Variable remanence acquisition efficiency in sediments containing biogenic and detrital magnetites: Implications for relative paleointensity signal recording

Abstract

AbstractWidespread geological preservation of biogenic magnetite makes it important to assess how such particles contribute to sedimentary paleomagnetic signals. We studied a sediment core from the South China Sea that passes the strict empirical criteria for magnetic “uniformity” used in relative paleointensity studies. Such assessments are based routinely on bulk magnetic parameters that often fail to enable identification of mixed magnetic mineral assemblages. Using techniques that enable component‐specific magnetic mineral identification, we find that biogenic and detrital magnetites occur in approximately equal concentrations within the studied sediments. We analyzed normalized remanence signals associated with the two magnetite components to assess whether co‐occurring biogenic and detrital magnetites record geomagnetic information in the same way and with the same efficiency. Paleomagnetic directions for the two components have no phase lag, which suggests that the biogenic and detrital magnetites acquired their magnetizations at equivalent times. However, we find that the biogenic magnetite is generally 2–4 times more efficient as the detrital magnetite in contributing to the natural remanent magnetization (NRM) despite their approximately equal magnetic contributions. Variations in the concentration and efficiency of remanence acquisition of the two components suggest that a significant part of the NRM is controlled by nongeomagnetic factors that will affect relative paleointensity recording. We recommend that methods suited to the detection of variable recording efficiency associated with biogenic and detrital magnetites should be used on a routine basis in relative paleointensity studies.