Using magnetic anisotropy to correct for paleomagnetic inclination shallowing in some magnetite-bearing deep-sea turbidites and limestones

Abstract

In sediments, remanence inclination I may be shallower than the inclination I H of the Earth's field in which they were deposited. Use of magnetic anisotropy to correct for this paleomagnetic inclination shallowing is tested for Quaternary turbidite muds from the Scotian Rise and for Cretaceous pelagic limestones from the Pacific Plate. Both are dominated by pseudo-single-domain magnetite carrying a stable remanence whose inclination I (determined by stepwise alternating field demagnetization) is shallower than I H, by an average of 12° for the turbidites and 17° for the limestones. Anhysteretic remanence (ARM), applied identically to various axes, is weakest (ARM min) perpendicular to bedding and strongest (ARM max) parallel to bedding, with little anisotropy in the bedding plane and ARM min ARM max averaging 0.84 for the turbidites and 0.87 for the limestones. Magnetite also dominates susceptibility with minimum ( K min) and maximum ( K max) susceptibility axes perpendicular and parallel to bedding, respectively, and with K min K max averaging 0.91 for the turbidites and 0.95 for the limestones. In theory, we expect to find a correlation between tan I and ARM min ARM max for a suite of specimens with the correlation line's prediction of tan I when ARM min ARM max = 1 yielding an I H estimate corrected for inclination shallowing. This correction method is also expected to succeed if ARM min ARM max is replaced by K min K max , provided susceptibility is dominated by multidomain or pseudo-single-domain magnetite. In practice, both anisotropies do succeed in correcting for inclination shallowing in three of the four turbidite cores. Both anisotropies would also likely succeed for the limestones (if there were enough samples at any one site) since correlating tanI tanI H in place of tan I for the five sites combined predicts tanI tanI H ∼ 1 when ARM min ARM max = 1 or K min K max = 1 . Hence, we recommend routinely measuring anisotropy in paleomagnetic studies of magnetite-bearing sediments and looking for a correlation between tan I and ARM min ARM max or K min K max . Remanence anisotropy is more laborious to measure but provides the more reliable estimate of inclination shallowing. In cases where no significant correlation is found, remanence anisotropy of the magnetite particles in the sediment may also have to be measured to correct for inclination shallowing.