TheHikurangi Trough(previously known as theHikurangi Trench)[1][a]is a sea floor feature of thePacific Oceanoff the north-eastSouth Islandand the east coast of theNorth Islandof New Zealand. It has been forming for about 25 million years and isturbidite-filled, particularly in its south. This characteristic can be used to distinguish it from the sediment-poor and deeperKermadec Trench,which is its continuation to the north. Sediment currently passing through the trough represents about 0.5% of the total sediment input to the world oceans. The trough has deep-seachemosyntheticecosystems that are unique.
Geography
editAlthough shallower than the trenches north of it, the Hikurangi Trough reaches depths of 3,000 metres (9,800 feet) as close as 80 kilometres (50 miles) from shore.[2]The southern trough structure is 6–10 km (3.7–6.2 mi) wide off the coast of northernCanterburywith an initial local depth of about 2,000 m (6,600 ft),[3]and towards its northern portions it has structures more like those found in oceanic trenches.[4]The trough widens about the latitude ofCook Straitto between 50–80 km (31–50 mi) up to theMāhia Peninsula.[5]There are various mappings of its northern limits as the Ruatōria debris avalanche was a poorly understood structure.[6]The limits of the trough are not defined by standard geographical works and various geological works have different mappings.[b]
A meandering depression within the trough like structure, theHikurangi Channel(also known asHikurangi Sea ChannelorHikurangi Seachannel), commences in the south and trends to the eastern side of the trough. By the latitude of Cook Strait the channel is about 80 m (260 ft) below the general level of the trough.[10]The seafloor Hikurangi Channel continues along its eastern side,[5]until at the Māhia Peninsula it breaks off towards the east through theHikurangi Plateauand discharges into the south-west Pacific abyssal plains.[11][12]The channel has a total length between 1,400–2,000 km (870–1,240 mi),[13]but only the initial 800 km (500 mi) portion is in the trough.[14]
The trough to the south has related structures such as the undersea Pūkākī, Okains and Pegasus canyons at the latitude ofPegasus Bay,[1][15]the Hurunui Canyon which is beyond theHurunui Rivermouth,[16]and the sudden coastal shelving to 823 m (2,700 ft) just south-east ofKaikōuraof theKaikōura Canyon,[17][4]where the sea floor drops to the depth of the trough. Extending from the hilly coast north of Kaikōura the undersea Kowhai canyons are to the west of the trough.[18]Towards the north eastern part of Cook Strait is the Cook Strait Canyon with part of the trough being 2,250 m (7,380 ft) deep. OffCape Palliser,a part of the trough is 2,236 m (7,336 ft) deep, less than theHikurangi Plateau's 2,588 m (8,491 ft) on the eastern side of the trough at this latitude. The Hikurangi Plateau extends east of the trough between theChatham Riseto the south and a point north of New Zealand'sEast Cape.On the east coast of the North Island between the Pāhaoa and Māhia canyons there is not direct drainage from the coast into the trough as the seafloor has a number of intermediate ridges and basins. In terms of being a definite trench with two sides, this is found on the sea floor north of the Māhia Peninsula but is broken by the Tuaheni submarine landslide at about 38°50'S,[19]and the submarine Ruatōria debris avalanche at about 38°S.[20]Beyond the Ruatōria debris avalanche theHikurangi Marginjoins the Ruatōria Scarp and then drops away from the undersea East Cape Ridge and commences trench–like structures to depths of 4,580 m (15,030 ft) assigned by many authorities to the Kermadec Trench which further north has a distinct transition to a much greater 7,436 m (24,396 ft) depth.[4][b]
Geology
editThe Hikurangi Trough is sediment filled as a result of being a key part of the eastern New Zealand oceanic sedimentary system for several million years.[21]The present North Island subduction and accretion that began in the middle and lateOligocene,caused thick sedimentary sequences to form in the then trench with enhanced abyssal erosion in the lateMiocene.[22]More recently it has sediment from the erosion of the uplifting mountains of the South Island of New Zealand such as theSouthern Alps,which formed from 6.4 million years ago.[22]This system currently contributes about 0.5% of the total sediment input to the world oceans.[13]The sediments in the trench are up to 5 km (3.1 mi) deep in the south and where they exit the Kaikōura Canyon have acoustic characteristics of gravelturbidites.[23]The turbidites thin to about 1 km (0.62 mi) in the north.[3]Gas hydrateshave been identified in the sediments and there are widespreadmethaneseeps.[24]Radiodatinganalysis of the carbonate rocks formed at such seeps show that the carbonate formation has been going on for periods between 2,360±70 yearsBPto 12,400±160 years BP.[25]
The active turbidite channel represented by the Hikurangi Channel, guidesturbidity currentsinto the path of the Pacific deep westernboundary currentto the north.[26]The Hikurangi Channel is known to be less than 3.5 million years old.[27]Initially the channel developed during the latePlioceneand extended along the Hikurangi Trench northwards.[22]The sediments are predominantly delivered by submarine canyons and slope gullies that cut across or circumvent obstructions to flow,[28]and the Kaikōura Canyon is known to be the dominant current active and longterm contributor in the case of the Hikurangi Trench.[3]Exceptionally, the2016 Kaikōura earthquakeprecipitated submarine mudslides and sediment flows that displaced about 850×106t (8.4×108long tons; 9.4×108short tons) of sediment into the trench from the Kaikōura Canyon,[29][30]with a turbidity current that travelled more than 680 km (420 mi) along the Hikurangi Channel. The furtherest marine core sampled in the channel so far revealed more than 16 cm (6.3 in) of fresh sediment.[31]The full analysis is nuanced, with flows from ten turbide triggering catchments but also flows that may be due to submarine land sliding from shaking associated with ground-motion amplitude peaks that produce failure in muddy sediments.[32]The catchment of the Cook Strait Canyon was a large contributor to the turbidite deposits from the earthquake.[31][c]
Tectonics
editTheHikurangi Marginsubductionzone is where the thick oceanicHikurangi Plateauhas been subducting for about 25 million years beneathcontinental crustof the oldIndo-Australian platepossibly without an intermediate continental crustmicroplate,although there are rotation features. The relative motion at the north is 6.1 cm/year (2.4 in/year) at East Cape and is down to 3.5 cm/year (1.4 in/year) atCape Turnagainwith regard to convergence. The net vector is 4.5 cm/year (1.8 in/year) to with a vector direction of 266° and 3.8 cm/year (1.5 in/year) in a direction of 259° at the Canterbury end.[6]This is split into convergence near the trench, strike-slip motion around the top of the forearc ridge, and extension in theTaupō Rift.[33]There is not a continuous trench with two sides at the margin and some complexity in the trench or trough like structures is due to the complex transitional tectonics and old subductingseamounts.[6]By contrast, the Kermadec andTonga trenchesrepresent the parts of theKermadec-Tonga subduction zonewhere theoceanic crustof thePacific plateis subducting beneath oceanic crust of theKermadecandTongamicroplates which also abut oceanic crust of the currentAustralian plate.[6]
Ecology
edit
Because of the delivery of waters by the Lower Circumpolar Deep Waters and current and coastal drainage and surface currents such as the East Cape Current a rich marine ecosystem exists in the coastal waters over the trench. The trench ecosystem itself has been much less studied as it is even deeper than the economically important deep marine fisheries.[34]The middle and southern Hikurangi Margin includes fisheries forhoki,alfonsinoandorange roughy.[35]The far south area of the trough near the Kaikōura Canyon, the Subtropical Front forms a convergence zone between the subtropical and subantarctic water masses represented by the Southland Current down to about 1,000 m (3,300 ft).[36]This region of ocean has been determined to be one of the most productive marine ecosystems on the planet. At this end of the trench, many deep water species are found close to the shore. This food source attracts thewhalesfor which the town ofKaikōurais famous. The 2016 Kaikōura earthquake profoundly disturbed this ecosystem, but there is evidence of recovery.[30]
In one study, biological sampling of the walls of the trough took place at six sites between 690 and 1,561 m (2,264 and 5,121 ft) deep.[35]Compared to similar sampling in the more northernBay of Plentythere were higher abundances in the Hikurangi Margin of manycrustaceansandwormssuch asmud dragons,ostracods,nematodeworms,copepods,tanaidacea,segmented worms,isopodsandamphipods.[37]
Methane seeps
editIn the trough itself, deep-seachemosyntheticecosystems are associated with methanecold seepsthat withauthigenesisusually createcarbonatemounds.[38]The single cell organisms responsible for these ecosystems are similar to those found at methane seeps worldwide.[39]Unlike other areas of the deep ocean no methane–associated mud volcanoes have been found on the margin.[40]
Bacterial mats, often white, are seen in video imaging.[40]Some of these systems differ from any others so far described worldwide.[41]For example, there are dense populations ofbristle wormsin dark sulphide-rich soft sediment microhabitats where there is a highfluxof both methane andsulphides.[40]These dark patches have distinctive pitting associated with the worm burrows.[42]Because bristle worms are found at cold seeps worldwide it is possible that the role of aerobic methanotrophy may have been underestimated in seafloor methane chemosynthetic ecosystems, as over 25% of themethaneescaping from the Hikurangi Margin cold seeps is metabolised aerobically.[43]
In general such methane seeps have anaerobic single cell organisms in the subsurface anoxic layers, with anaerobicmethanotrophicarchaeaand sulfate-reducingmyxococcota.[44]In highly reduced seep habitats the surface is dominated by bacterial mats, while where partialoxidationhas happened the bristle worm ecosystem previously mentioned is found associated with aerobicmethanotrophicGammaproteobacteria.Genetically, many of these have been characterised as related to the chemosyntheticendosymbiontsof marine invertebrates.[44]Sulfide is exploited as an energy resource by both the tubeworms and the clams.[25]With more advanced oxidation the ecosystem becomes dominated by chemosyntheticfrenulatetubeworms of the genusLamellibrachiaclosely related to those found in theLau Basin,[42]and clams, mussels, and variousDemospongiaesponges.[41][45]Thesaltwater clamsare of the genusCalyptogena,themusselsare in the deep-sea sub-familyBathymodiolinae.[45]
See also
editNotes
edit- ^The term trough is used predominantly in the article as this reflects the now accepted international name and that the trench is mainly filled with sediment
- ^abThe northern limits of the trough are ill-defined. Seismic geological studies are consistent with the trough being buried under the Ruatōria debris avalanche,[7]and there is an unnamed feature on the northern side of the Ruatōria debris avalanche south of the Ruatōria Scarp. Part of the Ruatōria Scarp could be associated with the subduction margin before it intercepts the geographical southernmost point of the Kermadec Trench as presently defined at 37° 45′ 16.27″ S by GEBCO.[1]The Marine Gazetteer defines the southern limit of the Kermadec Trench much further north as 35°30' S.[8]and there is different mapping in the historic geological literature.[6][9]
- ^No core sample was obtained at the outlet of the Kaikōura Canyon so the relative contributions of it and the Cook Strait Canyon at the time of the 2016 Kaikōura earthquake are ill defined.[31]
References
edit- ^abc"GEBCO:search:Hikurangi".Retrieved21 August2024.
- ^Lewis, Collott & Lallemand 1998,Fig.2.
- ^abcLewis, Collott & Lallemand 1998,p. 445.
- ^abc"Arcgis Ocean map:World Volcanoes compared to Tectonic Plate Boundaries".Retrieved19 August2024.
- ^abLewis, Collott & Lallemand 1998,pp. 444, 447.
- ^abcdeLewis, Collott & Lallemand 1998,p. 441.
- ^Barker et al. 2009,Figures 1, 10, 11.
- ^"Marine Gazetteer:Kermadec Trench".Retrieved19 August2024.
- ^Barker et al. 2009,Fig. 1.
- ^Lewis, Collott & Lallemand 1998,p. 444.
- ^Reyners, Eberhart-Phillips & Bannister 2011.
- ^Lewis, Collott & Lallemand 1998,pp. 442–4.
- ^abCarter, Carter & McCave 2004,p. 12.
- ^Lewis, Collott & Lallemand 1998,p. 450.
- ^"Marine Gazetteer:Okains Canyon".Retrieved19 August2024.
- ^"Marine Gazetteer:Hurunui Canyon".Retrieved19 August2024.
- ^"Marine Gazetteer:Kaikōura Canyon".Retrieved19 August2024.
- ^Lewis, Collott & Lallemand 1998,Fig. 3.
- ^Strachan et al. 2022,Figure 9.
- ^Strachan et al. 2022,Figure 2.
- ^Carter, Carter & McCave 2004,pp. 9–10.
- ^abcCarter, Carter & McCave 2004,p. 17.
- ^Lewis, Collott & Lallemand 1998,pp. 448, 445.
- ^Bowden et al. 2013,Gas hydrate extraction.
- ^abBowden et al. 2013,Succession.
- ^Carter, Carter & McCave 2004,p. 10.
- ^McArthur & Tek 2021,p. 883.
- ^McArthur & Tek 2021,Introduction.
- ^"Kaikōura earthquake provides world-first insight into submarine canyons".NIWA.17 October 2018.Retrieved22 August2024.
- ^abRowden 2018.
- ^abcHowarth et al. 2021,Fig.1, Fig. 3.
- ^Howarth et al. 2021,Fig. 3.
- ^Barker et al. 2009,2. Tectonic Setting.
- ^McCormack 2021.
- ^abRosli et al. 2016,Study area and sampling design.
- ^Bors et al. 2012.
- ^Rosli et al. 2016,Table 2..
- ^Bowden et al. 2013,Introduction, Fauna & habitats.
- ^Ruff et al. 2013,Abstract.
- ^abcBowden et al. 2013,Introduction.
- ^abRuff et al. 2013,Introduction.
- ^abBowden et al. 2013,Fauna & habitats.
- ^Ruff et al. 2013,Conclusion.
- ^abRuff et al. 2013,Bacterial diversity of Hikurangi margin sediments.
- ^abBowden et al. 2013,Habitat and community composition and structure.
Sources
edit- R. Wally Johnson; Jan Knutson; Stuart Ross Taylor; Australian Academy of Science (1989).Intraplate Volcanism: In Eastern Australia and New Zealand.Cambridge University Press.ISBN978-0-521-38083-6.
- Lewis, Keith B.; Collott, Jean-Yves; Lallemand, Serge E. (1998)."The dammed Hikurangi Trough: a channel-fed trench blocked by subducting seamounts and their wake avalanches (New Zealand-France GeodyNZ Project)"(PDF).Basin Research.10(4): 441–468.Bibcode:1998BasR...10..441L.doi:10.1046/j.1365-2117.1998.00080.x.
- Reyners, M.;Eberhart-Phillips, D.;Bannister, S. (2011)."Tracking repeated subduction of the Hikurangi Plateau beneath New Zealand".Earth and Planetary Science Letters.311(1): 165–171.Bibcode:2011E&PSL.311..165R.doi:10.1016/j.epsl.2011.09.011.Retrieved11 December2016.
- Barker, D.H.; Sutherland, R.; Henrys, S.; Bannister, S. (2009). "Geometry of the Hikurangi subduction thrust and upper plate, North Island, New Zealand".Geochemistry, Geophysics, Geosystems.10(2). Q02007.Bibcode:2009GGG....10.2007B.doi:10.1029/2008GC002153.
- Strachan, L.J.; Bailleul, J.; Bland, K.J.; Orpin, A.R.; McArthur, A.D. (2022). "Understanding sedimentary systems and processes of the Hikurangi Subduction Margin; from Trench to Back-Arc. Volume 1".New Zealand Journal of Geology and Geophysics.65(1): 1–16.Bibcode:2022NZJGG..65....1S.doi:10.1080/00288306.2022.2048032.
- McArthur, A.D.; Tek, D.E. (2021)."Controls on the origin and evolution of deep-ocean trench-axial channels"(PDF).Geology.49(8): 883–888.Bibcode:2021Geo....49..883M.doi:10.1130/G48612.1.ISSN0091-7613.
- Howarth, J.D.; Orpin, A.R.; Kaneko, Y.; Strachan, L.J.; Nodder, S.D.; Mountjoy, J.J.; Barnes, P.M.; Bostock, H.C.; Holden, C; Jones, K; Cağatay, M.N. (2021). "Calibrating the marine turbidite palaeoseismometer using the 2016 Kaikōura earthquake".Nature Geoscience.14(3): 161–7.Bibcode:2021NatGe..14..161H.doi:10.1038/s41561-021-00692-6.
- Carter, L.; Carter, R.M.; McCave, I.N. (2004). "Evolution of the sedimentary system beneath the deep Pacific inflow off eastern New Zealand".Marine Geology.205(1–4): 9–27.Bibcode:2004MGeol.205....9C.doi:10.1016/S0025-3227(04)00016-7.
- Ruff, S.E.; Arnds, J.; Knittel, K.; Amann, R.; Wegener, G.; Ramette, A.; Boetius, A. (2013)."Microbial communities of deep-sea methane seeps at Hikurangi continental margin (New Zealand)".PLOS One.8(9). e72627.doi:10.1371/journal.pone.0076869.PMC3800081.PMID24204691.
- Rowden, Ashley (27 June 2018)."Kaikoura Canyon submarine landslides erosion and deposition by turbidity currents".NIWA.Retrieved20 August2021.
- Bors, E.K.; Rowden, A.A.; Maas, E.W.; Clark, M.R.; Shank, T.M. (21 November 2012)."Patterns of Deep-Sea Genetic Connectivity in the New Zealand Region: Implications for Management of Benthic Ecosystems".PLOS One.7(11): e49474.doi:10.1371/journal.pone.0049474.hdl:1912/5732.PMC3504039.PMID23185341.
- McCormack, F. (2021)."The Kermadec Ocean Sanctuary: terraqueous territorialization and Māori marine environments"(PDF).Pacific Affairs.94(1): 77–96.doi:10.5509/202194177.Retrieved21 August2024.
- Rosli, N.; Leduc, D.; Rowden, A.A.; Clark, M.R.; Probert, P.K.; Berkenbusch, K.; Neira, C. (2016)."Differences in meiofauna communities with sediment depth are greater than habitat effects on the New Zealand continental margin: implications for vulnerability to anthropogenic disturbance".PeerJ.4.e2154.doi:10.7717/peerj.2154.PMC4941793.PMID27441114.
- Bowden, D.A.; Rowden, A.A.; Thurber, A.R.; Baco, A.R.; Levin, L.A.; Smith, C.R. (2013)."Cold Seep Epifaunal Communities on the Hikurangi Margin, New Zealand: Composition, Succession, and Vulnerability to Human Activities".PLOS One.8(10). e76869.Bibcode:2013PLoSO...876869B.doi:10.1371/journal.pone.0076869.PMID24204691.