Stable Isotope Values of the Mesoamerican Monsoon: δ18O and δ2H Values Reveal Climate Controls on Summer Rainfall Amount

Abstract:

Paleoclimate reconstructions using δ¹⁸O as a proxy for the isotopic composition of rainfall are based upon the mostly untested assumption that either rainfall amount or equilibration temperature are the main drivers modulating the isotopic composition of pluvial precipitation. Whilst a broad correlation between geographical location and driving mechanisms has been long recognized (i.e. amount effect is pervasive in tropical areas), further tests are required to determine the effect that different sources of moisture might impose on the isotopic composition of precipitation, particularly in areas where contributions from different ocean-basins might be significant, such as south Mexico.

Here, we present the δ¹⁸O and δ²H composition of summer rainfall collected throughout south, central and western Mexico, particularly from Veracruz, Puebla, Guerrero, Morelos, Mexico City, Jalisco, Michoacán and Querétaro states. The geographical and temporal extent of our sampling (2004, 2005, 2007, 2008, 2011) results in a large dataset comprising more than 600 samples and represents the base data to understand the atmospheric mechanisms modulating the isotopic composition of rainfall in Mexico. Our data span a range of 30‰ in δ¹⁸O, from high values nearest the Gulf of Mexico coast and during weak rainfall events, to lowest values in high-altitude central Mexico and during heavy rainfall events associated with tropical cyclones. Values on the Pacific Coast are intermediate, and likely reflect a contribution of both Gulf of Mexico and Pacific sources. Our data define a meteoric water line of δ²H = 7.92 × δ¹⁸O + 9.48, which indicate that most precipitation values formed close to isotopic equilibrium with water vapor. The two primary physiographic variables controlling δ¹⁸O values are distance from the Gulf of Mexico and altitude, which together explain about 70% of the variation in spatial δ¹⁸O values.