Scotland's pockmarks: understanding their formation and relationship to buried carbon in fjordic systems

Abstract

Pockmarks are concave seabed depressions formed by the venting of sub-seabed fluids into the water column. These fluids can contain greenhouse gases such as methane and carbon dioxide, formed by the microbial breakdown of organic carbon, thus creating a strong link between pockmarks and the marine carbon cycle. Pockmarks have been reported from the offshore waters around the UK, but until now no detailed study has been conducted on pockmarks in Scottish waters. It is important to understand the processes that lead to pockmark formation in order to assess their activity status and morphological evolution. Twelve study areas around western Scotland were selected, where pockmarks are observed, with three primary research aims: (i) to map the location and morphological characteristics of pockmarks; (ii) to map the distribution of sub-seabed gas and its spatial association with pockmarks; (iii) to characterise the physical and geochemical sediment stratigraphy from inside and outside pockmarks. Using statistical GIS methods, pockmark morphologies and distributions have been mapped and used to construct a classification system. The physical and geochemical stratigraphy of five sediment cores have also been analysed at a mm-scale resolution. 1015 previously unreported pockmarks have been mapped around western Scotland and separated into three distinct classes: common, elongated and deep; from which pockmark activity status can be inferred. This research has found that localised pockmark hot-spots are probably associated with areas of higher organic carbon within fjordic and extra-fjordic environments; these are not currently included in national carbon inventories. Finally, the geochemical analysis has shown that a shallowing of the redox zone inferred from Mn/ Fe ratio may provide a useful proxy for recent pockmark activity. These results will aid the assessment of pockmarks when considering geohazards and promote future research on the importance of pockmarks supporting vulnerable marine biotopes.