Wikipedia

South Pacific Gyre

TheSouthern Pacific Gyreis part of the Earth's system of rotating ocean currents, bounded by theEquatorto the north,Australiato the west, theAntarctic Circumpolar Currentto the south, andSouth Americato the east.[1]The center of the South Pacific Gyre is theoceanic pole of inaccessibility,the site on Earth farthest from any continents and productive ocean regions and is regarded as Earth's largest oceanic desert.[2]With an area of 37 million square kilometres, it makes up approximately 10% of the Earth's ocean surface.[3]Thegyre,as with Earth's other four gyres, contains an area with elevated concentrations ofpelagicplastics,chemical sludge,and otherdebrisknown as theSouth Pacific garbage patch.[4]

The South Pacific Gyre.

Sediment flux and accumulation

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Earth'strade windsandCoriolis forcecause the ocean currents in SouthPacific Oceanto circulate counterclockwise. The currents act to isolate the center of the gyre from nutrientupwelling,and few nutrients are transported there by the wind (eolian processes) because there is relatively little land in the Southern Hemisphere to supply dust to theprevailing winds.The low levels of nutrients in the region result in extremely lowprimary productivityin the ocean surface and subsequently very low flux oforganic materialsettling to the ocean floor asmarine snow.The low levels ofbiogenicand eolian deposition cause sediments to accumulate on the ocean floor very slowly. In the center of the South Pacific Gyre, the sedimentation rate is 0.1 to 1 m (0.3 to 3.3 ft) per million years. The sediment thickness (from basement basalts to the seafloor) ranges from 1 to 70m, with thinner sediments occurring closer to the center of the Gyre. The low flux of particles to the South Pacific Gyre causes the water there to be the clearest seawater in the world.[2]

Subseafloor biosphere

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Beneath the seafloor, themarine sedimentsand surrounding porewaters contain an unusualsubseafloor biosphere.Despite extremely low amounts of buried organic material,microbeslive throughout the entire sediment column. Average cell abundances and net rates ofrespirationare a few orders of magnitude lower than in any othersubseafloor biospherepreviously studied.[2]

The South Pacific Gyre subseafloor community is also unusual because it containsoxygenthroughout the entire sediment column. In other subseafloor biospheres, microbial respiration will break down organic material and consume all the oxygen near the seafloor leaving the deeper portions of the sediment column anoxic. However, in the South Pacific Gyre the low levels of organic material, the low rates of respiration, and the thin sediments allow the porewater to be oxygenated throughout the entire sediment column.[5]In July 2020,marine biologistsreported thataerobicmicroorganisms(mainly), in "quasi-suspended animation",were found inorganically poor sediments,up to 101.5 million years old, 250 feet below theseafloorof the region and could be thelongest-living life formsever found.[6][7]

Radiolytic H2:a benthic energy source

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Benthicmicrobes in organic-poor sediments in oligotrophic oceanic regions, such as the South Pacific Gyre, are hypothesized to metabolizeradiolytichydrogen(H2) as a primary energy source.[8][2][9]

The oceanic regions within the South Pacific Gyre (SPG), and other subtropical gyres, are characterized by low primary productivity in the surface ocean; i.e. they are oligotrophic. The center of the SPG is the furthest oceanic province from a continent and contains the clearest ocean water on Earth[2]with ≥ 0.14 mg chlorophyll per m3.[2]Carbon exported to the underlying deep ocean sediments via thebiological pumpis limited in the SPG, resulting in sedimentation rates that are orders of magnitude lower than in productive zones, e.g. continental margins.[2]

Typically, deep-ocean benthic microbial life utilizes the organic carbon exported from surface waters. In oligotrophic regions where sediments are poor in organic material, subsurface benthic life exploits other primary energy sources, such as molecular hydrogen (H2).[10][8][2][9]

Radiolysis of interstitial water

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Radioactive decayof naturally occurringuranium(238Uand235U),thorium(232Th), andpotassium(40K) in seafloor sediments collectively bombard the interstitial water withα,β,andγradiation. The irradiation ionizes and breaks apart water molecules, eventually yielding H2.The products of this reaction are aqueous electrons (e−aq), hydrogen radicals (H·), protons (H+), and hydroxyl radicals (OH·).[9]The radicals are highly reactive, therefore short-lived, and recombine to produce hydrogen peroxide (H2O2), and molecular hydrogen (H2).[10]

The amount of radiolytic H2production in seafloor sediments is dependent on the quantities of radioactive isotopes present, sediment porosity, and grain size. These criteria indicate that certain sediment types, such as abyssal clays and siliceous oozes, may have higher radiolytic H2production relative to other seafloor strata.[9]Also, radiolytic H2production has been measured in seawater intrusions into subseafloor basement basalts.[10]

Microbial activity

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The microbes best suited to utilize radiolytic H2are the knallgas bacteria,lithoautotrophes,that obtain energy by oxidizing molecular hydrogen via theknallgas reaction:[11]

H2(aq) + 0.5O2(aq) H2O (l)[12]

In the surface layer of sediment cores from oligotrophic regions of the SPG, O2is the primary electron acceptor used in microbial metabolisms. The O2concentrations decline slightly in surface sediment (initial few decimeters) and are unchanged to depth. Meanwhile, nitrate concentrations slightly increase downward or remain constant in sediment column at approximately the same concentrations as the deep water above the seafloor. Measured negative fluxes of O2in the surface layer demonstrate that a relatively low abundance of aerobic microbes that are oxidizing the minimally deposited organic matter from the ocean above. Extremely low cell counts corroborate that microbes exist in small quantities in these surface sediments. In contrast, a sediment cores outside of the SPG show rapid elimination of O2and nitrate at 1 meter below sea floor (mbsf) and 2.5 mbsf, respectively. This is evidence of much higher microbial activity, both aerobic and anaerobic.[9][2]

The production of radiolytic H2(electron donor) is stoichiometrically balanced with production of 0.5 O2(electron acceptor), therefore a measurable flux in O2is not expected in the substrate if both radiolysis of water and knallgas bacteria co-occur.[9][2]So, despite the known occurrence of radiolytic H2production, molecular hydrogen is below the detectable limit in the SPG cores, leading to the hypothesis that H2is the primary energy source in low-organic seafloor sediments below the surface layer.[9][2][8]

Water color

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Satellitedata images show that some areas in the gyre are greener than the surrounding clear blue water, which is frequently interpreted as areas with higher concentrations of livingphytoplankton.However, the assumption that greener ocean water always contains more phytoplankton is not always true. Even though the South Pacific Gyre contains these patches of green water, it has very little organism growth. Instead, some studies hypothesize that these green patches are a result of the accumulated waste of marine life. The optical properties of the South Pacific Gyre remain largely unexplored.[13]

Garbage patch

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This section is an excerpt fromSouth Pacific garbage patch.[edit]
The South Pacific Gyre can be seen in the lack of oceanic currents off the west coast of South America. Map of ocean currents circa 1943
This photo demonstrates the dispersal of plastic fragments of various sizes
Visualization of the flow pattern of ocean pollutants
TheSouth Pacific garbage patchis an area of ocean with increased levels ofmarine debrisand plastic particle pollution, within the ocean'spelagic zone.This area is in the South Pacific Gyre, which itself spans from waters east ofAustraliato theSouth Americancontinent, as far north as theEquator,and south until reaching theAntarctic Circumpolar Current.[14]The degradation of plastics in the ocean also leads to a rise in the level of toxics in the area.[15]The garbage patch was confirmed in mid-2017, and has been compared to theGreat Pacific Garbage Patch's state in 2007, making the former ten years younger. The South Pacific garbage patch is not visible on satellites, and is not a landmass. Most particles are smaller than a grain of rice.[16]A researcher said: "This cloud ofmicroplasticsextends both vertically and horizontally. It's more like smog than a patch ".[16]

References

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  1. ^"Anybody home? Little response in Pacific gyre".NBC News.Associated Press.22 June 2009.Retrieved3 January2021.
  2. ^abcdefghijkD'Hondt, Steven; et al. (July 2009)."Subseafloor Sediment In South Pacific Gyre One Of Least Inhabited Places On Earth".Proceedings of the National Academy of Sciences of the United States of America.106(28): 11651–11656.Bibcode:2009PNAS..10611651D.doi:10.1073/pnas.0811793106.PMC2702254.PMID19561304.
  3. ^Inc, Pelmorex Weather Networks (27 July 2020)."What lives in the Pacific's 'ocean desert'".The Weather Network.Retrieved31 December2022.{{cite web}}:|last=has generic name (help)
  4. ^Montgomery, Hailey (28 July 2017)."South Pacific Ocean Gyre Holds Massive Garbage Patch".Pelmorex Weather Networks.The Weather Network. Archived fromthe originalon 28 November 2020.Retrieved14 August2017.
  5. ^Fischer, J.P., et al."Oxygen Penetration deep into the sediment of the South Pacific Gyre"Biogeoscience (Aug. 2009): 1467(6).
  6. ^Wu, Katherine J. (28 July 2020)."These Microbes May Have Survived 100 Million Years Beneath the Seafloor – Rescued from their cold, cramped and nutrient-poor homes, the bacteria awoke in the lab and grew".The New York Times.Retrieved31 July2020.
  7. ^Morono, Yuki; et al. (28 July 2020)."Aerobic microbial life persists in oxic marine sediment as old as 101.5 million years".Nature Communications.11(3626): 3626.Bibcode:2020NatCo..11.3626M.doi:10.1038/s41467-020-17330-1.PMC7387439.PMID32724059.
  8. ^abcSauvage, J; et al. (2013). "Radiolysis and life in deep subseafloor sediment of the South Pacific Gyre".Goldschmidt 2013 Conference Abstracts:2140.
  9. ^abcdefgBlair, CC; et al. (2007). "Radiolytic Hydrogen and Microbial Respiration in Subsurface Sediments".Astrobiology.7(6): 951–970.Bibcode:2007AsBio...7..951B.doi:10.1089/ast.2007.0150.PMID18163872.
  10. ^abcDzaugis, ME; et al. (2016)."Radiolytic Hydrogen Production in the Subseafloor Basaltic Aquifer".Frontiers in Microbiology.7:76.doi:10.3389/fmicb.2016.00076.PMC4740390.PMID26870029.
  11. ^Singleton P, Sainsbury D (2001). "Hydrogen-oxidizing bacteria (the 'hydrogen bacteria'; knallgas bacteria)".Dictionary of Microbiology and Molecular Biology.3rd ed.
  12. ^Amend JP, Shock EL (2001)."Energetics of overall metabolic reactions of thermophilic and hyperthermophilic Archaea and Bacteria".FEMS Microbiology Reviews.25(2): 175–243.doi:10.1111/j.1574-6976.2001.tb00576.x.PMID11250035.
  13. ^Claustre, Herve; Maritorena, Stephane (2003). "The many shades of ocean blue. (Ocean Science)".Science.302(5650): 1514–1515.doi:10.1126/science.1092704.PMID14645833.S2CID128518190.
  14. ^"South Pacific Gyre – Correntes Oceânicas"– via Google Sites.
  15. ^Barry, Carolyn (20 August 2009)."Plastic Breaks Down in Ocean, After All And Fast".National Geographic Society. Archived fromthe originalon August 26, 2009.
  16. ^abNield, David (25 July 2017)."There's Another Huge Plastic Garbage Patch in The Pacific Ocean".Sciencealert.ScienceAlert.

Further reading

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