43979-AC8
Depositional Remanent Magnetization: Toward a Theoretical and Experimental Foundation
The geomagnetic field acts both as an umbrella, shielding us from cosmic radiation and as a window, offering one of the few glimpses of the inner workings of the Earth. Ancient records of the geomagnetic field can inform us about geodynamics of the early Earth and changes in boundary conditions through time. Thanks to its essentially dipolar nature, the geomagnetic field has acted as a guide, pointing to the axis of rotation thereby providing latitudinal information for both explorers and geologists. A complete understanding of the geomagnetic field requires not only a description of the direction of field lines over the surface of the Earth, but information about its strength as well. While directional information is relatively straightforward to obtain, intensity variations are much more difficult.
Magnetic grains tend to align with the ambient magnetic field as they fall through the water column and may become re-align post-depositionally as long as they remain sufficiently mobile. We are concerned here not with the myriad diagnetic processes but with the process of particle alignment in a viscous medium, generally termed "depositional remenant magnetization", but including physical re-alignment of particles via bioturbation and other syn-depositional processes.
The behavior of magnetic particles in a viscous medium has been considered for decades. When placed in a fluid, a magnetized particle is subjected to a hydrodynamic couple generated by fluid shear, tending to align particles on average with their long axes in the horizontal plane, a magnetic couple tending to align the magnetic moment with the ambient magnetic field, viscous drag and inertial forces tending to oppose motion and thermally inspired "Brownian" motions. In addition, magnetic particles are not isolated in most natural environments, but interact with each other and with other components in the sedimentary system, in particular, with clay particles. The degree of interaction with non-magnetic "matrix" components depends on the chemistry of the water including the pH and the salinity. When the magnetic particles are stuck to non-magnetic particles, for example, clay particles, their responsiveness to magnetic fields also depends on the interaction of the clay particles with each other, i.e., the tendency to flocculate. This latter depends on turbulence and chemistry of the water. All these factors mean that the magnetic response of the sediment is extremely difficult to predict in a quantitative fashion. Moreover, the response may change with changing sedimentary conditions and even relative paleointensity may be difficult to predict.
Nonetheless, sedimentary paleointensity has been a field of extremely active research. There are over 100 records of relative paleointensity in the litereature, with more appearing every month. What do these records mean and how do they relate to variations of the geomagnetic field? We require a much deeper understanding of depositional remanent magnetization to answer these questions and that is the topic of this proposal.
This two year grant has been primarily dedicated to the support of graduate student research. The student, Ritayan Mitra is now beginning his third year. During the last year he gave an invited talk at the fall AGU meeting and presented a poster at the Institute for Rock Magnetism's bi-annual meeting in Corsica (June, 2008). He is well on his way to finishing an important manuscript on the topic of depositional remanent magnetization. What he has found so far is 1) DRM is critically dependent on the process of flocculation,
2) flocculation is itself dependent on composition of the sediment and chemistry of the water, 3) sediments with inclination error will be poorly suited for relative paleointensity studies because inclination and intensity co-vary and are a complex function of the magnetic field vector, and 4) numerical models can predict the main features of the DRM but a quantitative mode of DRM that would allow prediction of paleointensity from sedimentary records is a long way off.
