Stable isotope signature of the Middle Holocene climatic change in the SW Iberia

Abstract

Between 7.5 kyr and 4 kyr a fluvial limestone tuffs system developed in the south-western corner of Iberia. This system is characterised by a segmented up to 60 meters thick limestone sequence dated by radiocarbon. The system is calcite dominated, and the other coeval mineral constituents comprise iron oxides and hydroxides and clay minerals. Other mineralogical components such as quartz and feldspar are seldom and detrital, from the surrounding Upper Carboniferous and Upper Triassic formations. Most of the tuffs are well banded, with a layering which ranges in thickness from the micrometric scale to the centimetric scale. The different centimetric layers were sampled and bulk δ18O, δ13C and 87Sr/86Sr were determined. The oxygen and carbon isotopes were used as recognized proxies for paleoenvironmental conditions and the Sr isotopes with the purpose of understanding the residence time of the meteoric waters in the surrounding carbonate formations. The main results point to: (i) the hydrologic conditions of the system were not suitable for the waters from which the limestone tuffs precipitated to equilibrate with the Mesozoic carbonate formations; (ii) the probability of having old carbon in those waters is very low increasing the reliability of the 14C age determinations; (iii) the composition in stable isotopes of the waters were probably close to the meteoric water composition for the period under consideration; (iv) between 7.5 kyr and 4 kyr the oxygen isotopic composition of the limestone tuffs show a gradual depletion in 16O with the values of δ18O increasing from -5 to -3.5 probably related to an increase in the importance of the evaporation in the hydrological system; (v) during the same time interval the 13C composition of the limestone tuffs remains crudely constant reflecting the stability of the vegetation cover. Acknowledgement: Financial support from HOLOCLIMA project (PTDC/CTE-GEX/71298/2006).