments may cause the artificial paleointensity variations. We could obtain a reliable paleointensity record from the studied sediments by evaluating separately the magnetizations carried by the two magnetic‐mineral components, which utilized the observations that the two magnetic‐mineral components have different resistance to an external magnetic field. Key Points: Magnetofossil and detrital partially oxidized unprotected magnetite are the major constituents of magnetic mineral assemblageLittle overlap in coercivity distribution of the two components enabled separate evaluation of relative paleointensity (RPI) recording efficiencyRPI recording efficiency of magnetofossil is lower than that of detrital unprotected magnetite/maghemite [ABSTRACT FROM AUTHOR]