4.2. Facies and carbon composition in the fjord; what is preserved and where?
We now present the OC content and composition within the fjord seabed, across the four previously-defined sub-environments (Fig. 3).
Sediment cores collected from the sandy channel floor andlobe have similar facies and OC composition, thus the OC composition of these sub-environments are described together. The channel floor and lobe contain 63 % carbon-poor (mean TOC = <0.05 %) coarse sand; 29% of carbon-rich (mean TOC = 3 %) fine sand; and 8 % of carbon-moderate (mean TOC = 0.5 %) mud (Fig. 5). δ13C and 14C ages revealed that fine sands contain young terrestrial woody debris (visible to the naked eye), buried under older biospheric soil organic matter in the muds (Hage et al., 2020; Fig. 6).
In contrast to the channel floor and lobe, the overbanks are dominated (80 %) by muddy deposits characterized by lower TOC values (0.35 % on average; Fig. 5). Fine sands found in the overbank cores have much lower TOC values compared to fine-sands in the channel floor and lobe. δ13C signatures range between -26 and -28 ‰, pointing to a terrestrial origin of the organic matter (Fig. 6). We note the absence of woody debris visible to the naked eye in the overbank cores, as opposed to the channel floor and lobe cores.
The top two meters of sediment recovered from the distal flat basin is made exclusively of muddy sediments characterized by a homogenous red (35 %) or grey facies (65 %). Grey muds have moderate TOC (0.65 % on average; Fig. 5) and a wide range of δ13C signatures, suggesting a mixed terrestrial and marine origin of organic matter (-22 to -26 ‰; Fig. 6). Red muds have high TOC (2.5 % on average; Fig. 5) with δ13C between -20.5 and -23 ‰ (Fig. 6) that point to a marine-dominated origin (MacDonald et al., 1991). Based on a binary mixing model using δ13C compositions of marine and terrestrial OC end-members, we estimate that about 54 % of the organic carbon found in the upper two meter sediment of the distal flat basin is of marine origin (Text S6, Table S6). 14C data and RPO thermograms on the red and grey muds in this distal site (Fig. 7) reveal that the OC in this muddy distal site is of similar age (500 to 110014C yr), compared to the organic matter found in the muddy deposits of the channel (997 14C yr; Fig. 6). Overall, OC associated with muds in the active channel and distal flat basin are older compared to the young OC associated fine sands found in the active channel (Fig. 6, Hage et al., 2020), due to mineral protection of old biospheric OC in muddy sediments (Hemingway et al., 2019).
In total, we estimate that the top two meters of sediments in the Bute turbidity current system comprise an annual OC burial rate ranging from 15.4 to 17.9 Kt OC/yr over decennial to centennial timescales, respectively (Table 2). We divide this budget between terrestrial and marine organic carbon contribution based on the mixing model applied to the distal flat basin samples (Text S6). In total, between 11.7 and 16.3 Kt OC/yr of terrestrial origin carbon are found in the fjord, in all of its sub-environments. Between 1.6 and 3.7 Kt OC/yr of marine origin carbon are found in the distal flat basin (Figs. 6 and 7, Table 2).
5. Discussion