Abstract

AGUILAR-RAMIREZ, César F. et al. Variation in isotopic composition of meteoric water along the central-northeastern section of the Mexican Fold-and-Thrust Belt, Sierra Madre Oriental. Bol. Soc. Geol. Mex [online]. 2017, vol.69, n.2, pp.447-463. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2017v69n2a9.

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 precipitationperiods. Sampling of meteoric water wascarried 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 WaterLine (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.