Biogeochemistry of Riverine Organic Matter Inputs to the Patagonian Fjords and Implications for Fjord Organic Carbon Budgets

Fjords are increasingly recognized as hotspots for organic carbon (OC) burial. The OC buried in fjords is of both marine and terrestrial origin, with a predominance of terrestrial OC in fjords worldwide. The proportions of marine and terrestrial OC in fjords are traditionally calculated using end-member modeling based on δ¹³C and/or N/C. However, characterizing the terrestrial end-member remains a challenge, with authors inconsistently using measurements obtained on land plants, soils, and/or river sediments. Here, we analyzed the TOC, δ¹³C, and N/C composition of soil samples, suspended river sediments, and bulk and grain-size fractions of river sediments from the main rivers discharging into the Patagonian fjords (44–48°S), to identify the processes that affect the biogeochemistry of the terrestrial organic matter reaching fjords via rivers. Radiocarbon measurements indicate that Patagonian rivers contain 0.18% petrogenic OC and variable concentrations of biospheric OC. Despite soil δ¹³C significantly decreasing with precipitation, δ¹³C in river sediments remains relatively stable around −27‰. In contrast, N/C in river sediments is highly variable, mostly due to a high contribution of petrogenic nitrogen in glacier-fed rivers. Furthermore, N/C varies significantly with sediment grain size, making it virtually impossible to define a fixed N/C value to represent the terrestrial end-member. By comparison, grain size has a limited influence on δ¹³C. Overall, our results support the use of riverine δ¹³C to define terrestrial OC in mixing models, regardless of the presence of glaciers in the watershed, and they suggest that the fraction of terrestrial OC buried in fjord sediments may have been underestimated.