A NOVEL METHOD TO RECONSTRUCT PAST CLIMATIC CHANGES USING STABLE ISOTOPES IN CAVE ICE

Stable isotope measurements on water samples from a 22.5 m long ice core, extracted from Scãrisoara Ice Cave (Romania), have shown that the major climatic events over the past 1500 years (DACP, MWP and LIA) are recorded by the cave ice. However, the stable isotope record yields larger than expected swings between warm and cold climates, possibly induced by the sampling and analysis technique (in this case, we have sampled the core at every 5 cm, regardless of its stratigraphy). In order to overcome this problem, we have developed a novel technique to reconstruct past climatic changes using stable isotopes in cave ice as proxies for air temperature.

The ice block is formed mainly of so-called 'lake ice,' in which a shallow lake formed mainly by seepage water freezes downward from the top to form a layer of stratified ice up to 15 cm thick. This possibly records (via its stable isotopic composition) late-summer through autumn air temperatures. High resolution stable isotope analyses of the core show a continuous enrichment of heavy isotopes in the ice and depletion in the remaining water. This pattern has been seen in both modern and old (ice core) ice. On a δ²H-δ¹⁸O plot, the samples of 'lake ice' align with a 'freezing slope,' which is lower (between 4 and 6) than that of the Local Meteoric Water Line (LMWL; δ²H = 8.14*δ¹⁸O+10.23). By intersecting this freezing slope with the LMWL we were able to reconstruct the initial isotopic composition of water before freezing. The values obtained are in close agreement we those measured before the onset of the freezing process, thus validating our method.

We further tested this method on ice samples from the MWP and LIA and the results showed that the previously observed swings have been removed from the record. We consider these rapid shifts (from high to low values) artifacts due to sampling, as some of the ice was possibly collected from the upper part of one annual layer, while the next one could have been taken from the lower part of the same layer, thus introducing an unwanted and false variation in the record.