Marine chemistry

Last updated
Total Molar Composition of Seawater (Salinity = 35) [1]
ComponentConcentration (mol/kg)
H
2O 		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

Marine chemistry, also known as ocean chemistry or chemical oceanography , is the study of chemical content in marine environments as influenced by plate tectonics and seafloor spreading, turbidity, currents, sediments, pH levels, atmospheric constituents, metamorphic activity, and ecology. Marine life has adapted to the chemistries unique to Earth's oceans, and marine ecosystems are sensitive to changes in ocean chemistry.

Contents

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.

Chemical oceanography is the study of the chemistry of Earth's oceans. An interdisciplinary field, chemical oceanographers study the distributions and reactions of both naturally occurring and anthropogenic chemicals from molecular to global scales. [2]

Due to the interrelatedness of the ocean, chemical oceanographers frequently work on problems relevant to physical oceanography, geology and geochemistry, biology and biochemistry, and atmospheric science. Many chemical oceanographers investigate biogeochemical cycles, and the marine carbon cycle in particular attracts significant interest due to its role in carbon sequestration and ocean acidification. [3] Other major topics of interest include analytical chemistry of the oceans, marine pollution, and anthropogenic climate change.

Organic compounds in the oceans

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. [4]

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. [5]

Chemical ecology of extremophiles

See also: Extremophile

The ocean is home to a variety of marine organisms known as extremophiles - organisms that thrive in extreme conditions of temperature, pressure, and light availability. Extremophiles inhabit many unique habitats in the ocean, such as hydrothermal vents, black smokers, cold seeps, hypersaline regions, and sea ice brine pockets. Some scientists have speculated that life may have evolved from hydrothermal vents in the ocean.

A diagram showing ocean chemistry around deep sea hydrothermal vents Deep Sea Vent Chemistry Diagram.svg
A diagram showing ocean chemistry around deep sea hydrothermal vents

In hydrothermal vents and similar environments, many extremophiles acquire energy through chemoautotrophy, using chemical compounds as energy sources, rather than light as in photoautotrophy. Hydrothermal vents enrich the nearby environment in chemicals such as elemental sulfur, H2, H2S, Fe2+, and methane. Chemoautotrophic organisms, primarily prokaryotes, derive energy from these chemicals through redox reactions. These organisms then serve as food sources for higher trophic levels, forming the basis of unique ecosystems.

Several different metabolisms are present in hydrothermal vent ecosystems. Many marine microorganisms, including Thiomicrospira, Halothiobacillus, and Beggiatoa , are capable of oxidizing sulfur compounds, including elemental sulfur and the often toxic compound H2S. H2S is abundant in hydrothermal vents, formed through interactions between seawater and rock at the high temperatures found within vents. This compound is a major energy source, forming the basis of the sulfur cycle in hydrothermal vent ecosystems. In the colder waters surrounding vents, sulfur-oxidation can occur using oxygen as an electron acceptor; closer to the vents, organisms must use alternate metabolic pathways or utilize another electron acceptor, such as nitrate. Some species of Thiomicrospira can utilize thiosulfate as an electron donor, producing elemental sulfur. Additionally, many marine microorganisms are capable of iron-oxidation, such as Mariprofundus ferrooxydans. Iron-oxidation can be oxic, occurring in oxygen-rich parts of the ocean, or anoxic, requiring either an electron acceptor such as nitrate or light energy. In iron-oxidation, Fe(II) is used as an electron donor; conversely, iron-reducers utilize Fe(III) as an electron acceptor. These two metabolisms form the basis of the iron-redox cycle and may have contributed to banded iron formations.

At another extreme, some marine extremophiles inhabit sea ice brine pockets where temperature is very low and salinity is very high. Organisms trapped within freezing sea ice must adapt to a rapid change in salinity up to 3 times higher than that of regular seawater, as well as the rapid change to regular seawater salinity when ice melts. Most brine-pocket dwelling organisms are photosynthetic, therefore, these microenvironments can become hyperoxic, which can be toxic to its inhabitants. Thus, these extremophiles often produce high levels of antioxidants. [6]

Plate tectonics

Magnesium to calcium ratio changes associated with hydrothermal activity at mid-ocean ridge locations MgCaRatioChanges.jpg
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. [7] 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. [8]

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. [9] A lower Mg/Ca ratio favors the precipitation of low-Mg calcite polymorphs of calcium carbonate (calcite seas). [7]

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). [7]

Experiments show that most modern high-Mg calcite organisms would have been low-Mg calcite in past calcite seas, [10] 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. [9] [10]

Human impacts

Marine pollution

This section is an excerpt from Marine pollution.[ edit ]

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. [11] It is a combination of chemicals and trash, most of which comes from land sources and is washed or blown into the ocean. This pollution results in damage to the environment, to the health of all organisms, and to economic structures worldwide. [12] 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. [13] 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. [14] Pathways of pollution include direct discharge, land runoff, ship pollution, bilge 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 change

Increased carbon dioxide levels, mostly from burning fossil fuels, are changing ocean chemistry. Global warming and changes in salinity [15] have significant implications for the ecology of marine environments. [16]

Acidification

This section is an excerpt from Ocean acidification.[ edit ]

Ocean acidification is the ongoing decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05. [17] Carbon dioxide emissions from human activities are the primary cause of ocean acidification, with atmospheric carbon dioxide (CO2) levels exceeding 410 ppm (in 2020). CO2 from the atmosphere is absorbed by the oceans. This chemical reaction produces carbonic acid (H2CO3) which dissociates into a bicarbonate ion (HCO−3) and a hydrogen ion (H+). The presence of free hydrogen ions (H+) lowers the pH of the ocean, increasing acidity (this does not mean that seawater is acidic yet; it is still alkaline, with a pH higher than 8). Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons. [18]

A change in pH by 0.1 represents a 26% increase in hydrogen ion concentration in the world's oceans (the pH scale is logarithmic, so a change of one in pH units is equivalent to a tenfold change in hydrogen ion concentration). Sea-surface pH and carbonate saturation states vary depending on ocean depth and location. Colder and higher latitude waters are capable of absorbing more CO2. This can cause acidity to rise, lowering the pH and carbonate saturation levels in these areas. Other factors that influence the atmosphere-ocean CO2 exchange, and thus local ocean acidification, include: ocean currents and upwelling zones, proximity to large continental rivers, sea ice coverage, and atmospheric exchange with nitrogen and sulfur from fossil fuel burning and agriculture. [19] [20] [21]

Deoxygenation

This section is an excerpt from Ocean deoxygenation.[ edit ]
Global map of low and declining oxygen levels in coastal waters (mainly due to eutrophication) and in the open ocean (due to climate change). The map indicates coastal sites where oxygen levels have declined to less than 2 mg/L (red dots), as well as expanding ocean oxygen minimum zones at 300 metres (blue shaded regions). Global areas of hypoxia.jpg
Global map of low and declining oxygen levels in coastal waters (mainly due to eutrophication) and in the open ocean (due to climate change). The map indicates coastal sites where oxygen levels have declined to less than 2 mg/L (red dots), as well as expanding ocean oxygen minimum zones at 300 metres (blue shaded regions).

Ocean deoxygenation is the reduction of the oxygen content in different parts of the ocean due to human activities. [23] [24] It occurs firstly in coastal zones where eutrophication has driven some quite rapid (in a few decades) declines in oxygen to very low levels. [23] This type of ocean deoxygenation is also called "dead zones". Secondly, there is now an ongoing reduction in oxygen levels in the open ocean: naturally occurring low oxygen areas (so called oxygen minimum zones (OMZs)) are now expanding slowly. [25] This expansion is happening as a consequence of human caused climate change. [26] [27] The resulting decrease in oxygen content of the oceans poses a threat to marine life, as well as to people who depend on marine life for nutrition or livelihood. [28] [29] [30] Ocean deoxygenation poses implications for ocean productivity, nutrient cycling, carbon cycling, and marine habitats. [31] [32]

Ocean warming exacerbates ocean deoxygenation and further stresses marine organisms, reducing nutrient availability by increasing ocean stratification through density and solubility effects while at the same time increasing metabolic demand. [33] [34] The rising temperatures in the oceans cause a reduced solubility of oxygen in the water, which can explain about 50% of oxygen loss in the upper level of the ocean (>1000 m). Warmer ocean water holds less oxygen and is more buoyant than cooler water. This leads to reduced mixing of oxygenated water near the surface with deeper water, which naturally contains less oxygen. Warmer water also raises oxygen demand from living organisms; as a result, less oxygen is available for marine life. [35]

Studies have shown that oceans have already lost 1-2% of their oxygen since the middle of the 20th century, [36] [37] and model simulations predict a decline of up to 7% in the global ocean O2 content over the next hundred years. The decline of oxygen is projected to continue for a thousand years or more. [38]

History

HMS Challenger (1858) HMS Challenger (1858).jpg
HMS Challenger (1858)

Early inquiries into marine chemistry usually concerned the origin of salinity in the ocean, including work by Robert Boyle. Modern chemical oceanography began as a field with the 1872–1876 Challenger expedition, which made the first systematic measurements of ocean chemistry.

Tools

Chemical oceanographers collect and measure chemicals in seawater, using the standard toolset of analytical chemistry as well as instruments like pH meters, electrical conductivity meters, fluorometers, and dissolved CO₂ meters. Most data are collected through shipboard measurements and from autonomous floats or buoys, but remote sensing is used as well. On an oceanographic research vessel, a CTD is used to measure electrical conductivity, temperature, and pressure, and is often mounted on a rosette of Nansen bottles to collect seawater for analysis. Sediments are commonly studied with a box corer or a sediment trap, and older sediments may be recovered by scientific drilling.

Marine chemistry on other planets and their moons

The chemistry of the subsurface ocean of Europa may be Earthlike. [39] The subsurface ocean of Enceladus vents hydrogen and carbon dioxide to space. [40]

See also

Related Research Articles

<span class="mw-page-title-main">Salinity</span> Proportion of salt dissolved in water

Salinity is the saltiness or amount of salt dissolved in a body of water, called saline water. It is usually measured in g/L or g/kg.

<span class="mw-page-title-main">Lysocline</span> Depth in the ocean below which the rate of dissolution of calcite increases dramatically

The lysocline is the depth in the ocean dependent upon the carbonate compensation depth (CCD), usually around 5 km, below which the rate of dissolution of calcite increases dramatically because of a pressure effect. While the lysocline is the upper bound of this transition zone of calcite saturation, the CCD is the lower bound of this zone.

<span class="mw-page-title-main">Seawater</span> Water from a sea or an ocean

Seawater, or sea water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5%. This means that every kilogram of seawater has approximately 35 grams (1.2 oz) of dissolved salts. The average density at the surface is 1.025 kg/L. Seawater is denser than both fresh water and pure water because the dissolved salts increase the mass by a larger proportion than the volume. The freezing point of seawater decreases as salt concentration increases. At typical salinity, it freezes at about −2 °C (28 °F). The coldest seawater still in the liquid state ever recorded was found in 2010, in a stream under an Antarctic glacier: the measured temperature was −2.6 °C (27.3 °F).

<span class="mw-page-title-main">Hydrothermal vent</span> Fissure in a planets surface from which heated water emits

Hydrothermal vents are fissures on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspots. Hydrothermal deposits are rocks and mineral ore deposits formed by the action of hydrothermal vents.

<span class="mw-page-title-main">Alkalinity</span> Capacity of water to resist changes in pH that would make the water more acidic

Alkalinity (from Arabic: القلوية, romanized: al-qaly, lit. 'ashes of the saltwort') is the capacity of water to resist acidification. It should not be confused with basicity, which is an absolute measurement on the pH scale. Alkalinity is the strength of a buffer solution composed of weak acids and their conjugate bases. It is measured by titrating the solution with an acid such as HCl until its pH changes abruptly, or it reaches a known endpoint where that happens. Alkalinity is expressed in units of concentration, such as meq/L (milliequivalents per liter), μeq/kg (microequivalents per kilogram), or mg/L CaCO3 (milligrams per liter of calcium carbonate). Each of these measurements corresponds to an amount of acid added as a titrant.

<span class="mw-page-title-main">Lost City Hydrothermal Field</span> Hydrothermal field in the mid-Atlantic Ocean

The Lost City Hydrothermal Field, often referred to simply as Lost City, is an area of marine alkaline hydrothermal vents located on the Atlantis Massif at the intersection between the Mid-Atlantic Ridge and the Atlantis Transform Fault, in the Atlantic Ocean. It is a long-lived site of active and inactive ultramafic-hosted serpentinization, abiotically producing many simple molecules such as methane and hydrogen which are fundamental to microbial life. As such it has generated scientific interest as a prime location for investigating the origin of life on Earth and other planets similar to it.

<span class="mw-page-title-main">Ocean acidification</span> Decrease of pH levels in the ocean

Ocean acidification is the ongoing decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05. Carbon dioxide emissions from human activities are the primary cause of ocean acidification, with atmospheric carbon dioxide levels exceeding 410 ppm. CO2 from the atmosphere is absorbed by the oceans. This chemical reaction produces carbonic acid which dissociates into a bicarbonate ion and a hydrogen ion. The presence of free hydrogen ions lowers the pH of the ocean, increasing acidity. Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.

<span class="mw-page-title-main">Calcite sea</span> Sea chemistry favouring low-magnesium calcite as the inorganic calcium carbonate precipitate

A calcite sea is a sea in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate. An aragonite sea is the alternate seawater chemistry in which aragonite and high-magnesium calcite are the primary inorganic carbonate precipitates. The Early Paleozoic and the Middle to Late Mesozoic oceans were predominantly calcite seas, whereas the Middle Paleozoic through the Early Mesozoic and the Cenozoic are characterized by aragonite seas.

<span class="mw-page-title-main">Aragonite sea</span> Chemical conditions of the sea favouring aragonite deposition

An aragonite sea contains aragonite and high-magnesium calcite as the primary inorganic calcium carbonate precipitates. The chemical conditions of the seawater must be notably high in magnesium content relative to calcium for an aragonite sea to form. This is in contrast to a calcite sea in which seawater low in magnesium content relative to calcium favors the formation of low-magnesium calcite as the primary inorganic marine calcium carbonate precipitate.

<span class="mw-page-title-main">Ocean</span> Body of salt water covering the majority of Earth

The ocean is the body of salt water that covers ~70.8% of the Earth. In English, the term ocean also refers to any of the large bodies of water into which the world ocean is conventionally divided. Distinct names are used to identify five different areas of the ocean: Pacific, Atlantic, Indian, Antarctic/Southern, and Arctic. The ocean contains 97% of Earth's water and is the primary component of the Earth's hydrosphere, thus the ocean is essential to life on Earth. The ocean influences climate and weather patterns, the carbon cycle, and the water cycle by acting as a huge heat reservoir.

<span class="mw-page-title-main">Oceanic carbon cycle</span> Ocean/atmosphere carbon exchange process

The oceanic carbon cycle is composed of processes that exchange carbon between various pools within the ocean as well as between the atmosphere, Earth interior, and the seafloor. The carbon cycle is a result of many interacting forces across multiple time and space scales that circulates carbon around the planet, ensuring that carbon is available globally. The Oceanic carbon cycle is a central process to the global carbon cycle and contains both inorganic carbon and organic carbon. Part of the marine carbon cycle transforms carbon between non-living and living matter.

<span class="mw-page-title-main">Shell growth in estuaries</span>

Shell growth in estuaries is an aspect of marine biology that has attracted a number of scientific research studies. Many groups of marine organisms produce calcified exoskeletons, commonly known as shells, hard calcium carbonate structures which the organisms rely on for various specialized structural and defensive purposes. The rate at which these shells form is greatly influenced by physical and chemical characteristics of the water in which these organisms live. Estuaries are dynamic habitats which expose their inhabitants to a wide array of rapidly changing physical conditions, exaggerating the differences in physical and chemical properties of the water.

<span class="mw-page-title-main">Marine biogeochemical cycles</span>

Marine biogeochemical cycles are biogeochemical cycles that occur within marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. These biogeochemical cycles are the pathways chemical substances and elements move through within the marine environment. In addition, substances and elements can be imported into or exported from the marine environment. These imports and exports can occur as exchanges with the atmosphere above, the ocean floor below, or as runoff from the land.

<span class="mw-page-title-main">Marine biogenic calcification</span> Shell formation mechanism

Marine biogenic calcification is the production of calcium carbonate by organisms in the global ocean.

<span class="mw-page-title-main">Justin B. Ries</span> American marine scientist

Justin Baker Ries is an American marine scientist, best known for his contributions to ocean acidification, carbon sequestration, and biomineralization research.

<span class="mw-page-title-main">Hydrothermal vent microbial communities</span> Undersea unicellular organisms

The hydrothermal vent microbial community includes all unicellular organisms that live and reproduce in a chemically distinct area around hydrothermal vents. These include organisms in the microbial mat, free floating cells, or bacteria in an endosymbiotic relationship with animals. Chemolithoautotrophic bacteria derive nutrients and energy from the geological activity at Hydrothermal vents to fix carbon into organic forms. Viruses are also a part of the hydrothermal vent microbial community and their influence on the microbial ecology in these ecosystems is a burgeoning field of research.

<span class="mw-page-title-main">Ocean acidification in the Arctic Ocean</span>

The Arctic ocean covers an area of 14,056,000 square kilometers, and supports a diverse and important socioeconomic food web of organisms, despite its average water temperature being 32 degrees Fahrenheit. Over the last three decades, the Arctic Ocean has experienced drastic changes due to climate change. One of the changes is in the acidity levels of the ocean, which have been consistently increasing at twice the rate of the Pacific and Atlantic oceans. Arctic Ocean acidification is a result of feedback from climate system mechanisms, and is having negative impacts on Arctic Ocean ecosystems and the organisms that live within them.

<span class="mw-page-title-main">Human impact on marine life</span>

Human activities affect marine life and marine habitats through overfishing, habitat loss, the introduction of invasive species, ocean pollution, ocean acidification and ocean warming. These impact marine ecosystems and food webs and may result in consequences as yet unrecognised for the biodiversity and continuation of marine life forms.

<span class="mw-page-title-main">Total inorganic carbon</span> Sum of the inorganic carbon species

Total inorganic carbon is the sum of the inorganic carbon species.

<span class="mw-page-title-main">Particulate inorganic carbon</span>

Particulate inorganic carbon (PIC) can be contrasted with dissolved inorganic carbon (DIC), the other form of inorganic carbon found in the ocean. These distinctions are important in chemical oceanography. Particulate inorganic carbon is sometimes called suspended inorganic carbon. In operational terms, it is defined as the inorganic carbon in particulate form that is too large to pass through the filter used to separate dissolved inorganic carbon.

References

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  29. ↑ Long, Matthew C.; Deutsch, Curtis; Ito, Taka (2016). "Finding forced trends in oceanic oxygen". Global Biogeochemical Cycles. 30 (2): 381–397. Bibcode:2016GBioC..30..381L. doi: 10.1002/2015GB005310 . ISSN   0886-6236. S2CID   130885459.
  30. ↑ Pearce, Rosamund (2018-06-15). "Guest post: How global warming is causing ocean oxygen levels to fall". Carbon Brief. Retrieved 2023-07-04.
  31. ↑ Harvey, Fiona (2019-12-07). "Oceans losing oxygen at unprecedented rate, experts warn". The Guardian. ISSN   0261-3077 . Retrieved 2019-12-07.
  32. ↑ Laffoley, D. & Baxter, J.M. (eds.) (2019). Ocean deoxygenation: Everyone's problem - Causes, impacts, consequences and solutions. IUCN, Switzerland.
  33. ↑ Bednaršek, N., Harvey, C.J., Kaplan, I.C., Feely, R.A. and Možina, J. (2016) "Pteropods on the edge: Cumulative effects of ocean acidification, warming, and deoxygenation". Progress in Oceanography, 145: 1–24. doi:10.1016/j.pocean.2016.04.002
  34. ↑ Keeling, Ralph F., and Hernan E. Garcia (2002) "The change in oceanic O2 inventory associated with recent global warming." Proceedings of the National Academy of Sciences, 99(12): 7848–7853. doi:10.1073/pnas.122154899
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  36. ↑ Bopp, L; Resplandy, L; Orr, JC; Doney, SC; Dunne, JP; Gehlen, M; Halloran, P; Heinze, C; Ilyina, T; Seferian, R; Tjiputra, J (2013). "Multiple stressors of ocean ecosystems in the 21st century: projections with CMIP5 models". Biogeosciences. 10 (10): 6625–6245. Bibcode:2013BGeo...10.6225B. doi: 10.5194/bg-10-6225-2013 . hdl: 11858/00-001M-0000-0014-6A3A-8 .
  37. ↑ Schmidtko, S; Stramma, L; Visbeck, M (2017). "Decline in global oceanic oxygen content during the past five decades". Nature. 542 (7641): 335–339. Bibcode:2017Natur.542..335S. doi: 10.1038/nature21399 . PMID   28202958. S2CID   4404195.
  38. ↑ Ralph F. Keeling; Arne Kortzinger; Nicolas Gruber (2010). "Ocean Deoxygenation in a Warming World" (PDF). Annual Review of Marine Science. 2: 199–229. Bibcode:2010ARMS....2..199K. doi:10.1146/annurev.marine.010908.163855. PMID   21141663. Archived from the original (PDF) on 2016-03-01.
  39. ↑ Greicius, Tony (2016-05-16). "Europa's Ocean May Have An Earthlike Chemical Balance". NASA. Retrieved 2022-05-22.
  40. ↑ "The Chemistry of Enceladus' Plumes: Life or Not?".
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