Abstract
Abstract. Fjords have been described as hotspots for carbon burial, potentially playing a key role within the carbon cycle as climate regulators over multiple timescales. Nevertheless, little is known about the long-term fate of the carbon that may become stored in fjordic sediments. One of the main reasons for this knowledge gap is that carbon arriving on the seafloor is prone to post-depositional degradation, posing a great challenge when trying to discriminate between an actual change in the carbon deposition rate and post-depositional carbon loss. In this study, we evaluate the use of modern benthic foraminifera as bio-indicators of organic carbon content in six voes (fjords) on the west coast of Shetland. Benthic foraminifera are known to be sensitive to changes in organic carbon content in the sediments, and changes in their assemblage composition therefore reflect synchronous variations in the quantity and quality of carbon reaching the seafloor. We identified four environments based on the relationship between benthic foraminiferal assemblages and organic carbon content in the sediments: (1) land-locked regions influenced by riverine and/or freshwater inputs of organic matter, namely the head of fjords with a restricted geomorphology; (2) stressed environments with a heavily stratified water column and sediments rich in organic matter of low nutritional value; (3) depositional environments with moderate organic content and mild or episodic current activity; and (4) marginal to coastal settings with low organic content, such as fjords with an unrestricted geomorphology. We conclude that foraminifera potentially provide a tool to disentangle primary organic carbon signals from post-depositional degradation and loss of organic carbon because of their environmental sensitivity and high preservation potential in the sedimentary record.
Highlights
Fjords sequester about 18 Mt of organic carbon (OC) annually in their sediments, partly acting to reduce the remineralisation of OC into the environment and buffering the release of carbon dioxide (CO2) to the atmosphere (e.g. Smith et al, 2015; Cui et al, 2016)
We evaluate the use of modern benthic foraminifera as bio-indicators of OC content in six voes on the west coast of Shetland whose sedimentation dynamics are likely influenced by the erosion of OC-rich peat from the surrounding catchments (Fig. 1)
The percentage of total carbon (TC) in west Shetland voes varies between 4.31 % and 14.32 % (Fig. 3d), with the highest percentage found at station MD15-05 (Clift Sound) and the lowest at station MD15-09 (Sand Sound)
Summary
Fjords sequester about 18 Mt of organic carbon (OC) annually in their sediments, partly acting to reduce the remineralisation of OC into the environment and buffering the release of carbon dioxide (CO2) to the atmosphere (e.g. Smith et al, 2015; Cui et al, 2016). A recent study by Smeaton et al (2017) estimated that the sediments of Scotland’s 111 major fjords hold 295 ± 52 Mt OC. The erosion of terrestrial organic matter (OM) is known to be a major source of OC in fjordic sediments (Cui et al, 2017), and Scottish peatlands hold 1620 Mt of OC (Chapman et al, 2009). Eroding peatlands may hold the potential to contribute significantly to OC burial and accumulation in Shetland’s voes and to the overall coastal OC store of this region. One of the main reasons for this knowledge gap is that OC is prone to post-depositional modification and degradation.
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