Magnetic and chemical changes in marine sediments

Abstract

A considerable amount of chemical knowledge of marine sediments has been acquired in recent years but has not yet been utilized by paleomagnetists. On the other hand, geochemists are often unaware of the usefulness of numerous magnetic techniques. In this review we try to bridge this gap, and in particular, we outline many of the chemical and magnetic principles that should allow paleomagnetists to better identify and understand chemical changes that affect the magnetic properties of marine sediments. The chemical principles include those for distinguishing the four major sources of sediments (continental, biological, authigenic/hydrogenous, volcanic/hydrothermal) from one another by determining elemental abundance distributions, as well as for investigating the stabilities of mineral phases relative to changes in pE and pH. The magnetic principles include the effects of authigenesis and diagenesis on magnetic properties, particularly on the direction and intensity of natural remanent magnetization (NRM). These principles are then applied to several sedimentary cores obtained from the Pacific Ocean. It is shown that although low‐temperature oxidation of titanomagnetites occurs in some of these cores, such oxidation has had only a minor effect on altering the NRM in most cases. On the other hand, some ferromanganese phases are magnetic and form authigenically in the marine environment. At least one of these phases, probably todorokite, sometimes carries a remanence sufficiently large to mask the depositional remanence carried by the titanomagnetite grains.