TEMPORAL BIASES IN THE FORMATION OF AUTHIGENIC AND BIOGENIC MINERALS AND THEIR IMPACT ON STABLE ISOTOPE-BASED PALEOALTIMETRY

The study of paleoelevation and paleorelief using stable isotope approaches relies extensively on utilizing isotopic data obtained from authigenic or biogenic minerals as a proxy for stable isotope ratios of ancient precipitation. It is well known, however, that these minerals do not form in equilibrium with precipitation, rather that form from ‘surface waters' that can be affected by a variety of hydrological processes. Equally important, these minerals also form over a range of geochemical conditions, and therefore potentially at different times of the year. For example, clays minerals form when soils are wet, carbonate minerals form when soils are drying, and vertebrates drink and form enamel at all times of the year. Both the hydrological processes that may effect oxygen isotope ratios of ‘surface waters' and possible seasonal biases in the timing of mineral formation need to be considered (1) when assuming the oxygen isotope ratio of low-elevation waters that are then compared with high-elevation proxy data, (2) when comparing isotopic data obtained from different minerals to make stable isotope maps (3) when applying empirical or model lapse rates based on the analysis of mean annual precipitation (4) when comparing proxy isotope data to modern isotope data of precipitation. Examples of problems that may arise if these variables are not considered are illustrated using published isotope data. Examples will also be given of the benefits of collecting isotopic data from multiple proxies, specifically those related to a more precise characterization of paleoclimate.