So far, no inventory or detailed studies have been conducted of pockmarks in Scottish fjords or the adjacent shelf seas west of the UK. In European waters, gas/fluid-escape related pockmarks have been identified from high-resolution bathymetry and geophysical data in all the shelf seas: in the Mediterranean ( Marinaro et al., 2006), Black Sea ( Çifi¸, Dondurur, & Ergün, 2003 Papatheodorou, Hasiotis, & Ferentinos, 1993), Baltic ( Whiticar & Werner, 1981), Barents Sea ( Hovland & Judd, 1988 Solheim & Elverhøi, 1993) and North Sea Basins ( Gafeira & Long, 2015a Judd & Hovland, 2007 Krämer et al., 2017). Pockmarks have been discovered in many locations on ocean floors worldwide and at a range of depths from >1000 m in the abyssal ocean ( Panieri et al., 2017 Pilcher & Argent, 2007) to much shallower settings on the continental shelf (<100 m), providing evidence of their wide bathymetric range. In some cases the presence of submarine structures made by leaking gases can be designated as a Special Area of Conservation (SAC), such as the European designated Scanner pockmark SAC in the North Sea ( Gafeira & Long 2015b JNCC, 2018). Furthermore the presence of gas release features are a significant geo-hazard to offshore developments ( Best et al., 2006).
Determining the age and activity status of pockmarks can be challenging, however there is a growing agreement that their distribution, density and morphology could be useful indicators of the gas-storage potential of the sediments beneath ( Hovland, Heggland, De Vries, & Tjelta, 2010 Krämer et al., 2017).
It is widely held that the presence of pockmarks at the seabed reflects the presence or former presence of gas-rich sub-surface sediments ( King & Maclean, 1970). They are typically formed by the focused migration and venting of carbon-rich fluids and gases, commonly methane, from the sub-seabed sediment into the water column ( Judd & Hovland, 2007). Pockmarks are classically described as conical shaped seabed depressions, but can vary greatly in shape and size. It is becoming increasingly apparent that the fjordic environments of western Scotland represent a more substantial carbon store (per square metre) than terrestrial equivalents ( Smeaton et al., 2016). (2017) demonstrate that the earlier estimates of organic carbon within Scotland’s 111 fjords or ‘sea lochs’ significantly underestimated the actual total, probably by around three orders of magnitude (0.34 Mt cf 640.7 Mt of C).
Two recent studies have provided the first quantitative estimates of total carbon storage buried within Scotland’s fjordic sediments ( Burrows et al., 2014 Smeaton et al., 2017) however, the results differ strongly. Despite fjordic environments being recognised over three decades ago as major carbon stores and globally important natural sequestration sites ( Syvitski, Burrell, & Skei, 1987), quantifying the carbon in fjordic and other marine sedimentary systems has remained a largely neglected topic. This work presents the first detailed mapping of pockmark fields in Scottish west coast waters and highlights the use of pockmarks as an indicator of the quantity, mobility and fate of stored carbon.įjords, firths and coastal seabed sediments are important stores of carbon ( Smeaton et al., 2016 Smith, Bianchi, Allison, Savage, & Galy, 2015). The recognition of separate pockmark classes could aid understanding of their age, activity and origin. A k-means clustering algorithm identifies three classes of pockmark morphology: deep, elongate and regular. We use morphological metrics and statistical procedures to classify and analyse the variety of pockmark forms. We map 1019 individual pockmarks in 12 different hydrographic areas covering ca. We use a Geographical Information System (GIS) to analyse multibeam echo-sounder bathymetric data and use a range of semi-automated tools to map seabed pockmarks in fjords and adjacent coastal waters around western Scotland. Pockmarks are seabed depressions that represent primary evidence of rapid biogenic/thermogenic gas build up and fluid release from seabed sediments to the water column.
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