MIDDLE LATE PLEISTOCENE RELATIVE PALEOINTENSITY OF THE GEOMAGNETIC-FIELD FROM LACUSTRINE SEDIMENTS, LAKE CHEWAUCAN, WESTERN UNITED-STATES

Detailed palaeomagnetic and rock magnetic studies of a 15 m succession of Middle/Late Pleistocene lacustrine sediments from ancient Lake Chewaucan, southern Oregon, western United States, indicate that the remanence-bearing grains are sufficiently uniform to be applicable to relative palaeointensity studies. We have used ARM, SIRM and chi for normalization of the NRM. All three parameters give essentially identical results in their relative stratigraphic variations, which indicates that the normalizations efficiently remove the effects of variation in magnetic mineral concentration. Patterns in grain-size variation, as indicated by small-scale quasi-cyclic fluctuations in hysteresis parameters, may be due to environmental changes such as lake-level variation. However, these fluctuations are within the acceptable range of grain sizes for palaeointensity studies and cannot be correlated with any of the features of the normalized remanence record. We therefore conclude that the large-scale variations in the normalized remanence record are due to geomagnetic palaeointensity fluctuations. Parts of the normalized remanence record, where firm chronological constraints exist, may correlate with features of relative palaeointensity records from deep-sea sediments. Our results also confirm the observation that low geomagnetic field intensities dominate during geomagnetic excursions. Further studies of relative palaeointensity of the geomagnetic field may enable the development of an independent time-scale which would make possible the direct correlation of palaeoclimate records from deep-sea and continental environments.