Pockmark

Abstract: Underwater gas and liquid escape from the seafloor has long been treated as a mere curiosity. It was only after the advent of the side-scan sonar and the subsequent discovery of pockmarks that the scale of fluid escape and the moon-like terrain on parts of the ocean floor became generally known. Today, pockmarks ranging in size from the ‘unit pockmark’ (1–10 m wide, < 0.6 m deep) to the normal pockmark (10–700 m wide, up to 45 m deep) are known to occur in most seas, oceans, lakes and in many diverse geological settings. In addition to indicating areas of the seabed that are ‘hydraulically active’, pockmarks are known to occur on continental slopes with gas hydrates and in association with slides and slumps. However, possibly their potentially greatest significance is as an indicator of deep fluid pressure build-up prior to earthquakes. Whereas only a few locations containing active (bubbling) pockmarks are known, those that become active a few days prior to major earthquakes may be important precursors that have been overlooked. Pockmark fields and individual pockmarks need to be instrumented with temperature and pressure sensors, and monitoring should continue over years. The scale of such research calls for a multinational project in several pockmark fields in various geological settings.

Abstract: Some pockmarks on the Norwegian continental shelf contain patches of cemented sediment that can provide hardgrounds attractive to a variety of benthonic organisms. Detailed examination of a sandstone sample from a pockmark in Norwegian Block 25/7 in the North Sea has revealed the presence of Mg calcite and aragonite cements, some of the latter forming botryoids. All cement types are characterized by extremely light carbon isotopic compositions, with a mean 13C value of -56.1 PDB, which shows that the cements contain carbonate produced by oxidation of biogenic methane. Oxidation occurred in both the oxic and anoxic diagenetic zones. Trace amounts of interstitial methane (mean 13C = -40.8%) and higher hydrocarbon gases (up to C5) with a C1/Cn ratio of 0.855-0.874 in the pockmark sediments indicate that some thermogenic methane may be mixed with the biogenic gas.