Scotland’s pockmarks: how and when did they form?

Pockmarks are roughly circular, concave, crater-like depressions in the sea-bed. They were first reported by King and MacLean (1970) offshore Nova Scotia (Canada), who suggested that gas and/or water from the underlying bedrock was released in sufficient quantities to put fine-grained marine sediment into suspension. Since then, pockmarks have been found worldwide: within lakes and estuaries, on open shelves and in deep oceans. Pockmarks are now considered to be the most abundant expression of gas seepage seen at seabed. This process of gas release is of great interest to geologists, but also has important implications for marine civil engineering and offshore industry (i.e. the renewable energy sector) as well as potentially impacting on marine life.

Higher-resolution multibeam echo-sounder surveys conducted over the last 10 years have identified increasing numbers of pockmarks around Scotland’s western and northern coast. Notable pockmark fields have been found within several sea lochs, including: Loch Eriboll, Loch Broom and the Minch (Stoker et al., 2006), areas east of the Small Isles (Howe et al., 2012) and the Firth of Lorn & Loch Linnhe (Howe et al., 2015). All these Scottish sea lochs occur within Precambrian to Early Palaeozoic metamorphic terrain which has subsequently been modified during the Quaternary – when extensive ice sheets carved out numerous basins into which late-glacial to Holocene sediments have collected.

The presence of deep basins and shallow sills has effectively trapped organic matter within the sediments since deglaciation resulting in the subsequent build-up and release of shallow biogenic gas. Importantly, the timing and evolution of pockmark formation and gas release can now be examined through the use of U-Th geochronology of methane-derived authigenic carbonates (MDACs) sampled from the sea-bed. Recent dating studies using this technique have resulted in high-impact journal publications (e.g. Cremiere et al., 2016).
Pockmark distribution is non-random; with their occurrence being apparently strongly controlled (Roy et al., 2016). Pockmark morphology also varies markedly with spatial location. This project would have twin aims:
1) to assess the degree of geological and bathymetric control on pockmark formation and morphology;
2) to examine the association between pockmark setting, rapid ice-sheet unloading and neotectonics (postglacial fault movement).
For instance in Loch Linnhe and the Firth of Lorn, pockmarks tend to form in lines reflecting the strongly faulted underlying geology of the area (Howe et al., 2015). In Loch Eriboll, pockmarks are subtle and very shallow; whilst around the Small Isles (Inner Hebrides), pockmarks are deeper and steeper-sided than any equivalent-sized pockmarks on the UK continental shelf (internal depths of >18 m). How can these differing morphological traits be explained? What do they mean for pockmark formation rate, gas escape, and sea-bed stability?

Key Research Questions:
 Does sea-bed geology determine pockmark distribution and morphology?
 Was pockmark formation triggered by rapid retreat of the last British Ice Sheet?
 Do Scotland’s pockmarks result from long-term reduced marine sedimentation above active seeps? Or did they form by episodic gas- release events and sediment expulsion?
 When did they form? And are Scottish pockmarks still active?
 What risks do they pose for marine infrastructure and engineering projects in UK waters?

For further information contact Dr Tom Bradwell [Email Address Removed] or Dr Joana Gafeira [Email Address Removed]