Influence of Magnetofossils on Paleointensity Estimations Inferred From Principal Component Analyses of First‐Order Reversal Curve Diagrams for Sediments From the Western Equatorial Pacific.

Relative abundance of magnetite originated from magnetotactic bacteria (magnetofossils) in sediments may influence relative paleointensity (RPI) estimations of the geomagnetic field, as some studies reported an inverse correlation between RPI and the ratio of anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization (kARM/SIRM), a proxy of the proportion of biogenic to terrigenous magnetic minerals as well as magnetic grain size. This study aims to evaluate the influence of magnetofossils on RPI estimations more selectively using first‐order reversal curve (FORC) diagrams. We studied three cores (KR0515‐PC4, MD982187, and MR1402‐PC1) from the western equatorial Pacific, among which large differences exist in the average natural remanent magnetization intensity normalized by ARM and kARM/SIRM. Principal component analyses (PCAs) were applied to FORC diagrams measured on bulk specimens from the three cores and silicate‐hosted magnetic inclusions extracted from Core MD982187, and three endmembers (EMs) were revealed (EM1: silicate‐hosted magnetic inclusions, EM2: other terrigenous, EM3: biogenic). EM3 proportions vary widely among the three cores. The average RPI decreases with increasing EM3 proportion, which is probably caused by higher ARM acquisition efficiency of magnetofossils due to small magnetostatic interactions. EM3 proportion correlates with kARM/SIRM, which confirms that kARM/SIRM represents the proportion of biogenic to terrigenous magnetic components. Core MR1402‐PC1 has the highest EM3 proportion, and its within‐core variation is small. From FORC‐PCA applied solely to this core, we infer that the configurations of biogenic magnetite chains such as bending and collapse may also influence kARM/SIRM and RPI estimations. Plain Language Summary: Information on the variations of the geomagnetic field intensity in the past (relative paleointensity [RPI]) is important for understanding, for example, how the geomagnetic field is generated in the Earth's core and whether the geomagnetic field has been related to evolution of life. Marine sediments potentially preserve continuous records of RPI, but it has been suspected that magnetic‐mineral composition changes of sediments may induce artificial RPI changes. This study aims to evaluate the influence of fossil magnetite produced by magnetotactic bacteria on paleointensity estimations. We used a technique based on magnetic hysteresis for estimating the proportion of biogenic magnetite to other terrigenous magnetic minerals in magnetic mineral assemblages, which was applied to sediment cores taken from the western equatorial Pacific. This technique enables to focus more on biogenic magnetite compared with previous estimations using a magnetic proxy. We confirmed that apparent RPI values inversely correlate with the proportion of the biogenic to terrigenous magnetic components, which implies that RPI is underestimated at a higher biogenic magnetite proportion. In addition, degradation of chain arrangements of magnetite originally produced by magnetotactic bacteria may also influence RPI. Our results will be useful for improving the reliability of paleointensity estimations. Key Points: Principal component analyses were applied to first‐order reversal curve diagrams from three sediment cores in western equatorial PacificRelative paleointensity (RPI; natural remanent magnetization/anhysteretic remanent magnetization [ARM]) decreases with increasing proportion of magnetofossils due to their higher ARM acquisition efficiencyBending and collapse of biogenic magnetite chains may also influence ARM susceptibility to saturation isothermal remanent magnetization ratio and RPI estimation [ABSTRACT FROM AUTHOR]