Boletín de la Sociedad Geológica Mexicana Volumen 69, núm. 2, 2017, p. 447 ‒ 463 http://dx.doi.org/10.18268/BSGM2017v69n2a9

Variación en la composición isotópica del agua meteórica a lo largo de la sección centro-noreste de la Sierra Madre Oriental

César F. Aguilar-Ramírez, Antoni Camprubí, Elisa Fitz-Díaz, Edith Cienfuegos-Alvarado, Pedro Morales-Puente

César F. Aguilar-Ramírez
This email address is being protected from spambots. You need JavaScript enabled to view it.
Programa de Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México; Ciudad Universitaria, 04510 Coyoacán, CDMX, México.

Antoni Camprubí
Elisa Fitz Díaz
Edith Cienfuegos Alvarado
Pedro Morales Puente
Instituto de Geología, Universidad Nacional Autónoma de México; Ciudad Universitaria, 04510 Coyoacán, CDMX, México

Abstract

We analyze the variations in isotopic composition of meteoric water along a transect in the Mexican Fold-Thrust Belt, located in the central–northeastern part of the Sierra Madre Oriental. The aim is to determine such variations as a function of 1) increasing topographic elevation and 2) the distance of rain inside the continent since evaporation occurrence of oceanic water from the Gulf of Mexico. Topographic and climate characteristics were considered in the studied section, which was chosen because it presents some advantages as follows. Deformation in this section exhibits orogenic wedge features, which generated an average slope between 1° and 1.5°, with no topographic barriers that obstruct the free circulation of wind. This allows us to evaluate significant isotopic variations of H and O as a function of topographic elevation. The section is located at constant latitude between 21° and 22° N. Furthermore, the average annual temperature decreases gradually from the foreland to the hinterland of the section during precipitation periods. Sampling of meteoric water was carried out by installing 12 rainwater collectors at heights between 2740 and 56 masl along the transect. Additionally, water samples were collected in rivers and springs during the 2012 rain season in zones of constant water runoff in 7 different localities. Obtained values from rain water vary between -4.89 and -11.75% for δ¹⁸O and between -25.35 and -80.34 ‰ for δ²H (relative to VSMOW)—These allowed to determine a Local Meteoric Water Line (LMWL) defined by the equation δ²H = δ¹⁸O * 8.15 + 15. With this, fractionation rate relative to elevation for O and H (δ¹⁸O y δ²H) were obtained at 2.19 ‰ km^-1 and 17.75 ‰ km^-1, respectively. The LMWL and fractionation rates obtained in this study can be used as regional-scale references in order to evaluate the role of meteoric water involved in atmospheric and shallow cortical processes.

Keywords: Isotopic composition, δ²H, δ¹⁸O, meteoric water, isotopic fractionation, stable isotopes.