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).