Origin, transport and burial of organic matter in the Whittard Canyon, North East Atlantic

Abstract

The Whittard submarine canyon (Celtic Sea, North East Atlantic) is one of the largest (~100km across, down to 4500m depth) and most complex underwater features in the North-Western European Margin having several branches all converging to the main channel. It is located >200km from the nearest coast, affected by complex hydrodynamics and is home to an array of diverse benthic ecosystems. Little is known about how submarine canyons differ from typical open-ocean environments. This study aims to improve the understanding of the canyon’s underlying role in biogeochemical cycling and carbon storage in relation to the sedimentological regime and specific geomorphic features. This study hypothesizes that the highly heterogeneous nature of the canyon’s physical landscapes may influence organic matter spatial patterns and the potential for carbon burial. This project attempted to assess this gap in knowledge by examining morphological (slope analysis, ruggedness), grain size characteristics, and biogeochemical properties (organic carbon, nitrogen, δ13C and δ15N isotopes) of 46 short cores (0-10cm) from a variety of depths, across nine main locations (Western, Western Middle, Acesta, Eastern Middle, Eastern Middle 2, Intersection, Eastern, Explorer and the Main channel branches). Grain sizes often appear multimodal, with a sandy/silty/clay predominance, with coarse material present at depths exceeding 1000m, indicating that diverse processes are at play within the system. Surficial mean molar C/N ratios are variable but often higher than typical marine ranges; 22 ± 15.62, given the distance from land, this is likely a consequence of reworking. Organic carbon contents generally exceeded the typical range expected from the deep-sea (>~0.5% of dry sediment) but are similar to other canyon systems that are closer to land, this may indicate an increased potential for carbon storage within the Whittard Canyon (Tyler et al., 2009b; Masson et al., 2010a). All samples within the central channel (Main Channel) at depths >3700m were organically enriched (>1.25%), this is likely an accumulation of organic material, funnelled from the upper canyon reaches. However, at one site of the Eastern branch (CE14009-005-450m), the lowest TOC content of 0.33% was recorded, indicating reduced input or the efficient recycling of organic matter. The majority of δ13C‰ values fell within the lighter isotopic range of ~-24‰ and ~-22‰, indicating phytoplankton and zooplankton derived organic matter. However, a heavier value was recorded at the connecting shelf of the Acesta and Eastern Middle branch CE16006-030-PSH12-511.37m (-12.76), indicating sulfide-oxidized carbon which is associated with the form II Rubisco pathway (Fry and Sherr, 1984; Levin and Michener, 2002; Hunter et al., 2013a). A mean δ15N signature of 3.32‰ ± 0.84 falls just below the deep-water average of ~5‰ value associated with internal cycling and assimilation by marine primary producers (Sigman and Casciotti, 2001). This is in line with previous research within the Whittard Canyon, where values of 4.09‰ ± 2.42 and 3.94‰ ± 0.67 for West and Eastern branches were recorded (Hunter et al., 2013a). Statistical analyses were carried out to decipher influences of morphology, location (i.e. branch), depth and potential anthropogenic activity. Previous research has indicated that the Eastern branch displays high biodiversity and faunal abundance, results from all disciplines explored in this study confirmed that the Eastern branch was the most heterogeneous, which may explain this phenomenon (Amaro et al., 2015; Gunton et al., 2015). Recent work suggests that anthropogenic (i.e. fishing) activities may impact natural processes, possibly affecting material transport, deposition and ecological functions within the Acesta and Eastern Middle branches, that would take many years to recover (Wilson et al., 2015). Because of this, this study proves useful as it provides one of the highest resolution accounts of recent conditions across this environmentally and biologically diverse canyon system.