An Instrumentally Calibrated Winter Temperature Record Reconstructed from C Isotopes In An Annually Laminated Stalagmite

Stalagmite SU-96-7 from NW Scotland grew continuously for ~1000 years up to 1996 AD, the year of its sampling. It contains a continuous annual fluorescence lamina chronology, and lamina widths have been shown to correlate with North Atlantic climate, and in particular the low frequency component of the NAO. We have recently undertaken 100 micron resolution micromilling of this stalagmite, analysing samples for d¹³C at annual to biannual resolution. The presence of a precisely constrained annual lamina chronology allows us to undertake calibration of the d¹³C record for the last 100 years. In addition, we have spent the last 3 years monitoring the cave system from which stalagmite SU-96-7 was sampled: this programme included drip and rainwater isotope sampling and cave climate monitoring. We investigated the relationship between stalagmite d¹³C and smoothed annual, seasonal and monthly climate parameters. For d¹³C, this revealed a statistically significant relationship between d¹³C and February temperature. A decadal smoothing yielded a correlation of r = -0.4, but the strongest correlation was with a smoothing of the preceding 24 yrs of February T (r = -0.8). February temperature is the coldest month of the year at the sample site, with average temperatures below 0 °C possible in cold years. We hypothesise that the more positive d¹³C of respired CO₂ in freezing winters, which is mixed in the soil before being recharged to the groundwater, where it is further mixed with autumn recharge waters before reaching the stalagmite. Despite the reduced degrees of freedom in the smoothed series, our 24 yr smoothed record is statistically significant and confirms that d¹³C in SU-96-7 can be used to provide a record of low frequency variability in winter temperature for the last millennium.