U-Th chronology and formation controls of methane-derived authigenic carbonates from the Hola trough seep area, northern Norway

Abstract

We investigated methane-derived authigenic carbonate (MDAC) crusts and nodules from a cold seep site on the northern Norwegian continental shelf in ca. 220m water depth to determine the timing and mode of their formation. Gas bubbling observed during remotely operated vehicle (ROV)-assisted sampling of MDAC crusts revealed ongoing seep activity. Authigenic carbonates were present as crusts on the seafloor and as centimetre-size carbonate-cemented nodules at several intervals within an adjacent sediment core. Aragonite-dominated mineralogy of the MDAC crusts suggests formation close to the seafloor at higher rates of sulphate-dependent anaerobic oxidation of methane (AOM). In contrast, dolomite-cemented nodules are consistent with the formation at the sulphate-methane-transition zone deeper within the sediment at lower rates of AOM. The δ13C-carbonate values of bulk rock and of micro-drilled aragonite samples vary between −22.2‰ and −34.6‰ (VPDB). We interpret the carbon in aragonite to be mainly derived from the anaerobic oxidation of thermogenic methane, with a minor contribution from seawater dissolved inorganic carbon (DIC). AOM activity is supported by high concentrations of AOM-related biomarkers of archaea (archaeol and 2-sn-hydroxyarchaeol) and sulphate-reducing bacteria (iso and anteiso-C15:0 fatty acids) in the crusts. The dolomite nodules exhibit higher δ13C-carbonate values (−12‰ VPDB) suggesting a smaller amount of methane-derived carbon, presumably due to the contribution of DIC migrating from depth, and lower AOM rates. The latter is supported by orders of magnitude lower concentrations of archaeol and sn-2-hydroxyarchaeol in the sediment interval containing the largest dolomite nodules. δ18O values of pure aragonite samples and dolomite nodules indicate the precipitation of carbonate close to isotopic equilibrium with seawater and no influence of gas hydrate-derived water. U-Th dating of two MDAC crusts shows that they formed between 1.61±0.02 and 4.39±1.63ka BP and between 2.65±0.02 and 4.32±0.08ka BP. We infer both a spatial and temporal change in methane flux and related MDAC formation at this seep site. These changes might be caused by regional seismic events that can affect pore pressure or re-activation of migration pathways thus facilitating fluid flow from deep sources towards the seabed.