Abstract
Marine sediments hold vast stores of organic carbon (OC). Techniques to spatially map sedimentary OC must develop to form the basis of seabed management tools that consider carbon-rich sediments. While the natural burial of carbon (C) provides a climate regulation service, the disturbance of buried C could present a significant positive feedback mechanism to atmospheric greenhouse gas concentrations. We present a regional Scottish case study that explores the suitability of integrating archived seafloor acoustic data (i.e., multibeam echosounder bathymetry and backscatter) with physical samples toward improved spatial mapping of surface OC in a dynamic coastal environment. Acoustic backscatter is a proxy for seabed sediments and can be collected over extensive areas at high resolutions. Sediment type is also an important predictor of OC. We test the potential of backscatter as a proxy for OC which may prove useful in the absence of exhaustive sediment data. Overall, although statistically significant, correlations between the variables OC, sediment type, and backscatter are relatively weak, likely reflecting a combination of limited and asynchronous data, sediment mobility over time, and complex environmental processing of OC in shelf sediments. We estimate linear mixed models to predict OC using backscatter and Folk sediment type as covariates. Our results show that incorporating backscatter in the model improves the precision of OC predictions by 14%. Backscatter discriminates between coarse and fine sediments, and therefore low and high OC regimes; however, was not able to discriminate amongst finer sediments. Although sediment type is a stronger predictor of OC, these data are available at a much lower spatial resolution and do not account for fine-scale variability. The resulting maps display varying spatial distributions of OC reflecting the different scales of the predictor variables, demonstrating a need for further methodological development. Backscatter shows considerable promise as a high-resolution predictor variable to improve the precision of surface OC maps, or to reduce the number of OC measurements required to achieve a specified precision. Applications of such maps have potential in improved C-stock estimates and the design of conservation and management strategies that consider marine sediments as valuable C stores.
Highlights
The marine environment has a significant role within the global carbon (C) cycle with 93% of the earth’s C stored and cycled here, providing an essential energy source for marine biodiversity (Nellemann et al, 2009)
Mapping the spatial distribution of sedimentary organic carbon (OC) at regional scales is important for targeted seabed management measures which can protect C-rich sediment stores, in addition to promoting biodiversity, and food provisioning services
We have utilised an existing MBES dataset to investigate the potential of acoustic backscatter as a proxy for sediment type and to map associated OC
Summary
The marine environment has a significant role within the global carbon (C) cycle with 93% of the earth’s C stored and cycled here, providing an essential energy source for marine biodiversity (Nellemann et al, 2009). Marine sediments are currently overlooked within current BC definitions and accounting frameworks because they do not directly sequester C via photosynthesis (Lovelock and Duarte, 2019) They are a regional and global repository for OC and act to bury, and remove, OC from the active C cycle over geological timescales (Smith et al, 2015; Sayedi et al, 2020). Activities that physically disturb seabed sediments cause the resuspension and exposure of buried OC to oxygen, which can be remineralised back to aqueous CO2 (Aller, 1994; Macreadie et al, 2019) The management of these activities, such as benthic trawling (Paradis et al, 2017), could be key to maintaining sediments as part of the suite of nature-based solutions for mitigating against climate change (Sala et al, 2021). Assessments of the spatial distribution of sedimentary OC are key to understanding the processes that influence how C is processed and where it is more likely to be accumulating, i.e., carbon hotspots (Diesing et al, 2021)
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