Ecology and geology of pockmarks

Pockmarks have now been recognised for nearly four decades, and are one of the most obvious and abundant structural features of the seafloor in some areas, yet there is still little knowledge of their influence on fauna or their exact ages and modes of formation. This thesis investigates mechanisms of pockmark formation in the recently discovered pockmarks in the Oslofjord and aspects of the benthic ecology of these ubiquitous topographic features in the North Sea and the Oslofjord. Piston cores collected from a pockmark in the Oslofjord indicated that their formation was probably initiated between 10.7 and 9.5 kyr BP. Thus, pockmarks may have initially formed in an ice-marginal marine setting, where mechanisms such as submarine melt water outflux, ice rafting and rapid isostatic uplift may be of strong relevance. The biostratigraphic and radiocarbon data collected indicate continuously low sedimentation rates inside the pockmark since its formation and that it has been actively seeping artesian ground water since this time. The influence of the pockmarks on the faunal communities in the Oslofjord was subtle. However, the abundances of key species inside pockmarks were significantly different to the surrounding seabed and as such I argue that the abundance of pockmarks in the Oslofjord has a considerable cumulative influence on overall faunal densities and populations. The megafaunal communities inside and outside of pockmarks in the North Sea were studied using remotely operated vehicles. The abundance, species richness and diversity increased from outside (background seabed) towards the centre of the pockmarks, probably due to habitat enrichment and morphological protection provided by the pockmark environment. The numbers of taxa present in the centre of pockmarks were approximately double those of the surrounding areas, and the centres had almost an order of magnitude more individuals than outside. Carbonate rocks provided alternative habitat for fauna: a complex hard substrate for colonisation and shelter in an otherwise homogeneous soft sediment environment. Despite fishing data showing the wider area to be intensively disturbed, large slow growing and vulnerable species, such as gorgonian corals, were found in the centre of the pockmarks indicating that they may offer important refuges from trawling activity. This thesis suggests that pockmarks are important structural features influencing biological abundance and richness, and highlights the need for more research investigating their effects on faunal communities at a range of scales (regional and global) and in different environmental settings