NOAA/WDS Paleoclimatology - Uamh an Tartair, Scotland 1000Yr Stalagmite SU967 Stable Isotope Data

STUDY NOTES: Stable isotope (d18O and d13C) data at near-annual resolution from a stalagmite from northwestern Scotland. Stalagmite SU967 was micro-milled at 100 micron resolution and analysed for delta 18O and 13C to obtain a record of these stable isotopes for the last 1000 years. The same stalagmite contains continuous annual laminae. The lamina width record has been previously shown to provide a climate record of rainfall amount and the North Atlantic Oscillation (Proctor et al. 2000, 2002; Trouet et al. 2009) and is archived at the WDC. In contrast, delta O-18 and 13-C cannot be interpreted as simple paleoclimate proxies. Stalagmite SU967 was milled at 100 micron resolution. Every fifth sample was extended to provide marker horizons. The stalagmite was reimaged using UV microscopy to enable a chronology for the stable isotope series to be built using the lamina width record. Each milled isotope sample integrates from <1 yr to c.10 years of speleothem accumulation, depending on growth rate. The calendar ages given are our best estimate of the start date for each mill, and therefore apply to subsequent years where growth rates are slow. Ages have the uncertainty of the original laminae chronology as archived at the WDC (see Proctor et al. 2002). Uamh an Tartair is part of the Cnoc nan Uamh cave system, 3 km east of Inchnadamph in Assynt, in Northwest Scotland. UK National Grid Reference NC 276 206
ABSTRACT SUPPLIED BY ORIGINATOR: 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 ~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 d18O but not for d13C, stalagmite d13C originally derives from soil CO2 produced predominantly by microbial respiration, modified by degassing-related kinetic fractionation, and d18O 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 d18O and precipitation amount or temperature are observed, but correlations are observed between rainfall d18O and 500 mb height at regional IAEA monitoring stations. However, no correlations are observed between stalagmite d18O and instrumental and reconstructed atmospheric circulation, preventing a simple palaeoclimate interpretation of the stalagmite d18O proxy. Stalagmite d13C has a stronger temporal autocorrelation than d18O, 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, d18O and d13C are discussed, and interpretations compared with other regional climate records. We conclude that, over the last millennium at this mid-latitude cave site, neither d18O nor d13C cannot be interpreted as a simple paleoclimate proxy.