Holocene snail shell isotopic record of millennial-scale hydrological conditions in western Mediterranean: Data from Bauma del Serrat del Pont (NE Iberian Peninsula)

摘要

Land snail shells are a common component in Mediterranean Holocene archaeological deposits, providing the opportunity to explore their potential as source of information concerning human behaviour and palaeoclimatic conditions. Many well-preserved shells of the caenogastropod Pomatias elegans were recovered along the Holocene succession of Bauma del Serrat del Pont (BSP), in the Iberian Peninsula. Their oxygen (δ~(18)Os) and carbon (δ~(13)Cs) isotopic compositions were analysed and compared with that of modern shells of the same species collected in two distinct areas near the archaeological site. Modern shells from Site A (shady) and B (sunny) show distinctly different oxygen isotopic ratios possibly due to the effect of micro-environmental conditions (e.g. temperature and relative humidity). Carbon isotopes, by contrast, reveal similar values. Isotope mass balance suggests a prevalent contribution of vegetation (~ 70%), integrated with foreign carbonates (~ 30%) to δ~(13)Cs of modern snails. Early-late Holocene shells (~ 9-2.5 cal ka BP) have lower δ~(18)Os compared with modern counterparts, which is consistent with prevailing wetter conditions compared with present day. The δ~(13)Cs reveals distinct hydrological regimes, wet and dry conditions, from early to late Holocene respectively. In general, shell isotopic records from western and central Mediterranean regions suggest wetter conditions during the middle Holocene, with a possible reduction in humidity from ~4 cal ka BP. The δ~(18)Os indicates a possible latitudinal difference in hydrological balance between Mediterranean and Atlantic Europe as inferred by previous studies of regional palaeoclimatic records. Carbon isotopes, by contrast, do not provide a clear climatic picture, probably due to the effect of distinct vegetation structure and composition. Comparisons with other environmental archives reinforce the concept of regional shell oxygen isotopic response to millennial-scale changes in hydrological condition over the western and central Mediterranean during the late Quaternary.