Extracting winter North Atlantic Oscillation information from a central European stalagmite

Abstract

This study explores the potential for extracting winter North Atlantic Oscillation (NAO) information from central European stalagmites using a high resolution calcite δ18O (δ18Ocal) time-series from a stalagmite (AH1) extracted from Atta cave in northwestern Germany. Samples milled at 25μm resolution were run through an isotope-ratio mass spectrometer in weight order. This highlighted the benefit of using a random sample order during isotope ratio analysis because drifts were identified with greater certainty than if the samples were run sequentially. The chronology, constructed using annual δ18Ocal cycles and verified by locating the radiocarbon bomb spike, hints that three isotope excursions may be related to events in the cave history: i) the discovery of AH1's cave passageway in 1985, ii) surface modification during World War Two and iii) the opening of the cave’s artificial cave entrance in 1920. However, it was not possible to locate old aerial photographs of the site and appropriate historical documentation, therefore these explanations remain speculative. A good visual correlation between δ18Ocal and meteoric precipitation δ18O (δ18Opr) cycles was established within the errors of the chronology, but a good quantitative correlation was not achievable probably because the winter δ18Opr signal was documented by too few δ18Ocal datapoints. Despite the poor quantitative correlation, the good visual δ18Ocal- δ18Opr correlation suggests an annual winter NAO reconstruction is possible using stalagmites from central Europe if ultra high temporal resolution is achieved and excellent chronological control is established. Additionally, this suggests that AH1 is amenable to detection of prolonged shifts in mean NAO state, provided trends associated with other processes are removed