High resolution δ~(18)O and δ~(13)C records from an annually laminated Scottish stalagmite and relationship with last millennium climate

摘要

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 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 δ~(18)0 but not for δ~(13)C, stalagmite δ~(13)C originally derives from soil CO_2 produced predominantly by microbial respiration, modified by degassing-related kinetic fractionation, and δ~(18)O 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 δ~(18)O and precipitation amount or temperature are observed, but correlations are observed between rainfall δ~(18)O and 500 mb height at regional IAEA monitoring stations. However, no correlations are observed between stalagmite δ~(18)O and instrumental and reconstructed atmospheric circulation, preventing a simple palaeoclimate interpretation of the stalagmite δ~(18)O proxy. Stalagmite δ~(13)C has a stronger temporal autocorrelation than δ~(18)O, indicative of soil mixing of respired CO_2 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, δ~(18)O and δ~(13)C are discussed, and interpretations compared with other regional climate records. We conclude that, over the last millennium at this mid-latitude cave site, neither δ~(18)O nor δ~(13)C cannot be interpreted as a simple paleoclimate proxy.