Palaeomagnetic and rock-magnetic studies of Holocene sediments from a maar lake in Central Mexico

MARCOS A. E. CHAPARRO; HARALD N. BÖHNEL; ROGER BYRNE; NORBERT R. NOWACZYK; ROBERTO S. MOLINA-GARZA; JUNGJAE PARK; JÖRG F.W. NEGENDANK

Bojnice Castle, Slovak Republic

Congreso; 11th Castle Meeting “New Trends in Geomagnetism Paleo, Rock and Environmental Magnetism”; 2008

Geophysical Institute of the Slovak Academy of Science

New palaeomagnetic data from a maar lake in Central Mexico is reported. A valuable contribution is expected because the need of data from continuous lake sediments records to describe the Holocene palaeomagnetic secular variation (PSV) in this region. Three sediment cores (of around 5 m in length) from Hoya San Nicolas maar in Guanajuato were studied for palaeomagnetic, rock-magnetic, palynological and geochemical analyses. Rock-magnetic results –from studies of magnetic susceptibility, hysteresis cycles, anhysteric and isothermal remanent magnetisation– suggest that the remanent magnetisation of the sediments is controlled by pseudo-single domain (titano)magnetite, which are suitable recorders of the geomagnetic field. The age-depth model implies average deposition rates of about 0.32 and 0.45 mm/yr and a basal age of about 11,600 cal. yr. BP. Magnetic susceptibility and natural remanent magnetisation logs were used to correlate the palaeomagnetic (declination and inclination) data of the three cores. Furthermore, a composite PSV record was obtained after applying a stretching and stacking process, and a chronology established with AMS dates on microscopic charcoal and stratigraphic correlations with other well dated Holocene records from Mexico and Guatemala. In this PSV curve, declination and inclination results show an oscillatory behaviour varying in a narrow range, and an interesting swing is observed around the basal ages (9,000-9,800 cal. yr. BP.). The San Nicolas PSV curve is in several ways similar to palaeomagnetic (master) curves from Europe and North America, especially with respect to inclination feature. Key words: lake sediments, Mexico, palaeomagnetic secular variation, Rock-magnetic parameters.