Hadal zone

Historically, the hadal zone was not recognized as distinct from the abyssal zone, although the deepest sections were sometimes called "ultra-abyssal". During the early 1950s, the Danish Galathea II and Soviet Vityaz expeditions separately discovered a distinct shift in the life at depths of {{Convert|6000-7000|m|ft|abbr=on}} not recognized by the broad definition of the abyssal zone.{{cite journal|author=Wolff|first=Torben|year=1959|title=The hadal community, an introduction|journal=Deep Sea Research|volume=6|pages=95–124|bibcode=1959DSR.....6...95W|doi=10.1016/0146-6313(59)90063-2}}{{Cite journal|last1=Jamieson|first1=Alan J.|author-link=Alan Jamieson|last2=Fujii|first2=Toyonobu|last3=Mayor|first3=Daniel J.|last4=Solan|first4=Martin|last5=Priede|first5=Imants G.|year=2010|title=Hadal trenches: the ecology of the deepest places on Earth (Review article)|url=http://izt.ciens.ucv.ve/ecologia/Archivos/ECO_POB%202010/ECOPO7_2010/Jamieson%20et%20al%202010.pdf|url-status=dead|journal=Trends in Ecology and Evolution|volume=25|issue=3|pages=190–197|doi=10.1016/j.tree.2009.09.009|archive-url=https://web.archive.org/web/20171225035341/http://izt.ciens.ucv.ve/ecologia/Archivos/ECO_POB%202010/ECOPO7_2010/Jamieson%20et%20al%202010.pdf|archive-date=25 December 2017|access-date=9 April 2017|pmid=19846236}} The term "hadal" was first proposed in 1956 by Anton Frederik Bruun to describe the parts of the ocean deeper than {{Convert|6000|m|ft|abbr=on}}, leaving abyssal for the parts at {{Convert|4000-6000|m|ft|abbr=on}}.{{cite journal|author=Bruun|first=Anton Frederik|author-link=Anton Frederik Bruun|date=16 June 1956|title=The Abyssal Fauna: Its Ecology, Distribution and Origin|journal=Nature|volume=177|issue=4520|pages=1105–1108|bibcode=1956Natur.177.1105B|doi=10.1038/1771105a0|s2cid=4182886}} The name refers to Hades, the ancient Greek god of the underworld. About 94% of the hadal zone is found in subduction trenches.[https://academic.oup.com/icesjms/article/79/4/1048/6576454?login=false Exponential growth of hadal science: perspectives and future directions identified using topic modelling]

Depths in excess of {{Convert|6000|m|ft|abbr=on}} are generally in ocean trenches, but there are also trenches at shallower depths. These shallower trenches lack the distinct shift in lifeforms and are therefore not hadal.{{cite book|title=The First Global Integrated Marine Assessment, World Oceans Assessment I|author=United Nations|publisher=Cambridge University Press|year=2017|isbn=978-1-316-51001-8|page=904|lccn=2017287717}}{{cite book|title=The Hadal Zone: Life in the Deepest Oceans|author=Jamieson|first=Alan|author-link=Alan Jamieson|publisher=Cambridge University Press|year=2015|isbn=978-1-107-01674-3|pages=18–21, 285–318|lccn=2014006998}}{{cite book|title=eLS|author=Jamieson|first=Alan J.|author-link=Alan Jamieson|publisher=John Wiley & Sons, Ltd|year=2011|isbn=978-0470016176|chapter=Ecology of Deep Oceans: Hadal Trenches|doi=10.1002/9780470015902.a0023606}} Although the hadal zone has gained widespread recognition and many continue to use the first proposed limit of {{Convert|6000|m|ft|abbr=on}}, it has been observed that {{Convert|6000-7000|m|ft|abbr=on}} represents a gradual transition between the abyssal and hadal zones, leading to the suggestion of placing the limit in the middle, at {{convert|6500|m|ft|abbr=on}}. Among others, this intermediate limit has been adopted by UNESCO.{{cite book|title=Marine Conservation Ecology|last1=Roff|first1=John|last2=Zacharias|first2=Mark|publisher=Earthscan|year=2011|isbn=978-1-84407-884-4}}{{cite book|url=https://unesdoc.unesco.org/ark:/48223/pf0000182451|title=Global Open Oceans and Deep Seabed (GOODS) – Biogeographic Classification|publisher=UNESCO|year=2009|editor-last=Vierros|editor-first=Marjo|series=IOC Technical Series|location=Paris|access-date=23 December 2017|editor-last2=Cresswell|editor-first2=Ian|editor-last3=Escobar Briones|editor-first3=Elva|editor-last4=Rice|editor-first4=Jake|editor-last5=Ardron|editor-first5=Jeff}} Similar to other depth ranges, the fauna of the hadal zone can be broadly placed into two groups: the hadobenthic species (compare benthic) living on or at the seabottom/sides of trenches, and the hadopelagic species (compare pelagic) living in the open water.{{cite book|title=The Living Ocean: Understanding and Protecting Marine Biodiversity|author=Thorne-Miller|first1=Boyce|last2=Catena|first2=John|publisher=John Wiley & Sons|year=1999|isbn=1-55963-678-5|edition=Second|page=57}}{{cite book|title=An Introduction to Marine Science|last1=Meadows|first1=P.S.|last2=Campbell|first2=J.I.|publisher=Wiley|year=1988|isbn=978-0-470-20951-6|edition=2nd|series=Tertiary Level Biology|page=7|lccn=87020603}}

File:Pelagiczone.svg

The deepest ocean trenches are considered the least explored and most extreme marine ecosystems. They are characterized by complete lack of sunlight, low temperatures, nutrient scarcity, and extremely high hydrostatic pressures. The major sources of nutrients and carbon are fallout from upper layers, drifts of fine sediment, and landslides. Most organisms are scavengers and detrivores. As of 2020, over 400 species are known from hadal ecosystems, many of which possess physiological adaptations to the extreme environmental conditions. There are high levels of endemism, and noteworthy examples of gigantism in amphipods, mysids, and isopods and dwarfism in nematodes, copepods, and kinorhynchs.{{cite book |last1=Ramirez-Llodra |first1=E | last2=Rowden |first2=AA | last3=Jamieson |first3=AJ |author3-link=Alan Jamieson| last4=Priede |first4=IG | last5=Keith |first5=DA |year=2020 |chapter=M3.6 Hadal trenches and troughs |chapter-url=https://global-ecosystems.org/explore/groups/M3.6 |editor1-last=Keith |editor1-first=D.A. |editor2-last=Ferrer-Paris |editor2-first=J.R. |editor3-last=Nicholson |editor3-first=E. |editor4-last=Kingsford |editor4-first=R.T. |title=The IUCN Global Ecosystem Typology 2.0: Descriptive profiles for biomes and ecosystem functional groups |location=Gland, Switzerland |publisher=IUCN | doi=10.2305/IUCN.CH.2020.13.en | isbn= 978-2-8317-2077-7|s2cid=241360441 }}

File:Alicella gigantea Japan Trench 2022.jpg) is found in the Hadal zone (collected from Japan Trench, 2022)]]

Marine life decreases with depth, both in abundance and biomass, but there is a wide range of metazoan organisms in the hadal zone, mostly benthos, including fish, sea cucumber, bristle worms, bivalves, isopods, sea anemones, amphipods, copepods, decapod crustaceans and gastropods. Most of these trench communities probably originated from the abyssal plains. Although they have evolved adaptations to high pressure and low temperatures such as lower metabolism, intra-cellular protein-stabilising osmolytes, and unsaturated fatty acids in cell membrane phospholipids, there is no consistent relationship between pressure and metabolic rate in these communities. Increased pressure can instead constrain the ontogenic or larval stages of organisms. Pressure increases ten-fold as an organism moves from sea level to a depth of {{Convert|90|m|abbr=on}}, whilst pressure only doubles as an organism moves from {{Convert|6000|to|11000|m|abbr=on}}.

Over a geological time scale, trenches can become accessible as previously stenobathic (limited to a narrow depth range) fauna evolve to become eurybathic (adapted to a wider range of depths), such as grenadiers and natantian prawns. Trench communities do, nevertheless, display a contrasting degree of intra-trench endemism and inter-trench similarities at a higher taxonomic level.

Only a relatively small number of fish species are known from the hadal zone, including certain grenadiers, cutthroat eels, pearlfish, cusk-eels, snailfish and eelpouts.{{cite journal|last1=Linley|first1=Thomas D.|last2=Gerringer|first2=Mackenzie E.|last3=Yancey|first3=Paul H.|last4=Drazen|first4=Jeffrey C.|last5=Weinstock|first5=Chloe L.|last6=Jamieson|first6=Alan J.|author6-link=Alan Jamieson|date=August 2016|title=Fishes of the hadal zone including new species, in situ observations and depth records of Liparidae|journal=Deep Sea Research Part I: Oceanographic Research Papers|volume=114|pages=99–110|bibcode=2016DSRI..114...99L|doi=10.1016/j.dsr.2016.05.003|doi-access=free}}{{cite journal |last1=Jamieson |first1=Alan J. |last2=Linley |first2=Thomas D. |last3=Eigler |first3=Shane |last4=Macdonald |first4=Tim |title=A global assessment of fishes at lower abyssal and upper hadal depths (5000 to 8000 m)|journal=Deep Sea Research Part I: Oceanographic Research Papers |date=1 December 2021 |volume=178 |pages=103642 |doi=10.1016/j.dsr.2021.103642 |bibcode=2021DSRI..17803642J |s2cid=239087034 |language=en |issn=0967-0637|doi-access= }} Due to the extreme pressure, the theoretical maximum depth for vertebral fish may be about {{convert|8000-8500|m|ft|abbr=on}}, below which teleosts would be hyperosmotic, assuming trimethylamine N-oxide requirements follow the observed approximate linear relationship with depth.{{cite journal|last1=Jamieson|first1=Alan J.|author-link=Alan Jamieson|last2=Yancey|first2=Paul H.|date=June 2012|title=On the Validity of the Trieste Flatfish: Dispelling the Myth|url=https://www.journals.uchicago.edu/doi/pdfplus/10.1086/BBLv222n3p171|url-status=live|journal=The Biological Bulletin|volume=222|issue=3|pages=171–175|doi=10.1086/BBLv222n3p171|jstor=41638633|pmid=22815365|s2cid=31549749|archive-url=https://web.archive.org/web/20191209070916/https://www.journals.uchicago.edu/doi/pdfplus/10.1086/BBLv222n3p171|archive-date=2019-12-09|url-access=subscription}}{{cite journal|last1=Yanceya|first1=Paul H.|last2=Gerringera|first2=Mackenzie E.|last3=Drazen|first3=Jeffrey C.|last4=Rowden|first4=Ashley A.|last5=Jamieson|first5=Alan|author5-link=Alan Jamieson|date=March 2014|title=Marine fish may be biochemically constrained from inhabiting the deepest ocean depths|url=https://www.pnas.org/content/pnas/111/12/4461.full.pdf|url-status=live|journal=PNAS|volume=111|issue=12|pages=4461–4465|bibcode=2014PNAS..111.4461Y|doi=10.1073/pnas.1322003111|pmc=3970477|pmid=24591588|archive-url=https://web.archive.org/web/20190704145853/https://www.pnas.org/content/pnas/111/12/4461.full.pdf|archive-date=2019-07-04|doi-access=free}} Some invertebrates do occur deeper, such as bigfin squid,{{Cite journal |last1=Jamieson |first1=Alan J. |last2=Vecchione |first2=Michael |date=2021-12-02 |title=Hadal cephalopods: first squid observation (Oegopsida, Magnapinnidae, Magnapinna sp.) and new records of finned octopods (Cirrata) at depths > 6000 m in the Philippine Trench |url=https://doi.org/10.1007/s00227-021-03993-x |journal=Marine Biology |language=en |volume=169 |issue=1 |pages=11 |doi=10.1007/s00227-021-03993-x |issn=1432-1793|url-access=subscription }}{{Cite web |author1=Brandon Specktor |date=2022-01-18 |title=World's deepest-dwelling squid spotted 20,000 feet under the sea |url=https://www.livescience.com/worlds-deepest-squid-philippine-trench |access-date=2024-06-30 |website=livescience.com |language=en}} certain polynoid worms, myriotrochid sea cucumbers, turrid snails and pardaliscid amphipods in excess of {{convert|10000|m|ft|abbr=on}}. In addition, giant protists known as Xenophyophora (foraminifera) live at these depths.[https://news.mongabay.com/2011/10/giant-one-celled-organisms-discovered-over-six-miles-below-the-oceans-surface/ Giant one-celled organisms discovered over six miles below the ocean’s surface]

The only known primary producers in the hadal zone are certain bacteria that are able to metabolize hydrogen and methane released by rock and seawater reactions (serpentinization),{{cite web|url=https://blogs.scientificamerican.com/artful-amoeba/what-lives-at-the-bottom-of-the-mariana-trench-more-than-you-might-think/|title=What Lives at the Bottom of the Mariana Trench? More Than You Might Think|author=Frazer|first=Jennifer|date=14 April 2013|website=Scientific American|url-status=dead|archive-url=https://web.archive.org/web/20190205034240/https://blogs.scientificamerican.com/artful-amoeba/what-lives-at-the-bottom-of-the-mariana-trench-more-than-you-might-think/|archive-date=5 February 2019}} or hydrogen sulfide released from cold seeps. Some of these bacteria are symbiotic, for example living inside the mantle of certain thyasirid and vesicomyid bivalves.{{cite journal|last1=Fujikura|first1=Katsunori|last2=Kojima|first2=Shigeaki|last3=Tamaki|first3=Kensaku|last4=Maki|first4=Yonosuke|last5=Hunt|first5=James|last6=Okutani|first6=Takashi|date=4 December 1999|title=The deepest chemosynthesis-based community yet discovered from the hadal zone, 7326 m deep, in the Japan Trench|url=https://www.int-res.com/articles/meps/190/m190p017.pdf|url-status=live|journal=Marine Ecology Progress Series|volume=190|pages=17–26|bibcode=1999MEPS..190...17F|doi=10.3354/meps190017|jstor=24854626|archive-url=https://web.archive.org/web/20190502063045/https://www.int-res.com/articles/meps/190/m190p017.pdf|archive-date=2 May 2019|doi-access=free}} Otherwise the first link in the hadal food web are heterotroph organisms that feed on marine snow, both fine particles and the occasional carcass.{{cite journal|last1=Blankenship|first1=Lesley E.|last2=Levin|first2=Lisa A.|author-link2=Lisa Levin|date=July 2007|title=Extreme food webs: Foraging strategies and diets of scavenging amphipods from the ocean's deepest 5 kilometers|journal=Limnology and Oceanography|volume=52|issue=4|pages=1685–1697|bibcode=2007LimOc..52.1685B|doi=10.4319/lo.2007.52.4.1685|jstor=4502323|doi-access=free}}

The hadal zone can reach far below {{convert|6000|m|ft|abbr=on}} deep; the deepest known extends to {{convert|10911|m|ft|abbr=on}}.{{cite web|url=https://oceanexplorer.noaa.gov/history/quotes/soundings/soundings.html|title=NOAA Ocean Explorer: History: Quotations: Soundings, Sea-Bottom, and Geophysics|publisher=NOAA, Office of Ocean Exploration and Research|access-date=2010-03-23}} At such depths, the pressure in the hadal zone exceeds {{convert|1100|atm|MPa psi|lk=on}}. Lack of light and extreme pressure makes this part of the ocean difficult to explore.

The exploration of the hadal zone requires the use of instruments that are able to withstand pressures of up to a thousand or more atmospheres. A few haphazard and non-standard tools have been used to collect limited, but valuable, information about the basic biology of a few hadal organisms. Manned and unmanned submersibles, however, can be used to study the depths in greater detail. Unmanned robotic submersibles may be remotely operated (connected to the research vessel by a cable) or autonomous (freely moving). Cameras and manipulators on submersibles allow researchers to observe and take samples of sediment and organisms. Failures of submersibles under the immense pressure at hadal zone depths have occurred. HROV Nereus is thought to have imploded at a depth of 9,990 meters while exploring the Kermadec Trench in 2014.

File:Bathyscaphe Trieste.jpg in 1958, used by Piccard and Walsh to reach Challenger Deep]]

The first manned exploration to reach Challenger Deep, the deepest known part of the ocean located in the Mariana Trench, was accomplished in 1960 by Jacques Piccard and Don Walsh.[http://library.thinkquest.org/04oct/00116/hadal.htm ThinkQuest] {{webarchive|url=https://web.archive.org/web/20070128202822/http://library.thinkquest.org/04oct/00116/hadal.htm |date=2007-01-28 }}. February 1, 2007. They reached a maximum depth of {{convert|10911|m|ft}} in the bathyscaphe Trieste.{{Cite news|url=http://www.guinnessworldrecords.com/news/60at60/2015/8/1960-deepest-manned-ocean-descent-393008|title=1960: Deepest Manned Ocean Descent|date=2015-08-19|work=Guinness World Records|access-date=2018-04-06|language=en-GB}}{{Cite web|url=https://web.whoi.edu/hades/about-hades/|title=About Hades|website=Hadal Ecosystem Studies|publisher=Woods Hole Oceanographic Institution|url-status=live|archive-url=https://web.archive.org/web/20190820163547/https://web.whoi.edu/hades/about-hades/|archive-date=2019-08-20|access-date=2018-04-06}}

James Cameron also reached the bottom of Mariana Trench in March 2012 using the Deepsea Challenger.{{cite web|url=https://www.nationalgeographic.com/news/2012/3/120325-james-cameron-mariana-trench-challenger-deepest-returns-science-sub/|title=James Cameron Completes Record-Breaking Mariana Trench Dive|last=Than|first=Ker|date=March 25, 2012|website=National Geographic|url-status=dead|archive-url=https://web.archive.org/web/20190919151244/https://www.nationalgeographic.com/news/2012/3/120325-james-cameron-mariana-trench-challenger-deepest-returns-science-sub/|archive-date=September 19, 2019}} The descent of the Deepsea Challenger reached a depth of {{convert|10908|m|ft}}, slightly less than the deepest dive record set by Piccard and Walsh.{{Cite web |date=2014-06-25 |title=DEEPSEA CHALLENGE – National Geographic Explorer James Cameron's Expedition |url=http://deepseachallenge.com/ |access-date=2022-01-01 |website= |archive-url=https://web.archive.org/web/20140625050833/http://deepseachallenge.com/ |archive-date=25 June 2014 |url-status=dead}} Cameron holds the record for the deepest solo dive.

In June 2012, the Chinese manned submersible Jiaolong was able to reach {{convert|7020|m|ft|abbr=on}} deep in the Mariana Trench, making it the deepest diving manned research submersible.{{Cite news|url=https://subseaworldnews.com/2012/07/10/jiaolong-reaches-7-000-meters-below-water/|title=Jiaolong Reaches 7.000 Meters Below Water|work=Subsea World News|access-date=2018-04-06|language=en-US}}{{Cite web|url=http://blogs.nature.com/news/2012/06/chinas-jiaolong-submersible-plunges-below-7000-metres.html|title=China's Jiaolong submersible plunges below 7,000 metres|last=Owens|first=Brian|date=25 Jun 2012|website=blogs.nature.com|language=en-US|url-status=live|archive-url=https://web.archive.org/web/20191112164107/http://blogs.nature.com/news/2012/06/chinas-jiaolong-submersible-plunges-below-7000-metres.html|archive-date=2019-11-12|access-date=2018-04-06}} This range surpasses that of the previous record holder, the Japanese-made Shinkai, whose maximum depth is {{convert|6500|m|ft|abbr=on}}.{{Cite web|url=https://www.jamstec.go.jp/e/about/equipment/ships/shinkai6500.html|title=Deep Submergence Research Vehicle – Shinkai 6500|publisher=JAMSTEC|url-status=live|archive-url=https://web.archive.org/web/20190518211231/https://www.jamstec.go.jp/e/about/equipment/ships/shinkai6500.html|archive-date=2019-05-18|access-date=2018-04-06}}

Few unmanned submersibles are capable of descending to maximum hadal depths. The deepest diving unmanned submersibles have included the Kaikō (lost at sea in 2003),{{Cite web|url=https://www.jamstec.go.jp/e/about/equipment/ships/kaiko.html|title=Remotely Operated Vehicle – Kaiko|publisher=JAMSTEC|url-status=live|archive-url=https://web.archive.org/web/20190902190540/https://www.jamstec.go.jp/e/about/equipment/ships/kaiko.html|archive-date=2019-09-02|access-date=2018-04-06}} the ABISMO,{{Cite press release|url=https://www.jamstec.go.jp/e/about/press_release/20080616/|title="ABISMO," Automatic Bottom Inspection and Sampling Mobile, Succeeds in World's First Multiple Vertical Sampling from Mid-ocean, Sea Floor and Sub-seafloor over Depth of 10,000 m in Mariana Trench|access-date=2018-04-06|date=2008-06-16|publisher=JAMSTEC|archive-date=2018-11-16|archive-url=https://web.archive.org/web/20181116171807/https://www.jamstec.go.jp/e/about/press_release/20080616/|url-status=live}} the Nereus (lost at sea in 2014),{{Cite press release|url=https://www.whoi.edu/press-room/news-release/Nereus-Lost/|title=Robotic Deep-sea Vehicle Lost on Dive to 6-Mile Depth|date=2014-05-10|publisher=Woods Hole Oceanographic Institution|url-status=live|archive-url=https://web.archive.org/web/20191209064433/https://www.whoi.edu/press-room/news-release/Nereus-Lost/|archive-date=2019-12-09|access-date=2018-04-06}} and the Haidou-1.{{Cite news|url=https://www.ndtv.com/world-news/chinas-unmanned-submersible-sets-new-national-record-1449082|title=China's Unmanned Submersible Sets New National Record|date=2016-08-23|access-date=2019-12-09|url-status=live|archive-url=https://archive.today/20191209065151/https://www.ndtv.com/world-news/chinas-unmanned-submersible-sets-new-national-record-1449082|archive-date=2019-12-09|publisher=NDTV|agency=Press Trust of India}}

{{clear}}

{{Portal|Ecology|Environment|Marine Life}}

• {{annotated link|Abyssal plain}}
• {{annotated link|Deep sea}}
• {{annotated link|Deep-submergence vehicle|Deep submergence vehicle}}
• {{annotated link|Abyssal zone}}
• {{annotated link|Sunlight zone}}; includes shallow waters and coral reefs

{{Reflist}}

• [http://www.spiegel.de/wissenschaft/natur/0,1518,583200,00.html Forscher filmen lebende Fische in Rekordtiefe (In German) from Spiegel 10/09/2008 about an expedition filming fish at a depth of more than 7,000 m]
• {{Cite AV media|url=https://www.youtube.com/watch?v=SuNYt3FVxjc |archive-url=https://ghostarchive.org/varchive/youtube/20211221/SuNYt3FVxjc |archive-date=2021-12-21 |url-status=live|title=Hadal highlights|date=11 January 2015|publisher=Schmidt Ocean Institute|via=YouTube}}{{cbignore}}

Category:Hydrology

Category:Oceanographical terminology

Category:Extreme points of Earth

Category:1950s neologisms