STABLE ISOTOPE COMPOSITIONS OF MODERN SURFACE WATER IN THE SIERRA MADRE OCCIDENTAL, MEXICO

The North American Monsoon is a major climate feature in southwestern USA and northwestern Mexico. Intense summer heating of the Mexico Plateau and surrounding area causes a low-pressure system, creating low-level moisture transport toward land from the Gulf of California and eastern Pacific. At the same time, easterly wind brings moisture from the Gulf of Mexico landward. Despite the fact that the monsoon brings more than 70% of the annual precipitation to northwestern Mexico, as summer precipitation, the timing and cause of its birth are not well understood. The high Sierra Madre Occidental plays a major role in dividing the moisture sources in Mexico. The monsoon mainly reaches its western slope and travels northward, while the eastern source governs precipitation to the east of the range. The change in moisture source and the high elevation of the Sierra Madre Occidental should influence the stable isotope compositions in surface water. To date, the seasonal variation and lapse rate of the surface water stable isotope compositions and their controlling factors in Mexico have not been studied. Here we study the δ¹⁸O, δD and d-excess values of modern surface water samples collected across the Sierra Madre Occidental in the wet season and integrate climatic and geographic data to understand their spatiotemporal patterns and controlling factors. Our samples were collected along two transects across the varying elevation of the Sierra Madre Occidental; further, one of the two transects extends to the east coast of Mexico. These data will show spatial variation of the isotope lapse rate as well as the influence of vapor sources on the isotope lapse rate. This study is the first to investigate the variability of modern surface water isotope compositions across the topography of northern Mexico. The new understanding has implications for paleoelevation reconstruction of the Sierra Madre Occidental and evolution of the North American Monsoon.