TEMPO OF METHANE DERIVED AUTHIGENIC CARBONATE FORMATION FROM THE NORTH SEA AND THE BARENTS SEA

The timing of past fluid discharge from CH₄-hydrate reservoirs into the ocean, and possibly the atmosphere, can be constrained through U-Th dating of methane derived authigenic carbonate (MDAC) crusts from seafloor seepage sites. We present U-Th data from MDACs recovered from the North Sea and Barents Sea using a remotely operated vehicle. 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. Compared to other records typically dated using the U-Th system (e.g. speleothems or corals) the main challenges to obtaining accurate age information from MDACs are (1) the isolation of pure carbonate via microsampling and (2) accurate correction for the isotopic composition of the detritus.

In this study we have targeted both the relatively pure late-stage cavity-filling CaCO₃ that provide minimum age estimates for each crust, and the earlier crust forming CaCO₃ that occurs distributed throughout the sediment matrix. The late stage cavity filling CaCO₃ ages are relatively insensitive to detrital Th and U corrections. In contrast the matrix forming CaCO₃ is disseminated at a scale which makes micro-sampling high-purity materials problematic, requiring a substantial correction for detrital content in order to derive meaningful temporal information. 2-40 mg subsamples were microdrilled from individual carbonate domains. Late-stage infills showed [²³⁰Th/²³²Th]_AR of 0.8-200, with ²³²Th concentrations of 4-800 ppb, while early carbonate cements gave [²³⁰Th/²³²Th]_AR of 0.8-3 with 1000-7000 ppb of ²³²Th. Modelled initial δ²³⁴U were close to expected values for modern/Holocene seawater. Intra-crust dates range between 1 – 13.2 ka. Based on analyses of matrix-forming CaCO₃ and multiple generations of cavity fills individual, up to 15 cm thick crust 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. Up to 2 cm thick layered cavity fills revealed resolvable growth histories, on the order of 1 kyr. Future work will focus on improving the accuracy of U-Th dates from matrix-forming CaCO₃ to better constrain the onset of CH₄ emissions at different localities in the North Sea and Barents Sea.