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] 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.[11]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.[12] 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.[13] 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[14] have significant implications for the ecology of marine environments.[15]

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.[16] 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.[17]

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.[18][19][20]

Deoxygenation

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).[21]

Ocean deoxygenation is the reduction of the oxygen content in different parts of the ocean due to human activities.[22][23] It occurs firstly in coastal zones where eutrophication has driven some quite rapid (in a few decades) declines in oxygen to very low levels.[22] 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.[24] This expansion is happening as a consequence of human caused climate change.[25][26] 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.[27][28][29] Ocean deoxygenation poses implications for ocean productivity, nutrient cycling, carbon cycling, and marine habitats.[30][31]

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.[32][33] 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.[34]

Studies have shown that oceans have already lost 1-2% of their oxygen since the middle of the 20th century,[35][36] 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.[37]

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.