5.4. Comparison with larger, deep-sea turbidity current systems
To our knowledge, only one study has provided OC balance budgets between river source and turbidity current sub-environments within a single system. This is the Congo River to deep-sea Fan (Baudin et al., 2020), where burial rates are calculated over centennial timescales that are comparable to the longer timescales considered in this study. Using a source to sink approach, Baudin et al. (2020) found that 33 to 69 % of the annual OC delivered by the Congo River is buried in the Congo turbidity current system. This burial efficiency is similar to, if slightly lower than, our estimate in Bute Inlet (50 to 70 %), highlighting the difficulty in closing budgets between river source and marine sinks fed by turbidity currents, particularly in open marine systems such as the Congo system (Baudin et al., 2020). Within these two turbidity current systems, both the Congo and Bute Inlet submarine canyon-channels show minimal long-term (> 100 year) OC accumulation (Fig. 8, Baudin et al., 2020). We note however that under-sampling of the Congo Canyon and Channel occurred in past studies due to the difficulty with sampling sandy deposits (Baudin et al., 2020). This under-sampling of the canyon and channel may result in underestimation of OC burial rates in the Congo Channel, similarly to the Bute Inlet channel where only 30 cm long sediment cores could be retrieved. In both Congo and Bute Inlet systems, the lobe holds most of the total buried OC.
Besides this example from the Congo system, work in Gaoping Canyon offshore Taiwan shows a terrestrial OC preservation efficiency of > 70% in marine sediments. The Gaoping submarine canyon is supplied by river floods (hyperpycnal) and dilute surface plumes (hypopycnal) inputs (Kao et al., 2014). This high OC burial efficiency results from this mountainous island being a hotspot for sediment and OC production, with frequent typhoons that deliver terrestrial OC (e.g. woody debris associated with sands) through the submarine canyon to the deep-sea (Hilton et al., 2008, West et al., 2011, Liu et al., 2016). Another example of high OC burial efficiency comes from the Bay of Bengal where almost no loss of OC was observed between the Ganges-Brahmaputra River inputs and the Bengal Submarine Fan (Galy et al., 2007). Coarse turbidity current deposits from the channel-levees of the Bengal Fan submarine system were also shown to have abundant terrestrial woody fragments throughout the last 19 My (Lee et al., 2019). These OC-rich turbidity current deposits are interpreted to result from high-magnitude, low-frequency events (e.g. floods, cyclones; Lee et al.; 2019).
Based on these examples, it is plausible that rare and extreme events may also affect the OC burial efficiency and distribution in Bute Inlet. It was recently shown that most turbidity currents (~90 %) dissipate within the shallowest-water (< 200 m depth and < 12 km along channel from the delta) part of Bute Inlet, whereas less frequent (~10%) events rework this material and progressively shuffle it downstream to the lobe (Heijnen et al., in review). We further suspect that rare long runout events can flush material to the distal flat basin, as evidenced by thick sandy accumulations found at >3 m depth in 8 m long piston cores collected in the distal basin at the location of core 15 (Fig. 3, Heerema, 2021). Based on our OC burial efficiency estimates (i.e., 50 to 70 % over decennial to centennial timescales, respectively), it appears that OC burial efficiency is higher when considering longer timescales, which are more likely to integrate some large events than shorter timescales.
An example of rare and large event occurred in the Bute Inlet area on 28 November 2020 when a glacial lake outburst flood took place at Elliot Creek, which is a tributary of Southgate River (Fig. 1). This outburst flood caused an exceptionally large landslide involving 15 million cubic meters of terrestrial material, some of which was released into the Southgate River, ultimately discharging into Bute Inlet (Geertsema et al., in review,https://blogs.agu.org/landslideblog/2020/12/16/bute-inlet-landslide/). The impact of this event on the OC burial in Bute Inlet is yet to be determined and compared with the pre-event OC fluxes presented in this study, but it may help to understand sediment and OC delivery in most infrequent but large magnitude events.
6. Conclusion
This study provides the first complete source-to-sink budget for organic carbon (OC) in a river-fed fjord turbidity current system, showing how OC is distributed within the upper <2 m of sediment. OC fluxes are estimated for the fjord’s two river sources (Homathko and Southgate Rivers, which provide 94 % of the water and sediment discharge to the fjord), and compared to OC burial fluxes in different seafloor sub-environments. We estimate that the annual OC export from both rivers to the fjord is 23 ± 5 Kt C/yr, although this may not capture the full annual range of streamflow variability. The annual terrestrial OC burial rate of the entire fjord is estimated to range from 12 to 16 Kt OC/yr. This suggests that a relatively high terrestrial OC burial efficiency (60 ± 10 %) occurs within the fjord. Terrestrial OC is distributed as follows across the fjord seafloor; 63 ± 14 % of the total terrestrial OC burial occurs in the sand-dominated channel floor and lobe (covering 17 % of the fjord seafloor area), whereas 37 ± 14 % occurs in the mud-dominated overbank and distal flat basin (covering 83 % of the fjord seafloor area). Therefore, hydrodynamic fractionation of OC by turbidity currents leads to variable burial efficiencies in different sub-environments of the fjord. This study helps to understand how OC is buried within fjords over short (decennial to centennial) timescales. A comparison to other (non-fjord) turbidity current systems suggests that these systems consistently have high (50 to 100 %) terrestrial OC burial efficiency (Galy et al., 2007, Liu et al., 2016, Hilton et al., 2017; Baudin et al., 2020). Turbidity current systems may thus play a globally important role in the highly efficient delivery and burial of mainly terrestrial OC, which affects atmospheric CO2levels over geological timescales (Berner, 1982) and food resources for modern benthic ecosystems (Włodarska‐Kowalczuk et al., 2019).
Data availability: Discharge data for the Homathko River and Southgate River are available from https://wateroffice.ec.gc.ca; station 08GD004 and station 08GD010, respectively. Multibeam bathymetric data are held by the Geological Survey of Canada and Canadian Hydrographic Survey. Sediment core and river sample locations are provided in the supplementary material of this study, as well as organic geochemistry measurements made on each sample (Tables S1, S2, S7 and S8).
7. Acknowledgements
We acknowledge that this work took place in the unceded traditional territory of the Homalco First Nation. We thank the captain and crew of the CCGS Vector (Canada) for sample collection. S.H. acknowledges funding by the IAS postgraduate grant scheme, a Research Development funds offered by Durham University, and the NOCS/WHOI exchange program. S.H. has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 899546. The field campaign and geochemical analyses were supported by Natural Environment Research Council Grants NE/M007138/1, NE/W30601/1 and NE/M017540/1. M.J.B.C. was funded by a Royal Society Research Fellowship (DHF\R1\180166). M.A.C. was supported by the U.K. National Capability NERC CLASS program (NE/R015953/1) and NERC Grants (NE/P009190/1 and NE/P005780/1). C.J.H. and M.S.H. were funded by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska- Curie grant agreement No. 721403 - ITN SLATE. E.L.P. was supported by a Leverhulme Early Career Fellowship (ECF-2018-267).