is estimated to range 20-70% of carbon content of the oceans, being higher near river outlets and lower in the open ocean. Marine life is largely similar in biochemistry to terrestrial organisms, except that they inhabit a saline environment. One consequence of their adaptation is that marine organisms are the most prolific source of halogenated organic compounds.
Seafloor spreading on mid-ocean ridges is a global scale ion-exchange system. Hydrothermal vents at spreading centers introduce various amounts of iron, sulfur, manganese, silicon and other elements into the ocean, some of which are recycled into the ocean crust. Helium-3, an isotope that accompanies volcanism from the mantle, is emitted by hydrothermal vents and can be detected in plumes within the ocean. Spreading rates on mid-ocean ridges vary between 10 and 200 mm/yr. Rapid spreading rates cause increased basalt reactions with seawater. The magnesium/calcium ratio will be lower because more magnesium ions are being removed from seawater and consumed by the rock, and more calcium ions are being removed from the rock and released to seawater. Hydrothermal activity at ridge crest is efficient in removing magnesium. A lower Mg/Ca ratio favors the precipitation of low-Mg calcite polymorphs of calcium carbonate. Slow spreading at mid-ocean ridges has the opposite effect and will result in a higher Mg/Ca ratio favoring the precipitation of aragonite and high-Mg calcite polymorphs of calcium carbonate. Experiments show that most modern high-Mg calcite organisms would have been low-Mg calcite in past calcite seas, meaning that the Mg/Ca ratio in an organism's skeleton varies with the Mg/Ca ratio of the seawater in which it was grown. The mineralogy of reef-building and sediment-producing organisms is thus regulated by chemical reactions occurring along the mid-ocean ridge, the rate of which is controlled by the rate of sea-floor spreading.
Climate change
Increased carbon dioxide levels, resulting from anthropogenic factors or otherwise, have the potential to impact ocean chemistry. Global warming and changes in salinity have significant implications for ecology of marine environments. One proposal suggests dumping massive amounts of lime, a base, to reverse the acidification and "increase the sea's ability to absorb carbon dioxide from the atmosphere".
A planetary scientist using data from the Cassini spacecraft has been researching the marine chemistry of Saturn's moonEnceladus using geochemical models to look at changes through time. The presence of salts may indicate a liquid ocean within the moon, raising the possibility of the existence of life, "or at least for the chemical precursors for organic life".
