High resolution δ¹⁸O and δ¹³C 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 ~1000yrs until 1996AD, 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 δ¹⁸O but not for δ¹³C, stalagmite δ¹³C originally derives from soil CO₂ produced predominantly by microbial respiration, modified by degassing-related kinetic fractionation, and δ¹⁸O 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 δ¹⁸O and precipitation amount or temperature are observed, but correlations are observed between rainfall δ¹⁸O and 500mb height at regional IAEA monitoring stations. However, no correlations are observed between stalagmite δ¹⁸O and instrumental and reconstructed atmospheric circulation, preventing a simple palaeoclimate interpretation of the stalagmite δ¹⁸O proxy. Stalagmite δ¹³C has a stronger temporal autocorrelation than δ¹⁸O, 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, δ¹⁸O and δ¹³C are discussed, and interpretations compared with other regional climate records. We conclude that, over the last millennium at this mid-latitude cave site, neither δ¹⁸O nor δ¹³C cannot be interpreted as a simple paleoclimate proxy.