Ángel Carrancho

Age and Date for Early Arrival of the Acheulian in Europe (Barranc de la Boella, la Canonja, Spain)

The first arrivals of hominin populations into Eurasia during the Early Pleistocene are currently considered to have occurred as short and poorly dated biological dispersions. Questions as to the tempo and mode of these early prehistoric settlements have given rise to debates concerning the taxonomic significance of the lithic assemblages, as trace fossils, and the geographical distribution of the technological traditions found in the Lower Palaeolithic record. Here, we report on the Barranc de la Boella site which has yielded a lithic assemblage dating to ∼1 million years ago that includes large cutting tools (LCT). We argue that distinct technological traditions coexisted in the Iberian archaeological repertoires of the late Early Pleistocene age in a similar way to the earliest sub-Saharan African artefact assemblages. These differences between stone tool assemblages may be attributed to the different chronologies of hominin dispersal events. The archaeological record of Barranc de la Boella completes the geographical distribution of LCT assemblages across southern Eurasia during the EMPT (Early-Middle Pleistocene Transition, circa 942 to 641 kyr). Up to now, chronology of the earliest European LCT assemblages is based on the abundant Palaeolithic record found in terrace river sequences which have been dated to the end of the EMPT and later. However, the findings at Barranc de la Boella suggest that early LCT lithic assemblages appeared in the SW of Europe during earlier hominin dispersal episodes before the definitive colonization of temperate Eurasia took place.

Magnetic fingerprint of tsunami-induced deposits in the Ixtapa-Zihuatanejo Area, Western Mexico

The Pacific coast of Mexico has repeatedly been exposed to destructive tsunamis. Recent studies have shown that rock magnetic methods can be a promising approach for identification of tsunami- or storm-induced deposits. We present new rock magnetic and anisotropy of magnetic susceptibility (AMS) results in order to distinguish tsunami deposits in the Ixtapa–Zihuatanejo area. The sampled, 80 cm-deep sequence is characterized by the presence of two anomalous sand beds within fine-grained coastal deposits. The lower bed is probably associated with the 14 March 1979 Petatlán earthquake (M W = 7.6), whereas the second one formed during the 21 September 1985 Mexico earthquake (M W = 8.1). Rock magnetic experiments discovered significant variations within the analysed sequence. Thermomagnetic curves reveal two types of behaviour: one in the upper part of the sequence, after the occurrence of the first tsunami, and the other in the lower part of the sequence, during that event and below. Analysis of hysteresis parameter ratios in a Day plot also allows us to distinguish two kinds of behaviour. The samples associated with the second tsunami plot in the pseudo-single-domain area. In contrast, specimens associated with the first tsunami and the time between both tsunamis display a very different trend, which can be ascribed to the production of a considerable amount of superparamagnetic grains, which might be due to pedogenic processes after the first tsunami. The studied profile is characterized by a sedimentary fabric with almost vertical minimum principal susceptibilities. The maximum susceptibility axis shows a declination angle D = 27°, suggesting a NNE flow direction which is the same for both tsunamis and normal currents. Standard AMS parameters display a significant enhancement within the transitional zone between both tsunamis. The study of rock magnetic parameters may represent a useful tool for the identification and understanding of tsunami deposits.

Thermomagnetic monitoring of lithic clasts burned under controlled temperature and field conditions: Implications for archaeomagnetism

Resumen Se presenta un estudio combinado termico y magnetico sobre un conjunto de clastos liticos de diferentes litologias tallados experimentalmente (silex, cuarcita, caliza, arenisca y obsidiana), calentados bajos condiciones de campo y temperatura controladas. El objetivo principal de este estudio es evaluar la viabilidad de uso de estas materias primas, comunmente encontradas en yacimientos arqueologicos prehistoricos, para fines arqueomagneticos. Los analisis del magnetismo de las rocas comprendieron la medida de la susceptibilidad magnetica a bajo campo, curvas de adquisicion progresiva de la magnetizacion remanente isoterma (IRM), ciclos de histeresis y curvas termomagneticas de los clastos liticos tanto antes como despues del calentamiento experimental. Todas las litologias salvo la obsidiana, registraron un incremento de hasta dos ordenes de magnitud en sus parametros dependientes de la concentracion magnetica, indicando la formacion de nuevos minerales ferrimagneticos. Las muestras de obsidiana y arenisca son los portadores de la remanencia mas fiables, seguidos de caliza, silex y cuarcita. Los valores de susceptibilidad magnetica muestran diferencias significativas entre litologias. La magnetizacion remanente isoterma demostro ser tambien altamente discriminatoria asi como los parametros de histeresis a temperatura ambiente. Las principales alteraciones macroscopicas fueron cambios de coloracion, rubefacciones, depresiones circulares (potlids) en los silex y la formacion masiva de fisuras internas en los especimenes de obsidiana. La tecnica de paleointensidad multiespecimen fue aplicada en muestras representativas proporcionando resultados satisfactorios para las muestras de obsidiana y arenisca. Se discute la aplicabilidad arqueologica de los resultados asi como tambien su relevancia geomagnetica.

Rock-magnetic and paleomagnetic results from the Tepic-Zacoalco rift region (western Mexico)

A rock-magnetic and paleomagnetic investigation was carried out on eleven Pleistocene and Pliocene 40Ar/39Ar dated lava flows from the Tepic-Zacoalco rift region in the western sector of the Trans-Mexican Volcanic Belt (TMVB) with the aim of obtaining new paleomagnetic data from the study region and information about the Earth’s magnetic field recorded in these rocks. Rock-magnetic experiments including measurement of thermomagnetic curves, hysteresis parameters and isothermal remanence acquisition curves were carried out to find out the carriers of remanent magnetisation and to determine their domain structure. Although some samples were characterised by the presence of a single ferromagnetic phase (magnetite), in most cases more phases were observed. Analysis of hysteresis parameters showed a mixture of single domain and multidomain particles, the fraction of the latter varying between 40% and 80%. Paleomagnetic results were obtained in all sites, although in 7 sites characteristic remanence directions and remagnetisation circles had to be combined in order to calculate site means. The six Pliocene sites not showing intermediate polarity yielded a paleomagnetic pole (latitude ϕ = 81.1°, longitude λ = 94.3°) which roughly agrees with the expected one. Paleomagnetic directions do not indicate significant vertical-axis block rotations in the western TMVB area. Reversed polarities observed can be correlated to the Gilbert chron, normal polarities to the Gauss chron or the Brunhes chron and intermediate polarities to the Cochiti-Gilbert or the Gilbert-Gauss transition. The reversed or intermediate polarity magnetisation recorded in one of the sites (542 ± 24 ka) corresponds either to the West Eifel 4 or the West Eifel 5 excursion, while the reversed polarity observed in the other site (220 ± 36 ka) very likely provides new evidence for the Pringle Falls excursion or the event recorded in the Mamaku ignimbrite.

A comparison of Thellier‐type and multispecimen paleointensity determinations on Pleistocene and historical lava flows from Lanzarote (Canary Islands, Spain)

project CGL2012-32149 (Ministerio de Econom ia y Competitividad, Spain), project 320/2011 (Ministerio de Medio Ambiente y Medio Rural y Marino, Spain) and the European Regional Development Fund (ERDF).

A detailed paleomagnetic and rock-magnetic investigation of the Matuyama-Brunhes geomagnetic reversal recorded in the tephra-paleosol sequence of Tlaxcala (Central Mexico)

Geomagnetic reversals are global phenomena, for about 50 years the paleomagnetists attempted to acquire as many detailed records as possible using the magnetic memory of sediments and lava flows. Yet, transitional field behavior remains poorly characterized largely because of sporadic aspect of volcanic eruptions. In some specific cases, paleosols such as those developed from alluvial or aeolian sediments, may also record the variations of the Geomagnetic Field across the polarity changes. Here, we report a detailed paleomagnetic and rock-magnetic investigation on some radiometrically dated chromic luvisols located in Central Mexico carrying detrital or chemical remanent magnetization. The research was developed in order i) to demonstrate the primary origin of the magnetic remanence and ii) to show that paleosoils are good candidates to provide a high resolution record of the behavior of geomagnetic field during reversals. The lower part of the paleosoil sequence shows a clearly defined reverse polarity magnetization followed by geomagnetically unstable transitional field and ended by normal polarity remanence. Our AMS and rock magnetic data suggest that magnetization is acquired during the initial stage of soil formation in context of active volcanic activity since magnetic fabric is essentially sedimentary and reverse and normal polarity paleodirections are almost antipodal. Titanomagnetites are identified as main magnetic carriers of rock-magnetic measurements including thermomagnetics and hysteresis cycles. We propose that the transition recorded in this study correspond to the B-M boundary, considering the K-Ar datings available at the sequence bottom and that the chromic luvisols are potentially good recorders of the paleosecular variation. The identification of the B-M boundary within the studied sequence has fundamental significance for improving the chronological scale of Tlaxcala paleosol-sedimentary sequence and its correlation with the global proxies.