Tracing the export of terrestrial biospheric carbon from source-to-sink through molecular 14C analyses in two large Alpine catchments

The residence time of organic carbon (OC) in terrestrial reservoirs, particularly soils and freshwater systems, plays a crucial role in modulating the dynamics of the global carbon cycle. Radiocarbon (¹⁴C) is an invaluable tool for tracing the time since the biosynthesis of organic matter, enabling the quantification of carbon residence times in these terrestrial pools. While the majority of carbon fixed through terrestrial primary productivity rapidly returns to the atmosphere, a stabilized fraction of OC escapes (re-)mineralization. This OC may subsequently be exported from terrestrial ecosystems and buried in marine and terrestrial sedimentary sequences over longer timescales, effectively sequestering atmospheric CO₂.

Mineral association has been identified as a key mechanism of this stabilization. Consequently, source-specific biomarkers targeting terrestrial, mineral-associated OC are of particular interest for tracking especially resistant OC species. In our study, we apply compound-specific ¹⁴C analysis on leaf wax fatty acids (n-alkanoic acids). These long-chain fatty acids (C24+) are exclusively produced by vascular plants. Moreover, their highly hydrophobic nature promotes mineral association, making them ideal molecular markers of stabilized soil OC that can be traced through export and burial.

We employ a source-to-sink approach, targeting mineral soil profiles, fluvial sediment, and lake sediment within two Alpine sediment routing systems: the Alpine Rhine and Alpine Rhone catchments. Additionally, we analyze selected depths from well-dated deltaic sediment cores spanning the past 120 years to estimate catchment-averaged transit times of long-chain fatty acids and to assess temporal variability in these trends.

Initial results indicate significant pre-aging of OC in soil profiles, Δ¹⁴C from -100‰ to below -500‰, combined with rapid and efficient fluvial export of our target compounds. Sediment core data reveal millennial-scale catchment transit times for long-chain fatty acids. Further, they show the impact of anthropogenic disturbances, which have led to an increase in the age of exported soil OC across the investigated period.