Comparison of the Koenigsberger ratio averaged over time in different sediment types can shed light on the paleomagnetic recording efficiency of those sediment types. The contribution of the geomagnetic paleointensity to the ratio is assumed here to be averaged out over the examined interval. This is likely to be a good first order approximation as the paleointensity varies at a scale of thousands to tens or hundreds of thousands of years, and the intervals examined are typically over a million years.
Time-averaged Koenigsberger ratios for eight ODP sites are given in Table T2 and Figure F10. The time-averaged Koenigsberger ratio is calculated quite easily from the GHMT logs: the mean absolute value for the remanent anomaly and the mean value of the induced anomaly are calculated and their ratio is taken. We attempted to select log intervals of the same sediment type for this analysis (Table T2). To enable comparison, the ratios have been corrected for latitude, by assuming the geomagnetic paleointensity at the surface is due to an axial dipole.
The ratios for the silty clays from Holes 1095B and 1096C are similar to each other but about five times as large as for the diamict of Hole 1103A on the continental shelf. The diamict logged in Leg 188 Hole 1166A (Prydz Bay) (O'Brien, Cooper, Richter, et al., 2000) has a similarly low ratio. The glacial and preglacial coarse-grained sediments at Site 1166 also have low ratios. The silty clays of Deep Sea Drilling Project (DSDP) Leg 162 Hole 987E have values comparable to the silty clays logged during Leg 178. Hole 986C has a high ratio of 1.6 (no core paleomagnetic results were obtained from this hole, because a strong drill string overprint). Both these DSDP Leg 162 holes yielded good GHMT polarity stratigraphies by the correlation analysis method (Higgins et al., 1999). DSDP Leg 145, Hole 884E has the highest ratio of the examined holes, at 2.02. GHMT logs from this hole gave an excellent polarity stratigraphy (Thibal et al., 1995).
The main result is that finer-grained sediments have higher Koenigsberger ratios than coarser-grained sediments, with the implication that the finer-grained sediments record the paleofield more efficiently. This result is expected, given the known mechanism of depositional remanence acquisition described above. This result partly explains why it was difficult to establish a polarity stratigraphy for Hole 1103A. The other reasons were the presence of numerous spikes in the record, the depositional environment, and the lack of a priori biostratigraphic dates.
The Koenigsberger results are also important from the point of view of paleointensity studies, in which it is important to establish that the paleomagnetic recording efficiency does not change significantly down the section under study. This is checked by establishing that the magnetic mineral type and size fall within a certain range (e.g., Tauxe, 1993). The remanence intensity normalized by some measure of ferrimagnetic mineral concentration (e.g., susceptibility) can then be taken to be a measure of the field paleointensity.
The ratios presented here vary by a factor of ~10, depending on the sediment type. This is likely because a larger average ferrimagnetic grain size leads to less efficient paleomagnetic recording. Ratios from just the fine-grained sediment types are more constant, varying by a factor of ~2. Paleointensity records are usually generated from this kind of fine-grained sediment, and this study offers background information about the variability of the paleomagnetic recording efficiency. It is important that the recording efficiency is as constant as possible in the particular sediment interval under study. More determinations of the Koenigsberger ratio from different sites will be useful to add weight to these initial analyses.