Contrasting ages and dynamics of unsaturated zone carbon pools at a mid-latitude karst system

Abstract. The sources and turnover of dissolved inorganic (DIC) and dissolved organic carbon (DOC) in the unsaturated zone of terrestrial ecosystems remain poorly understood and contribute to significant gaps in understanding terrestrial carbon export dynamics. Karst systems, and caves in particular, offer a window into the subsurface, allowing direct observation and sampling of percolating water solutions. In this study, we characterized the radiocarbon (¹⁴C) composition of DIC and DOC in subsurface drip and river water in Milandre cave, Switzerland, over a period of two years, and assessed longer-term (multidecadal) carbon cycle dynamics using secondary cave carbonate deposits (speleothems). The results reveal significant age differences between carbon pools, with DIC being consistently younger than DOC. Variations in total organic carbon (TOC) concentrations and ¹⁴C suggest carbon pools of different ages contribute to the subsurface DOC pool with significant short-term (monthly or shorter) variability. The younger DIC pool indicates a rapidly cycling carbon pool under open system carbonate dissolution conditions, i.e. with low bedrock contribution to the isotopic signature. A well-defined ¹⁴C bomb peak present in the carbonate fraction of an actively growing stalagmite confirms the trend of rapidly cycling DIC on multidecadal scales. The speleothem organic carbon fraction is several thousand years older with a muted ¹⁴C bomb peak, showing that the isotopic signature of cave water DOC is faithfully recorded in speleothems over longer time periods. These findings highlight the complexity of carbon cycling in karst environments and the importance of catchment-level assessments of carbon export signatures to better constrain the terrestrial carbon budget.