Beyond the Bulk: Magnetic Property Variations With Sediment Grain Size. Insights Into Process and Provenance

Abstract Bulk (whole sample) magnetic properties of three‐hundred‐sixty‐two samples from five North Atlantic Ocean sediment cores are compared to the magnetic properties of five grain‐size fractions separated from the same samples. The relative abundance and magnetic properties of the size fractions are highly variable both within a core and among locations. Clay often dominates the sediment texture but contains low concentrations of magnetic minerals whose composition varies little from site‐to‐site. Silts are more magnetically variable, contain much higher ferrimagnetic mineral concentrations, are magnetically coarser than clays, and retain strong imprints of their geological source. The medium silt size fraction is a strong source discriminator, dictates bulk sediment magnetic property variations, and helps characterize a new magnetic source fingerprint for Laurentia. In contrast, bulk sediment magnetic properties are an average of the properties of the individual fractions weighted by the magnetic concentration of those fractions and their abundance. Bulk sample averaging is observed across magnetic methods, which mutes individual grain‐size property variations that convolve source signatures and transport driven texture variations. Grain‐size‐specific approaches can help to restrict textural variance to isolate source signatures, track sediment texture changes independently of magnetic variation and provenance changes, facilitate process‐based interpretations, and provide sedimentology‐based insights into these important paleo‐archives and their recorded paleo‐environmental changes.