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The Southern Pacific Gyre is part of the Earth's system of rotating ocean currents, bounded by the
Equator to the north,
Australia to the west, the
Antarctic Circumpolar Current to the south, and
South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...
to the east.
The center of the South Pacific Gyre is the
oceanic pole of inaccessibility
Oceanic may refer to:
*Of or relating to the ocean
*Of or relating to Oceania
**Oceanic climate
**Oceanic languages
**Oceanic person or people, also called "Pacific Islander(s)"
Places
* Oceanic, British Columbia, a settlement on Smith Island, ...
, the site on Earth farthest from any continents and productive ocean regions and is regarded as Earth's largest oceanic desert.
The
gyre
In oceanography, a gyre () is any large system of circulating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity, horizontal friction and vertical friction dete ...
, as with Earth's other four gyres, contains an area with elevated concentrations of
pelagic plastics,
chemical sludge, and other
debris
Debris (, ) is rubble, wreckage, ruins, litter and discarded garbage/refuse/trash, scattered remains of something destroyed, or, as in geology, large rock fragments left by a melting glacier, etc. Depending on context, ''debris'' can refer to ...
known as the
South Pacific garbage patch
The South Pacific garbage patch is an area of ocean with increased levels of marine debris and plastic particle pollution, within the ocean's pelagic zone. This area is in the South Pacific Gyre, which itself spans from waters east of Australia ...
.
Sediment flux and accumulation
Earth's
trade winds and
Coriolis force cause the ocean currents in South
Pacific Ocean
The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contin ...
to circulate counterclockwise. The currents act to isolate the center of the gyre from nutrient
upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted surface water. The nut ...
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 the
prevailing winds
In meteorology, prevailing wind in a region of the Earth's surface is a surface wind that blows predominantly from a particular direction. The dominant winds are the trends in direction of wind with the highest speed over a particular point on ...
. The low levels of nutrients in the region result in extremely low
primary productivity
In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through c ...
in the ocean surface and subsequently very low flux of
organic material
Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
settling to the ocean floor as
marine snow
In the deep ocean, marine snow (also known as "ocean dandruff") is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to ...
. The low levels of
biogenic and eolian deposition cause sediments to accumulate on the ocean floor very slowly. In the center of the South Pacific Gyre, the sedimentation rate is 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 cause the water there to be the clearest seawater in the world.
Subseafloor biosphere
Beneath the seafloor, the
marine sediments and surrounding porewaters contain an unusual
subseafloor biosphere. Despite extremely low amounts of buried organic material,
microbes
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
live throughout the entire sediment column. Average cell abundances and net rates of
respiration
Respiration may refer to:
Biology
* Cellular respiration, the process in which nutrients are converted into useful energy in a cell
** Anaerobic respiration, cellular respiration without oxygen
** Maintenance respiration, the amount of cellul ...
are a few orders of magnitude lower than in any other
subseafloor biosphere previously studied.
The South Pacific Gyre subseafloor community is also unusual because it contains
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
throughout 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. In July 2020,
marine biologists reported that
aerobic
Aerobic means "requiring air," in which "air" usually means oxygen.
Aerobic may also refer to
* Aerobic exercise, prolonged exercise of moderate intensity
* Aerobics, a form of aerobic exercise
* Aerobic respiration, the aerobic process of cel ...
microorganisms (mainly), in "quasi-
suspended animation
Suspended animation is the temporary (short- or long-term) slowing or stopping of biological function so that physiological capabilities are preserved. It may be either hypometabolic or ametabolic in nature. It may be induced by either endogen ...
", were found in
organically poor sediments, up to 101.5 million years old, 250 feet below the
seafloor
The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean
The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth an ...
of the region and could be the
longest-living life forms ever found.
Radiolytic H2: a benthic energy source
Benthic microbes in organic-poor sediments in oligotrophic oceanic regions, such as the South Pacific Gyre, are hypothesized to metabolize
radiolytic hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
(H
2) as a primary energy source.
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
with ≥ 0.14 mg chlorophyll per m
3.
Carbon exported to the underlying deep ocean sediments via the
biological pump
The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sediments.Sigman DM & GH ...
is limited in the SPG, resulting in sedimentation rates that are orders of magnitude lower than in productive zones, e.g. continental margins.
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 (H
2).
Radiolysis of interstitial water
Radioactive decay of naturally occurring
uranium (
238U and
235U),
thorium (
232Th), and
potassium (
40K) in seafloor sediments collectively bombard the interstitial water with
α,
β, and
γ radiation. The irradiation ionizes and breaks apart water molecules, eventually yielding H
2. The products of this reaction are aqueous electrons (e
−aq), hydrogen radicals (H·), protons (H
+), and hydroxyl radicals (OH·).
The radicals are highly reactive, therefore short-lived, and recombine to produce hydrogen peroxide (H
2O
2), and molecular hydrogen (H
2).
The amount of radiolytic H
2 production 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 H
2 production relative to other seafloor strata.
Also, radiolytic H
2 production has been measured in seawater intrusions into subseafloor basement basalts.
Microbial activity
The microbes best suited to utilize radiolytic H
2 are the knallgas bacteria,
lithoautotrophes, that obtain energy by oxidizing molecular hydrogen via the
knallgas reaction:
H
2 (aq) + 0.5O
2 (aq) H
2O (l)
In the surface layer of sediment cores from oligotrophic regions of the SPG, O
2 is the primary electron acceptor used in microbial metabolisms. The O
2 concentrations 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 O
2 in 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 O
2 and 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.
The production of radiolytic H
2 (electron donor) is stoichiometrically balanced with production of 0.5 O
2 (electron acceptor), therefore a measurable flux in O
2 is not expected in the substrate if both radiolysis of water and knallgas bacteria co-occur.
So, despite the known occurrence of radiolytic H
2 production, molecular hydrogen is below the detectable limit in the SPG cores, leading to the hypothesis that H
2 is the primary energy source in low-organic seafloor sediments below the surface layer.
Water color
Satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioi ...
data 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 living
phytoplankton. 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.
Garbage patch
References
Further reading
*
{{Ocean
South Pacific
Oceanic gyres