Can remanence anisotropy detect paleomagnetic inclination shallowing due to compaction? A case study using Cretaceous deep‐sea limestones

We studied 35 Cretaceous limestone specimens from five Pacific plate Deep Sea Drilling Project sites. Inclination I N of the natural remanence is on average 17° shallower than the average 44° expected paleofield inclination I . Anhysteretic remanence (ARM) applied identically to various axes was found to be weakest (ARM min ) perpendicular to bedding and strongest (ARM max ) parallel to bedding. The average ARM min /ARM max of 0.87 as well as the inclination shallowing of 17° likely originated from sediment compaction rotating the long axes of magnetite grains toward the bedding plane. This origin is theoretically and experimentally consistent with the average fractional compaction of 0.6 experienced by our sediments (estimated from their porosity). A compaction origin is also supported by the significant correlation found between tan I N /tan I and ARM min /ARM max . The correlation line's slope of 2.3±0.7 agrees with theory, taking into account our observation that ARM given perpendicular to the long axes of magnetite grains has on average ∼ 0.37 times the intensity of ARM given axially. These results suggest that compaction‐induced inclination shallowing may be detected in a suite of fine‐grained magnetite‐bearing sediments by looking for a correlation between tan I N and ARM min /ARM max (having shown that ARM anisotropy is foliated in the bedding plane). This correlation line's prediction of I N when ARM min /ARM max = 1 should estimate I corrected for inclination shallowing.