Understanding key geological processes and controls on cold-water coral habitat development in submarine canyons

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O'Reilly, Luke
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University College Cork
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Cold-water corals are sessile, filter-feeding organisms that baffle water flow inducing sedimentation around their framework. Through geological time, should environmental conditions permit, they can produce positive topographic features on the seafloor called mounds through successive and persistent reef development. These reef ecosystems are considered biodiversity “hotspots” between 200 and 1000 m in the Atlantic Ocean. They are regarded as vulnerable marine ecosystems, providing essential ecosystem services. Over the past two decades, a considerable body of information has been accumulated on understanding the temporal development of CWC reef and mound formation. However, this research is limited in resolution, the range of study sites and datasets analysed. Here, an assessment of the temporal variation of CWC reefs and mounds situated in the west Porcupine Bank (wPB) and Porcupine Bank Canyon (PBC) is presented as well as background palaeoenvironmental information from an off-mound core. Previous studies of the spatial distribution of reefs and mounds reveal that they are dispersed across a variety of geomorphological settings in the region, including the canyon head, along the canyon lip and on the bank. This research broadly aims to understand the temporal distribution of the coral habitats within these contrasting settings. In 2015 and 2016, the QuERCi I and QuERCi II research cruises attempted coring the substrates of the canyon using traditional methods (i.e. gravity and box-corers). However, the acquired cores were insufficient in size and lacked an understanding of what habitat they were taken from. As such, 2 more research cruises (CoCoHaCa I and CoCoHaCa II) were carried out in 2017 and 2018 using sophisticated novel coring systems (ROV-vibrocoring). These methods proved successful, and cores were acquired through various CWC habitats in the canyon (mound summits, flanks, bank, slope and foot of the slope) and presented herein. This data includes novel 3-dimensional computed tomography (CT) derived imagery alongside traditional sedimentological approaches. The CT imagery was used to classify reef and mound formation/cessation. The cores were then split, sampled and investigated using a series of analytical techniques. The phases of formation/cessation were first constrained using radiocarbon dating and the cores were subsequently examined using grain size analysis to interpret the hydrodynamic regime. Stable isotope analysis on planktic and benthic foraminifera was then used to investigate paleoenvironmental conditions, which were contextualized by benthic foraminifera assemblages. An off-mound core was examined to elucidate the impact of the (de)glaciation of the British-Irish Ice Sheet (BIIS) on the wPB. Analysis of the core revealed that several fluxes of ice-rafted debris were deposited to the site. It was found that bottom currents became sluggish during stadial phases. Evidence for iceberg scouring in the core was also identified. Two coral bearing cores acquired from mound summits of variable distance to the canyon were then analysed. It was found that mound growth was twice as fast on the canyon lip than mounds 1 km away on the wPB. Multiproxy data revealed that a high food signal occurs closer to the canyon. This suggests that submarine canyons play a key role in enhancing particle supply and therefore influences coral growth and mound developmenton the margin. The radiocarbon dates acquired from coral bearing cores on the wPB suggest that corals occupied the bank since at least 45.1 ka BP. This finding subsequently resulted in revising our understanding of CWC re-expansion into the NE Atlantic during favourable climatic conditions, highlighting the crucial role played by submarine canyons. Findings outlined in this thesis provides the scientific community with new insights into the tolerances of cold-water corals during ecological tipping points. Furthermore, it highlights the need to investigate other submarine canyons occupied by CWCs in the NE Atlantic using ROV-vibrocoring.
Marine geology , Submarine canyons , Cold-water corals , Ice-rafted debris , Micropaleontology , Planktic foraminifera , Benthic foraminifera , Grain size analysis , Computed tomography , Stable isotopes , Carbon dating , Sedimentology , Holocene , Pleistocene , Special Area of Conservation , Porcupine Bank Canyon
O’Reilly, L. G. R. 2022. Understanding key geological processes and controls on cold-water coral habitat development in submarine canyons. PhD Thesis, University College Cork.