Extending the applications of sediment profile imaging to geochemical interpretations using colour

Abstract

Whilst Sediment Profile Imaging (SPI) is a very widely used technique in the regulatory assessment of seabed environmental health, and in the study of seafloor sediment-biology interactions, the potential for SPI images to be used in a geochemical context has not been rigorously assessed. Here we have examined relationships between colour and geochemistry in a sediment core collected from the Celtic Sea, North West European Shelf, that was digitally imaged and on which detailed geochemical analyses were also performed. Average oxygen penetration depth was 4.08 ± 0.72 mm, (n = 5), whilst the apparent redox potential discontinuity (aRPD) as determined by sediment colour change was at 78 mm. As iron (oxyhydr)oxides decreased with depth, black sulfide phases increased, and the aRPD most closely correlated with this geochemical change rather than the oxygen penetration depth. Colour analysis of the image showed a clear correlation of brightness with black FeS (acid volatile sulfide). There was a general correlation of iron oxide phases with orange colour in the upper part of the sediment profile, whilst.in the lower part of the core the orange oxide phases appeared to be obscured by the black FeS present. The sulfide-brightness relationship indicates colour analysis can provide an estimate of FeS, and potentially the carrying capacity for toxic metals such as cadmium, zinc and copper as sulfides in this type of sediment. Additionally, detailed geochemical analyses of SPI cores may provide new insights into the activity and impacts of infauna and the link with sediment biogeochemical cycles of carbon and nutrients.