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

Abstract

<jats:p>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.</jats:p>

The Formation of Authigenic Carbonates at a Methane Seep Site in the Northern Part of the Laptev Sea

“…For example, isotopic values of biogenic (microbial) methane vary from −50% to −110% , while carbonates that formed from this methane type typically have δ 13 C values ranging from −40% to −60% [13,30,32,42]. Furthermore, microbial activity during AOM has a fractionation effect for δ 13 C, consequentially offsetting δ13C methane by approximately 20% to 60% [1,66].…”

“…The presence of seawater cations (Ca 2+ , Mg 2+ ) and elevated carbonate alkalinity promote the precipitation of authigenic carbonates [7,19], which show a wide range of mineralogical compositions. The occurrence of Mg-calcite and aragonite in cold seep marine environments has been reported by various authors [1,4,20]. Other carbonate minerals, such as dolomite, ankerite, ikaite, and glendonite, are significantly less common [21,22].…”

“…Methane cold seeps are a widespread phenomenon that occurs on the shelves and continental slopes of inland and marginal seas around the world [1][2][3][4][5][6][7], including the arctic seas [8][9][10]. They represent areas of the seafloor where methane-rich fluids in chemical non-equilibrium with seawater are transported through the sub-seafloor environment and are released at the bottom-sediment-seawater interface [11][12][13].…”

Foraminifera associated with cold seeps in marine sediments

“…It has been confirmed that MDAC can capture methane-derived 13 C-depleted (may lower than 60 ‰ PDB) and 18 O-enriched signatures (may higher than 8.9 ‰ PDB) from seep environments (Naehr et al, 2007;Feng et al, 2018;Buckman et al, 2020). Foraminifera fossil shells in sediments are susceptible to MDAC secondary overgrowth after diagenetic alteration, which will obscure the original information contained in the shells (e.g.…”

“…Elevated carbonate alkalinity promoted by AOM results in the formation of authigenic carbonate minerals. Typically, methanederived authigenic carbonate (MDAC) is a universal mineral indicator in seep areas and is primarily composed of acicular aragonite, high-Mg calcite, and dolomite (Boetius et al, 2000;Judd et al, 2007;Naehr et al, 2007;Buckman et al, 2020;Judd et al, 2020). However, in some cases, Fe-and Mn-carbonates may also be present (Suess, 2020).…”