Using the radiocarbon bomb spike to constrain the age of soil organic carbon delivered to Lake Constance sediments.

The residence time of carbon in terrestrial ecosystems, such as soils and freshwater, sets the pace of the terrestrial carbon cycle. Understanding export pathways and turnover times of soil organic carbon (OC_Soil) is crucial to assess responses to climate and land use changes. Our study aims to quantify the average turnover time of OC_Soil in the catchment of perialpine Lake Constance. Lake sedimentary sequences integrate organic carbon from their catchment and aquatic primary productivity. They act as both burial sites of organic carbon (OC) and time series archives of catchment processes. Thus, they can bridge the gap between plot scale observations, e.g., on soil carbon turnover, and observations made at the outlet of major river systems.

Sedimentary organic carbon sources include aquatic primary productivity, OC_Soil, and rock-derived (petrogenic) OC. Stable carbon isotopes (δ¹³C) can be used to identify the relative contribution of these pools. The 5700-year half-life of radiocarbon (¹⁴C) coupled with the atmospheric nuclear bomb spike in the early 1960s can be used to infer the age and turnover rate of the OC pools on millennial to annual timescales. Bulk OC isotope analysis of a varved sediment core spanning the past ca. 110 years at quasi-annual resolution was used to constrain the age of soil carbon delivered to Lake Constance. We combined the geochemical data with a Markov-Chain-Monte-Carlo-based approach to identify the most probable age structure of aquatic and soil-derived OC components and to quantify their respective contributions in addition to petrogenic OC.

Radiocarbon analysis of sedimentary bulk OC reveals a well-defined but muted bomb spike in the early 1960s. However, bulk Δ¹⁴C_OC values remain below 0‰, implying a predominance of aged OC. Based on the δ¹³C-based three-component linear mixing model, we found these values to be the result of an OC mixture containing approx. 40% pre-aged soil carbon and up to 20% fossil petrogenic carbon.  Accounting for these inputs, we estimated that soil-derived OC delivered to Lake Constance is centennial in age, implying interim storage prior or subsequent to erosion from the landscape.