3.5. Calculation of organic carbon burial rates in fjord
sediments
The OC burial rates (kt/yr) in the fjord are calculated using Equation 1
(Table 2; Baudin et al., 2020) as follows:
A * TOC * ρ * (1- φ) * SR . (Equation 1)
Where A is the surface area of a sub-environment (m2),
TOC is the total OC content (%) averaged between samples from a given
sub-environment, ρ is the sediment density (kg/m3), φ
is the porosity (%), and SR is the sedimentation rate (m/yr).
Sedimentations rates (SRs) typically vary when measured over different
timescales (Sadler, 1981), especially in a highly active turbidity
current system (Prior et al., 1987, Heijnen et al., 2020), making them
challenging to quantify. Thus, SRs were estimated herein based on two
independent approaches that provide ranges of SRs (Texts S4 and S5). The
first approach uses differences between two bathymetric surveys obtained
in 2008 and 2018 and thus holds for a decennial timescale (Heijnen et
al., in review, Table 2, Text S4). The second approach uses210Pb and 137Cs dating methods
applied to sediment cores collected in overbank and distal flat basin
settings (Syvitski et al., 1988; Heerema, 2021; Text S5), and thus holds
for a centennial timescale. Based on these two approaches and
timescales, we use ranges of SRs for each sub-environment as follows. SR
in the channel ranges from -16 cm/yr (i.e. erosional) to 0.7 cm/yr (i.e.
slightly depositional). SR in the lobe ranges from 10 to 18 cm/yr,
implying a large accumulation of sediment over both decennial and
centennial timescales. SR in the overbank area varies between 2 to 2.3
cm/yr. Finally, SR in the distal flat basin ranges from 1 to 2.3 cm/yr.
We note that the first approach (i.e., 11 yr time-lapse bathymetric
analysis) highlights zones of erosion and deposition that migrate
upstream significantly (100 to 450 m per year) due to the presence of
active knickpoints in the channel (Heijnen et al., 2020). This explains
why the channel is net-erosive on a decennial timescale (SR =
~ -16 cm/yr). The 30 cm long sediment cores used in this
study thus probably represent deposits that were a few days to weeks
old, as direct monitoring shows that over 100 turbidity currents can
occur in one freshet (Chen et al., 2021; Pope et al., in review). The
top of these deposits were likely reworked again by turbidity currents
in the following days to weeks, progressively moving the sediment down
the channel. The channel thus acts as a conduit through which sand and
associated OC is shuffled to the lobe, in multiple stages over several
weeks to several decades (Heijnen et al., in review). Time-lapse
bathymetry also shows that the terminal lobe is built up from a small
number of large magnitude ‘channel flushing’ events (Heijnen et al., in
review), resulting in a locally high SR (SR = 18 cm/yr, Table 2) over
decennial timescales. It should be kept in mind that this bathymetric
method underestimates the depositional volume in areas of slow
deposition (overbanks), as cm-thin drapes of sediment cannot be resolved
by this method. The second approach (i.e., 210Pb and137Cs dating) postulates that the entire Bute
submarine system is net aggrading over centennial timescales (Syvitski
et al., 1988). Thus, the channel floor is assumed to aggrade at the same
rate as the adjacent overbank areas, rather than being strongly
erosional. This is consistent with a rather shallow (~20
m relief) channel, as prolonged erosion in a submarine channel that is
likely at least hundreds of years old would have carved a much deeper
conduit. This assumption of overall aggradation results in a slow, but
positive, SR in the channel (SR = 0.07 cm/yr). This SR in the channel is
then assumed to be balanced by less aggradation in the lobe (SR = 10
cm/yr), such that the total sediment budget within the various
sub-environments on the fjord seafloor then balances the rate at which
sediment is supplied by the rivers (Table 1).
In total, all OC burial rates in the submarine system are provided as
ranges (Table 2) based on the two approaches used to estimate SR.
4. Results
Below we provide the OC content and composition for both rivers and the
fjord sediments separately. We note that comparison between TC and TOC
on all samples revealed the absence of carbonates within both river and
fjord samples (Fig. 4).
Carbon composition supplied by both rivers; what is
coming in?
Coarse sand samples collected from the riverbank and delta areas have
relatively low TOC (mean TOC = 0.35 %), and δ13C
values (-27 to -28 ‰) indicating a terrestrial origin (Hecky and
Hesslein, 1995). TOC is moderately high (mean TOC = 0.8 %) in the fine
sands collected in the river waters, banks and deltas; whereas
δ13C values are low (-25 to -29 ‰) and point again to
a terrestrial origin (Hecky and Hesslein, 1995). TOC is highest in muddy
sediments (mean TOC = 3.1%, Fig. 5) collected in the river plume at the
fjord head. δ13C signatures for these river plume
samples are unusually high (-12 to -20 ‰; Fig. 6), despite the absence
of carbonates (Fig. 4). These high δ13C values are
interpreted to be linked to bacterioplankton producing extra cellular
polymeric substances (EPS; Albright, 1983), and this will be further
discussed in Section 5.1.
In total, we estimate that about 23.4 ± 5.2 Kt OC/yr are delivered
annually by the Homathko and Southgate Rivers. This is based on the
estimated sediment discharge (suspended and bedload) and on the average
TOC content measured between samples collected in both rivers in October
2017 (Table 1).