Succession and stable isotope composition of gastropods in Holocene semi-arid alluvial sequences (Bardenas Reales, Ebro Basin, NE Spain): Palaeoenvironmental implications

Three alluvial units, ranging from the middle to late Holocene in age, were distinguished in Bardenas Reales Natural Park (Ebro Basin, NE Spain) based on morphosedimentary analysis and radiocarbon data. In this sensitive Mediterranean alluvial system, the succession and isotopic composition of gastropods greatly contribute to establishing the prevailing palaeoenvironmental conditions. The oldest unit (4763±87 to 2848±55 cal. yr BP) represents fluvial channels and swamped areas, includes the greatest proportion of aquatic gastropods (Ancylus fluviatilis and Lymnaea truncatula) and marsh species (Vallonia pulchella) and presents the most negative values of δ18O (mean −2.21‰ PDB). The intermediate unit (1403±60 to 836±65 cal. yr BP) shows a high rate of alluvial activity related to irregular torrential rainfall, with sedimentary features of aridity, and this unit includes only dry open ground gastropods (Xeroplexa sp., Chondrinidae indet. and Granopupa granum) and presents the least negative values of δ18O (mean −1.17‰ PDB). The youngest unit (191±97 to 127±82 cal. yr BP) exhibits a recurrence of flooding events, mostly includes marsh (Truncatellina callicratis and Lauria cylindracea) and aquatic (Lymnae truncatula) species and shows a very heterogeneous isotopic signal, with −1.83‰ PDB as the mean value of δ18O. As a consequence, a palaeoenvironmental pattern comprised of an early wetter/colder phase followed by a warmer/drier period and, finally, an increase in both cold conditions and hydrological variability, is deduced for the middle to late Holocene in the Ebro Basin. Gastropod shell δ13C values, related to vegetation, can also be linked to palaeoenvironmental changes. The succession and stable isotope composition of gastropods in alluvial sequences are revealed as very sensitive indicators of rapid Holocene climatic changes in fragile semi-arid landscape systems.