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).