Relative efficiencies of remanence acquisition in biogenic and detrital magnetite

Abstract:

Paleomagnetic analyses have been carried out on sedimentary rocks for over 65 years. Throughout much of this time, sedimentary magnetization acquisition has been associated with the depositional remanent magnetization and post-depositional remanent magnetization concepts, where dominantly detrital magnetic particles align passively with the geomagnetic field either during or after deposition. We are currently learning a great deal of new information about how sediments acquire a remanent magnetization. In recent years, biogenic magnetite has been recognized as an important source of sedimentary paleomagnetic signals, with common co-occurrence of biogenic and detrital particles. Detrital and biogenic magnetite particles will have different grain size distributions and, therefore, are likely to have different efficiencies of alignment with the geomagnetic field. The two particle types should then be expected to contribute differentially to remanence normalizations used in relative paleointensity investigations. We have assessed the relative efficiency or remanence acquisition for both detrital and biogenic magnetite in various sediments and find that biogenic magnetite is generally 2-4 times more efficient than detrital magnetite in contributing to sedimentary natural remanent magnetizations. Temporal variations in the concentration and efficiency of remanence acquisition of the two components will introduce an important non-geomagnetic factor that is important to consider in relative paleointensity studies, particularly if the relative proportion of the two magnetite components varies stratigraphically within a studied sedimentary sequence. Our results are the first to provide an assessment of variable remanence acquisition in sediments and demonstrates that much remains to be learned about sedimentary magnetization acquisition despite the longevity and importance of this subject.