High Resolution Monitoring of Organic Matter at Milandre Cave, Switzerland. Implications for Future Paleoecosystem Proxies.

Terrestrial ecosystems, including soil and the biosphere, represent important reservoirs of carbon sources and cycling (IPCC, 2000). However, the reaction of terrestrial ecosystems to the changing climate remains poorly constrained. Over the past 20 years, interest in the organic matter (OM) fraction of speleothems, typically comprising 0.01-0.3% of the total carbon (Blyth et al., 2016), has increased due to its potential to offer information about past ecosystems. The sources of speleothem OM are not fully understood and are likely to be a combination of contributions from overlying vegetation, soil, microbial activity within the karst system, and cave fauna. Due to the link that the inner cave environment has with the karst, the signal of non purgeable organic carbon (NPOC) sourced from the overlying soil, vadose zone, or within the cave itself may be preserved within speleothems (Blyth et al., 2013). Hence, the isotopic characterisation (𝛿¹³C and ¹⁴C) of stalagmite NPOC has the potential to give information about past ecological and climactic state of the surrounding region (Blyth et al., 2013).

Presented here are the first results of a high-resolution process study of organic and inorganic carbon fluxes in the Milandre cave (Switzerland), whereby the main carbon source reservoirs will be monitored for two years. The total organic carbon content of cave waters ranges from 0.6 -1.3mg/L. The 𝛿¹³C of CO₂ in gas samples from atmospheric air (-9.24 ‰), soil air (-12.68 - -27.20‰), gas well air (-24.97- -25.78‰), and cave air (-14.29 - -25.32‰) were analysed. The soil air, well air and cave air have 𝛿¹³C values which range from close to atmospheric 𝛿¹³C to the most 𝛿¹³C depleted cave air end member which suggests differing levels of gas mixing throughout the system. Ultimately, this information will be used to constrain the source of speleothem NPOC and allow the assessment of its suitability as a proxy for ecosystem change.   

 

Blyth, A., Hartland, A. and Baker, A., 2016. Organic proxies in speleothems – New developments, advantages and limitations. Quaternary Science Reviews, 149, pp.1-17.

Blyth, A., Smith, C., and Drysdale, R., 2013. A new perspective on the 13C signal preserved in speleothems using LC-IRMS analysis of bulk organic matter and compound specific stable isotope analysis. Quaternary Science Reviews, 75, pp. 143-149.

IPCC, 2000: Land Use, Land-Use Change and Forestry. (R.Watson, I.Noble, B Bolin, N.H. Ravindranath,D.J. Verardo and D.J. Dokken (eds.)).  Cambridge University Press, UK, pp.375