Distribution and classification of pockmarks on the seabed around western Scotland

Allan Audsley,Tom Bradwell,John A Howe,John M Baxter

doi:10.1080/17445647.2019.1676320

Abstract

ABSTRACT 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 km2. We use morphological metrics and statistical procedures to classify and analyse the variety of pockmark forms. A k-means 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.

Highlights

Fjords, firths and coastal seabed sediments are important stores of carbon (Smeaton et al, 2016; Smith, Bianchi, Allison, Savage, & Galy, 2015)
This work forms part of a wider study to investigate whether high-resolution hydro-acoustic bathymetry data can be used to reveal the location, extent and fate of carbon stored in seabed sediments around western Scotland
By clipping the raster surface to the regions that contained the features of interest, we reduced the mapping time and removed the effect large basins have on the mapping process

Summary

Introduction

Firths and coastal seabed sediments are important stores of carbon (Smeaton et al, 2016; Smith, Bianchi, Allison, Savage, & Galy, 2015). A range of seabed gas-release and instability features have been identified from MBES imagery in the fjords and coastal waters of West Scotland, often coupled with other subseabed acoustic data (Arosio & Howe, 2018; Hillman, Gorman, & Pecher, 2015; Stoker et al, 2006; Stoker & Bradwell, 2009). This paper maps the distribution of seabed pockmarks from MBES data in a range of offshore settings around Western Scotland, all of which contain substantial Quaternary sediment sequences. This work forms part of a wider study to investigate whether high-resolution hydro-acoustic bathymetry data can be used to reveal the location, extent and fate of carbon stored in seabed sediments around western Scotland

Study sites

Data and methods

Pockmark classification

Discussion and conclusions