Scottish Continental Shelf Research Articles

Regional variation in directed swimming by Atlantic salmon smolts leaving Scottish waters for their oceanic feeding grounds—a modelling study

Abstract An understanding of the migratory trajectories and behaviours of Atlantic salmon post-smolts during the early marine phase of their life cycle is needed for effective conservation and marine planning. A simulation study of post-smolt migration from Scottish shores was carried out, combining a high-resolution hydrodynamic model of the Scottish continental shelf with a Lagrangian particle tracking model. Various different notional swimming behavioural scenarios were simulated, including current-orientated and compass-directed movement across a range of swimming speeds. Current-following behaviours did not facilitate migration towards feeding grounds in the North Atlantic. In addition, modelled trajectories resulting from directed-swimming behaviours imply that populations around Scotland need to adopt different locally adapted migratory strategies to successfully reach their feeding grounds.

Onset and demise of coral reefs, relationship with regional ocean circulation on the Wyville Thomson Ridge

Results from a vibrocore collected on the northern edge of the Scottish continental shelf at around 300 m water depth, on the Wyville Thomson Ridge, enable to reconstruct the history of cold water coral (CWC) reef growth and demise during the Holocene period. We report on significant age differences between U/Th and 14C dates obtained on pristine well-preserved CWC (Lophelia pertusa), which may reflect an diagenetic process that standard quality tests have failed to highlight. Additional 14C dates derived from bivalve fragments (Venus sp.) and foraminifera (Cibicides refulgens) samples show a gradual ageing with core depth but with significant age inversions in the lower section of the sediment core, which we consider reflects sedimentary mixing. We thus chose to derive an independent age model using planktonic foraminifera Globigerina bulloides stable isotope profiles. The vibrocore record is divided into 3 phases: 1- Mixed sediment deposits of glacial age corresponding to the base of the core with ages older than 13 cal ka BP, 2- The end of the deglacial/early Holocene between 13 and 9 cal ka BP and 3- Finally, the Holocene period from around 9 cal ka BP with abundant Lophelia pertusa fossils. Siliciclastic grain size and clay mineralogical composition show two significant shifts at around 13 and 9 cal ka BP indicating changes in sedimentary sources and transport associated with the dynamics and flow patterns of surface currents during the deglaciation and Holocene. Our results show that the onset of CWC reef growth on the Wyville Thomson Ridge occurred around 9 cal ka BP and was associated with a shift in flow patterns of surface currents in this area. This change of circulation patterns induced favourable sedimentological and hydrological conditions for corals to grow, and is associated with large scale modifications of North Atlantic circulation patterns at the end of the deglaciation.

The Relationship between Phytoplankton Distribution and Water Column Characteristics in North West European Shelf Sea Waters

Phytoplankton underpin the marine food web in shelf seas, with some species having properties that are harmful to human health and coastal aquaculture. Pressures such as climate change and anthropogenic nutrient input are hypothesized to influence phytoplankton community composition and distribution. Yet the primary environmental drivers in shelf seas are poorly understood. To begin to address this in North Western European waters, the phytoplankton community composition was assessed in light of measured physical and chemical drivers during the “Ellett Line” cruise of autumn 2001 across the Scottish Continental shelf and into adjacent open Atlantic waters. Spatial variability existed in both phytoplankton and environmental conditions, with clear differences not only between on and off shelf stations but also between different on shelf locations. Temperature/salinity plots demonstrated different water masses existed in the region. In turn, principal component analysis (PCA), of the measured environmental conditions (temperature, salinity, water density and inorganic nutrient concentrations) clearly discriminated between shelf and oceanic stations on the basis of DIN∶DSi ratio that was correlated with both salinity and temperature. Discrimination between shelf stations was also related to this ratio, but also the concentration of DIN and DSi. The phytoplankton community was diatom dominated, with multidimensional scaling (MDS) demonstrating spatial variability in its composition. Redundancy analysis (RDA) was used to investigate the link between environment and the phytoplankton community. This demonstrated a significant relationship between community composition and water mass as indexed by salinity (whole community), and both salinity and DIN∶DSi (diatoms alone). Diatoms of the Pseudo-nitzschia seriata group occurred at densities potentially harmful to shellfish aquaculture, with the potential for toxicity being elevated by the likelihood of DSi limitation of growth at most stations and depths.

The Holocene palaeoceanographic history of Muck Deep, Hebridean shelf, Scotland: has there been a change of wave climate in the past 12 000 years?

Abstract. Depressions (known as deeps) on the Scottish continental shelf are sediment sinks and they, thus, provide important sources of information on past oceanography and climate. Muck Deep is a partially infilled relict glacial valley cut in the continental shelf. A 70 cm core shows an upper buff sandy mud layer (0–4500 BP, radiocarbon years) overlying a dark grey mud layer (4500–12 000 BP). The foraminiferal faunas suggest that the upper sandy layer formed under higher energy conditions comparable to those existing now, where storm waves from southwesterly gales are responsible for transporting material from the adjacent shelf into the deeps. The grey mud formed under lower energy conditions as a consequence of lower storm wave influence on the sea floor (therefore, reduced transport of tests from the shelf into the deep). Because of the interaction of changing sea-levels and glacial rebound, water depths over Muck Deep were 35 m greater at 14–15 ka and have never been less than now. Therefore, the likely explanation of the lower energy deposits at the base of the core is related to the impact of storm waves on sediment transport from the adjacent shelves. Either water depths over the adjacent shelves were too great to be influenced by storm waves or the wave climate (direction) differed at that time.

A model of the spring migration into the North Sea by Calanus finmarchicus overwintering off the Scottish continental shelf

A particle tracking model was used to investigate the annual spring invasion of the North Sea by Calanus finmarchicus copepodites which overwinter in deep water off the Scottish continental shelf. Flow fields generated by a hydrodynamic model (HDM) were used to simulate the advection of zero drag particles representing the copepods. Particles were released simultaneously from a regular lattice of start positions at a given depth (D1), and ascended at a fixed rate (dD/dt) until they reached a final depth (D2) in the surface layers. The proportion of particles reaching target areas in the northern North Sea was relatively insensitive to a moderate degree of variation (±20%) around chosen default values of the vertical migration parameters (D1, D2 and dD/dt), derived from field data. The inclusion of horizontal diffusion velocities and diel vertical migration in surface layers did not significantly affect the results. Sensitivity to wind direction was investigated by applying flow fields from HDM runs with different wind forcing scenarios. For the default vertical migration parameters, only north‐westerly winds resulted in particles entering the North Sea from release locations north of the Iceland–Scotland Ridge, where dense aggregations of overwintering copepods were encountered during field surveys. The particle tracking model predicted that the major routes for the spring Calanus invasion into the North Sea were the East of Shetland Atlantic Inflow and the Norwegian Trench Atlantic Inflow, which agreed with seasonal trends observed in Continuous Plankton Recorder data. Overall, despite its relative simplicity, particle tracking was confirmed as a robust tool to explore the causal mechanisms behind the annual invasion of the North Sea by C. finmarchicus emerging from diapause in the deep waters off the Scottish continental shelf.

Decadal variability in the composition of Faroe Shetland Channel bottom water

Two standard sections across the deep water channel separating the Faroese Plateau from the Scottish continental shelf have been surveyed regularly since the start of the 20th century. There have been significant changes in the characteristics of surface, intermediate and deep water masses during this period. At intermediate depths, the presence of Norwegian Sea Arctic Intermediate Water (NSAIW) was evident as a salinity minimum during the first decade of the century. During the decades 1960–1980 this salinity minimum disappeared, and only four water types were identified in the Channel. Since 1980 the salinity of the intermediate water has again decreased, due to changes in the atmospheric forcing over the Nordic Seas, and it is again evident on a θS curve as a distinct minimum. The salinity of the bottom water in the Channel has also decreased (0.01/decade) linearly since the mid-1970s, although at a slower rate than the intermediate water (0.02/decade). The decline in salinity of the bottom water cannot be accounted for by changes in the salinity of upper Norwegian Sea Deep Water (NSDW), which Faroe Shetland Channel Bottom Water (FSCBW) has traditionally been assumed to be composed of. There is evidence that the upper level of NSDW has become deeper outside the Channel owing to a reduced supply from the Greenland Sea. This has resulted in a change in the composition of FSCBW, from being approximately 60% NSDW during the period 1970–1985 to 40% NSDW since 1990. Thus, the thermohaline circulation of the Nordic Seas has lost its deep water connection. The associated freshening of FSCBW has propagated out through the Channel into the North Atlantic and has resulted in a reduction of the salinity (0.02/decade) and transport (1–7%/decade) of Iceland Scotland Overflow Water (ISOW) into the North Atlantic.

Evidence for Heinrich event 1 in the British Isles

In the north Irish Sea basin (ISB), sedimentary successions constrained by AMS 14C dates obtained from marine microfaunas record three major palaeoenvironmental shifts during the last deglacial cycle. (i) Marine muds (Cooley Point Interstadial) dated to between 16.7 and 14.7 14C kyr BP record a major deglaciation of the ISB following the Late Glacial Maximum (LGM). (ii) Terminal outwash and ice-contact landforms (Killard Point Stadial) were deposited during an extensive ice readvance, which occurred after 14.7 14C kyr BP and reached a maximum extent at ca.14 14C kyr BP. At this time the lowlands surrounding the north ISB were drumlinised. Coeval flowlines reconstructed from these bedforms end at prominent moraines (Killard Point, Bride, St Bees) and indicate contemporaneity of drumlinisation from separate ice dispersal centres, substrate erosion by fast ice flow, and subglacial sediment transfer to ice-sheet margins. In north central Ireland bed reorganisation associated with this fast ice-flow phase involved overprinting and drumlinisation of earlier transverse ridges (Rogen-type moraines) by headward erosion along ice streams that exited through tidewater ice margins. This is the first direct terrestrial evidence that the British Ice Sheet (BIS) participated in Heinrich event 1 (H1). (iii) Regional mud drapes, directly overlying drumlins, record high relative sea-level (RSL) with stagnation zone retreat after 13.7 14C kyr BP (Rough Island Interstadial). Elsewhere in lowland areas of northern Britain ice-marginal sediments and morainic belts record millennial-scale oscillations of the BIS, which post-date the LGM advance on to the continental shelf, and pre-date the Loch Lomond Stadial (Younger Dryas) advance in the highlands of western Scotland (ca. 11–10 14C kyr BP). In western, northwestern and northern Ireland, Killard Point Stadial (H1) ice limits are reconstructed from ice-flow lines that are coeval with those in the north ISB and end at prominent moraines. On the Scottish continental shelf possible H1-age ice limits are reconstructed from dated marine muds and associated ice marginal moraines. It is argued that the last major offshore ice expansion from the Scottish mountains post-dated ca. 15 14C kyr BP and is therefore part of the H1 event. In eastern England the stratigraphic significance of the Dimlington silts is re-evaluated because evidence shows that there was only one major ice oscillation post-dating ca.18 14C kyr BP in these lowlands. In a wider context the sequence of deglacial events in the ISB (widespread deglaciation of southern part of the BIS → major readvance during H1 → ice sheet collapse) is similar to records of ice sheet variability from the southern margins of the Laurentide Ice Sheet (LIS). Well-dated ice-marginal records, however, show that during the Killard Point readvance the BIS was at its maximum position when retreat of the LIS was well underway. This phasing relationship supports the idea that the BIS readvance was a response to North Atlantic cooling induced by collapse of the LIS. © 1998 John Wiley & Sons, Ltd.

The tephrochronology and radiocarbon dating of North Atlantic, Late-Quaternary sediments: an example from the St. Kilda Basin

Abstract A sequence of disseminated basaltic tephras of Icelandic provenance has been investigated in sediments of Late Quaternary age recovered from the St Kilda Basin, on the Scottish continental shelf. The tephras were deposited from gradually melting rafted pack ice, transported on an anti-clockwise surface current originating to the north of Iceland. The presence of these ice-rafted tephras extends the zone of this current activity well beyond its previously documented western limit, demonstrating current impingement on the UK continental shelf. The evidence of ice-rafting, together with the biostratigraphy and a series of AMS 14 C dates, confirm that this deposition occurred during the Younger Dryas chronozone. Electron probe microanalysis (EPMA) of glass-shard geochemistry is used to ralate the St Kilda tephras to tephras found in marine and terrestrial deposits throughout the North Atlantic area, and to possible volcanic centres in Iceland. The joint role of tephrochronology and radiocarbon dating is discussed in relation to the comparative reliability of marine and terrestrial timescales. Problems with the chronology of the terrestrial equivalents of these tephras in Northern Iceland are highlighted.

Late Devensian and Flandrian palaeoenvironmental changes on the Scottish continental shelf west of the Outer Hebrides

AbstractExamination of two radiocarbon‐dated vibrocores taken from south of St Kilda at a water depth of about 155 m, a short distance within the maximum position of the Late Devensian (Dimlington Stadial) ice sheet, suggests that the St Kilda Basin became free of glacier ice after 15250 yr BP. Sedimentation in a shallow, low energy, high arctic, muddy environment continued until after 13500 yr BP. There followed a higher energy temperate episode during which water depths were roughly about 40 m: this is correlated with the latter part of the Windermere Interstadial and with the warmer interval known in shallow Scottish seas about or a little before 11 000 yr BP. The Loch Lomond (Younger Dryas) Stadial is marked in the vibrocores by the return of muddy sediments and a cold‐water fauna. Relatively shallow water conditions seem to have persisted into the earliest Flandrian, when the water depth was still roughly 60 m, corresponding to a sea‐level in the area 90–100 m below present. It is suggested that pack ice was widespread in the northeast Atlantic before the Windermere Interstadial and also during the Loch Lomond Stadial, when it transported shards of Icelandic volcanic ash into the St Kilda basin. Estimates of sea‐surface temperature for the last part of the Windermere Interstadial are close to those derived from the deep‐sea record for the same period.

The theoretical development of machair in the Hebrides

Abstract The current theory of machair development presupposes that the source of the parent material is pre‐Flandrian offshore sand banks. New work indicates that the source is more likely to be from the shell‐producing fauna living on the Scottish continental shelf. This implies that machair did not form as a ‘single event’, but rather that it has steadily increased in volume during the Flandrian. The machair landforms are controlled by rainfall and their present relative distributions are due to agricultural activities in the prehistoric period.

Geological inheritance and crystal dynamics of the northwest Scottish continental shelf

Deep Seismic reflection profiles have been combined with conventional Seismic surveys across the northwest Scottish continental shelf to interpret basin development in the region. Post-Caledonian basins are built upon partially reactivated older structures which have strongly influenced their internal structure and stratigraphy. We show that the Minch Basin, along with several others, developed in the hangingwall of the partially reactivated Outer Isles Fault, a planar fault dipping at about 30° ESE that reaches nearly to the Moho. The internal structure of these hangingwall basins is compared with appropriate scaled analogue models to show that they have developed by passive hangingwall collapse above a footwall block that moved towards the WNW. This geometry is incorporated into a lithospheric dynamic model with northwest extension towards the Rockall Trough from a fixed lithosphere beneath the Scottish Highlands. This model requires non-linear lithosphere extension which is accommodated by movements on faults and block rotation in the upper crust, extension and thinning of the lower crust along a network of anastomosing ductile shears, extensional movement of the Flannan Fault in the upper mantle and horizontal displacement along the Moho beneath the Outer Hebrides. This dynamic model has wider application for other extensional regions.

Modelling of storm-driven shelf waves north of Scotland—I. Idealized models

Storm-driven currents observed over the Scottish continental shelf were described by Gordon and Huthnance (1987 , Continental Shelf Research, 7, 1015–1048). Their suggestion of continental shelf-wave responses is investigated here using semi-analytic and numerical models of a straight continental shelf. Grid resolution, boundary conditions, truncations and differences of depth profile are considered with the semi-analytic model, from which dispersion curves and wave forms are derived. These forms are closely matched by the fully numerical model forced by specified elevations at one end. Further numerical calculations are described, for idealized wind forcing along or across limited regions of the shelf or slope. Localized forcing of short duration gives currents of the type observed, being rotary (clockwise) with roughly 22 h period. More extensive alongshelf forcing gives predominantly alongshelf currents with a strength tending to follow the forcing; the second form of observed response. Frictional effects are considered. A companion paper ( Flather and Proctor, 1988 , in preparation) describes the results of numerical simulations using realistic meteorological forcing and bathymetry.

Storm-driven continental shelf waves over the Scottish continental shelf

Currents and winds were observed over a 3-year period at two stations on the Scottish continental shelf, near the shelf break east and west of the Shetland Islands. Two types of responses to severe winter storms were observed. The first type was an “oscillatory response” where the currents rotated almost uniformly in the same sense as an inertial motion but with a frequency of about0.6 f (f is the inertial frequency), or a period of about 23 h. The second type of response, a “quasi-steady response”, was an along-isobath current that flowed as long as the wind blew and stopped when the wind stopped. Both responses were barotropic. Typical peak wind stress was 1 Pa or more, and response amplitudes were 0.2–0.4 m s −1. The oscillatory responses were forced by relatively short-duration winds (typically a half day) while the quasi-steady responses were forced by longer-period winds. Both responses appeared to be lowest-mode continental shelf waves (CSW's) but from different places on the dispersion curve. The oscillatory response appears to be a zero-group-velocity CSW which is predicted to “resonate” near the observed frequency. The quasi-steady response is a low-frequency, low-wavenumber CSW in the non-dispersive regime where velocities are constant. We present a model for CSW generation at all frequencies, with particular emphasis on results for the “resonant” regime. This model predicts the dominant characteristics of both the oscillatory and quasi-steady responses and makes reasonable the fact that these dominant characteristics are so well-defined in our data. Baroclinic responses were not observed in severe storms because the storms occurred in the months from December to April when the nearby continental shelf waters are well-mixed, surface to bottom.

Submerged rock platforms on the continental shelf west of Sula Sgeir

Synopsis Two major submerged erosion platforms have been identified on the north-western Scottish continental shelf to the west of Sula Sgeir. The lower, at c. −155 m, is up to 6 km broad but can only be traced for c. 15 km laterally. The upper platform, at c. −125 m, varies between 0.8 and 3 km in width and has been traced for over 50 km in a NNE–SSW direction. The platforms are attributed to marine erosion during periods of low sea level, each platform perhaps being subjected to erosion on more than one occasion. The −155 m platform is apparently the older and was formed prior to extensive glacial erosion of the shelf. It is assigned a Late Tertiary or Early Pleistocene age. The −125 m platform post-dates glaciation of the middle shelf but in major part pre-dates the Late Devensian glacial maximum. It is assigned a Middle to Late Pleistocene age. The island of Sula Sgeir and the neighbouring sea stacks were formed when sea level was c. −40 m relative to that of the present.

Late Quaternary glaciation and environmental change on St. Kilda, Scotland, and their palaeoclimatic significance

On Hirta, the largest island of the St. Kilda archipelago near the western edge of the Scottish continental shelf, evidence has been found for four cold periods and one intervening mild period. During the most recent cold period (the Loch Lomond Stadial), two protalus ramparts were formed. Prior to this there occurred two periods of valley glaciation separated by a mild interval during which the local vegetation was dominated by grasses and sedges. For the most recent valley glaciation (= Late Devensian glacial maximum) a mean July temperature of 4°C is inferred. At this time intensive periglacial processes were operative down to present sea level. At some earlier, pre‐Dcvensian, time St. Kilda was invaded by the Scottish ice sheet.

Bioclastic carbonate sedimentation on a high-latitude, tide-dominated shelf; Northeast Orkney Islands, Scotland

ABSTRACT Shell-sands and gravels cover much of the shallow Orkneys shelf at 59°N, accumulating locally into 30-m-high banks at rates up to 540 g/m2/yr (67 cm/l,000 yr). Overall, the Orkney shelf sedimentation rate is approximately 10 cm/l,000 yr, compared with 3 cm/1,000 yr for the entire Scottish continental shelf. Major sandbanks are located off headlands that produce circulation loops in the tidal flow. Regional sandwave orientations reveal a clockwise transport of sediment around the islands, probably resulting from storm-wave reinforcement of the tidal asymmetry combined with the net inflow of Atlantic water into the North Sea. Carbonate production is high from the Modiolus epifauna. Modiolus shell gravels, commonly containing Glycymeris, pass laterally into comminuted shell-sands. Within the euphotic zone (down to 40 m) dead shells are weakened by echinoid biting, algal boring, and limpet grazing. Boring by fungi and clionid sponges and grazing by chitons are common but not depth-restricted. Sediments contain 89-95% carbonate on the level bottom offshore, but 94-99% in sandwaves. Mean values for the main skeletal components are--bivalves 46%, barnacles 18%, bryozoans 11%, and serpulids 7%. Calcareous algal gravels occur in sheltered areas less than 20 m deep. Bryozoa typify lower-energy offshore environments, while more durable barnacle and serpulid debris is concentrated in sandwave fields. The sediments are dominantly calcitic and have a high preservation potential.

Shell-boring algae on the Scottish continental shelf: identification, distribution, bathymetric zonation

ABSTRACT3050 dead molluscan shells and fragments of calcareous algae were collected from 104 benthic stations at depths from 2 to 160 m in the Firth of Clyde, the Firth of Lorne and on the Orkney shelf, British Isles. The following micro-algal borings were recognised and are described: (i) Rhodophyta: Conchocelis, (ii) Chlorophyta: Ostreobium quekettii, Eugomontia sacculata, Phaeophila sp. and (iii) Cyanophyta: Plectonema terebrans, Hyella spp.These endolithic algae are bathymetrically zoned. Eugomontia, Phaeophila and the cyanophytes characterize the upper part of the photic zone (2–20 m). Shallow sheltered environments are typified by a high density of algal borings, most of them aligned perpendicular to shell surfaces. The lower photic zone is characterized by Ostreobium and Conchocelis whose borings are parallel to shell surfaces. The photic limit is about 40 m for the open Orkney shelf (59°N), and about 22 m for the partly enclosed firths (56°N).The Orkney population is dominated by two chlorophyte taxa: rhodophytes and cyanophytes are absent. This may be due to higher tidal energy conditions.

Sub-tidal motion on the Scottish continental shelf, August–September 1971

Simultaneous 28-day records of current and bottom pressure from four positions spanning 300 km near the shelf edge, and sea level at five coastal sites, show sub-tidal variations forming a coherent but limited response to the moderate wind stress over the shelf.

Biogenic carbonate sediments on the Scottish continental shelf and on Rockall Bank

Biogenic carbonate sediments on the Scottish continental shelf and on Rockall Bank (northeast Atlantic) are largely composed (in the fraction > 2 mm) of bivalve fragments, serpulid tubes (attached forms and Ditrupa arietina), barnacle plates, and bryozoans. Other components include fragments from regular and irregular echinoids, and from articulate and inarticulate brachiopods. In inshore Shetland waters, barnacles and attached serpulids are the dominant components, while west of Orkney and west of the Outer Hebrides bivalves tend to be dominant. On the outer shelf the polychaete Ditrupa may be the dominant component. On Rockall Bank bryozoans are the dominant components in the shallow waters close to Rockall Island and on Bryony Bank. Elsewhere bivalves are generally the dominant component. Radiocarbon dating indicates that these shell gravels are of Holocene age. In broad general terms, the faunal composition of these gravels can be related to local living faunas.