Radiocarbon and stable carbon isotopic insights into provenance and cycling of carbon in Lake Superior

This study applies radiocarbon and stable carbon isotopic distributions to investigate carbon sources and cycling within Lake Superior. We report the radiocarbon (Δ¹⁴C) and stable carbon isotope (δ¹³C) values and the carbon concentrations within dissolved organic carbon (DOC), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) in the lake's western basin water column. Samples were taken during spring mixing and late-summer thermal stratification over a 2-yr period (2007-2009). Distinct processes operating in the surface (photosynthesis) and deep waters (sediment resuspension and pore-water intrusion) control the relative contribution of modern and ancient DOC and POC in the water column. The terrigenous carbon input to the open lake POC varied from 13% ± 4% during late summer stratification to 9% ± 3% during spring mixing, with most of the terrestrial carbon being ¹⁴C-enriched (modern). The DIC reservoir cycles rapidly, with a bulk Δ¹⁴C_DIC value that records atmospheric radiocarbon levels from 3 yr prior to sampling. The DOC pool recycles on a longer time scale than does the DIC, with a DOC residence time of ≤ 60 yr. The suspended POC was in most cases older than co-occurring DOC, most likely as a result of resuspension of lake sediments.