Biogeochemical cycles

Marine biogeochemical cycles

The dominant feature of the planet viewed from space is water – oceans of liquid water flood most of the surface while water vapour swirls in atmospheric clouds and the poles are capped with ice.

Taken as a whole, the oceans form a single marine system where water – the "universal solvent" ^[418] – dissolves nutrients and substances containing elements such as oxygen, carbon, nitrogen and phosphorus. These substances are endlessly cycled and recycled, chemically combined and then broken down again, dissolved and then precipitated or evaporated, imported from and exported back to the land and the atmosphere and the ocean floor. Powered both by the biological activity of marine organisms and by the natural actions of the sun and tides and movements within the Earth's crust, these are the marine biogeochemical cycles.^[419]^[420]

Marine nitrogen cycle
Marine phosphorus cycle

Sediments and biogenic ooze

Sediments at the bottom of the ocean have two main origins, terrigenous and biogenous. Terrigenous sediments account for about 45% of the total marine sediment, and originate in the erosion of rocks on land, transported by rivers and land runoff, windborne dust, volcanoes, or grinding by glaciers.

Biogenous sediments account for the other 55% of the total sediment, and originate in the skeletal remains of marine protists (single-celled plankton and benthos organisms). Much smaller amounts of precipitated minerals and meteoric dust can also be present. Ooze, in the context of a marine sediment, does not refer to the consistency of the sediment but to its biological origin. The term ooze was originally used by John Murray, the "father of modern oceanography", who proposed the term radiolarian ooze for the silica deposits of radiolarian shells brought to the surface during the Challenger Expedition.^[422] A biogenic ooze is a pelagic sediment containing at least 30 percent from the skeletal remains of marine organisms.

More information Main types of biogenic ooze, type ...

Main types of biogenic ooze
type	mineral forms	protist responsible	name of skeleton	description
Siliceous ooze	SiO₂ quartz glass opal chert	diatoms	frustule	Individual diatoms range in size from 0.002 to 0.2 mm.^[423]
Siliceous ooze	SiO₂ quartz glass opal chert	radiolarians	skeleton	Radiolarians are protozoa with diameters typically between 0.1 and 0.2 mm that produce intricate mineral skeletons, usually made of silica
Calcareous ooze	CaCO₃ calcite aragonite limestone chalk	foraminiferans	test	There are about 10,000 living species of foraminiferans,^[424] usually under 1 mm in size.
		coccolithophores	coccolith	Coccolithophores are spherical cells usually less than 0.1 mm across, enclosed by calcareous plates called coccoliths.^[425] Coccoliths are important microfossils. They are the largest global source of biogenic calcium carbonate, and make significant contributions to the global carbon cycle.^[426] They are the main constituent of chalk deposits such as the white cliffs of Dover.

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An elaborate mineral skeleton of a radiolarian made of silica.
Diatoms, major components of marine plankton, also have silica skeletons called frustules.
Coccolithophores have plates or scales made with calcium carbonate called coccoliths
Calcified test of a planktic foraminiferan

A diatom microfossil from 40 million years ago
Diatomaceous earth is a soft, siliceous, sedimentary rock made up of microfossils in the form of the frustules (shells) of single cell diatoms (click to magnify).
Illustration of a Globigerina ooze
Shells (tests), usually made of calcium carbonate, from a foraminiferal ooze on the deep ocean floor

Land interactions

Land interactions impact marine life in many ways. Coastlines typically have continental shelves extending some way from the shore. These provide extensive shallows sunlit down to the seafloor, allowing for photosynthesis and enabling habitats for seagrass meadows, coral reefs, kelp forests and other benthic life. Further from shore the continental shelf slopes towards deep water. Wind blowing at the ocean surface or deep ocean currents can result in cold and nutrient rich waters from abyssal depths moving up the continental slopes. This can result in upwellings along the outer edges of continental shelves, providing conditions for phytoplankton blooms.

Water evaporated by the sun from the surface of the ocean can precipitate on land and eventually return to the ocean as runoff or discharge from rivers, enriched with nutrients as well as pollutants. As rivers discharge into estuaries, freshwater mixes with saltwater and becomes brackish. This provides another shallow water habitat where mangrove forests and estuarine fish thrive. Overall, life in inland lakes can evolve with greater diversity than happens in the sea, because freshwater habitats are themselves diverse and compartmentalised in a way marine habitats are not. Some aquatic life, such as salmon and eels, migrate back and forth between freshwater and marine habitats. These migrations can result in exchanges of pathogens and have impacts on the way life evolves in the ocean.