Origin of High Density Seabed Pockmark Fields and Their Use in Inferring Bottom Currents

Abstract

Some of the highest density pockmark fields in the world have been observed on the northwest Australian continental shelf (>700/km 2) where they occur in muddy, organic-rich sediment around carbonate banks and paleochannels. Here we developed a semi-automated method to map and quantify the form and density of these pockmark fields (~220,000 pockmarks) and characterise their geochemical, sedimentological and biological properties to provide insight into their formative processes. These data indicate that pockmarks formed due to the release of gas derived from the breakdown of near-surface organic material, with gas accumulation aided by the sealing properties of the sediments. Sources of organic matter include adjacent carbonate banks and buried paleochannels. Polychaetes biodiversity appears to be affected negatively by the conditions surrounding dense pockmark fields since higher biodiversity is associated with low density fields. While regional bi-directionality of pockmark scours corresponds to modelled tidal flow, localised scattering around banks suggests turbulence. This multi-scale information therefore suggests that pockmark scours can act as proxy for bottom currents, which could help to inform modelling of benthic biodiversity patterns.

Distribution and classification of pockmarks on the seabed around western Scotland

“…The elongation and vertical relief (Figure 4) of the pockmarks were chosen as key morphological metrics to explore possible feature classes. Pockmark elongation most likely represents the effects of local hydrographic conditions Hovland, 1983;Picard et al, 2018), whereas the depth or vertical relief of the pockmark more closely relates to activity status of pockmarks; with deeper pockmarks likely to have been active for longer periods of time or to have experienced a more intense venting of gas (forcing more sediment into suspension), alternatively the sediment characteristics of the area might allow deeper and steep pockmark walls to form without collapsing. In either case it is likely that the different hydrographic conditions and history of gas venting have formed pockmarks with different morphologies.…”

“…Elongate forms are seen to be proportionately more common within Loch Broom, Little Loch Broom, North Minch, Loch Slapin and Inner Sea of Hebrides. In all these localities it is likely that higherthan-average or sustained bottom currents have acted to scour and sculpt features in the direction of prevailing currents (Hovland, 1983;Picard et al, 2018). Interestingly, deep pockmarks are rare or absent in those settings where elongated forms are relatively common, possibly related to several variables such as a reduced activity status; where pockmarks do not have the same capacity to have a single large venting episode.…”

Geomorphometric Characterization of Pockmarks by Using a GIS-Based Semi-Automated Toolbox

“…These features are not distinguished using bathymetric data alone, and when identified, their vertical relief is frequently underestimated. For that type of setting, a semi-automated approach based on the BPI, as presented by [49], could be more effective. These features are not distinguished using bathymetric data alone, and when identified, their vertical relief is frequently underestimated.…”

Charting the Course for Future Developments in Marine Geomorphometry: An Introduction to the Special Issue

“…The results are to be used for marine spatial planning on the Eastern Brazilian Shelf. Picard et al [39] supported this theme with a study of hydrodynamics patterns by documenting the use of semi-automated methods to map and quantify the form and density of pockmark fields in one of the regions with the highest concentration of those features in the world: the Northwest Australian continental shelf. Whilst regional bi-directionality of pockmark scours corresponded to the modelled tidal flow, localized scattering around banks suggested turbulence regimes.…”