Characterizing organic carbon source, age, and degradation state in Northern Patagonia fjord surface sediments

Resumen: Fjords are a globally significant carbon sink with the highest burial rates of organic carbon (OC) per unit area of all marine depositional systems. As many fjords face the impacts of global change, understanding carbon cycling in these estuaries is crucial. We estimated OC sources and degradation state in Northern Patagonia (41°S to 47°S) fjord sediments using radiocarbon, stable isotopes, and chemical biomarkers. Surface sediment OC content ranged from 0.15 to 5.34%. Majority of this OC was modern, although there was a widespread but low petrogenic OC (OC_petro) content of 0.13, 95% confidence interval [0.08–0.18%]. This OC_petro can be the dominant and even sole source of OC to Northern Patagonia sediments in regions with low overall OC contents. There were steep gradients of modern terrestrial OC (OC_terr) contributions moving from fjords (∼70%) to broader bays and channels (∼6%), indicating these fjords act as OC_terr sinks. There was an apparent decoupling of OC_terr and marine OC (OC_mar) degradation in sediments, reflected by lignin and amino acid biomarkers. This decoupling likely results from the preferential association of OC_mar with minerals, illustrated by high OC_mar to surface area ratios (OC/SA; 0.38 mg C m⁻²) compared to terrestrial OC/SA (0.13 mg C m⁻²), a continuous supply of OC_mar compared to point source inputs of OC_terr, as well as the greater sensitivity of OC_mar to aerobic degradation. Overall, the Northern Patagonia fjord system is a sink of OC_terr, and OC cycling within these fjords will continue to evolve with anthropogenic global change.