The Croker Carbonate Slabs: extensive methane-derived authigenic carbonate in the Irish Sea—nature, origin, longevity and environmental significance

Abstract

The Croker Carbonate Slabs, in the UK sector of the Irish Sea, has shallow (70 to 100 m) water, strong (> 2 knot) tidal currents, coarse mobile surficial sediments and the most extensive methane-derived authigenic carbonate (MDAC) known in European waters. Multi-disciplinary studies (2004 to 2015) were commissioned specifically to document the benthic habitat, and have resulted in the designation of this site as a Marine Protected Area (MPA) under the European Commission’s Habitats Directive as an example of “Submarine structures formed by leaking gases”. However, this paper is focussed on the geoscience aspects of the site: the mineralogy and isotopic composition of the MDAC, its formation and age. It considers the implications of these findings with respect to the timing of the deglaciation of the area since the Last Glacial Maximum (LGM), and the environmental implications of the seepage of methane from the site over a period of at least 17,000 years. Carbon isotope ratios (δ13 C − 34 to − 54‰) confirm that the carbonate minerals (high-Mg calcite and aragonite) result from the anaerobic oxidation of methane. Widespread shallow gas within post-glacial sediments is sourced from underlying coal-bearing Carboniferous strata. Geophysical (side-scan sonar and multi-beam echo sounder) and visual surveys show that the MDAC occurs as isolated lumps, continuous pavements, and cliffs < 6 m tall, which post-date the post-glacial sediments, but are in places covered by a veneer of coarse mobile surficial sediments. U-Th dates (17,000 ± 5500 to 4000 ± 200 BP) suggest continual MDAC formation since the last glacial maximum, and constrain the postglacial sea level rise in this part of the Irish Sea; the site must have been submarine before MDAC formation started, whether or not methane was escaping. Visual and acoustic evidence of gas seepage is limited, but methane concentrations in the water are high (< 21.4 nmol l−1) and suggest present-day export to the atmosphere. It is also implied that significant methane release to the atmosphere occurred immediately after the retreat of the ice that covered the site during the LGM until 21.9 to 20.7 ka BP.

Methane Derived Authigenic Carbonate (MDAC) Aragonite Cemented Quaternary Hardground from a Methane Cold Seep, Rathlin Basin, Northern Ireland: δ13C and δ18O Isotopes, Environment, Porosity and Permeability

“…The complex methane-AOM-sulphate reaction promotes the growth of carbonate cemented hardground pavements, which have been reported from a number of locations within the Irish Sea [2,[4][5][6][7][8][9][10][11][12], the best three documented examples being Holden's Reefs (Tremadog Bay, offshore Wales), the Codling Fault Zone (Irish Sea, Ireland) and the Croker Carbonate Slabs (Irish Sea, Ireland) [2]. The last occurs at a water depth of 70 to 100 m, covers an area of some 500,000 m 2 , and may have been a continuously active methane seep for 17,000 years [2]. MDAC phases include acicular aragonite and high-magnesium calcite, with associated pyrite [2].…”

“…Methane-derived authigenic carbonate (MDAC) forms within sediments near to the seafloor, within the sulphate-methane transition zone (SMTZ), through the mixing of methane derived by thermocatalytic cracking of kerogen, or the microbial decomposition of organic material in shallower muddy sediments, interacting with SO 4 2¯f rom seawater; with anaerobic oxidation of methane (AOM) and sulphate reduction by sulphate reducing bacteria (SRB's) and methane utilising archaea (Figure 2) [1,2]. Methane seep carbonates are known from around the world [3], for instance offshore Costa Rica, Norwegian Sea, Black Sea, the Mediterranean, and are diverse niche ecological refugia as well as indicators of deep, leaking hydrocarbon reserves [3].…”

“…Methane seep carbonates are known from around the world [3], for instance offshore Costa Rica, Norwegian Sea, Black Sea, the Mediterranean, and are diverse niche ecological refugia as well as indicators of deep, leaking hydrocarbon reserves [3]. The complex methane-AOM-sulphate reaction promotes the growth of carbonate cemented hardground pavements, which have been reported from a number of locations within the Irish Sea [2,[4][5][6][7][8][9][10][11][12], the best three documented examples being Holden's Reefs (Tremadog Bay, offshore Wales), the Codling Fault Zone (Irish Sea, Ireland) and the Croker Carbonate Slabs (Irish Sea, Ireland) [2]. The last occurs at a water depth of 70 to 100 m, covers an area of some 500,000 m 2 , and may have been a continuously active methane seep for 17,000 years [2].…”

“…The last occurs at a water depth of 70 to 100 m, covers an area of some 500,000 m 2 , and may have been a continuously active methane seep for 17,000 years [2]. MDAC phases include acicular aragonite and high-magnesium calcite, with associated pyrite [2]. Other authors have also reported high-magnesium calcite (up to 48 mol% MgCO 3 ) and dolomite from the Croker site [4,5], while the Codling Fault Zone has been described as aragonite dominated [11].…”

Geological and geotechnical constraints in the Irish Sea for offshore renewable energy

“…pockmarks) which destabilise the seabed (Yuan et al, 1992). Alternatively, in sandier conditions, seeping gas can precipitate carbonate minerals at the seabed forming methane-derived authigenic carbonate which cements the seafloor forming hard substrates (Judd et al, 2019;O'Reilly et al, 2014;Van Landeghem et al, 2015).…”