ENVIRONMENTAL FACTORS CONTROLLING THE δ13C AND δ18O VARIATIONS OF RECENT FLUVIAL TUFAS: A 12-YEAR RECORD FROM THE MONASTERIO DE PIEDRA NATURAL PARK (NE IBERIAN PENINSULA)

A 12-year study (2000-2012) of the stable isotopes of recent fluvial tufas has been performed in the Monasterio de Piedra Natural Park (River Piedra, NE Iberian Peninsula), an area of Mediterranean climate with intense tufa deposition since the Pleistocene. The biannual monitoring of the tufa calcite delta C-13 and delta O-18 signatures and of the water delta O-18 value has demonstrated a clear seasonal pattern for the tufa-calcite oxygen isotope composition, with less negative values for cool periods, which is consistent with regional temperature oscillations. The tufa calcite delta C-13 values only occasionally exhibit a pattern, which is inconclusive and opposite to the delta O-18 clear pattern. The water delta O-18 signature has a distinct seasonal pattern, also opposite to the tufa-calcite delta O-18 clear pattern. The temperature calculations from the tufa-calcite delta O-18 values and average water delta O-18 values agree with the seasonal pattern of measured water temperatures, with a mean difference of 2.3 degrees C between the calculated and measured temperatures. The anomalous values recorded for both the water and the tufa-calcite oxygen isotope composition during a two-year period are characterized by smoothing of the seasonal variations of the tufa calcite and by reversal of the water pattern. There is also an increase in the differences between the measured and estimated temperatures. The detected anomaly is roughly synchronous with a change in the isotopic signature of the regional precipitation delta O-18 values. This contribution demonstrates that tufas in this depositional and climatic context are good indicators of seasonal temperature oscillations, and that tufas can also record other interannual environmental changes such as variations in the isotopic composition of precipitation. These results can be extrapolated to ancient tufa systems developed in similar contexts, which would expand the potential of tufas as high-resolution records of paleoenvironmental conditions.