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| 103 | A nalysis of the magnetic record of burnt archaeological materials can provide interesting data, especially with regard to chronology. The Earth’s magnetic field undergoes changes in direction and intensity that vary both spatially and temporally around the surface of the Earth and that can end up being registered via various mechanisms (mainly thermal) in diverse archaeological materials. One of the phenomena responsible for these changes, of direct interest to archaeology, is the one known as secular variation of the earth’s magnetic field (SV), the principle on which archaeomagnetic dating is based. Archaeomagnetism is an interdisciplinary branch of geophysics that studies directional variations (declination [d] and inclination [I]) and changes in intensity (F) in the magnetic field in given areas by means of the analysis of the magnetic record in archaeological materials (Aitken 1990). SV measures regional changes in direction and intensity of the magnetic field on the order of 102 to 103 years. Thanks to direct recordings of SV in western Europe over the past four centuries (Alexandrescu, Courtillot, and Le Mouël 1997), we know that the magnetic field has varied over the past 150 years at a rate of ~0.20°/yr for middle latitudes (McElhinny and McFadden 2000). In addition to being an erratic phenomenon, SV has a regional character, considered inhomogeneous for regions of greater than 500 km in radius. Logically, in order to determine past changes in the Earth’s magnetic field, it is necessary to use geological and/or archaeological materials susceptible to having registered these variations in magnetization over time. The archaeomagnetic method is based on the fact that a material heated to temperatures over the Curie point of ferromagnetic materials present in its composition (e.g., 580°C for magnetite), acquires a thermoremanence (TRM) at the moment of cooling. This TRM represents an instant in the history of the Earth’s magnetic field and can be very stable through time if the material remains in situ and undergoes no further thermal and/or chemical events that would reset the direction and intensity of the original magnetism. However, secondary ChAPter seVen Preliminary Archaeomagnetic and rock-magnetic results from the holocene Fire lenses in el mirón Cave Ángel Carrancho and Juan José Villalaín translated by lawrence guy straus | 104 | Chapter Seven With the goal of extending the application of archaeomagnetic methods to older archaeological materials and adding new SV data to the Iberian curve, we present here the results from a study of burnt Holocene levels in El Mirón Cave (Ramales de la Victoria, Cantabria ). The presence of many burnt levels that are well dated by radiocarbon and apparently well preserved indicates that this is a good context in which to obtain new archaeomagnetic data. In addition we conducted detailed rock-magnetic experiments to determine the properties of natural remanent magnetization (NRM) aswellasthenatureandstabilityofmineralogicalphases that carry the archaeomagnetic signal. The study of the magnetic properties (composition, concentration, and granulometry of the magnetic minerals) of these burnt facies can provide information relevant to the use and preservationofthefirestructuresandlensesintheHolocene strata of the site. the Archaeological site and Chronological Framework El Mirón Cave (43°14’48” N; 3°27’5” W) is located in the upper valley of the Río Asón on the northern edge of the Cantabrian Cordillera (Spain) at about 260 m above present sea level. The archaeological excavations directed by L. G. Straus and M. R. González Morales since 1996 have revealed a long cultural sequence with levels spanning the late Middle Paleolithic through the early Bronze Age (plus traces of medieval and modern human activity). The site has 65 radiocarbon dates, ranging from 41,000 BP (uncal.) to Ad 1400 (Straus and González Morales 2003, 2007, and unpublished data), making it one of the most complete and bestdated cave site sequences for the late Upper Pleistocene and Holocene in western Europe. The excavations have been centered on two areas, each about 9–10 m2 , in the vestibule of the cave, connected by a 9 × 1 m trench (figure 7.1). Our interest in this work lies in the major presence of burning episodes in the Holocene stratigraphy of the cave, specifically in the Neolithic, Chalcolithic, and early Bronze Age levels. The Neolithic has been identified in the outer vestibule (OV) Cabin area (Levels 10–8) and in the western half of the Mid-Vestibule (Levels 303.3–303) Trench and consists principally of clearly burned layers defined by magnetizations (e.g., viscous remanent magnetization...

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