Marine chemistry

Marine chemistry, also known as ocean chemistry or chemical oceanography, 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.

**Total Molar Composition of Seawater (Salinity = 35)**[1]
Component	Concentration (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
C_T	0.00206
Br⁻	0.000844
B_T (total boron)	0.000416
Sr²⁺	0.000091
F⁻	0.000068

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

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 provide energy through a process called chemosynthesis.

Plate tectonics

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.[6] 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.[7]

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

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).[6]

Experiments show that most modern high-Mg calcite organisms would have been low-Mg calcite in past calcite seas,[9] 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.[8][9]

Human impacts

Marine pollution

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.[10] 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.[11] 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.[12] 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[13] have significant implications for the ecology of marine environments.[14]

Acidification

Ocean acidification is the 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.[15] 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 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.[16]

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 unit is equivalent to a tenfold change in hydrogen ion concentration).[17] 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.[18][19][20]

Deoxygenation

Global map of low and declining oxygen levels in the open ocean and coastal waters. The map indicates coastal sites where anthropogenic nutrients have resulted in oxygen declines to less than 2 mg/L (red dots), as well as ocean oxygen minimum zones at 300 metres (blue shaded regions).[21]

Ocean deoxygenation is the reduction of the oxygen content of the global oceans and coastal zones due to human activities[22][23] as a consequence of anthropogenic emissions of carbon dioxide[24][25] and eutrophication-driven excess production.[22] It is manifest in the increasing number of coastal and estuarine hypoxic areas, or dead zones, and the expansion of oxygen minimum zones (OMZs) in the world's oceans. The decrease in oxygen content of the oceans has been fairly rapid and poses a threat to all aerobic marine life, as well as to people who depend on marine life for nutrition or livelihood.[26][27][28][29]

Oceanographers and others have discussed what phrase best describes the phenomenon to non-specialists. Among the options considered have been ocean suffocation (which was used in a news report from May 2008[30]), "ocean oxygen deprivation",[31] "decline in ocean oxygen", "marine deoxygenation", "ocean oxygen depletion" and "ocean hypoxia". The term “Ocean Deoxygenation” has been used increasingly by international scientific bodies because it captures the decreasing trend of the world ocean's oxygen inventory.[22]

History

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.