5.2. How much and what type of organic is buried in the
Bute Inlet sub-environments?
The surficial sediments from the Bute turbidity current system bury a
total of 15.4 – 17.9 Kt OC/yr, with 11.7 to 16.3 Kt OC/yr being of
terrestrial origin. 76 to 91 % of the OC buried in Bute Inlet is thus
terrestrial. This proportion is higher compared to previous estimates
made in other fjords where 42 to 65 % of the OC was found to be
terrestrial (Cui et al., 2016, Smeaton & Austin, 2020). This further
highlights the importance of turbidity currents for distributing
terrestrial OC in river-fed fjords, and the necessity to include
turbidity current processes when assessing the burial of terrestrial OC
globally.
There are a number of uncertainties behind OC fluxes calculated in Bute
Inlet. First, we do not have sediment cores in the external parts of the
overbanks, close to the fjord’s sidewalls. Therefore, we extrapolate the
OC composition found in the overbanks close to the channel to the entire
overbank area. These more distal sites in the overbank may either be
poorer in terrestrial OC compared to the sites close to the channel fed
by the rivers, or richer in OC due to smaller rivers and streams and
potential landslides sourced from the fjord steep sidewalls. Second, we
assume that the 30 cm thick (i.e. maximum a few weeks old) deposits
recovered from the channel are reproduced over centennial timescales
despite the rapidly migrating knickpoints in the channel (Prior et al.,
1987, Heijnen et al., 2020). We could expect that channel-floor fine
sands, which are rich in OC, are excavated several times before reaching
their ultimate burial location on the lobe, decreasing their OC content
over time (Heijnen et al., in review). However, we do not see any
evidence of OC content decrease along the channel transect, at least
over the very recent (30 cm below seafloor) deposits observed in this
study. We suspect that the channel is an efficient conduit for the
delivery of terrestrial OC to the lobe, but this needs to be confirmed
with longer sediment cores that are difficult to collect in sandy
channel floors (as piston corers tend to fail to penetrate sandy
seabed).
Despite these uncertainties, the data collected in Bute Inlet allow us
to derive for the first time a detailed OC burial budget in fjord
sediments controlled by turbidity currents (Fig. 8). Together the
channel and lobe, dominated by sands (Fig. 5) and only covering 17 % of
the fjord’s seafloor area, comprise 48 to 77 % of the total annual
terrestrial OC burial flux in Bute Inlet. This terrestrial OC is
predominantly young, associated with fine sands and buried beneath a
layer of mud (Hage et al., 2020, Fig. 6). The overbanks, dominated by
mud (Fig. 5) and covering 47 % of the fjord’s seafloor area, comprise
15 to 25 % of the total annual terrestrial OC burial flux, with low TOC
in the fine sands. The settling velocities of waterlogged woody debris
have been shown to be relatively rapid due to their size and density
(Waterson et al., 2008, Hoover et al., 2010, McArthur et al., 2016),
such that this woody debris is carried mainly along the channel floor.
This concentration of woody debris along the channel floor could explain
the low TOC values seen within the fine-sands of the overbanks. The
distal flat basin, exclusively made of mud in the surficial (200 cm)
sediment and covering 36 % of the fjord’s seafloor area, comprises the
remaining 6 to 22 % of the total annual terrestrial OC burial flux. The
abundance of mud in the surficial sediment of the distal flat basin is
explained by the limited number of turbidity currents reaching this site
in present times (Prior et al., 1987; Chen et al., 2021; Pope et al., in
review).