Stable isotope compositions of surface water in Mexico between 22–26 °N

Abstract

We study the δ18O, δD, and d-excess values of 205 surface water samples collected in a wet and a dry season from the west to the east coast of Mexico between 22 and 26 °N to understand the controlling factors of isotopic patterns and lapse rates. The samples include lake, pond, river, spring, and tap waters. The δ18O (δD) values range from −13.2‰ (−97.2‰) to +20.3‰ (+74.0‰), and decrease from both coasts to higher elevations in the mountains adjacent to the coasts. The lake and pond waters have the largest stable isotopic variations and gentlest surface water slopes (<5.5) in both seasons, suggesting surface evaporation. Our moisture back trajectory analysis indicates that the strongest evaporation occurs in the Mexican Plateau, where dry season moisture from the east and west mix, and up to 17% of the moisture in the wet season precipitation is recycled. The river waters in the dry season and the spring waters in both seasons also have gentle surface water slopes (∼6.6), suggesting some degree of evaporation. On the Sierra Madre Occidental, the δ18O isotopic lapse rate of river and spring water is generally low, −1.3‰/km in the wet season and −1.8‰/km in the dry season, most likely because of sub-cloud evaporation of rain at high elevation. Our results highlight that isotopic lapse rates vary greatly because of changes in local climate conditions, and these changes are not necessarily contemporaneous at high and low elevations. Therefore, evaporation of ground and river water in the geologic past should be considered in stable isotope-based paleoclimate reconstructions, and a range of isotopic lapse rates should be used in stable isotope paleoaltimetry.