¹⁴C Variation of Dissolved Lignin in Arctic River Systems

Assessing permafrost-release signals in arctic rivers is challenging due to mixing of complex carbon components of contrasting ages. Compound-specific ¹⁴C analysis of terrestrially derived molecules may reduce the influence of mixed carbon sources and potentially provide a closer examination on the dynamics of permafrost-derived carbon in arctic rivers. Here we employed a recently modified method to determine radiocarbon contents of lignin phenols, as a classic tracer for terrestrial carbon, isolated from the dissolved organic matter (DOM) of two arctic river systems that showed contrasting seasonal dynamics and age components in DOM. While dissolved lignin had relatively invariant ¹⁴C contents in the Mackenzie, it was more concentrated and ¹⁴C-enriched during spring thaw but relatively diluted and ¹⁴C-depleted in the summer flow or permafrost thaw waters in the Kolyma. Remarkably, the covariance between dissolved lignin concentrations and its ¹⁴C contents nicely followed the Keeling plot, indicating mixing of a young pool of dissolved lignin with an aged pool of a constant concentration within the river. Using model parameters, we showed that although the young pool had similarly modern ages in both rivers, Kolyma had a much higher concentration of aged dissolved lignin and/or with older ages. With this approach, our study not only provided the first set of ¹⁴C data on dissolved lignin phenols in rivers but also demonstrated that the age and abundance of the old DOM pool can be assessed by radiocarbon dating of dissolved lignin in arctic rivers related to permafrost release.