Ocean chemistry

Total Molar Composition of Seawater (Salinity = 35)[1]
Component Concentration (mol/kg)
H
2
O
53.6
Cl
0.546
Na+
0.469
Mg2+
0.0528
SO2−
4
0.0282
Ca2+
0.0103
K+
0.0102
CT 0.00206
Br
0.000844
BT (total boron) 0.000416
Sr2+
0.000091
F
0.000068

Ocean chemistry, also known as marine chemistry, is influenced by plate tectonics and seafloor spreading, turbidity currents, sediments, pH levels, atmospheric constituents, metamorphic activity, and ecology. The field of chemical oceanography studies the chemistry of marine environments including the influences of different variables. Marine life has adapted to the chemistries unique to earth's oceans, and marine ecosystems are sensitive to changes in ocean chemistry.

The impact of human activity on the chemistry of the earth's oceans has increased over time, with pollution from industry and various land-use practices significantly affecting the oceans. Moreover, increasing levels of carbon dioxide in the earth's atmosphere have led to ocean acidification, which has negative effects on marine ecosystems. The international community has agreed that restoring the chemistry of the oceans is a priority, and efforts toward this goal are tracked as part of Sustainable Development Goal 14.

Marine chemistry on earthEdit

Organic compounds in the oceansEdit

Colored dissolved organic matter (CDOM) is estimated to range 20-70% of carbon content of the oceans, being higher near river outlets and lower in the open ocean.[2]

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.[3]

Chemical ecology of extremophilesEdit

 
A diagram showing ocean chemistry around deep sea hydrothermal vents

The ocean provides special marine environments inhabited by extremophiles that thrive under unusual conditions of temperature, pressure, and darkness. Such environments include hydrothermal vents and black smokers and cold seeps on the ocean floor, with entire ecosystems of organisms that have a symbiotic relationship with compounds that provided energy through a process called chemosynthesis.

Plate tectonicsEdit

 
Magnesium to calcium ratio changes associated with hydrothermal activity at mid-ocean ridge locations

Seafloor spreading on mid-ocean ridges is a global scale ion-exchange system.[4] 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.[5]

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.[6] A lower Mg/Ca ratio favors the precipitation of low-Mg calcite polymorphs of calcium carbonate (calcite seas).[4]

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 (aragonite seas).[4]

Experiments show that most modern high-Mg calcite organisms would have been low-Mg calcite in past calcite seas,[7] 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.[6][7]

Human impactsEdit

Marine pollutionEdit

Marine pollution occurs when substances used or spread by humans, such as industrial, agricultural and residential waste, particles, noise, excess carbon dioxide or invasive organisms enter the ocean and cause harmful effects there. The majority of this waste (80%) comes from land-based activity, although marine transportation significantly contributes as well.[8] Since most inputs come from land, either via the rivers, sewage or the atmosphere, it means that continental shelves are more vulnerable to pollution. Air pollution is also a contributing factor by carrying off iron, carbonic acid, nitrogen, silicon, sulfur, pesticides or dust particles into the ocean.[9] The pollution often comes from nonpoint sources such as agricultural runoff, wind-blown debris, and dust. These nonpoint sources are largely due to runoff that enters the ocean through rivers, but wind-blown debris and dust can also play a role, as these pollutants can settle into waterways and oceans.[10] Pathways of pollution include direct discharge, land runoff, ship pollution, atmospheric pollution and, potentially, deep sea mining.

The types of marine pollution can be grouped as pollution from marine debris, plastic pollution, including microplastics, ocean acidification, nutrient pollution, toxins and underwater noise. Plastic pollution in the ocean is a type of marine pollution by plastics, ranging in size from large original material such as bottles and bags, down to microplastics formed from the fragmentation of plastic material. Marine debris is mainly discarded human rubbish which floats on, or is suspended in the ocean. Plastic pollution is harmful to marine life.

Climate changeEdit

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.[11] 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".[12][13][14]

Ocean acidificationEdit

Ocean acidification is the ongoing decrease in the pH value of the Earth's oceans, caused by the uptake of carbon dioxide (CO
2
) from the atmosphere.[15] The main cause of ocean acidification is the burning of fossil fuels. Ocean acidification is one of several effects of climate change on oceans. Seawater is slightly basic (meaning pH > 7), and ocean acidification involves a shift towards pH-neutral conditions rather than a transition to acidic conditions (pH < 7).[16] The concern with ocean acidification is that it can lead to the decreased production of the shells of shellfish and other aquatic life with calcium carbonate shells, as well as some other physiological challenges for marine organisms. The calcium carbonate shelled organisms can not reproduce under high saturated acidotic waters. An estimated 30–40% of the carbon dioxide from human activity released into the atmosphere dissolves into oceans, rivers and lakes.[17][18] Some of it reacts with the water to form carbonic acid. Some of the resulting carbonic acid molecules dissociate into a bicarbonate ion and a hydrogen ion, thus increasing ocean acidity (H+ ion concentration).

Between 1751 and 1996, the pH value of the ocean surface is estimated to have decreased from approximately 8.25 to 8.14,[19] representing an increase of almost 30% in H+ ion concentration in the world's oceans (note the pH scale is logarithmic so a change of one in pH unit is equivalent to a ten fold change in H+ ion concentration).[20][21] Increasing acidity is thought to have a range of potentially harmful consequences for marine organisms such as depressing metabolic rates and immune responses in some organisms and causing coral bleaching.[22] By increasing the presence of free hydrogen ions, the additional carbonic acid that forms in the oceans ultimately results in the conversion of carbonate ions into bicarbonate ions. Ocean alkalinity (roughly equal to [HCO3] + 2[CO32−]) is not changed by the process, or may increase over long time periods due to carbonate dissolution.[23] This net decrease in the amount of carbonate ions available may make it more difficult for marine calcifying organisms, such as coral and some plankton, to form biogenic calcium carbonate, and such structures become vulnerable to dissolution.[24] Ongoing acidification of the oceans may threaten future food chains linked with the oceans.[25][26]

Marine chemistry on other planets and their moonsEdit

A planetary scientist using data from the Cassini spacecraft has been researching the marine chemistry of Saturn's moon Enceladus using geochemical models to look at changes through time.[27] 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".[27][28]

See alsoEdit

ReferencesEdit

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