HR: 1340h
AN: GP43B-0863    [Abstracts]
TI: Does Lithology Influence Relative Paleointensity Records? A Statistical Analysis on South Atlantic Pelagic Sediments
AU: von Dobeneck, T
EM: dobeneck@uni-bremen.de
AF: University of Bremen, Department of Geosciences, P.O.box 330 440, Bremen, 28334 Germany
AU: * Franke, C
EM: cfranke@uni-bremen.de
AF: University of Bremen, Department of Geosciences, P.O.box 330 440, Bremen, 28334 Germany
AU: * Franke, C
EM: cfranke@uni-bremen.de
AF: Utrecht University, Paleomagnetic Laboratory 'Fort Hoofddijk', Budapestlaan 17, Utrecht, 3584 CD Netherlands
AB: The relative paleointensity (RPI) method assumes that the intensity of Post Depositional Remanent Magnetization (PDRM) depends exclusively on the magnetic field strength and the concentration of the magnetic carriers. Sedimentary remanence is regarded as an equilibrium state between aligning geomagnetic and randomizing interparticle forces. Just how strong these mechanical and electrostatic forces are, depends on many petrophysical factors related to mineralogy, particle size and shape of the matrix constituents. We therefore test the hypothesis that variations in sediment lithology modulate RPI records. For ninety selected Late Quaternary sediment samples from the subtropical and subantarctic South Atlantic Ocean a combined paleomagnetic and sedimentological dataset was established. Misleading alterations of the magnetic mineral fraction were detected by a routine Fe/kappa test (Funk et al., 2004). Samples with any indication of suboxic magnetite dissolution were excluded from the dataset. The parameters under study include carbonate, opal and terrigenous content, grain size distribution and clay mineral composition. Their bi- and multivariate correlations with the RPI signal were statistically investigated using standard techniques and criteria. While several of the parameters did not yield significant results, clay grain size and chlorite correlate weakly and opal, illite and kaolinite correlate moderately to the NRM/ARM signal used here as a RPI measure. The most influential single sedimentological factor is the kaolinite/illite ratio with a Pearson's coefficient of 0.51 and 99.9$%$ significance. We find that kaolinite has a positive and illite a negative effect on magnetic alignment, while smectite is more indifferent. This is certainly related to the contrasting unit-layer charges of the three clay minerals, eventually also to their crystalline versus flaky structure and low versus medium to high plasticity Our regionally restricted results also indicate an inhibition of magnetic particle alignment by the presence of siliceous microfossils. This could be simply due to the porous structure of diatoms, radiolaria and sponge spicules which favor a high initial porosity followed by extreme postdepositional compaction. A three-member regression model suggests that all considered matrix effects can make up over 50$%$ of the observed RPI dynamics.
DE: 9325 Atlantic Ocean
DE: 3675 Sedimentary petrology
DE: 3022 Marine sediments--processes and transport
DE: 1521 Paleointensity
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting