High resolution delta O-18 and delta C-13 records from an annually laminated Scottish stalagmite and relationship with last millennium climate

High-resolution (annual to decadal) stable isotope records of oxygen and carbon are analysed from an annually laminated stalagmite from NW Scotland. The sample, which was deposited for similar to 1000 yrs until 1996 AD, has previously provided annual resolution climate reconstructions of local rainfall and regional winter North Atlantic Oscillation (wNAO) from variations in annual growth rate. For our stalagmite, for which modern cave monitoring demonstrates that equilibrium deposition is highly likely for delta O-18 but not for delta C-13, stalagmite delta C-13 originally derives from soil CO2 produced predominantly by microbial respiration, modified by degassing-related kinetic fractionation, and delta O-18 from the composition of infiltrating water during periods of infiltrating water. Both the presence of fluorescent laminae and modern drip-water monitoring demonstrate a drip hydrology that comprises both event and storage components. Over the instrumental period, no correlations between stalagmite or rainfall delta O-18 and precipitation amount or temperature are observed, but correlations are observed between rainfall delta O-18 and 500 mb height at regional IAEA monitoring stations. However, no correlations are observed between stalagmite delta O-18 and instrumental and reconstructed atmospheric circulation, preventing a simple palaeoclimate interpretation of the stalagmite delta O-18 proxy. Stalagmite delta C-13 has a stronger temporal autocorrelation than delta O-18, indicative of soil mixing of respired CO2 and significant variability between drips and at different times; correlations with instrumental climate data are therefore not possible. The relative timing of changes in growth rate, delta O-18 and delta C-13 are discussed, and interpretations compared with other regional climate records. We conclude that, over the last millennium at this mid-latitude cave site, neither delta O-18 nor delta C-13 cannot be interpreted as a simple paleoclimate proxy. (C) 2010 Elsevier B.V. All rights reserved.