Palaeosecular Variation Recorded on Lake Sediments from South Argentina

Paleomagnetic studies carried out on the bottom sediments from lake El Trebol, South Argentina are described. Two cores, of length 7.5 m and 11 m, were collected and subsampled. The pairs of magnetic susceptibility and intensity of remanent magnetization profiles show a good correlation with the sedimentological macroscopic descriptions, and also with those of shorter cores previously studied. The stability of the natural remanent magnetization was investigated by alternating field demagnetization and the stable remanent magnetization was isolated. Declination and inclination logs were consistent with the correlations. Radiocarbon dating from earlier studies have enabled us to construct a secular variation record from South Argentina for the past 17, 000 years. The declination and inclination logs show a significant anomaly at about 15, 500 yr b.p., which is accompanied by a decrease in both intensity of magnetization and modified Koenigsberger ratio, but without any change in susceptibility. These results suggest the possibility that a geomagnetic excursion or very short reversal of the geomagnetic field was recorded. Excursions at about this time have been recorded in several other parts of the world.