Geomagnetic secular variation and the dating of Quaternary sediments

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.