In large industrialized cities, tons of particles containing heavy metals are released into the environment and accumulate on street surfaces. Such particles cause a potential risk to human health due to their composition and size. The heavy metal contamination levels, main emission sources, and human health risks were identified in 482 samples of street dust. Heavy metal concentrations were obtained by microwave-assisted acid digestion and ICP-OES. The results indicated that street dust in Mexico City is contaminated mainly with Pb, Zn, and Cu, according to the contamination factor and the geoaccumulation index. The pollution load index of the street dust was made with the concentrations of Pb, Zn, Cu, Cr, and Ni. The main sources of Pb, Zn, Cu, and Cr are anthropic, probably due to vehicular traffic. The highest levels of Cr and Pb in urban dust represent a health risk for children. Contamination limits were proposed for heavy metals in street dust of Mexico City. These limits might be useful to generate and apply public policies to decrease anthropic emissions of the heavy metals studied, particularly Cr and Pb.

Thellier paleointensity experiments are carried out on the Early Cretaceous Arapey Formation, which belongs to the Parana Magmatic Province (PMP) in northern Uruguay. Twenty‐seven samples out of 225 analyzed yield reliable absolute intensity determinations. For these samples the NRM fraction f used for determination ranges between 0.33 to 0.83 and the quality factor q ranges from 4.5 to 14.1, being generally greater than 5. The flow‐mean paleointensity values obtained in this study range from 16.3 ± 2.3 to 41.5 ± 3.8 μT, with corresponding virtual dipole moments (VDMs) from 3.4 ± 0.6 to 8.2 ± 0.8 (1022 Am2). This corresponds to mean VDM value of 6.4 ± 2.3 × 1022 Am2. 40Ar/39Ar dating on the PMP sequence by Feraud et al. (1999) gave a mean age of 132.2 ± 0.5 Ma. Our results in conjunction with previous studies (Kosterov et al., 1998; Goguitchaichvili et al., 2002) suggest that the paleostrength during the early Cretaceous might be as variable and strong as Brunhes data and not anomalously low as previously suggested. The scatter observed in our study is similar to recent findings by Granot et al. (2007) for the Troodos Ophiolite.

Palabras clave: contaminación, kriging indicador, norma oficial mexicana, plomo, vanadio RESUMEN Siete millones de personas mueren al año por cáncer relacionado con la contaminación ambiental. El polvo urbano (PU) contiene partículas < 10 µm que afectan la salud humana. En la Ciudad de México (CDMX) existe sólo un estudio de PU. El objetivo de este estudio fue identificar las zonas de la CDMX con mayor probabilidad de contaminación por metales pesados (MP) en el PU. Se recolectaron 89 muestras de PU y se midió la concentración de MP por fluorescencia de rayos X. El análisis espacial se realizó mediante interpolación con kriging indicador. El porcentaje de la superficie con mayor probabilidad de que los MP superen los límites máximos permitidos (LMP) fueron: Cr (LMP 280 mg/kg), 0.55 %, suroeste; Cr (LMP 64 mg/kg), 89 %, toda la CDMX y la mayoría de las áreas conurbadas (AC); Cu, 17 %, 11 delegaciones y AC del norte; Ni, 13 %, centro y suroeste; Pb (LMP 140 mg/kg), 17.5 %, delegaciones céntricas y AC del noroeste; Pb (LMP 400 mg/kg), 2.2 %, delegaciones céntricas y AC del noroeste; V, 26 %, delegaciones céntricas y del sur, así como AC del noreste; Zn, 21 %, delegaciones céntricas y AC del norte. Los elementos con mayor número de sitios y superficie contaminada son: Cr, Zn, Cu y Pb. Los sitios con mayor superficie de sobreposición de MP son aquellos con asociaciones entre Cu y Zn (15 %), y Cu y Pb (13 %). Las delegaciones Cuauhtémoc, Venustiano Carranza y Gustavo A. Madero son las más contaminadas con metales pesados. ABSTRACTSeven million people die per year of cancer related to environmental pollution. Urban dust (UD) contains particles smaller than 10 μm that affect human health. In Mexico City (CDMX) only one study on UD has been conducted. The objective of this study was to identify the areas most likely to be contaminated by heavy metals (HM) contained in the UD of CDMX. Eighty nine UD samples were collected; the analysis of the HM concentration was made by X-ray fluorescence, and spatial analysis by interpolation with kriging indicator. The percentage of the area with the highest probability that HMs exceeded the maximum permitted limits (MPL) were: Cr (MPL 280 mg/kg), 0.55 %, southwest of the CDMX; Cr (MPL 64 mg/kg), 89 %, all CDMX and most of the conurbated areas (CA); Cu, 17 %, 11 municipalities and the northern CA; Ni, 13 %, center and southwest of the CDMX; Pb (MPL 140 mg/kg), 17.5 %, centric municipalities and the northwest CA; Pb (MPL 400 mg/kg), 2.2 %, centric municipalities and the northwest CA; V, 26 %, centric and southern municipalities and the northeast CA; Zn, 21 %, centric municipalities and the northern CA. Elements with a greater number of sites and contaminated surface are: Cr, Zn, Cu and Pb. The largest HM overlapping sites are those with Cu and Zn associations (15 %), and Cu and Pb associations (13 %). The municipalities Cuauhtémoc, Venustiano Carranza and Gustavo A. Madero are the most contaminated with heavy metals.

The geomagnetic field variations on the continent of Africa are still largely undeciphered for the past two millennia. In spite of archaeological artefacts being reliable recorders of the ancient geomagnetic field strength, only few data have been reported for this continent so far. Here we use the Thellier-Coe and calibrated pseudo-Thellier methods to recover archaeointensity data from Burkina Faso and Ivory Coast (West Africa) from well-dated archaeological artefacts. By combining our 18 new data with previously published data from West Africa, we construct a reference curve for West Africa for the past 2000 years. To obtain a reliable curve of the archaeointensity variation, we evaluate a penalized smoothing spline fit and a stochastic modelling method, both combined with a bootstrap approach. Both intensity curves agree well, supporting the confidence in our proposed intensity variation during this time span, and small differences arise from the different methodologies of treating data and uncertainties. Two prominent peaks at around 740 AD and 1050 AD appear to be common in ours and several reference curves from other locations, indicating a general westward movement from China to Hawaii of a rather stable feature of the geomagnetic field. However, independent smaller peaks that do not correlate in different locations may hint to localized expressions of the geomagnetic field as a result of temporarily varying non-dipole sources.Archaeological artefacts have already in the 1930s been proven to be excellent recorders of the geomagnetic field (GMF) for the past 10 millennia 1 . This type of material acquires a thermoremanent magnetization (TRM) at the time of its heating and subsequent cooling that is proportional to the ancient GMF intensity, or archaeointensity. This TRM is dependent on many factors including the ancient heating temperature, the type of magnetic mineral carrying the magnetization, and the reheating process.Since the beginning of archaeomagnetism, retrieving archaeointensities has been considered a demanding process. Several methods have been developed, for example the thermal Thellier-family methods 2-4 , the microwave technique 5,6 , the multispecimen methods 7,8 and non-heating methods 9,10 . One of the non-heating methods, the relative pseudo-Thellier technique 11 , makes use of alternating magnetic fields to demagnetize the samples and has recently been successfully calibrated and applied to volcanic rocks 12-14 . However, the calibrated pseudo-Thellier method has never been applied to archaeological artefacts before. The Thellier-family methods are considered as the most reliable of all techniques, because they have been used since nearly 100 years and include many quality assessments. These methods can comprise up to 50 time-consuming heating steps. On the contrary, the pseudo-Thellier method is much faster and avoids chemical alteration of the samples.Archaeological artefacts have been collected from many locations world wide to construct palaeosecular variation curves, or reference c...

We present here new paleointensity and geochronology results from Early Cretaceous volcanic rocks of Sierra Chica de Cordoba (Argentina). The new K-Ar isotopic ages of 5 samples range from 136 to 122 Ma. Twenty five samples from 7 individual flows yielded acceptable paleointensity estimates. The mean paleointensity values per flow are ranging from 53.0 ± 1.9 to 25.4 ± 2.6 μT and the corresponding Virtual Dipole Moments (VDMs) are ranging from 9.3±1.3 to 4.6±0.5 (10 22 Am 2 ). This corresponds to the mean value of 7.3±1.7×10 22 Am 2 , which is compatible to the present geomagnetic axial dipole. Currently available selected paleointensity data from 80 to 130 Ma suggest that geomagnetic field strength frequently fluctuated before and during the Cretaceous Normal Superchron while the magnetic polarity maintained stable. The mean paleointensities derived from Córdoba lavas agree remarkably well with those obtained from the Paraná Magmatic Province (133-132 Ma). This reinforces the hypothesis about the unreliability of 'Mesozoic Dipole Low'.