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 to the east. The center of the South Pacific Gyre is the oceanic pole of inaccessibility, the site on Earth furthest from any continents and productive ocean regions and is regarded as Earth's largest oceanic desert. With an area of 37 million square kilometres, it makes up roughly 10% of the Earth's ocean surface. The gyre, as with Earth's other four gyres, contains an area with elevated concentrations of pelagic plastics, chemical sludge, and other debris known as the South Pacific garbage patch.
Sediment flux and accumulation Earth's trade winds and Coriolis force cause the ocean currents in South Pacific Ocean to circulate counterclockwise. The currents act to isolate the center of the gyre from nutrient upwelling, 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. The low levels of nutrients in the region result in very low primary productivity in the ocean surface and subsequently extremely low flux of organic material settling to the ocean floor as marine snow. 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 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.
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 live throughout the entire sediment column. Average cell abundances and net rates of respiration are a few orders of magnitude lower than in any other subseafloor biosphere previously studied. The South Pacific Gyre subseafloor community is also unusual since it contains oxygen 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 microorganisms (mainly), in "quasi-suspended animation", were found in organically poor sediments, up to 101.5 million years old, 250 feet below the seafloor 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 (H2) 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 m3. Carbon exported to the underlying deep ocean sediments via the biological pump is limited in the SPG, resulting in sedimentation rates that are orders of magnitude lower than in productive zones, e.g. continental margins. Deep-ocean benthic microbial life typically 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).
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 H2. 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 (H2O2), and molecular hydrogen (H2). The amount of radiolytic H2 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 H2 production relative to other seafloor strata. Also, radiolytic H2 production has been measured in seawater intrusions into subseafloor basement basalts.
Microbial activity The microbes best suited to utilize radiolytic H2 are the knallgas bacteria, lithoautotrophes, that obtain energy by oxidizing molecular hydrogen via the knallgas reaction:
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