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. 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. We map 1019 individual pockmarks in 12 different hydrographic areas covering ca. 2019 km 2 . We use morphological metrics and statistical procedures to classify and analyse the variety of pockmark forms. A kmeans clustering algorithm identifies three classes of pockmark morphology: deep, elongate and regular. The recognition of separate pockmark classes could aid understanding of their age, activity and origin. 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. ARTICLE HISTORY

The Loch Lomond Stadial (LLS) (12.9 – 11.7 ka BP) was the last climatic episode in which Scotland possessed a significant ice mass. Loch and Glen Etive represent one of the best-preserved glacial landsystems in western Scotland, having formed a conduit for ice draining from high on Rannoch Moor. Loch Etive is a glacially over-deepened trough 6 km north of Oban, now a sea loch 30 km long and up to 1.5 km wide, extending from the Firth of Lorn NE towards Rannoch Moor. It is a key site in which to understand the glacial processes that shaped the Western Highlands during the LLS, leading to full glacial retreat by 11.4 ka BP. Based on a comprehensive high-resolution multibeam bathymetric sonar survey, the morphology of the seabed was analysed using ArcGIS to create a series of detailed geomorphological maps. Morphological interpretations, backscatter and seismic reflection data from the loch were combined to reconstruct the deglaciation of Loch Etive. The presence of transverse ridges interpreted as recessional moraines and submarine eskers indicates a dynamic glacial retreat from the maximum seaward extent west of the present-day coastline. At its maximum extent, the glacier formed a proglacial delta generating large volumes of meltwater that flowed over the rock sill of the present-day Falls of Lora. The confluence of glacial ice masses, flowing from the Pass of Brander and tributary corrie glaciers, created a complex glaciofluvial environment in the outer basin of Loch Etive. This is reflected in the formation of a large submarine esker. In the inner loch basin, the presence of recessional moraines at intervals suggests a dynamic glacier, responding to seasonal temperature fluctuations, possibly subsequently retreating northeastwards c. 50 m annually as the LLS ended.

Sub-seabed gas is commonly associated with seabed depressions known as pockmarks—the main venting sites for hydrocarbon gases to enter the water column. Sub-seabed gas accumulations are characterized by acoustically turbid or opaque zones in seismic reflection profiles, taking the form of gas blankets, curtains or plumes. How the migration of sub-seabed gas relates to the origin and distribution of pockmarks in nearshore and fjordic settings is not well understood. Using marine geophysical data from Loch Linnhe, a Scottish fjord, we show that shallow sub-seabed gas occurs predominantly within glaciomarine facies either as widespread blankets in basins or as isolated pockets. We use geospatial ‘hot-spot’ analysis conducted in ArcGIS to identify clusters of pockmarks and acoustic (sub-seabed) profile interpretation to identify the depth to gas front across the fjord. By combining these analyses, we find that the gas below most pockmarks in Loch Linnhe is between 1.4 m and 20 m deep. We anticipate that this work will help to understand the fate and mobility of sedimentary carbon in fjordic (marine) settings and advise offshore industry on the potential hazards posed by pockmarked seafloor regions even in nearshore settings.