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