Quantifying Marine Sedimentary Carbon: A New Spatial Analysis Approach Using Seafloor Acoustics, Imagery, and Ground-Truthing Data in Scotland

William E. N. Austin,Dayton Dove,Rhys Cooper,Urška Demšar,Corallie Hunt,Craig Smeaton

doi:10.3389/fmars.2020.00588

Abstract

Marine sediments are important repositories of particulate organic matter, effectively burying organic carbon (OC) over geological timescales thus providing a climate regulation service. However, the spatial distribution of this marine sedimentary OC store is not well constrained. In this study we leverage a high resolution multibeam echosounder (MBES) survey taken at Loch Creran, a model fjordic site on the west coast of Scotland, to develop a new methodology for predicting the distribution of OC in surface sediments. Using an integrated approach, we combine use MBES survey, video imagery and ground-truthing data to produce a high-resolution (2 m x 2 m) map of surficial carbon and calculate a 10 cm stock. We find that the backscatter survey reliably uncovers a heterogeneous seabed and that OC correlates strongly with the MBES backscatter signal as a function of sediment composition. We estimate that there are approximately 12,346 ± 2,677 tonnes of OC held within the top 10 cm of mixed sediments across the MBES survey area (7.96 km2; 60% of the total area), upscaled to 20,577 ± 4,462 t of OC across Loch Creran (13.27 km2). Normalised by area, we find that mixed fine sediments with small fractions of sand and gravel are more effectivehold more carbon OC stores than homogenous fine sediments. This initial work proposes a novel methodological approach to using high resolution MBES surveys to improve the spatial mapping of sedimentary C carbon (C) and identification of C hotspots, enabling consideration of this resource in sedimentary carbon accounting, seabed management and climate mitigation strategies.

Highlights

The continental shelf is a net sink for carbon dioxide (CO2) with substantial long-term stores of C found in seabed sediments (Bauer et al, 2013)
While the integration of multibeam echosounder (MBES), video and ground-truth data is commonly used within benthic habitat mapping studies (De Falco et al, 2010; Haris et al, 2012; Brown et al, 2019) here we extend this idea to develop a novel methodology to map the spatial variability of organic carbon (OC)
We found that high levels of Organic matter (OM) caused artifacts within the particle size results, by masking the presence of sand grains (Supplementary Figure 1), which could be responsible for the scattering we see in the fine-grained sediments

Summary

Introduction

The continental shelf is a net sink for carbon dioxide (CO2) with substantial long-term stores of C found in seabed sediments (Bauer et al, 2013). The North-West European continental shelf is predicted to store 476 Mt of OC in surficial sediments, there are large uncertainties. Quantifying Marine Sedimentary Carbon within this estimate (ranging between 230 and 882 Mt) (Diesing et al, 2017). With a growing understanding of the role of the shelf seas in carbon storage (Kröger et al, 2018), sediment maps detailing OC have the potential to inform marine management strategies for the preservation of sedimentary C, which is vulnerable to disturbance, natural or otherwise (Almroth-Rosell et al, 2012; Van De Velde et al, 2018). Associated uncertainty estimates are high over such large scales due to the low-density ground-truth data, for instance within the northern North Sea (Diesing et al, 2017)

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