Trends in the Long‐Term Variation of Environmental Magnetism of North Pacific Pelagic Clay: Beyond the Two‐Component Interpretations

Abstract Pelagic clays are complex mixtures of continental dust, volcanic dust, and marine precipitates such as biogenic, hydrothermal, and hydrogenic minerals. Nevertheless, the magnetic properties of the North Pacific clays have often been explained by two components: biogenic magnetite and continental dust. To better characterize the magnetic variations, we measure and compare the long‐term records from wide areas in the central and western North Pacific. The rock magnetic variations are modeled as a mixing of four endmembers. The first endmember (EM1) is characterized by low magnetostatic interactions, which is interpreted to be biogenic magnetite. The second endmember (EM2) shows relatively high coercivity typical of modern continental dust. The third endmember (EM3) reveals smaller proportions of high‐coercivity (>300 mT) minerals than EM2. EM3 is often dominant between ca. 25 and 5 Ma, and its contribution is higher near the western pacific subduction zones. Previous mineralogical and geochemical data suggest increasing volcanic contributions are associated with EM3 at one site. For other sites, direct evidence for the origin of EM3 is currently lacking. The fourth magnetic endmember (EM4′) corresponds to higher contribution of high‐coercivity (>300 mT) minerals, and its contribution is high at around 50–40 Ma. This signal may represent continental dust distinct from modern Asian dust. Our results show that the environmental magnetism of pelagic sediments contains overlooked major variations, which would benefit paleoceanographic and tectonic studies.