The effects of megafaunal burrows on radiotracer profiles and organic composition in deep-sea sediments: preliminary results from two sites in the bathyal north-east Atlantic

Megafaunal burrows were detected in boxcores from two sites in the bathyal north-east Atlantic. Burrow contents were analysed to assess their significance to sediment radiotracer profiles and organic composition. At 1100 m depth, burrow openings up to 3 cm diameter occurred at a density of approximately 5 m−2. Burrows at 12-18 cm sediment depth extending horizontally for up to 35 cm and linked to the surface by vertical shafts were provisionally attributed to echiuran worms, although no occupants were found in situ. In one example the horizontal burrow section was filled with green slurry, for which scanning electron microscopy, 210Pbexcess and organic content all indicated a phytodetrital origin. At 1920 m depth no large burrow openings were found in five boxcores examined, but large subsurface biogenic structures were present. Galleries at 15-26 cm depth were traced horizontally for up to 30 cm, but contained no occupants or filling. Extended linear bands of faecal pellets were found in three boxcores at 13-17 cm depth. Excess 210Pb content indicated that most of these structures resulted from surface deposit feeding. Faecal pellet bands may partially explain the occurrence of subsurface peaks detected in profiles of 210Pbexcess at this site. Results suggest that 'caching' of phytodetritus and subsurface deposition of faeces are two mechanisms for the rapid, deep burial of relatively fresh organic matter, but the significance of these processes to sediment geochemistry cannot be quantified without much information on the distribution, identity and abundance of burrowing megafauna in the deep sea.