1974 Theses Doctoral

Magnetic mineralogy and magnetic properties of deep-sea sediments

Kent, Dennis V.

The magnetic mineralogy and magnetic properties of seven deep-sea sediment cores were investigated in an attempt to gain a better understanding of the fundamental processes controlling the intensity and direction of their natural remanent magnetism (NRM). The principal findings are these:

(i) The magnetic mineralogy of these sediments was often complex and difficult to describe. This complexity was usually due to the presence in individual samples of titanomagnetites with a range of oxidation states and titanium contents. Other magnetic minerals may also occur in deep-sea sediments but were not detected in the cores studied. A complex magnetic mineralogy can be expected considering the number of possible sediment sources.

(ii) The remanence in the investigated deep-sea sediments as carried predominantly by small concentrations of particles of titanomagnetite, generally less than several microns in sizeTHese may occur as discrete grains or be included in larger nonferrimagnetic minerals. More than one member of the titanomagnetite series may be present in the sediment but not all necessarily contribute to the natural remanent magnetism. However, parameters such as susceptibility, isothermal remanence or anhysteretic remanence that may be used to normalize the NRM intensity for variations in magnetic concentration will receive a contribution from all magnetic minerals present in the sediment.

(iii) The titanomagnetites in some sediments have suffered various degrees of low-temperature oxidation. These cation-deficient forms often carry large unstable secondary magnetizations, particularly viscous remanence. There was no obvious correlation between gross sediment lithology and the occurrence of cation-deficient titanomagnetites. However, sediment which has accumulated at rates less than about 3mm/1000 years seemed to possess relatively large concentrations of these oxidized magnetic minerals as well as large soft magnetizations of secondary origin. Sediment with deposition rates greater than 3mm/1000 years had less oxidized titanomagnetite and small soft magnetizations. Slow accumulation rates would allow longer exposure times to oxygenated bottom waters for the detrital titanomagnetites and therefore become more oxidized. These conditions might also allow direct authigenic formation of maghemite.

(iv) The stable natural remanent magnetization of deep-sea sediment can be attributed in large part to detrital remanent magnetization (DRM). This magnetism was probably acquired soon after deposition as a result of interstitial rotations of magnetic grains in response to the geomagnetic field. The statistical alignment of the magnetic grains was preserved as the sediment became consolidated, thereby inhibiting further grain rotation. By analogy with laboratory experiments, disturbance of the sediment by benthic burrowing organisms can assist the process of alignment of the magnetic particles in a magnetic field. The depth to which these organisms, if present, can penetrate in the sediment column may determine the difference between sediment age and the detrital remanence.

(v) Post-depositional DRM produced in the laboratory was similar in intensity and stability to the NRM of the sediment core. Each remanence had good magnetic stability and received a proportionate contribution from the entire range of coercivities present. Furthermore, the post-depositional DRM recorded the direction of the ambient magnetic field without systematic errors. The apparent success in producing a remanence in the laboratory whose characteristics were similar to the NRM of the same sediment may provide a means of obtaining paleointensity values from sediments.