3.3. Geochemistry on river and fjord samples
Total carbon was measured on all river and fjord samples using an LECO elemental analyzer. All samples were also analyzed for total organic carbon (TOC) content after trace amount of inorganic carbon (carbonate) were removed using HCl and rinsing with milli-Q water. No effervescence was observed during this acidification step, confirming minimal carbonates in the system (Fig. 4). TOC measurements allowed for quantifying the total amount of OC delivered by the rivers and buried in the different fjord sub-environments. In order to constrain the type (terrestrial or marine) of OC, all samples were analyzed for carbon stable isotopes (δ13C) using an isotope ratio mass spectrometer. Terrestrial organic matter derived from C3 plants has an average δ13C between -26 and -28 ‰ (Hecky and Hesslein, 1995) and can reach values as low as -31 ‰, which correspond to young woody debris (Hage et al., 2020). In contrast, marine organic matter preserved in northeast Pacific sediments has an average δ13C of -21.5 ‰ (MacDonald et al., 1991).
Previous work in Bute Inlet has used a wider range of methods (e.g. radiocarbon Δ14C dating and separation of OC mixtures by ramped pyrolysis-oxidation RPO) on selected river and sandy channel samples (Hage et al., 2020). We complement these previous detailed measurements with new 14C and RPO measurements on three samples from the distal sub-environment (Fig. 7). The RPO method identifies POC fractions based on thermal lability by heating each sample from 20 to 1000 °C in an oxygenated carrier gas, thus sequentially combusting POC into CO2. The CO2 collected between temperature inter­vals (“fractions”) is measured for carbon stable isotopes (δ13C) and radiocarbon activity (14C/12C). These isotope measurements on separated fractions enabled characterization of the distribution of POC source and age within individual samples (Hemingway et al., 2017).