4th International Submarine Canyon Symposium (INCISE2018)
5-7 November 2018, Shenzhen, CHINA
This is the detail of Organic matter and sediment characteristics in the Whittard Canyon, North East Atlantic: Preliminary results
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Emma Smith, Annette Wilson, Martin White, Veerle Huvenne, Jason Kirby, Kostas Kiriakoulakis
Organic matter and sediment characteristics in the Whittard Canyon, North East Atlantic: Preliminary results
The Whittard Canyon System (WCS; Celtic Sea, North East Atlantic) is a large (100 km across, 4500 m deep), dendritic system of smaller branches, the major ones are hereby labelled Western Middle, West/East Middle, Eastern Middle, Eastern and Far Eastern branches. WCS is situated in an area of high primary productivity away from terrestrial influence (~300 km from land) and characterised by complex hydrodynamic processes, supporting diverse ecosystems such as extensive cold-water coral (CWC) reefs and bivalve Acesta excavata beds. Sedimentological (grain size), morphological and geochemical (OC and N, δ13C and δ15N, lipids – not shown here) analyses are being carried out on a large number of POM samples (n=20) from the benthic boundary layer (BBL <15 m above bottom) and underlying short (< 20cm) sediment core samples (n=44). These were collected over a range of depths, from several branches, in spring and summer of 2014 to 2016. The aim was to assess the characteristics of material transport, carbon storage and deep-sea ecosystem functioning within the WCS, the latter particularly in relation to organic matter (OM) origin, transformations and nutritional quality.
The sediments were dominated by silt and sand in most branches except the Far East branch where larger clay fractions were evident, suggesting varying and distinct hydrodynamic regimes within WCS. Preliminary OC results showed that most sediments are within typical deep sea values, i.e. 0.2%-0.6% OC of dry sediment. However, within the A. excavata rich area of the Western/East Middle branch at 681 m depth OC was measured as high as 2.7%. The reason for this exceptionally high value is unclear but it is worth noting that this area is known to be affected by thick nepheloid layers, possibly induced by trawling in the upper reaches of the canyon. BBL POM had typical deep sea OC and N concentrations at all sites sampled, ranging from 2.7 to 43.53 µg L-1 and 1.18 to 7.16 µg L-1, respectively. Molar C/N ratios of BBL POM ranged from 1.6 – 8.28, mostly showing a marine signal. The lower C/N values (<5) probably reflect inputs from (re)suspended inorganic nitrogen trapped in clay minerals. The marine signal of surficial sedimentary OM matter is also supported by the range of δ13C and δ15N values (-11.5‰ to 25.2‰ and 0.1‰ to 4.4‰ respectively measured from 30 core tops). However there seems to be a decoupling of BBL POM δ13C and δ15N isotope values from the underlying sediments, as the former have lower δ13C and higher δ15N values (-31.38‰ to -23.4‰ and 7.6‰ to 19.9‰) than the latter; the differences are significant (t-test; <0.05). The reason for this observation is unclear but could be related to nitrogen limitation and increased reworking of POM in the benthic boundary layer. Overall preliminary results emphasize the complexity of the WCS in terms of material transport and organic matter origin and transformations, highlighting a decoupling of sedimentary processes from (re)suspended overlying particles and possible influences of anthropogenic practices in certain parts of it.
Session 3: Patterns and heterogeneity in submarine canyons