Magnetic mineral alteration induced by diffusive oxygen and cation fluxes in ridge flank sediments

The project focuses on magnetic mineral alteration induced by diffusive oxygen and cation fluxes in ridge flank sediments. A special target of this investigation are thin sediment covers on relatively young, porous basaltic crust with active, hydrothermal circulation. Beside the normal, downward oxygen diffusion at the seabed, ridge flank sediments in such settings experience an upward directed diffusive exchange of oxygen and other elements from the underlying crust. As a consequence the sediment undergoes a double change of redox conditions from deposition under oxic conditions to successively reducing conditions during burial back to oxic conditions induced by upward oxygen diffusion. Past changes of bottom water oxygenation result in multiple non-steady redox fronts propagating through the sediment column. Diagnetic effects such as dissolution, (re-)precipitation and oxidation of redox senstive, magnetic minerals are inferred under such conditions. This may impair the preservation of paleomagnetic and environmental-magnetic signals resulting in misinterpretations and flawed age-depth estimation. Our goal is to understand the underlying processes and to find ways to assess and to solve the related problems.

We investigate sediment cores from the Clarion-Clipperton-Zone in the eastern equatorial Pacific that show increasing pore-water oxygen concentrations near the sediment-crust interface. A multitude of rock- and sediment magnetic measurements accompanied with electron-microscopic investigation, element analysis and geochemical modelling are used to identify, quantify and explain the diagenetic effects experienced by the magnetic carrier minerals under such conditions.

Hypotheses:

1) The magnetic properties of the sediments significantly change concomitant to downcore oxygen profiles.

2) The investigation of magnetic mineral alteration may be used to trace past changes of bottom water oxygenation.

Contacts: Adrian Hoefken, Tilo von Dobeneck