Case study: Blue carbon in Scottish marine sedimentary environments

Continental shelf and Exclusive Economic Zone sedimentary carbon stores

The growing interest in sedimentary carbon (C) in the marine environment, sometimes referred to as Blue Carbon, stems not from the direct sequestration of carbon dioxide (CO₂) from the atmosphere, but rather from the receipt of carbon from in situ biological processes and large carbon subsidies from other environments. These carbon subsidies can derive from marine biological, terrestrial biological and geological sources. Globally, marine sediments are estimated to bury approximately 160 million tonnes organic carbon per year (Mt OC yr^-1)(Hedges et al., 1997; Smith et al., 2014) with 42% of this total captured by non-deltaic continental shelf sea sediments. Globally significant Scottish marine carbon stores can be compared to equivalent Scottish terrestrial carbon stores, such as peatlands, soils and living forest (Figure 1).

Figure 1: Schematic showing that Scottish marine sediment carbon stores (top 10 cm) presented as CO₂ equivalent amounts and shown against CO₂ equivalents for Scottish peatland (top 10 cm), other soils (top 10 cm) and forests (living trees) (source: https://blogs.gov.scot/marine-scotland/2020/02/24/new-blue-carbon-reports/).

A recent report (Smeaton et al., 2020) sets out to provide an improved estimate of the carbon held within Scotland’s marine sedimentary environments. Using a standardised framework, first-order estimates of Scotland’s surficial (top 10 cm) marine sedimentary carbon stock were undertaken using open-access data sources. The surficial sediments (top 10 cm) of the mapped area (554,755 km² of the total area of 618,744 km²) of the continental shelf and Exclusive Economic Zone seabed adjacent to Scotland holds an estimated 1,515 ± 252 Mt C. Most of this carbon is in the form of calcium carbonate, with an estimated 1,294 ± 161 Mt inorganic carbon (IC) being held within the surficial sediments (Figure 2).

Figure 2: Spatial distribution of inorganic carbon (IC) across Scotland’s continental shelf sediments (Smeaton et al., 2020). Scottish Marine Regions and Offshore Marine Regions outlined in white.

A significantly lower quantity of carbon in these surface sediments is stored in the organic (OC) form, with an estimated 221 ± 92 Mt OC currently held within the top 10 cm of sediment within the mapped area of the continental shelf and Exclusive Economic Zone (Figure 3).

Figure 3: Spatial distribution of organic carbon (OC) across Scotland’s continental shelf and Exclusive Economic Zone sediments (Smeaton et al., 2020). Scottish Marine Regions and Offshore Marine Regions outlined in white.

Scottish sedimentary carbon “hot-spots”

Significant new stores of sedimentary carbon (295.6 ± 52 Mt OC) have recently been quantified within the post-glacial sedimentary sequences of Scottish sea lochs (fjords) (Smeaton et al., 2017); it is estimated that a further 4.16 ± 0.5 Mt OC is stored in the surficial sediment (top 10 cm) of Scotland’s sea lochs (Smeaton & Austin, 2019). These organic rich sea loch sediments cover a significantly smaller area (2,052 km²) than their continental shelf counterparts, yet their carbon density (20.27 ± 3.67 tonnes OC ha^-1) far exceeds that found on Scotland’s continental shelf (area < 200 m water depth) (4.2 ± 1.8 tonnes OC ha^-1). These results highlight that fjords are globally significant “hot spots” for carbon burial (Smith et al., 2015) and recent work by Smeaton & Austin (2017) highlights that Scottish sea lochs provide effective traps for terrestrial carbon within coastal seas, with important implications for national carbon accounting.