New Mexico Geological Society Annual Spring Meeting — Abstracts

Carbon-14, oxygen-18, deuterium, and noble gases were measured in 35 wells and springs in the Ojo Alamo Sandstone, Nacimiento Formation, and San Jose Formation of the central San Juan Basin, New Mexico. Carbon-14 dating was performed by five methods of correcting for dilution of groundwater during recharge by nonradioactive carbon. Despite some differences between the isotopic exchange processes considered by each method, the correction techniques provided a reasonably consistent range of groundwater ages. The calculated ages ranged from modern to greater than 35,000 years BP and formed a hydrologically plausible age distribution pattern. Oxygen-18 and deuterium concentrations exhibited significant correlations with groundwater ages. Paleoclimatic temperatures were calculated from the shifts in stable isotope concentrations relative to the meteoric water line. Noble gas paleothermometry provided some independent verification of the stable isotope temperatures. The temperature pattern indicates a climatic waryning beginning at 23,000 BP continuing until approximately 17,000 BP. This correlates well with the early melting of San Juan Mountain glaciers. Although data are limited, the Holocene period appears to be characterized by the continuation of cooler temperatures until around 7,000 BP when climatic warming occurred. Stable isotope shifts were also used to estimate relative groundwater recharge rates and .provided reasonable correlation with lake level fluctuations estimated by independent methods. Indications are that higher than normal recharge occurred prior to 22,000 BP and again during the period from 19,000 to 17,000 BP. Typical Holocene recharge rates appear to be comparable to modern ones. An important advantage of the stable isotope methods is that both temperature and precipitation, as reflected by groundwater recharge, can be separated and studied as independent factors, which is not usually possible by paleobotanical techniques.