Full article available in PDF version.

Abstract We have been studying the effects of diagenesis on the paleomagnetic signal of lacustrine sediments by examining tephra layers found in unaltered and altered states in the same diagenetic environment. In this paper, we report rock magnetic data from seven such layers in six diagenetic environments ranging from saline-alkaline to mildly alkaline. The effects of diagenesis can be complicated and varied but certain patterns are evident. These patterns indicate that both the physical state of the magnetic grains and the geochemistry of the porewaters are important in determining the effects of diagenesis. In some cases, diagenesis simply reduces the intensity of the original magnetization, but in others it produces a new magnetization that completely overwhelms the original one. Measurements of various rock magnetic parameters have allowed us to relate the changes in intensity to changes in the particle-size distribution of the magnetic carriers. In particular, decreases in intensity appear to be associated with selective dissolution of the fine-grained magnetic carriers or with a general reduction in the quantity of magnetic carriers of all grain sizes; increases in intensity seem to result from a reduction in the numbers of coarse-grained magnetic carriers. Plots of the demagnetization behavior of natural and saturation isothermal remanent magnetizations may be useful in discriminating between unaltered and altered material, especially when used in conjunction with other rock magnetic information.

Long-term changes in the inclination and declination of the magnetic field at a site are manifestations of geomagnetic secular variation. If a master curve of secular variation is available, then correlation of the paleomagnetic record of undated Quaternary sediments with the master curve can lead to determination of the age of the sediments. Because secular variation is coherent only over distances on the order of a few thousand kilometers, separate master curves must be developed for each region. Historical records, lava flows, and archaeological sites can all provide information about secular variation, but only rapidly deposited sediments can provide the continuous record needed to construct a master curve. The quality of the sedimentary secular variation record depends, however, on the processes by which the sediments acquire their magnetization. These processes create inherent limitations on the agreement in space and time between records. So-called “second-generation” paleomagnetic studies of lacustrine sequences are now yielding credible master curves. These studies are characterized by careful attention to coring procedures, good stratigraphic control, a firm chronologic framework, replicate paleomagnetic sampling, and auxiliary rock magnetic studies. Sediments from lakes in Oregon and Minnesota have provided master curves for western and east-central North America, respectively. Analysis of these master curves shows that dating of sedimentary sequences by geomagnetic secular variation is feasible and that it can provide new opportunities for high-resolution studies of climatologic and sedimentologic processes.