Figure Captions
Figure 1 : A. Location of Bute Inlet in British Columbia (Canada). B. Bute Inlet is fed by Homathko and Southgate Rivers. Watershed areas were delimited by Gonzalez et al. (2018)
Figure 2 : Bathymetry of the head of Bute Inlet (collected in 2008 by the CCGS Vector), also showing locations of sediment samples collected in Homathko and Southgate Rivers in 2017, and offshore in fjord sediments in 2016.
Figure 3 : Submarine morphology and sediment cores collected in the Bute turbidity current system. Boxes A to D: zoom-ins showing the parts of the system bounded by overbank. E & F: Sediment core set. 30 cm long cores were collected using a box coring system; 200 cm long cores were collected using a piston coring system.
Figure 4 : comparison between total carbon (including inorganic and organic carbon) and total organic carbon content measured on all samples collected in rivers and fjord.
Figure 5 : Facies and total organic carbon (TOC) content within rivers, submarine channel and lobe, overbank, and distal flat basin. Pie charts represent the contribution (in %) of each of the facies to a given sub-environment.
Figure 6 : Total Carbon content (TC) versus carbon stable isotopes (δ13C) measured on all samples collected in the rivers and in the Bute turbidity current system. δ13C values are reported relative to Vienna Pee-Dee Belemnite (VDBP). Radiocarbon dates are expressed as reservoir age offsets in 14C years (following Soulet et al., 2016). The combination of bulk measurement of carbon, stable isotopes and radiocarbon isotopes allowed five carbon pools to be identified. (1) Extra polymeric substances associated with bacterioplankton in the river plumes at the surface of the fjord waters (Albright, 1983). (2) Marine carbon produced in the distal site. (3) Young terrestrial carbon in the form of woody debris almost exclusively buried in the sandy submarine channel. (4) Old terrestrial biospheric organic carbon aged in soils. (5) Petrogenic (rock-derived) organic carbon associated with coarse sand.
Figure 7 : Separation of OC mixtures by ramped oxidation (RPO) for three samples collected in the distal flat basin (Core 15 in Fig. 3). Two facies are identified in this core: muddy sediment with a reddish colour and muddy sediments with a grey colour and organic debris. Black lines show distribution of activation energy (Ea) (thermogram; Hemingway et al., 2017). Blue squares show radiocarbon ages (in fraction modern, Fm, a measurement of the deviation of the 14C/12C ratio of a carbon fraction from “modern”). Red dots show carbon stable isotopes (δ13C, in ‰). Blue bars represent the Ea range to which each red dot and blue square applies.
Figure 8 : Summary illustration (not to scale) showing total organic carbon (OC) fluxes from rivers to seafloor sediment in Bute Inlet. OC fluxes are given as the average value between sediment budgets estimated using two approaches (see Methods; Heijnen et al., in review, Syvitski et al., 1988, Heerema, 2021). Error bars correspond to the range between the two approaches. Sediment and OC are shuffled stepwise down the channel before reaching the lobe due to migrating knickpoints. The channel is thus net erosive over decennial timescales, with patches of erosion (E) and deposition (D) between knickpoints. Over longer timescales (>100’s yr), the channel is interpreted as being neutral to slowly aggrading.
Table 1 : Homathko and Southgate Rivers characteristics and estimates of annual organic carbon fluxes. a: Syvtiski and Farrow (1983). b: see Text S1 for suspended load estimates in both rivers. c: no bedload estimate was found in the literature for the Southgate River. The bedload was thus estimated based on the Homathko River using the ratio between the two river watersheds as a scaling factor (Text S2).
Table 2 : Sediment budget and organic carbon (OC) fluxes in the sub-environments of the Bute turbidite system over 10 and 100 yr timescales. a: sediment volumes derived from repeated bathymetric surveys between 2008 and 2018 (Heijnen et al.. in review). Sedimentation rates are obtained by dividing the sediment volume by the surface area of a given sub-environment. Volume uncertainties are based on vertical accuracy of the multibeam surveys of 0.5 % of the water depth. hence the 600 m deep lobe is greatly affected (Heijnen et al.. 2020). b: Assumed sedimentation rates in the channel and lobe are based on c and Baudin et al. (2020; see Suppl. Material). c: Sedimentation rates in the overbank and distal basin are based on 210Pb and 137Cs dating (Syvistki et al.. 1988. Heerema. 2020; Suppl. Material). OC annual fluxes are obtained as follows: OC flux = Sediment volume x TOC x (1-Porosity) x Density
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