Oxygen and carbon isotope signatures in late Neogene horse teeth from Spain and application as temperature and seasonality proxies

Abstract

Here we describe a data set of 249 stable O and C isotope compositions from fossil horse teeth from the Upper Neogene of Central Spain and discuss their potential as environmental proxies. It has been shown before that horse tooth enamel δ 18O is a good proxy for local meteoric water δ 18O H2O, which, in turn, bears a distinct relation to temperature during precipitation formation at mid to high latitudes. We discuss the reconstructed δ 18O H2O values, and apply modern δ 18O H2O — temperature equations for the Iberian Peninsula to these values. Results for the Early Pliocene suggest mean annual temperatures up to 2–3 °C higher than today, fitting pollen-based estimates fairly well. Temperatures 5 °C higher than today are predicted for the early Late Miocene (11–9 Ma). These estimations are feasible given the local occurrence of crocodilians and the presence of fruit-eating hominids in Iberia at that time. The estimated amount of cooling since then is similar to the estimated decrease in NW Europe based on plants. The stable carbon isotope record indicates the virtual absence of C4 plants, pointing to an environment consisting of C3 plants under slightly water-stressed conditions. The long-term δ 13C trend follows the expected decrease in average global δ 13C CO2 predicted from planktonic foraminifers. Within-tooth variability is supposed to reflect seasonal change, but the amplitude is expected to have been significantly damped by internal time-averaging effects related to the mixing with atmospheric O 2, reservoir dynamics, enamel maturation and sampling procedure. The exact degree of damping is difficult to estimate, but theoretical considerations suggest values between 50 and 75%, depending on temporal resolution and element used (C/O). Reconstructed seasonal temperature ranges for Central Spain are equal or slightly less than the modern range. Within-tooth δ 13C variation is more difficult to explain, as it can be attributed to multiple factors, such as intra-annual changes in δ 13C CO2, micrometeorology, plant response and diet. The occurrence of both positive and negative intra-tooth δ 18O– δ 13C correlations may be related to orbitally-forced variations in seasonality.