South Pacific Gyre

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 extremely low primary productivity in the ocean surface and subsequently very 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 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. 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 In the surface layer of sediment cores from oligotrophic regions of the SPG, O2 is the primary electron acceptor used in microbial metabolisms. The O2 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 O2 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 O2 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 H2 (electron donor) is stoichiometrically balanced by the production of 0.5 O2 (electron acceptor), therefore a measurable flux in O2 is not expected in the substrate if both radiolysis of water and knallgas bacteria co-occur. So, despite the known occurrence of radiolytic H2 production, molecular hydrogen is below the detectable limit in the SPG cores, leading to the hypothesis that H2 is the primary energy source in low-organic seafloor sediments below the surface layer. == Water color ==

Satellite 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 ==