Utilizing continental hydrothermal systems as a record of ancient precipitation oxygen isotopes: examples from the Miocene San Juan Mountains

Abstract:

The use of water isotopes, both O and H, is a crucial component of paleoclimate and hydroclimate reconstructions. Yet, a continuing issue for accessing this record in the deeper past is untangling large scale patterns from seasonal effects, diagenesis, and other confounding record in sedimentary and paleobontanical archives. We take advantage of the well known and studied behavior of hydrothermal alteration of igneous rocks by meteoric water to access an alternative archive. Our approach is to take a spread (10x10 km) of whole rock data, interpolate between these data to make a contoured cross-section, and apply an inverse model to "undo" the aqueous alteration and calculate the original, meteoric water oxygen isotope composition. We have applied this technique in the San Juan Mountains, Colorado USA, which preserve a series of calderas and hydrothermal systems ranging in age from 25-15Ma. Our analysis suggests a 7‰ depletion in precipitation oxygen isotope values between 20-15 Ma, and tentatively ascribe this depletion to surface uplift during this time. More broadly, this technique has the advantage of reflecting climatic timescale shifts in oxygen isotope values, since these hydrothermal systems are generally active for 1000s of years. Thus, we average out any affects of seasonality. In addition, the high-temperature nature of the hydrothermal oxygen isotope exchange renders the rocks more resilient to later isotope exchange, which is often problematic for sedimentary archives that are formed at low, surface temperatures. This approach should be able to be applied widely across much of western North America, and anywhere where there are hydrothermal systems driven by meteoric water, and should serve as a compliment to other paleohydrological techniques.