Speleothem Climate Capture:A Holocene Reconstruction of Northern Iberian Climate and Environmental Change

An extensive 4 year cave monitoring program has been undertaken at Asiul Cave, a previously unstudied site in Cantabria (Spain). Monitoring indicates that speleothem forming drip waters are sourced from winter rainfall and that the isotopic composition of these waters is influenced by the amount of rainfall. Modern carbonate deposits accurately preserve the isotopic composition of the karst water from which they have formed, indicating that older speleothem deposits should be ideal for the reconstruction of palaeoclimatic conditions, including importantly palaeorainfall amount reconstruction. Two speleothem samples were therefore removed from the cave and analysed for a suite of geochemical proxies. Coeval oxygen isotope records from Asiul Cave indicate that northern Iberia has experienced considerable deviations in rainfall during the last 12,500 years. These high resolution records are strongly coupled with changes in other regionally important climate archives, helping to add to our understanding of northern Iberian climate evolution. The Asiul speleothem records however, go beyond explaining local changes in environmental conditions by exhibiting a strong coupling between atmospheric conditions, in the form of the North Atlantic Oscillation (NAO) and North Atlantic Ocean circulation. These speleothem archives indicate that the NAO controls not only the positioning of atmospheric storm tracks throughout Europe; but through interactions with the surface layer of the ocean can cause major changes in oceanic circulation. These NAO controlled changes in North Atlantic Ocean circulation have been shown to cause significant cooling within the northern North Atlantic and the southerly transport of ice rafted debris, with a millennial periodicity of ~1500 years. The Asiul cave speleothem record is one of the first convincing archives of a millennial scale NAO system which has the capacity to force changes in oceanic circulation. These speleothems also act to extend existing archives of the NAO back into the Younger Dryas; by doing so the Asiul records challenge our current understanding of NAO dynamics and the exact timing of initial NAO development