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).