Understanding key geological processes and controls on cold-water coral habitat development in submarine canyons
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Date
2022
Authors
O'Reilly, Luke
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Publisher
University College Cork
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Abstract
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.
Description
Keywords
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
Citation
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.