Geomagnetic secular variations over the last 6500 years as recorded by sediments from the lakes of south Argentina

Regional type curves depicting secular variations in declination and inclination through the last 6700 calendar years constructed for southwestern Argentina (about 41 degreesS, 71 degrees 30'W) by stacking palaeomagnetic data derived from lake sediment cores are described and analysed. The data consist of records from four lakes which have been previously studied (El Trebol, Brazo Campanario, Moreno and Escondido), and are here integrated. Within-lake correlation was based on magnetic susceptibility (chi) and intensity of the natural remanent magnetization (NRM). Interlake correlation was derived from declination and inclination patterns and a set of radiocarbon age determinations. The secular variation (SV) of the geomagnetic field was studied using spectral analysis and precession analysis of the magnetic vector. The spectrum obtained from spectral analysis of declination (D) and inclination (I) at the same time shows major peaks at periodicities of about 3400, 1600 and 1200 yr for both parameters. In order to test the stability of the periodicities obtained, spectral analysis has been applied to every 3500-yr time span at intervals of 500 yr for both records. Patterns of circularity during the last 6700 yr appear to recur with a near constant interval of about 2000 yr. Both open and closed loops are associated with clockwise and counter-clockwise circularity.