Use of small-amplitude paleomagnetic fluctuations for correlation and dating of continental climatic changes

Abstract

Recent advances in field and laboratory methods have made possible the study of paleomagnetic fluctuations recorded with high resolution in lake sediments of relatively small amplitude and short period (∼10 2 years to ∼10 4 years). Foremost among these methods are sediment-coring techniques that allow azimuthally oriented long cores (⩽25 m) to be raised with small orientation errors (⩽ ± 3°), and the use of cryogenic magnetometers to measure weakly magnetized ( M ⩽ 10 −9 A m 2) sediment samples accurately. With these methods, records of inclination, declination, and relative paleointensity (NRM/ARM) have been obtained in North America that span the last ∼20,000 years. Substantial agreement in declination and inclination features is seen on a scale of 10 3 km in North America for the last ∼10,000 years. These paleomagnetic features can be useful in determining the relative age of paleoclimatic features, particularly in those situations where 14C dating is unreliable. Similar agreement is seen for the last 4000 years between relative-paleointensity estimates obtained from Pennsylvania lake sediments and absolute-paleointensity estimates obtained from 14C-dated lava flows and potsherds from the western United States. Similar paleointensity fluctuations to those occurring from 0 to 4000 yr B.P. in the western United States are observed for the last 12,500 yr B.P. in lake sediments from New York and Pennsylvania. Such relative-paleointensity records from lake sediments, if independently confirmed, considerably extend the range and resolution of paleointensity data and have important implications for the study of secular-variation models, variations in atmospheric radiocarbon concentration, and the proposed relationships between geomagnetism and climate. An absolute secular-variation chronology for the varved sediments of Elk Lake, Minnesota, can be used to “date” magnetically the secular-variation records of unvarved sediments elsewhere in North America.