Spatial Relationships between Pockmarks and Sub-Seabed Gas in Fjordic Settings: Evidence from Loch Linnhe, West Scotland

Sub-seabed gas is commonly associated with seabed depressions known as pockmarks—the main venting sites for hydrocarbon gases to enter the water column. Sub-seabed gas accumulations are characterized by acoustically turbid or opaque zones in seismic reflection profiles, taking the form of gas blankets, curtains or plumes. How the migration of sub-seabed gas relates to the origin and distribution of pockmarks in nearshore and fjordic settings is not well understood. Using marine geophysical data from Loch Linnhe, a Scottish fjord, we show that shallow sub-seabed gas occurs predominantly within glaciomarine facies either as widespread blankets in basins or as isolated pockets. We use geospatial ‘hot-spot’ analysis conducted in ArcGIS to identify clusters of pockmarks and acoustic (sub-seabed) profile interpretation to identify the depth to gas front across the fjord. By combining these analyses, we find that the gas below most pockmarks in Loch Linnhe is between 1.4 m and 20 m deep. We anticipate that this work will help to understand the fate and mobility of sedimentary carbon in fjordic (marine) settings and advise offshore industry on the potential hazards posed by pockmarked seafloor regions even in nearshore settings.

Towards a Sedimentary Carbon Stock Estimate for Scotland's EEZ: A Tiered Mapping Approach.

<p>Coastal and shelf sediments trap and bury significant quantities carbon (Berner, 1982) and provide an conditions allowing for the long-term storage of carbon. Through burying this carbon these sediments potentially provide a climate mitigation services. Currently our understanding of the spatial distribution of C within the surficial sediments of  coastal and shelf seas is limited. Using Scotland’s EEZ as a natural laboratory in conjunction with the tiered seabed mapping methodology developed by Smeaton and Austin (2019), we show that coastal and shelf sediments are highly heterogenous in both sediment type and C content. The tiered approach utilised in this study is ideally suited to global applications where data availability may differ significantly. Improved spatial mapping of seabed C will provide policy makers with a new tool for the targeted management and protection of these globally important C stores.</p><p>Berner, R. A., 1982, Burial of organic carbon and pyrite sulfur in the modern ocean: Its geochemical and environmental significance.Am. J. Sci.282,451–473 (1982)</p><p>Smeaton, C. and Austin, W.E.N., 2019. Where’s the Carbon: Exploring the Spatial Heterogeneity of Sedimentary Carbon in Mid-Latitude Fjords. Frontiers in Earth Science, 7, p.269.</p>

Assessing the Influences of Large-Scale Disturbance on Sedimentary Marine Carbon Stores

<p>Shelf and coastal seas hold vast quantities of sedimentary carbon which contribute to atmospheric carbon dioxide removal and long-term carbon storage. However, the stability and resilience of this key component of global natural capital remains poorly quantified, particularly under anthropogenic stressors. Demersal trawling activity is the most significant cause of widespread anthropogenic disturbance to the seabed, leading to massive sediment resuspension events and wide scale impact to benthic communities. The impacts of trawling on benthic ecosystems and biodiversity are well reported and understood within the literature (e.g. Jones, 1992; Rijnsdorp et al., 2016); however, a knowledge gap remains regarding the post-trawl fate of sedimentary carbon (van de Velde et al., 2018).</p><p>In order to gain a better understanding of the post-disturbance effects of carbon cycling in marine sediments, an experimental trial to mimic fishing impacts was created. Over a 21-day period, a series of closed-tank incubation experiments investigating the impact of simulated benthic fishing gear penetration depth in soft sediments was conducted. Here, marine sediments underwent an artificial disturbance event every 24 hours, with a series of varying depth regimes used. We hypothesise that the large-scale resuspension events caused by trawling may contribute towards an enhancement in localised carbon cycling, and thus a reduction in the net carbon storage within these sediments. The aim of this experiment was to better understand the biogeochemical processes which occur in marine sediments during massive resuspension events, with a particular emphasis on the fate of resuspended organic carbon matter and its potential vulnerability. Dissolved organic carbon and various macronutrients of interest (e.g. PO<sub>4</sub>, SiO<sub>2</sub>, NH<sub>4</sub>, NO<sub>2</sub>, NO<sub>3</sub>) were also measured.</p><p>Jones, J.B., 1992. Environmental impact of trawling on the sea bed: a review. New Zeal. J. Mar. Freshw. Res. 26, 59–67. https://doi.org/10.1080/00288330.1992.9516500org/10.1080/00288330.1992.9516500</p><p>Rijnsdorp, A.D., Bastardie, F., Bolam, S.G., Buhl-Mortensen, L., Eigaard, O.R., Hamon, K.G., Hiddink, J.G., Hintzen, N.T., Ivanović, A., Kenny, A., Laffargue, P., Nielsen, J.R., O’Neill, F.G., Piet, G.J., Polet, H., Sala, A., Smith, C., Van Denderen, P.D., Van Kooten, T., Zengin, M., 2016. Towards a framework for the quantitative assessment of trawling impact on the seabed and benthic ecosystem. ICES J. Mar. Sci. 73, i127–i138. https://doi.org/10.1093/icesjms/fsv207</p><p>van de Velde, S., Van Lancker, V., Hidalgo-Martinez, S., Berelson, W.M., Meysman, F.J.R., 2018. Anthropogenic disturbance keeps the coastal seafloor biogeochemistry in a transient state. Sci. Rep. 8. https://doi.org/10.1038/s41598-018-23925-y</p>

Terrestrial Vegetation Drives Methane Production in the Sediments of two German Reservoirs

Inland waters and reservoirs in particular are significant sources of methane to the atmosphere. However, little information is available on the extent to which organic carbon from terrestrial vegetation or from internal photosynthesis fuels the methane production. This limits our ability to constrain methane emissions efficiently. We studied the isotopic composition (13C, 14C) of pelagic and sedimentary carbon sources in two small German reservoirs. The methane was enriched by radiocarbon with isotopic ranges (∆14C 5‰ to 31‰) near to fresh terrestrial organic carbon (OC, 17‰ to 26‰). In contrast, potential source OC produced by internal photosynthesis was characterized by negative ∆14C values (−30‰ and −25‰) as derived from signatures of inorganic carbon in the reservoirs. The particulate OC in stream supplies (terrestrial OC) was also 14C depleted in almost all cases, but highly variable in ∆14C (−131‰ to 42‰). Although the import of terrestrial OC was lower than the amount of OC produced by reservoir-internal photosynthesis, we conclude that the methane production was predominantly fuelled by catchment vegetation. The utilized terrestrial OC was of contemporary origin, fixed within years to decades before sampling and supplemented with reservoir-internal or aged terrestrial OC. Our results indicate that terrestrial biomass is an important driver of methane production in reservoirs receiving significant imports of terrestrial OC.