RECENT ADVANCES IN THE USE OF PALEOMAGNETISM FOR DATING SEDIMENTS

The Earth's magnetic field varies over many different time scales. Over tens of years to hundreds of years, the direction of the field can change by as much as 30 or 40 degrees at mid-latitudes. These changes, known as paleosecular variation, are coherent over distances of at least a few thousand kilometers, and records of paleosecular variation from different sites in the same region can be compared and compiled into a master curve that can then be used to provide a chronology for a new site. In recent years, researchers have used paleomagnetic records from Holocene lacustrine sequences to develop high-quality master curves for several regions of the world. They have also shown that when master curves from lacustrine sequences are compared to paleomagnetic records from nearby marine environments with high deposition rates, the agreement is usually quite remarkable. Thus, the use of master curves of paleosecular variation for the dating of both lacustrine and marine sediments deposited during the past 10,000 years is well-established.

However, the Earth's magnetic field is a vector which has a magnitude as well as a direction. Determining the magnitude (or intensity) of the field is more difficult than determining its direction because the strength of magnetization of a sediment reflects both the intensity of the magnetic field at the time of deposition and the concentration of magnetic material in the sediment. In the past fifteen years, methods have been developed to separate the influence of these two factors, and it is now possible to obtain reliable records of the paleointensity variations of the Earth's magnetic field. These records show that the field varies on time scales on the order of a few thousand to a few tens of thousands of years and that these variations are globally coherent. Moreover, composite records of paleointensity variations now span the last million years. Although these records were developed from and have been applied mainly to marine sediments, the global coherence of the paleointensity signal means that paleointensity dating can also be used for lacustrine sediments deposited during the past 1 million years.