On its shoreline areScotland to the north,England to the east,Wales to the southeast,Northern Ireland and theRepublic of Ireland to the west. The Irish Sea is of significant economic importance to regional trade, shipping and transport, as well as fishing andpower generation in the form ofwind power andnuclear power plants. Annual traffic between Great Britain and Ireland is over 12 million passengers and 17 million tonnes (17,000,000 long tons; 19,000,000 short tons) of traded goods.
The Irish Sea joins theNorth Atlantic at both its northern and southern ends. To the north, the connection is through the North Channel between Scotland and Northern Ireland and theMalin Sea. The southern end is linked to the Atlantic through theSt George's Channel between Ireland andPembrokeshire, and the Celtic Sea. It is composed of a deeper channel about 300 km (190 mi) long and 30–50 km (20–30 mi) wide on its western side and shallower bays to the east. The depth of the western channel ranges from 80 m (260 ft) to 275 m (900 ft).
Cardigan Bay in the south, and the waters to the east of the Isle of Man, are less than 50 m (160 ft) deep. With a total water volume of 2,430 km3 (580 cu mi) and a surface area of 47,000 km2 (18,000 sq mi), 80% is to the west of the Isle of Man. The largest sandbanks are theBahama andKing William Banks to the east and north of the Isle of Man and theKish Bank, Codling Bank, Arklow Bank and Blackwater Bank near the coast of Ireland. The Irish Sea, at its greatest width, is 200 km (120 mi) and narrows to 75 km (47 mi).[9]
On the North. The Southern limit of the Inner Seas off the West Coast of Scotland, defined as a line joining the South extreme of theMull of Galloway (54°38'N) in Scotland and Ballyquintin Point (54°20'N) in Northern Ireland.
The Irish Sea has undergone a series of dramatic changes over the last 20,000 years as thelast glacial period ended and was replaced by warmer conditions. At the height of the glaciation, the central part of the modern sea was probably a longfreshwater lake. As theice retreated 10,000 years ago, the lake reconnected to the sea.
Because Ireland has neither tunnel nor bridge to connect it with Great Britain, the vast majority of heavy goods trade is done by sea. Northern Ireland ports handle 10 million tonnes (9,800,000 long tons; 11,000,000 short tons) of goods trade with the rest of theUnited Kingdom annually; the ports in the Republic of Ireland handle 7.6 million tonnes (7,500,000 long tons; 8,400,000 short tons), representing 50% and 40% respectively of total trade by weight.
ThePort of Liverpool handles 32 million tonnes (31,000,000 long tons; 35,000,000 short tons) of cargo and 734,000 passengers a year.[16]Holyhead port handles most of the passenger traffic fromDublin andDún Laoghaire ports, as well as 3.3 million tonnes (3,200,000 long tons; 3,600,000 short tons) of freight.[17]
Ports in the Republic handle 3,600,000 travellers crossing the sea each year, amounting to 92% of all Irish Sea travel.[18]
Ferry connections from Wales to Ireland across the Irish Sea includeFishguard Harbour andPembroke toRosslare, Holyhead to Dún Laoghaire and Holyhead to Dublin. From Scotland,Cairnryan connects with bothBelfast andLarne. There is also a connection betweenLiverpool and Belfast via the Isle of Man or direct fromBirkenhead. The world's largestcar ferry,Ulysses, is operated byIrish Ferries on the Dublin Port–Holyhead route;Stena Line also operates between Britain and Ireland.
"Irish Sea" is also the name of one of theBBC'sShipping Forecast areas defined by the coordinates:
The British shipLCT 326 sank in the Irish sea and was discovered in March 2020. In September 2021, the British Navy shipHMS Mercury was discovered; it sank in 1940. The British shipSS Mesaba was sunk by the Imperial German NavyU-118 in 1918 and discovered in 2022.[20] This ship is well known for sailing near theTitanic and for attempting to warn the Titanic about dangerous icebergs.
TheCaernarfon Bay basin contains up to 7 cubic kilometres (1.7 cu mi) ofPermian andTriassic syn-rift sediments in an asymmetricalgraben that is bounded to the north and south by LowerPaleozoicmassifs.[21] Only two exploration wells have been drilled so far, and there remain numerous undrilled targets in tiltedfault blockplays.[22] As in the East Irish Sea Basin, the principal target reservoir is the Lower TriassicSherwood Sandstone, top-sealed by younger Triassicmudstone andevaporites.[21] Wells in the Irish Sector to the west have demonstrated that pre-riftWestphaliancoal measures are excellent hydrocarbonsource rocks and are at peak maturity for gas generation (Maddox et al., 1995).[23][21]Seismic profiles clearly image thesestrata continuing beneath a basal Permian unconformity into at least the western part of the Caernarfon Bay Basin.[21]
The timing of gas generation presents the greatest exploration risk.[24] Maximum burial of, and primary gas migration from, the source rocks could have terminated as early as the Jurassic, whereas many of the tilted fault blocks were reactivated or created duringPaleogene inversion of the basin.[24] However, it is also possible that a secondary gas charge occurred during regional heating associated with intrusion of Paleogene dykes, such as those that crop out nearby on the coastline of north Wales. Floodpage et al. (1999) have invoked this second phase of Paleogene hydrocarbon generation as an important factor in the charging of the East Irish Sea Basin’s oil and gas fields.[25] It is not clear as yet whether aeromagnetic anomalies in the southeast of Caernarfon Bay are imaging a continuation of the dyke swarm into this area too, or whether they are instead associated with deeply buried Permian syn-rift volcanics.[21] Alternatively, the fault block traps could have been recharged by exsolution of methane from formation brines as a direct result of the Tertiary uplift (cf. Doré and Jensen, 1996).[26]
TheCardigan Bay Basin forms a continuation into British waters of Ireland’s North Celtic Sea Basin, which has two producing gas fields.[27] The basin comprises a south-easterly deepening half-graben near the Welsh coastline, although its internal structure becomes increasingly complex towards the southwest.[28] Permian to Triassic syn-rift sediments are less than 3 km thick and overlain by up to 4 km of Jurassic strata, and locally up to 2 km of Paleogene fluvio-deltaic sediments.[27] The basin has a proven petroleum system, with potentially producible gas at the Dragon discovery and oil shows in three other wells.[29] Reservoir targets include Lower Triassic Sherwood Sandstone, Middle Jurassic shallow marine sandstones and limestones (Great Oolite), and Upper Jurassic fluvial sandstone (Dragon Field reservoir).[30]
The most likely hydrocarbon source rocks are Early Jurassic marine mudstones.[27] These are fully mature for oil in the west of the UK sector and gas-mature in the adjacent Irish sector.[27] Gas-prone Westphalian pre-rift coal measures may also exist locally at depth.[27] The basin underwent two phases of Tertiary compressive uplift; maximum burial ended by the Late Cretaceous, or earlier if Cretaceous strata were never deposited.[27] The Dragon discovery proves hydrocarbons were locally retained despite Tertiary deformation.[29] Untested stratigraphic trap potential also exists near synsedimentary faults in the Middle Jurassic interval.[29]
The Liverpool Bay Development was BHP’s largest operated asset.[31] It comprises the integrated development of five offshore oil and gas fields in the Irish Sea:[32]
Douglas oil field Hamilton gas field Hamilton North gas field Hamilton East gas field Lennox oil and gas field
Oil is produced from the Lennox and Douglas fields.[32] It is then treated at the Douglas Complex and piped 17 km to an oil storage barge.[31] Gas is produced from the Hamilton, Hamilton North and Hamilton East reservoirs.[32] After initial processing at the Douglas Complex the gas is piped to the Point of Ayr terminal.[31] It is then piped to PowerGen’s CCGT station at Connah’s Quay.[33] PowerGen is the sole purchaser of gas from Liverpool Bay.[31]
The Liverpool Bay Development comprises four offshore platforms, offshore storage and loading facilities, and the Point of Ayr terminal.[34] Production started as follows: Hamilton North (1995), Hamilton (1996), Douglas (1996), Lennox (oil only, 1996), Hamilton East (2001).[31] First contract gas sales were in 1996.[31]
The quality of water in Liverpool Bay was historically affected by sewage sludge dumping,[35] which became illegal in December 1988.[36]
With 210 billion cubic metres (7.5 trillion cubic feet) of gas and 176 million barrels (28,000,000 m3) of oil estimated by operators (DTI, 2001), the basin is in a mature exploration phase.[37] EarlyNamurian basinal mudstones are the source rocks.[38] Production is from fault-bounded Lower Triassic Sherwood Sandstone reservoirs, sealed by Triassic mudstones and evaporites.[39] Future exploration focuses on extending this play, with additional potential in Carboniferous fluvial sandstones.[40] This requires intraformational mudstone seals because no top-seal exists where reservoirs subcrop the base Permian unconformity in the east; Carboniferous strata crop out at the seabed in the west.[41]
Previous drilling in theKish Bank Basin confirmed petroleum potential with oil shows in wells and hydrocarbon seeps from airborne surveys.[42] New 2-D seismic analysis revealed a large undrilled Lower Triassic structural closure ~10 kilometres (6 mi) offshore Dublin.[43] The Dalkey prospect may be oil-prone, given prolific Lower Triassic reservoirs in the nearby eastern Irish Sea.[44] It may contain ~870 million barrels (140,000,000 m3) of oil in place.[45] The prospect carries significant risk; partners are advancing a focused work programme to mitigate these risks.[46] Its shallow-water location near shore may reduce drilling and development costs.[46]
The information on theinvertebrates of the seabed of the Irish Sea is rather patchy because it is difficult to survey such a large area, where underwater visibility is often poor and information often depends upon looking at material brought up from the seabed in mechanical grabs. However, the groupings of animals present depend to a large extent on whether the seabed is composed ofrock,boulders,gravel,sand,mud or evenpeat. In the softsediments seven types of community have been provisionally identified, variously dominated bybrittle-stars,sea urchins, worms,mussels,tellins,furrow-shells, and tower-shells.
Parts of the bed of the Irish Sea are very rich in wildlife. The seabed southwest of the Isle of Man is particularly noted for its rarities and diversity,[50] as are thehorse mussel beds of Strangford Lough.Scallops andqueen scallops are found in more gravelly areas. In the estuaries, where the bed is more sandy or muddy, the number of species is smaller but the size of their populations is larger.Brown shrimp,cockles and edible mussels support localfisheries in Morecambe Bay and the Dee Estuary and the estuaries are also important as nurseries forflatfish,herring andsea bass. Muddy seabeds in deeper waters are home to populations of theDublin Bay prawn, also known as "scampi".[51]
The open sea is a complex habitat in its own right. It exists in three spatial dimensions and also varies over time and tide. For example, where freshwater flows into the Irish Sea in river estuaries its influence can extend far offshore as the freshwater is lighter and "floats" on top of the much larger body of salt water until wind and temperature changes mix it in. Similarly, warmer water is less dense and seawater warmed in the inter-tidal zone may "float" on the colder offshore water. The amount of light penetrating the seawater also varies with depth and turbidity. This leads to differing populations ofplankton in different parts of the sea and varying communities of animals that feed on these populations. However, increasing seasonal storminess leads to greater mixing of water and tends to break down these divisions, which are more apparent when the weather is calm for long periods.
Plankton includes bacteria, plants (phytoplankton) and animals (zooplankton) that drift in the sea. Most are microscopic, but some, such as the various species ofjellyfish andsea gooseberry, can be much bigger.
Diatoms anddinoflagellates dominate the phytoplankton. Although they are microscopic plants, diatoms have hard shells and dinoflagellates have littletails that propel them through the water. Phytoplankton populations in the Irish Sea have a spring "bloom" every April and May, when the seawater is generally at its greenest.
Crustaceans, especiallycopepods, dominate the zooplankton. However, many animals of the seabed, the open sea and the seashore spend their juvenile stages as part of the zooplankton. The whole plankton "soup" is vitally important, directly or indirectly, as a food source for most species in the Irish Sea, even the largest. The enormousbasking shark, for example, lives entirely on plankton and theleatherback turtle's main food is jellyfish.
A colossal diversity ofinvertebratespecies live in the Irish Sea and its surrounding coastline, ranging from flower-like fan-worms to predatoryswimming crabs to largechameleon-likecuttlefish.[51]Some of the most significant for other wildlife are the reef-building species like the inshore horse mussel of Strangford Lough, the inter-tidal honeycomb worm of Morecambe Bay,Cumbria andLancashire, and the sub-tidal honeycomb worm of theWicklow Reef. These build up large structures over many years and, in turn, provide surfaces, nooks and crannies where other marine animals and plants may become established and live out some or all of their lives.
There are quite regular records of live and stranded leatherback turtles in and around the Irish Sea. This species travels north to the waters off theBritish Isles every year following the swarms of jellyfish that form its prey. Loggerhead turtle,ridley sea turtle andgreen turtle are found very occasionally in the Irish Sea but are generally unwell or dead when discovered. They have strayed or been swept out of their natural range further south into colder waters.[52][53]
The estuaries of the Irish Sea are of international importance for birds. They are vital feeding grounds onmigration flyways forshorebirds travelling between theArctic and Africa. Others depend on the milder climate as a refuge when continental Europe is in the grip of winter.[51]
Twenty-one species ofseabird are reported as regularly nesting on beaches or cliffs around the Irish Sea. Huge populations of thesea duck,common scoter, spend winters feeding in shallow waters off eastern Ireland, Lancashire and North Wales.[51]
The common orharbour seal and thegrey seal are both resident in the Irish Sea. Commonseals breed in Strangford Lough, grey seals in southwest Wales and, in small numbers, on the Isle of Man. Grey seals haul out, but do not breed, offHilbre andWalney islands,Merseyside, theWirral, St Annes, Barrow-in-Furness Borough, and Cumbria.[51]
The Irish Sea has been described byGreenpeace as the mostradioactively contaminated sea in the world with some "eight million litres ofnuclear waste" discharged into it each day fromSellafield reprocessing plants, contaminating seawater, sediments and marine life.[56]
Low-levelradioactive waste has been discharged into the Irish Sea as part of operations at Sellafield since 1952. The rate of discharge began to accelerate in the mid- to late 1960s, reaching a peak in the 1970s and generally declining significantly since then. As an example of this profile, discharges ofplutonium (specifically241Pu) peaked in 1973 at 2,755 terabecquerels (74,500 Ci)[57] falling to 8.1 TBq (220 Ci) by 2004.[58] Improvements in the treatment of waste in 1985 and 1994 resulted in further reductions inradioactive waste discharge although the subsequent processing of a backlog resulted in increased discharges of certain types of radioactive waste. Discharges oftechnetium in particular rose from 6.1 TBq (160 Ci) in 1993 to a peak of 192 TBq (5,200 Ci) in 1995 before dropping back to 14 TBq (380 Ci) in 2004.[57][58] In total 22 petabecquerels (590 kCi) of241Pu was discharged over the period 1952 to 1998.[59] Current rates of discharge for manyradionuclides are at least 100 times lower than they were in the 1970s.[60]
Analysis[61][62] of the distribution of radioactive contamination after discharge reveals that mean sea currents result in much of the more soluble elements such ascaesium being flushed out of the Irish Sea through the North Channel about a year after discharge. Measurements of technetium concentrations post-1994 has produced estimated transit times to the North Channel of around six months with peak concentrations off the northeast Irish coast occurring 18–24 months after peak discharge. Less soluble elements such as plutonium are subject to much slower redistribution. Whilst concentrations have declined in line with the reduction in discharges they are markedly higher in the eastern Irish Sea compared to the western areas. The dispersal of these elements is closely associated with sediment activity, withmuddy deposits on the seabed acting as sinks, soaking up an estimated 200 kg (440 lb) ofplutonium.[63] The highest concentration is found in the eastern Irish Sea in sediment banks lying parallel to the Cumbrian coast. This area acts as a significant source of wider contamination as radionuclides are dissolved once again. Studies have revealed that 80% of current seawater contamination by caesium is sourced from sediment banks, whilst plutonium levels in the western sediment banks between the Isle of Man and the Irish coast are being maintained by contamination redistributed from the eastern sediment banks.
The consumption of seafood harvested from the Irish Sea is the main pathway for exposure of humans to radioactivity.[64] Theenvironmental monitoring report for the period 2003 to 2005 published by theRadiological Protection Institute of Ireland (RPII) reported that in 2005 average quantities of radioactive contamination found in seafood ranged from less than 1 Bq/kg (12 pCi/lb) for fish to under 44 Bq/kg (540 pCi/lb) for mussels.[65] Doses of man-made radioactivity received by the heaviest consumers of seafood in Ireland in 2005 was 1.10 μSv (0.000110 rem).[66] This compares with a corresponding dosage of radioactivity naturally occurring in the seafood consumed by this group of 148 μSv (0.0148 rem) and a total average dosage in Ireland from all sources of 3,620 μSv (0.362 rem).[67] In terms of risk to this group, heavy consumption of seafood generates a 1 in 18 million chance of causing cancer. The general risk of contracting cancer in Ireland is 1 in 522. In the UK, the heaviest seafood consumers in Cumbria received a radioactive dosage attributable to Sellafield discharges of 220 μSv (0.022 rem) in 2005.[68] This compares to average annual dose of naturally sourced radiation received in the UK of 2,230 μSv (0.223 rem).[69]
Discussions of linking Britain to Ireland began in 1895,[70] with an application for £15,000 towards the cost of carrying out borings and soundings in the North Channel to see if a tunnel between Ireland and Scotland was viable. Sixty years later,Harford Montgomery Hyde,Unionist MP for North Belfast, called for the building of such a tunnel.[71] A tunnel project has been discussed several times in theIrish parliament.[72][73][74][75] The idea for a 34-kilometre (21 mi) long rail bridge or tunnel continues to be mooted. Several potential projects have been proposed, including one between Dublin and Holyhead put forward in 1997 by the British engineering firm Symonds. At 80 km (50 mi), it would have been by far thelongest rail tunnel on earth with an estimated cost approaching £20 billion.[76]
An offshore wind farm was developed on theArklow Bank,[77]Arklow Bank Wind Park, about 10 km (6.2 mi) off the coast ofCounty Wicklow in the south Irish Sea. The site currently has seven GE 3.6 MWturbines, each with 104-metre (341 ft) diameterrotors, the world's first commercial application of offshore wind turbines over threemegawatts in size. The operating company,Airtricity, has indefinite plans for nearly 100 further turbines on the site.
DuringWorld War I the Irish Sea became known as "U-boat Alley", because the U-boats moved their emphasis from theAtlantic to the Irish Sea after the United States entered the war in 1917.[82][83]
The Irish Sea figures prominently in theMabinogion. In thesecond branch of the Mabinogi the Irish Sea is crossed from the south toHarlech byMatholwch, the Irish King, who has come to seek the hand ofBranwen ferch Llŷr, sister ofBendigeidfran, King of the Island of the Mighty. Branwen and Matholwch marry, but when she becomes abused by Matholwch, her brother crosses the sea from Wales to Ireland to rescue her. Within the story the Irish Sea is said to be shallow; in addition, it contains two rivers, the Lli and the Archan.[84]
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