Magnetic Signatures Associated with Early Diagenesis (Invited)

Magnetic minerals are sensitive indicators of sedimentary redox conditions. In oxic environments, detrital magnetic minerals can provide records of diverse climatically driven processes. In suboxic and anoxic environments, iron-bearing minerals undergo changes associated with progressive microbially mediated organic carbon degradation during early diagenetic iron and sulfate reduction. Iron reduction causes dissolution of detrital magnetic minerals and increases dissolved iron concentrations in sedimentary pore waters. Sulfate reduction decreases sulfate to near-zero values and increases dissolved sulfide. The sulfide then reacts with dissolved iron to form iron sulfides (mackinawite, greigite and pyrite) at the expense of detrital magnetic minerals (e.g., magnetite). Magnetic measurements enable identification of magnetite dissolution and the presence of ferrimagnetic greigite, which help to reconstruct diagenetic processes. Many variations on the standard steady state early diagenetic scheme have been detected using magnetic techniques. For example, major changes in sedimentation rate can cause jumps in the positions of redox boundaries, which can be detected by preservation of relict detrital magnetic mineral assemblages. Greigite growth tens of meters below the sulfate-methane transition is not predicted in standard models of sedimentary pyrite formation and has important implications for paleomagnetic recording. Methane migration through sediments in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetizes sediments. Non-steady state diagenesis often occurs in environments with climatically driven cyclical variations in biological productivity and/or bottom water ventilation. Magnetic properties vary systematically in such sedimentary cycles and can produce useful signatures concerning paleo-ocean ventilation. Magnetotactic bacteria biomineralize magnetic particles (magnetite or greigite) within cellular membranes to enable navigation using the geomagnetic field. These organisms generally require redox gradients and live near the oxic-anoxic interface in stratified waters or sediments. Preservation of fossil magnetite or greigite magnetosomes within sediments provides useful and contrasting information about past early diagenetic conditions. Environmental magnetic analysis can therefore provide valuable information concerning a variety of early diagenetic processes.