Freshwater tufa record from Spain supports evidence for the past interglacial being wetter than the Holocene in the Mediterranean region

摘要

The sedimentation of spring-fed tufa deposits is subject to two main factors: the ground water discharge from the spring and the supersaturation of such waters with respect to carbonate at the surface. In Trabaque Canyon (central Spain), several spring tufa deposits were formed during the Pleistocene and Holocene, mostly during mild periods. They were linked to the outflow of ground waters that in each period cropped at a different site along the canyon, depending on the intersection of the water table and the thalweg. Recent lowering of the water table due to changes in land uses have resulted in a downstream shift of the spring location along the bottom of the valley, confirming the relationship between the water table level and the spring location. The elevation of the deposits is independent of their age and geomorphologic evidence suggests that although over ten metres of tufa has been repeatedly deposited and eroded, there was not a lowering of the base level in the canyon since the last interglacial. Thus, it is possible to compare the water table levels during the last two interglacial periods. Due to the link of the spring and the tufa deposits, the location of the latter has been considered an indicator of ancient water table and consequently as a proxy of the recharge by rainwater to the aquifer. Geomorphic comparison of deposits from the previous interglacial to those from the Holocene indicates that the former are at higher elevations along the valley, suggesting that this period was wetter than the Holocene. Comparison of Trabaque Canyon record with other paleo-hydrological reconstructions from Southern Europe and the Mediterranean agree, supporting the scarce number of continental records in which interglacial comparisons are possible in the region.