Climatic variations during the Holocene inferred from radiocarbon and stable carbon isotopes in a high-alpine cave.

A novel technique making use of laser ablation coupled online to accelerator mass spectrometry (LA-AMS) allows analyzing the radiocarbon (¹⁴C) concentration in carbonate samples continuously at high spatial resolution within very short analysis times. This new technique can provide radiocarbon data similar to the spatial resolution of stable carbon (C) isotope measurements by isotope-ratio mass spectrometry (IRMS) and, thus, can help to interpret d¹³C signatures, which otherwise are difficult to understand due to numerous processes contributing to changes in C-isotope changes ratios. In this work we analyzed d¹³C and ¹⁴C on the Holocene stalagmite SPA 127 from the high-alpine Spannagel Cave (Austria). Combined stable carbon and radiocarbon profiles allow to identify three growth periods characterized by different d¹³C signatures: (i) the period > 8 ka BP is characterized by relatively low d¹³C values with small variability combined with a comparably high radiocarbon reservoir effect (expressed as dead carbon fraction, dcf) of around 60%. This points towards C contributions of host rock dissolution and/or from an old organic matter (OM) reservoir in the karst potentially mobilized due to the warm climatic conditions of the early Holocene. (ii) Between 3.8 – 8 ka BP a strong variability in d¹³C reaching values from -8 to +1‰ with a generally lower dcf was observed. The d¹³C variability is most likely caused by changes in gas exchange processes in the cave, which are induced by reduced drip rates as derived from reduced stalagmite growth rates. Additionally, the lower dcf indicates that the OM reservoir is contributing less to stalagmite growth in this period possibly as a result of reduced precipitation or because it is exhausted. (iii) In the youngest section between 2.4 – 3.8 ka BP, comparably stable and low d¹³C values combined with an increasing dcf reaching up to 50% are again hinting towards a contribution of an aged OM reservoir in the karst. [ABSTRACT FROM AUTHOR]