Constraints on the timing and duration of methane derived authigenic carbonate formation in the North Sea and the Barents Sea from U-Th dating

Abstract:

Assessment of the tempo of past methane discharges from sub-surface reservoirs, and their relationship to external forcings such as ice-sheet collapse, is in part hindered by the lack of robust age constraints. Methane-derived authigenic carbonate (MDAC) crusts exhibiting characteristic ¹³C-depleted isotopic signatures were collected from several seepage sites on the Norwegian continental shelf, including sites in the North Sea, the Norwegian Sea and the Barents Sea and a combined petrological, stable isotope and geochronology study has been carried out. MDAC samples exhibited a range of mineralogical compositions and contexts, from carbonate-cemented silt, sand, and gravel, to cavities lined with relatively pure (>90%) late-stage aragonitic infills. The U-Th dating results constrain the main episode of carbonate crust formation in the Barents and Norwegian seas during the time interval between 14 and 5 ka. Based on analyses of matrix-forming CaCO₃ and multiple generations of cavity fills individual, up to 15 cm thick crusts may represent at least 3.3 kyr of growth. Cavity fills representing focussed fluid flow post-date the main interval of crust formation by ca. 2 kyr and sub-sampling of these layered cavity fills revealed resolvable growth histories, on the order of 1 kyr. The dating of the main phases of MDAC formation suggest that the methane seepage along the northern Norwegian margin was coincident with the collapse of the Scandinavian ice sheet and deglaciation of the area. The precipitation of studied North Sea carbonate crusts occurred more recently, from 6 to 1 ka, suggesting that their formation is unrelated to the glacial history of the area and gas hydrate stability. These data, combined with published data from other MDAC sites (e.g., Gulf of Mexico, Hydrate Ridge, Sea of Japan) demonstrate the utility of constraining the temporal evolution of hydrate and/or hydrocarbon system related fluid flow from the subsurface to the ocean/atmosphere, and assessing variation within and between different basins and areas (e.g., glaciated vs. non-glaciated terranes).