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).