An investigation into the influence of methane seepage on calcium, calcium carbonate and other elemental concentrations in shallow water sediments in Dunmanus Bay, Ireland.

In the marine environment, hydrocarbon seeps are known to be common and recognisable features and occur in shallow water and in the deep ocean. These seeps tend to be located at distinctive geotectonic, geochemical and biological interfaces where gas-rich fluids (e.g. methane) migrate upwards to the seabed. Seep characteristics range from diffuse seafloor venting to more focused low emission gas escape and these characteristics can be associated with morphological features on the seabed such as pockmarks and mud volcanoes. These seeps can host microbial communities that thrive on the venting methane and are also associated with methane-derived authigenic carbonates which under certain conditions can precipitate as concretions. A pockmark field was first discovered in Dunmanus Bay, in Co.Cork, Ireland, in 2007 during multibeam mapping of the Bay. In shallow water depths of around 40 m, 60 pockmarks have been mapped with the largest reaching up to 20 m in diameter and up to 1 m in depth. The overall lithology of the Bay is characterised by coarse to medium sand, with mud to fine sand dominating closer to the mouth of the Bay. The pockmark field is located in an area of relatively finer particle size. Surveys show evidence of minor subsurface gas seepage in the region and also provide evidence of gas accumulations specific to the fine-grained muddy seabed in the pockmark field. In addition to this, a number of faults cross the region and these are thought to be likely pathways for gas migration to shallow sediments. iii The aims of this research are twofold: (1) To investigate the influence of methane seepage on calcium and calcium carbonate concentrations and (2) to determine if methane seepage influences the concentration and distribution patterns of major and trace elements in the Bay. To investigate calcium and calcium carbonate concentrations, three 6m vibrocores were collected. One core was collected from within a pockmark, one from beside a pockmark but still in the gas seepage area and another core taken for comparison away from the pockmark field. To determine elemental concentrations and distribution patterns for major elements (Ca, Fe, Ti, Sr) and trace elements (Ba, Mn, Zr, Sc, Rb) twenty-two surficial sediments were collected throughout the bay and within the pockmark field. All samples were analysed for their geochemical and physical properties. In conjunction with Particle Size Analysis, advanced analytical techniques such as Field-Portable X-ray Flourescence (FP-XRF), Bench XRF (XRF), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM-EDS), were used. In addition, statistical analysis was carried out on the FP-XRF results data from the surficial sediments. All the results obtained were used to provide an insight in to marine calcification and elemental concentration and distribution patterns in seep environments. The results show a decrease in Ca concentrations with depth and indicate that no obvious precipitation of CaCO3 minerals occurs. This is due to a combination of factors which include (1) a low methane seepage rate, (2) microbially mediated reactions in the sulphate-methane-transition-zone that affect the calcium carbonate equilibrium and (3) the possibility of lateral migration of seawater (carrying CO2) iv from depth through a permeable gravel layer located approximately 4 meters below the seafloor. The concentration and spatial distribution patterns of major elements (Ca, Fe, Ti, Sr) and trace elements (Ba, Mn, Zr, Sc, Rb) suggest that all were associated with all sediment types. Levels of concentration of Fe, Ti and Ba were broadly similar to levels in the soils in the surrounding landmass. Ca concentrations were high throughout the Bay, suggesting a large biogenic influence in the sediment. Sr followed similar trends to Ca. Mn was lower in the surficial sediments compared to regional soil concentration. Zr and Sc had higher concentrations in the pockmark field compared to the rest of the Bay and seem to be associated with the fine-grained sediments which occur here due to the high surface to volume ratios and adsorption capacities of their grains. Statistical analysis indicated that there was no significant difference between medians in the pockmark field and the rest of the Bay for Ca, Ti, Ba and Zr while statistical analysis confirmed that there was significant difference between the two areas for Fe, Sr, Mn, Sc and Rb.