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

Jim Buckman,Helen Lewis,Zeyun Jiang,Terry Donnelly,Alastair Ruffell

doi:10.3390/geosciences10070255

Abstract

A block of sandstone retrieved by divers from near Rathlin Island, Co. Antrim, Northern Ireland, represents an aragonite cemented sand formed during the Quaternary. Strongly negative δ13C of the aragonite cement (−50 to −60‰ δ13C) indicates that the hardground was formed by the anaerobic oxidation of methane (AOM), resulting in the formation of a methane-derived authigenic carbonate (MDAC) hardground. Such hardgrounds have previously been recorded as forming extensive pavements in deeper waters in the mid Irish Sea (e.g., Croker Carbonate Slabs), although the latter also contains high-magnesium calcite. Sand was initially deposited as part of a storm lag deposit, with a reworked bivalve and gastropod fauna. This sand was then colonised by a probable crustacean fauna, producing horizontal open dwelling burrows (Thalassinoides). After aragonite cementation, the hardground was colonised by boring bivalves, with slightly negatively elevated levels of δ13C. Finally, the hardground was colonised by an encrusting fauna (bryozoans, calcareous algae and serpulids), by then in warmer seas. Continued depleted levels of δ13C present within the encrusting fauna (−1 to −5‰ δ13C) indicate continued methane generation and seepage, which may still be active to the present day, and to the possibility of shallow gas reserves. The δ18O values change between macro-infauna vs. encrusters, indicating a warming in water temperature, reflecting glacial and post-glacial environments. The aragonite cemented sandstone has a highly variable porosity, with large vugs (open burrows and borings), smaller mouldic porosity within gastropods and bivalves and complex micro-porosity associated with acicular aragonite cements. Overall permeability was recorded at the 2.5 to 23 Darcies level, reflecting the highly variable vuggy porosity, although matrix permeability was around 100 mD and controlled by the MDAC fabric. Actual permeability will likely be controlled by the extent to which larger pores are interconnected. The sea around the Rathlin Island area contains a diverse fauna, which is worthy of future study in the context of cold seep and MDAC pavement formation.

Highlights

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 SO4 2 ̄ from seawater; with anaerobic oxidation of methane (AOM) and sulphate reduction by sulphate reducing bacteria (SRB’s) and methane utilising archaea (Figure 2) [1,2]
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
We demonstrated the complex interaction of methane expulsion, sulphur-reduction and a palimpsest of faunal inhabitation that will add to the understanding of methane-derived authigenic carbonate (MDAC) [51]

Summary

Introduction

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 SO4 2 ̄ from seawater; with anaerobic oxidation of methane (AOM) and sulphate reduction by sulphate reducing bacteria (SRB’s) and methane utilising archaea (Figure 2) [1,2]. Holden’s Reefs (Tremadog Bay, offshore Wales), the Codling Fault Zone (Irish Sea, Ireland) and the Croker Carbonate Slabs (Irish Sea, Ireland) [2]. Other authors have reported high-magnesium calcite (up to 48 mol% MgCO3 ) and dolomite from the Croker site [4,5], while the Codling Fault Zone has been described as aragonite dominated [11]. The Croker site records MDAC with a depleted δ13 C of between −34 to −54‰, which indicates a thermogenic methane origin from the cracking of kerogen, with a possible Carboniferous source [1,2]. Material collected from offshore, Rathlin Island, Northern Ireland, indicates an extension of the known range of MDAC cementation within the Irish Sea area, and comes from shallower water depths than those previously described

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