STABLE ISOTOPE GEOCHEMISTRY AND HYDROLOGY OF THE LAKE JUNíN WATERSHED, PERU

Variations in oxygen and hydrogen isotopes in response to orographic effects across the Eastern Cordillera are well understood on the windward side of mountains, where elevation drives a predictable fractionation during precipitation. Changes in these isotopic values are recorded in paleo climatic proxies such as glaciers and have been studied extensively to reconstruct past climatic and topographic evolution. However, uncertainty in the fractionation effects of rainout and vapor transport across the Andean Plateau has limited these reconstructions. Specifically, isotopic effects in precipitation across the Central Cordillera are scarcely documented. This paper presents a modern hydrological dataset consisting of 73 sample sites collected in the Lake Junín watershed of the Central Cordillera, Peru, during the austral winter from May to September. Stable isotopic evidence including δ¹⁸O, δD, δ¹³C, and δ¹³C-DOC and basic water parameters such as pH, pCO2, TA, DO%, and dBars were collected and measured from streams, springs, and rivers in the watershed to evaluate spatial variations and observe geochemical trends. These data show that δ¹⁸O ranges from 1.50 to -15.48‰ and δD ranges from -23.3 to -121.0‰, while δ¹³C ranges from -22.11 to -32.29‰. These values did not show a progressive spatial trend from east to west or north to south but all samples fall below values of the global meteoric water line (GMWL). The LMWL has a slope of about 5.5 and is characteristic of arid to semi-arid conditions in which the vapor phase that rains out over the mountains has been enriched in light isotopes by numerous cycles of evapotranspiration over the Amazon Basin. This leaves the resulting surface waters highly enriched in lighter isotopes relative to other meteoric water despite the preferential rainout of heavier isotopes during precipitation. Anion and cation concentrations were also measured and revealed that these waters can be categorized as Ca-HCO₃ type water typical of surface waters that are dissolving limestone and calcium bearing aluminum silicates. Further analysis and interpretation will be done to more accurately characterize the geochemical conditions and origins of the surficial waters in the Lake Junín Watershed, which are vital to understanding this intricate geochemical system.