C01_03

Combining radiocarbon and stable C isotopes of stalagmites to gain novel insights into geochemical processes at Spannagel Cave

Welte C¹, Fohlmeister J³, Wertnik M², Spötl C⁴

¹LIP, ETH Zurich, Zurich, Switzerland, ²Geological Institute, ETH Zurich, Zurich, Switzerland, ³Federal Office for Radiation Protection, Berlin, Germany, ⁴University of Innsbruck, Innsbruck, Austria

Stable carbon (C) isotope records from stalagmites are readily available as they are often measured alongside stable oxygen isotopes (δ¹⁸O). Their interpretation, however, remains challenging due to myriad processes contributing to changes in the C-isotope ratio. Spatially resolved radiocarbon (¹⁴C) data can help to interpret ¹³C signatures[1],  but are rarely available due to expensive and time-consuming analysis. Rapid and continuous analysis of ¹⁴C concentration in carbonate samples at spatial resolution down to 100 μm is now possible using LA-AMS (laser ablation accelerator mass spectrometry).

Combined δ¹³C and ¹⁴C profiles (expressed as dead carbon fraction, dcf) allowed to hypothesize on the interplay of regional climate and contribution of an old organic C reservoir to stalagmite growth at Spannagel Cave, Austria, in a previous study[2]. Here, we present LA-AMS results from a second Holocene stalagmite from Spannagel Cave (SPA 128). Both stalagmites show large and fast variations in the dcf and δ¹³C. SPA 128 has a generally higher dcf (~50%) and a more negative δ¹³C signal that point towards continuous contribution of an old organic C reservoir to the stalagmite C. Even though, the observed signals cannot be explained conclusively so far, it can be stated that the stable oxygen isotopes agree well in both stalagmites. This is an encouraging finding for future studies making use of δ¹⁸O as climate proxy.

[1]D. Rudzka et al., (2011) GCA 75, 4321-4339.

[2] C. Welte et al., (2021). Clim. Past 17, 2165–2177.