Galia González-Hernández

RADIOCARBON LABORATORY AT THE NATIONAL AUTONOMOUS UNIVERSITY OF MEXICO: FIRST SET OF SAMPLES AND NEW 14C INTERNAL REFERENCE MATERIAL

This contribution reports the first set of samples and the creation of an internal reference material at the recently opened Radiocarbon Laboratory at the National Autonomous University of Mexico (UNAM). Samples for the initial measurements were selected from archaeological and dating projects on Teotihuacn, one of the largest and best-studied Mesoamerican urban and ceremonial centers. The 14C dates were compared to results obtained by 2 other laboratories in order to assess the results obtained at UNAM and validate the adopted methodology. As part of the quality assurance protocol, an internal reference material was created that consists of charred wood from the Teotihuacn site with a 14C activity in the value range expected for samples from Mesoamerican archaeological sites. Results from 7 analyses have a mean of 1750 16 BP (80.43 0.16 pMC).

Temporal and Spatial Variations of Atmospheric Radiocarbon in the Mexico City Metropolitan Area

The Mexico City Metropolitan Area (MCMA) produces a complex mixture of gases and aerosols from diverse sources, including burning of fossil fuels, biomass, and wastes, with a significant biogenic contribution. We present the first results of ongoing projects to study temporal and spatial variations of 14 CO 2 in the area. Temporal variations reconstructed from tree rings of Taxodium mucronatum indicate a considerable radiocarbon depletion, in accordance to the vast amount of fossil fuels burnt inside Mexico Valley, with values between 62 and 246‰ lower than background values for the 1962–1968 period, and lower by 51–88‰ for the 1983–2010 period. The lower dilution found for the last decades might indicate an increase in enriched 14 CO 2 sources. Results from the spatial distribution, as revealed from integrated CO 2 samples and grasses from six points within the MCMA collected during the 2013 dry season, show variations between sites and sample types. For integrated CO 2 samples, values range from 35.6‰ to 54.0‰, and for grasses between –86.8‰ and 40.7‰. For three of the sampling points, the grasses are significantly depleted, by up to ~133‰, as compared to the corresponding integrated CO 2 sample. This may result from differences in the carbon assimilation period and exposure to different CO 2 sources. Higher-than-background Δ 14 C values were found for all integrated CO 2 samples, presumably resulting from 14 C-enriched CO 2 derived from forest fires in the mountains during the sampling period. Results obtained so far confirm the complexity of the 14 C cycle in the MCMA. DOI: 10.2458/azu_rc.57.18360

Modern Radiocarbon Levels for Northwestern Mexico Derived from Tree Rings: A Comparison with Northern Hemisphere Zones 2 and 3 Curves

The radiocarbon variation for northwestern Mexico during the period 1950-2004 was studied by accelerator mass spectrometry (AMS) and liquid scintillation counting (LSC) analyses of tree rings. Two tree-ring sequences of Pseudotsuga menziesii, sampled in a site isolated from urban centers and active volcanoes (26.18°N, 106.3°W, 3000 m asl), were dendrochronologically dated and separated in annual rings prior to 14C analysis. Results obtained show a similar profile to the values reported for the Northern Hemisphere (NH), having significant correlation coefficients with the compilation curves for NH zone 2 (r = 0.987, p < 0.001) and NH zone 3 (r = 0.993, p < 0.001). The maximum peak is centered at 1964.5 with a ∆14C value of 713.15 ± 9.3‰. The values obtained for the period 1958-1965 are lower than zone 2 values and higher than zone 3 values. For the period 1975-2004, the values obtained are higher than the NH compilation curve and other NH records. We attribute the first divergence to the North American monsoon that may have carried 14C-depleted air from the south during the summer months; the second divergence may be attributable to 14C-enriched biospheric CO2.

Changes in CO2 Emission Sources in Mexico City Metropolitan Area Deduced from Radiocarbon Concentrations in Tree Rings

We present radiocarbon ( 14 C) in tree rings from Mexico City and a reconstruction of fossil CO 2 concentrations for the last five decades, as part of a research program to understand the 14 C dynamics in this complex urban area. Background values were established by 14 C concentrations in tree rings from a nearby clean area. Agreement between background and NH-zone 2 values indicate Taxodium mucronatum is a good biomonitor for annual atmospheric 14 C variations. Values for the urban tree rings were significantly lower than background values, indicating a 14 C depletion from fossil CO 2 emissions. There is an increasing trend of fossil CO 2 between 1960 and 1990, in agreement with the population growth and the increasing demand for fossil fuels in Mexico City. Between 1990 and 2000, there is an apparent decrease in fossil CO 2 concentration, increasing again after 2000. The decrease in 2000, despite being of the same magnitude as the overall uncertainty, may reflect environmental policies that improved the energy efficiency and reduced CO 2 emissions in the area. The increase in fossil CO 2 concentration between 2000 and 2010 may be attributable to the significant growth of motor vehicle usage in Mexico City, which made transportation the main energy-demanding and -emitting sector.