High resolution bathymetry of the eastern slope of the Porcupine Seabight

doi:10.1016/S0025-3227(03)00093-8

Marine Geology

Volume 198, Issues 1–2, 30 June 2003, Pages 27-54

https://doi.org/10.1016/S0025-3227(03)00093-8 Get rights and content

Abstract

The topography of the eastern margin of the Porcupine Seabight was surveyed in June 2000 utilizing swath bathymetry. The survey was carried out during RV Polarstern cruise ANT XVII/4 as part of the GEOMOUND project. The main objective was to map and investigate the seafloor topography of this region. The investigated area contains a variability of morphological features such as deep sea channels and giant mounds. The survey was planned and realized on the basis of existing data so as to guarantee the complete coverage of the margin. In order to achieve a resolution of the final digital terrain model (DTM) that meets the project demands, data processing was adjusted accordingly. The grid spacing of the DTM was set to 50 m and an accuracy better than 1% of the water depth was achieved for 96% of the soundings.

Introduction

A high precision multibeam survey, covering the eastern slope of the Porcupine Seabight, was performed with the German RV Polarstern during the cruise ANT XVII/4 within the framework of the GEOMOUND project (GEOlogy of carbonate MOUNDs). The survey was carried out between 11 and 16 June 2000. The GEOMOUND project focuses on the geological evolution of giant, deep-water carbonate mounds in the Porcupine Basin and the Rockall Trough off western Ireland. A number of large carbonate mounds have been earlier identified in the Porcupine Basin by seismic profiles (Hovland et al., 1994). In order to understand their structure and genesis as well as the processes which exist in conjunction with these mounds, the bathymetry of the region has to be known in detail. For this reason, the survey was performed in combination with sub-bottom profiling.

The goals of the survey were primarily to create a large scale bathymetric chart of the mound and the canyon areas along the slope, to create a mosaicked sonar image from the multibeam sidescan data, and finally to investigate the scientific benefit of calibrated backscatter data derived from multibeam measurements. Calibrated backscatter data can be used to derive information on the seafloor surface, for example, on grain size. Changes in the seafloor roughness can be linked to bathymetric features which will be the topic of a separate publication. This contribution focuses on the swath bathymetry and its results.

Section snippets

Methods and data

The area of investigation is part of the European continental margin west of Ireland. Prior to track planning, the existing bathymetric data in this region were evaluated. The survey lines were planned mainly on basis of the depth contours extracted from the GEBCO Digital Atlas 1997 (GDA 97) (GEBCO, 1997). Due to considerable morphological variabilities within the area of investigation and in order to minimize depth changes along the survey lines, the tracks were planned parallel to the slope.

Morphology of the continental margin

The Porcupine Seabight west of Ireland extends from approximately 49° to 52° N lat and from 14° to 11° W long. In this section, a general bathymetric description of the eastern margin of the Porcupine Seabight will be given. The northern and southern boundaries are 50°25′N and 51°40′N, respectively (see Fig. 6). During the multibeam survey only the major parts of the slope were systematically surveyed. Due to time constraints, the margin could not be completely covered from bottom to top.

The

Discussion

The area of the Belgica mound province has a dimension of approximately 40×30 km for the north–south and east–west directions, respectively. A lack of mound occurrences exists around 51°30′N separating the Belgica province into a northern and a southern mound cluster. Two scenarios are possible to explain this situation. Either the conditions for settling and growth of the corals were unfavorable (for example, missing hard ground or insufficient nutrient flow) or mounds exist in that area but

Conclusions

The multibeam data were thoroughly cleaned and processed. Overall the multibeam data are of high quality. The standard deviation is mostly better than 1% of the water depth and underlines this statement. The high accuracy level could be obtained by continuously overlapping of adjacent survey lines and a high accuracy navigation using differential GPS.

The final DTM has been adjusted to completely cover the investigated area. If future investigations require a more detailed view on the data, the

Acknowledgements

We are grateful to the crew of RV Polarstern for support during cruise ANT XVII/4 and to R. Usbeck for operating the CTD device. Financial support of these investigations was granted by the European Commission (Contract No. EVK3-CT-1999-00016).

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The Porcupine Seabight is a north-south trending basin in the NE Atlantic located approximately 150 km off the south-west coast of Ireland (Fig. 1A) and encompasses the Belgica Mound Province (BMP). The BMP, located on the eastern flank of the Porcupine Seabight (Fig. 1A), is comprised of numerous CWC carbonate mounds, most of which are dominated by the framework-building scleractinians Lophelia pertusa and Madrepora oculata (e.g. Huvenne et al., 2002; Beyer et al., 2003). Moreover, the BMP is partially enclosed by a Special Area of Conservation designated under the EU Habitats Directive.
Scleractinian cold-water corals (CWCs), such as Lophelia pertusa and Madrepora oculata, build complex frameworks and carbonate mound habitats in the deep-sea and are regarded as deep-sea biodiversity hotspots. However, there is a paucity of research regarding temporal change in the biological composition of these CWC habitats and analysis of this change is critical, especially considering the anthropogenic influences impacting upon these vital habitats. This study is the first to analyse temporal biological change on a CWC mound at a small temporal (4 years) and large spatial (entire mound) scale. Video datasets were collected over the Piddington Mound of the Moira Mounds, Porcupine Seabight, offshore Ireland, in 2011 and 2015. 0.25 m² quadrats were placed on 510 screenshots from each year (1020 total). Species were identified and quantified. CTD data were also analysed to investigate potential contributory causes of change. There was a highly significant (P < 0.01) decline in average M. oculata percentage cover from 2011 (1.37 ± 3.79%) to 2015 (0.70 ± 2.76%); however, there was not a significant change (P = 0.74) in average L. pertusa percentage cover in 2011 (2.33 ± 7.52%) when comparing it with 2015 (5.28 ± 7.88%). Additionally, there was an increase in average percentage cover of two poriferan species, Aphrocallistes sp. and Hexadella sp., from 2011 (1.18 ± 2.35%; 0.21 ± 0.42%, respectively) to 2015 (2.15 ± 3.48%; 0.33 ± 0.55%, respectively) (P < 0.01 for both species). Shannon-Weiner and Pielou's Species Evenness diversity indices were generally higher in 2011 than in 2015. This overall decline in diversity and increase in poriferan abundances is attributed to the decline in M. oculata, which may have been influenced by altered environmental conditions in 2015, as indicated by the CTD data. Moreover, differing biological and ecological responses of L. pertusa and M. oculata to environmental changes were considered and discussed as possible contributors to the results observed for these two species. If M. oculata percentage cover declines linearly at the current rate (approximately 0.17% per year, if a constant decline in each of the four years), then by 2019, this species may be nearly absent from the Piddington Mound, which would have major effects on species’ abundances, distributions, and biodiversity. Therefore, further research and continuous monitoring of the temporal change in biological composition within CWC reef/mound habitats are critical.
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High resolution bathymetry of the eastern slope of the Porcupine Seabight

Abstract

Introduction

Section snippets

Methods and data

Morphology of the continental margin

Discussion

Conclusions

Acknowledgements

Mar. Pet. Geol.

Mar. Geol.

Speed of sound in seawater at high pressures

J. Acoust. Soc. Am.

Modern swathe sounding and sub-bottom profiling technology for research applications: The Atlas Hydrosweep and Parasound Systems

Mar. Geophys. Res.

Hydrosweep: New era in high precision bathymetric surveying in deep and shallow water

Mar. Geod.