CV10_28 Irish Sea cruise report
O'Reilly, Shane S. and Szpak, Michal and Monteys, Xavier and Kelleher, Brian (2010) CV10_28 Irish Sea cruise report. Technical Report. Geological Survey of Ireland.
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Cruise CV10_28 mobilized and demobilized from Howth, Co. Dublin, on the 2nd and 7th June 2010 respectively, and totalled over 112 operational hours. The objectives of this cruise were primarily to investigate and sample previously identified methane-related seepage structures in the Irish Designated Seabed Zone (IDSZ) of the Irish Sea, to carry out sampling grids of Dublin Bay and of the region of the IDSZ from Dublin Bay to Dundalk Bay for multidisciplinary geochemical and microbiological studies, and also to sample the water column for dissolved organic matter studies. A total of 113 sampling stations were successfully sampled: 46 grabs; 55 boxcores (0.25m2); 5 gravity cores (1m); and 7 vibrocores (3m). Gas-related seepage features successfully investigated were methane-derived authigenic carbonates (MDAC) in the Codling Fault Zone (CFZ), the Lambay Deep mud diapir, and also a shallow pockmark in the northen mudbelt region. Four videolines with over six hours of video footage were acquired in the MDAC target sites. Video footage and image stills of clusters of MDAC slabs, nodules and outcroppings, as well as indications enhanced anaerobic oxidation of methane (AOM) were successfully obtained. These features were subsequently successfully ground-truthed, sampled and preliminary geochemical analysis was carried out, thereby proving MDAC mounds in the CFZ are distinct features caused by active sub-surface seepage of methane. The Lambay Deep mud diapir was investigated by videoline deployment, whereby no distinct differences in seabed structure or biodiversity were observed. Fine sandy muds with relatively high macrofaunal biodiversity characterized seabed both inside and outside of Lambay Deep. The feature and surrounding seabed was ground-truthed by vibrocore and boxcore, and results supported the videoline findings. Sub-surface sediment (0-3m) was characterized by homogenous well-sorted fine sandy mud, with low infauna and shell presence, and little or no evidence of sub-surface anoxia. The mudbelt pockmark was investigated by videoline deployment and also subsequently ground-truthed by gravity core and boxcore. Videoline and sampling evidence suggests that the pockmark and reference samples from the surrounding seabed is uniform, and characterized by very fine mud with little sub-surface zonation. Previous sub-bottom profiling of the feature suggests no evidence of gas seepage or accumulations in the vicinity of the pockmark. The weather and sampling conditions were overall favourable throughout the cruise but the lack of dynamic positioning aboard the Celtic Voyager and also the restriction on the vibrocore to slack water and day-time use resulted in some sampling limitations and incomplete secondary objectives. Overall CV10_28 achieved the vast majority of its objectives, and a comprehensive foundation for numerous studies and peer-reviewed publications has been established.
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