Characterizing storm event riverine dissolved organic carbon through ramped temperature oxidation

Abstract:

Rivers link the various components of the global carbon cycle by transporting dissolved organic matter (DOM), and transforming its composition from headwater to ocean. Storms produce marked changes in the dissolved carbon pool, by increasing its lability and allochthonous portions. We present data from several sites along the Connecticut River and its northern VT headwaters. Sites were selected to include both forested and wetland-dominated headwater watersheds and larger river systems. Furthermore, we analyzed both a baseflow and post-storm water sample. After extracting DOM using solid phase extraction (PPL columns), we applied a novel ramped-temperature oxidation analysis (so-called ramped Pyrox analysis) by submitting the sample to a constant temperature ramp of 5°C /min from 25 to 600°C and cryogenically collecting the evolved CO₂ using a dual trapping system connected to a vacuum line. The concentration of evolved CO₂ gas was monitored over time as well as the ¹³C/¹²C isotope ratio for several discrete portions of the temperature range. Previous studies have indicated that the oxidation temperature of evolved CO₂ is correlated with radiocarbon age, and that isotope composition can depend on discharge and distance downstream, among other factors (Rosenheim et al., 2013; Rosenheim and Galy, 2012). In addition, we compare these samples to a powdered, bulk, natural DOM standard available from the International Humic Substances Society. This sample was collected from the humic and fulvic acid-rich Suwannee River in southeastern Georgia (see figure). The mass-weighted average isotope ratio for the Suwanee sample is -28.1‰ on the VPDB scale, consistent with a dominant C3 plant source. Our data provide a detailed characterization of the reactivity and stable isotope distribution of riverine dissolved organic carbon, which we will discuss in the context of the pulse-shunt concept (Raymond et al., 2016).