Supplementary material to "Stalagmite carbon isotopes suggest deglacial increase in soil respiration in Western Europe driven by temperature change"

The temperate region of Western Europe underwent dramatic climatic and environmental change during the last deglaciation. Much of what is known about the terrestrial ecosystem response to deglacial warming stems from pollen 15 preserved in sediment sequences, providing information on vegetation composition. Other ecosystem processes, such as soil respiration, remain poorly constrained over past climatic transitions, but are critical for understanding the global carbon cycle and its response to ongoing anthropogenic warming. Here we show that speleothem carbon isotope (δCspel) records may retain information on local soil respiration, and allow its reconstruction over time. While this notion has been proposed in the past, our study is the first to rigorously test it, using a combination of multi-proxy geochemical analysis (δC, Ca isotopes, and 20 radiocarbon) on three speleothems from Northern Spain, and quantitative forward modelling of processes in soil, karst, and cave. Our study is the first to quantify and remove the effects of prior calcite precipitation (PCP, using Ca isotopes) and bedrock dissolution (using the radiocarbon reservoir effect) from the δCspel signal to derive changes in respired δC. Coupling of soil gas pCO2 and δC via a mixing line describing diffusive gas transport between an atmospheric and a respired end member allows modelling of changes in soil respiration in response to temperature. Using this coupling and a range of other parameters 25 describing carbonate dissolution and cave atmospheric conditions, we generate large simulation ensembles from which the results most closely matching the measured speleothem data are selected. Our results robustly show that an increase in soil pCO2 (and thus respiration) is needed to explain the observed deglacial trend in δCspel. However, the Q10 (temperature sensitivity) derived from the model results is higher than current measurements, suggesting that part of the signal may be https://doi.org/10.5194/cp-2021-3 Preprint. Discussion started: 4 February 2021 c © Author(s) 2021. CC BY 4.0 License.