G03_03

Sedimentary radiocarbon signatures reveal persistent input of soil organic matter into Lake Constance

Mittelbach B¹, White  M¹, Rhyner T¹, Blattmann T¹, Haghipour N¹, Wessels M³, Dubois N², Eglinton T¹

¹ETH Zurich, Zürich, Switzerland, ²Eawag, Dübendorf, Switzerland , ³Institut für Seenforschung der LUBW, Langenargen, Germany

Inland waters play a crucial role in the global carbon cycle, as the sequestration of organic carbon (OC) in lake sediments constitutes a sink of atmospheric CO₂. Notably, the source of this sequestered OC has important implications for carbon cycling and climate. The burial of recently synthesized terrestrial and aquatic biospheric OC and of aged, soil-derived OC represents a drawdown of atmospheric CO₂. In contrast, erosion, transport, and reburial of rock-derived OC exert no net effect on atmospheric CO₂ levels.

Radiocarbon can be used to differentiate between these different sources of OC and constrain proportions of recent, pre-aged,  and fossil carbon. Moreover, the  20th-Century  radiocarbon “bomb spike” offers the possibility to resolve organic matter turnover and transport on decadal timescales. We combine ¹⁴C and stable δ¹³C isotope signatures of bulk OC from sediment cores retrieved from perialpine Lake Constance to assess the nature and dynamics of OC accumulation over the last century. Information about OC source isotope signatures was gathered from soil and bedrock profiles in the catchment as well as sediment traps in the lake. 

Our results show a muted, but distinct, bomb spike with Δ¹⁴C values of bulk OC increasing from -250‰ to -100‰ in the early 1960s. A linear mixing model reveals that soil-derived, pre-aged carbon comprises the largest contributor to the bulk sedimentary OC pool. We attribute the presence of the bomb spike signal to inputs of aquatic biomass, arguing for a rapid incorporation of bomb carbon into the lake DIC pool.