| dc.description.abstract | Submarine Groundwater Discharge (SGD) is an important process affecting ocean chemistry by transporting critical solutes, including trace elements, nutrients, and dissolved organic and inorganic carbon from the continental shelf to the open ocean. In the Arctic, SGD is proposed as one of the key processes transporting methane to the fjords and continental shelf. Therefore, the interplay between Arctic cryosphere dynamics and SGD alters the transport of methane over geological timescales. With the ongoing global warming, it is important to understand these interactions. In this study, the controlling factors of precipitation of authigenic minerals are studied by analysing the authigenic crusts formed at bedrock outcrops on the walls of two submarine canyons. Permeable strata within the exposed bedrock succession serve as groundwater discharge conduits, transporting methane to the seafloor resulting in formation of crusts of authigenic carbonate (primarily calcite) and sulphate (barite) precipitates, along with microbial mats. The isotopic composition of carbonates (δ18O and δ13C) and organic matter (δ15N and δ13C) were determined, along with the mineralogical composition of the samples by using X-ray diffraction, micro X-ray fluorescence, and scanning electron microscope energy-dispersive spectrometer analysis. Scanning electron microscopy backscatter images were used to obtain insight into the textural relationships and dynamics of authigenic precipitation. All samples exhibit extremely negative δ13C values in both carbonates and organic matter. The results indicate that barite-rich samples contain authigenic carbonate, with δ18O values that are slightly less positive and δ13C values that are slightly less negative compared to carbonate-rich samples. Organic matter is less abundant in barite-rich samples and exhibits less negative δ13C values and lower δ15N values compared to carbonate-rich samples. Based on these results it could be concluded that multiple factors control the precipitation of the authigenic precipitates. The very negative δ13C values of both carbonates and organic matter indicate a methane derived origin. Anaerobic oxidation of methane is likely to have played a key role, by creating a high alkaline zone favouring carbonate precipitation. Barite precipitation is primarily controlled by the availability of dissolved barium in discharging groundwater rather than by anaerobic oxidation of methane coupled with sulphate reduction. Barite precipitation occurs deeper in the authigenic crust compared to carbonates at the base of the sulphate-methane transition zone due to its low solubility. Carbonate precipitation likely happens closer to the surface-water interface. Overall, it was found that the mineral precipitation environment is dynamic, likely influenced by variations in methane flux that cause fluctuations in the thickness and position of the sulphate-methane transition zone along the canyon wall. The mineralogy of the carbonates is affected by environmental factors, such as methane flux and temperature. In this study, primarily calcite was identified, consistent with low methane flux and low temperature. However, the presence of minor Mg-calcite and aragonite suggests variable flux conditions and a dynamic precipitation environment. The changing (micro)environmental conditions likely affect precipitation not only temporarily but also spatially along the canyon walls. | |
