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 inclin ation through the last 6700 calendar years constructed for southwestern Arg entina (about 41 degreesS, 71 degrees 30'W) by stacking palaeomagnetic data derived from lake sediment cores are described and analysed. The data cons ist of records from four lakes which have been previously studied (El Trebo l, Brazo Campanario, Moreno and Escondido), and are here integrated. Within-lake correlation was based on magnetic susceptibility (chi) and inte nsity of the natural remanent magnetization (NRM). Interlake correlation wa s derived from declination and inclination patterns and a set of radiocarbo n age determinations. The secular variation (SV) of the geomagnetic field was studied using spect ral analysis and precession analysis of the magnetic vector. The spectrum o btained from spectral analysis of declination (D) and inclination (I) at th e 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 periodicitie s obtained, spectral analysis has been applied to every 3500-yr time span a t 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-cloc kwise circularity.