Marine 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

Marine chemistry,also known asocean chemistryorchemicaloceanography,is the study of chemical content in marine environments as influenced byplate tectonicsandseafloor spreading,turbidity,currents,sediments,pHlevels,atmosphericconstituents,metamorphic activity,andecology.Marine lifehas adapted to the chemistries unique toEarth's oceans,andmarine ecosystemsare 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 ofcarbon dioxide in the Earth's atmospherehave led toocean 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 ofSustainable Development Goal 14.

Chemicaloceanographyis the study of thechemistryof Earth'soceans.An interdisciplinary field, chemical oceanographers study the distributions and reactions of both naturally occurring andanthropogenic chemicalsfrom molecular to global scales.^[2]

Due to the interrelatedness of the ocean, chemical oceanographers frequently work on problems relevant tophysical oceanography,geologyandgeochemistry,biologyandbiochemistry,andatmospheric science.Many chemical oceanographers investigatebiogeochemical cycles,and themarine carbon cyclein particular attracts significant interest due to its role incarbon sequestrationandocean acidification.^[3]Other major topics of interest includeanalytical chemistryof the oceans,marine pollution,andanthropogenic 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 ofhalogenated organic compounds.^[5]

Chemical ecology of extremophiles

The ocean is home to a variety of marine organisms known asextremophiles– organisms that thrive in extreme conditions of temperature, pressure, and light availability. Extremophiles inhabit many unique habitats in the ocean, such ashydrothermal vents,black smokers,cold seeps,hypersaline regions, andsea 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 seahydrothermal vents

In hydrothermal vents and similar environments, many extremophiles acquire energy throughchemoautotrophy,using chemical compounds as energy sources, rather than light as inphotoautotrophy.Hydrothermal vents enrich the nearby environment in chemicals such aselemental sulfur,H₂,H₂S,Fe²⁺,andmethane.Chemoautotrophic organisms, primarily prokaryotes, derive energy from these chemicals throughredox reactions.These organisms then serve as food sources for highertrophic levels,forming the basis of unique ecosystems.

Several different metabolisms are present in hydrothermal vent ecosystems. Many marine microorganisms, includingThiomicrospira,Halothiobacillus,andBeggiatoa,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 thesulfur cyclein hydrothermal vent ecosystems. In the colder waters surrounding vents, sulfur-oxidation can occur using oxygen as anelectron acceptor;closer to the vents, organisms must use alternate metabolic pathways or utilize another electron acceptor, such as nitrate. Some species ofThiomicrospiracan utilize thiosulfate as an electron donor, producing elemental sulfur. Additionally, many marine microorganisms are capable of iron-oxidation, such asMariprofundus 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 anelectron 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 tobanded 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

Seafloor spreading onmid-ocean ridgesis a global scaleion-exchangesystem.^[7]Hydrothermal vents at spreading centers introduce various amounts ofiron,sulfur,manganese,siliconand other elements into the ocean, some of which are recycled into theocean 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 increasedbasaltreactions with seawater. Themagnesium/calciumratio 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 calcitepolymorphsofcalcium 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 ofreef-buildingand 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

Marine pollutionoccurs when substances used or spread by humans, such asindustrial,agriculturalandresidential waste,particles,noise,excesscarbon dioxideorinvasive organismsenter theoceanand cause harmful effects there. The majority of this waste (80%) comes from land-based activity, althoughmarine transportationsignificantly 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 therivers,sewageor the atmosphere, it means thatcontinental shelvesare more vulnerable to pollution.Air pollutionis also a contributing factor by carrying off iron, carbonic acid,nitrogen,silicon, sulfur,pesticidesor dust particles into the ocean.^[13]The pollution often comes fromnonpoint sourcessuch as agriculturalrunoff,wind-blowndebris,and dust. These nonpoint sources are largely due to runoff that enters the ocean through rivers, but wind-blowndebrisand 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 frommarine debris,plastic pollution,includingmicroplastics,ocean acidification,nutrient pollution,toxins and underwater noise. Plastic pollution in the ocean is a type of marine pollution byplastics,ranging in size from large original material such as bottles and bags, down tomicroplasticsformed 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 tomarine life.

Climate change

Increasedcarbon dioxidelevels, mostly from burningfossil fuels,are changing ocean chemistry.Global warmingand changes insalinity^[15]have significant implications for theecologyof marineenvironments.^[16]

Acidification

Ocean acidificationis the ongoing decrease in thepHof the Earth'socean.Over the past 200 years, the rapid increase in anthropogenicCO₂(carbon dioxide)production has led to an increase in theacidityof the Earth's oceans. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05.^[17]Carbon dioxide emissionsfrom human activities are the primary cause of ocean acidification, withatmospheric carbon dioxide (CO₂) levelsexceeding 410 ppm (in 2020). CO₂from theatmosphereis absorbed by the oceans. This chemical reaction producescarbonic acid(H₂CO₃) whichdissociatesinto abicarbonate ion(HCO−3) and ahydrogen ion(H⁺). The presence of free hydrogen ions (H⁺) lowers the pH of the ocean, increasingacidity(this does not mean thatseawateris acidic yet; it is stillalkaline,with a pH higher than 8).Marine calcifying organisms,such asmollusksandcorals,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. There are several other factors that influence the atmosphere-ocean CO₂exchange, and thus local ocean acidification. These includeocean currentsandupwellingzones, proximity to large continental rivers,sea icecoverage, and atmospheric exchange withnitrogenandsulfurfromfossil fuelburning andagriculture.^[19]^[20]^[21]

Deoxygenation

Global map of low and declining oxygen levels in coastal waters (mainly due toeutrophication) and in theopen ocean(due toclimate change). The map indicates coastal sites where oxygen levels have declined to less than 2 mg/L (red dots), as well as expanding oceanoxygen minimum zonesat 300 metres (blue shaded regions).^[22]

Ocean deoxygenationis the reduction of theoxygen contentin different parts of theoceandue to human activities.^[23]^[24]There are two areas where this occurs. Firstly, it occurs incoastal zoneswhereeutrophicationhas driven some quite rapid (in a few decades) declines in oxygen to very low levels.^[23]This type of ocean deoxygenation is also calleddead zones.Secondly, ocean deoxygenation occurs also in the open ocean. In that part of the ocean, there is nowadays an ongoing reduction in oxygen levels. As a result, the naturally occurring low oxygen areas (so calledoxygen minimum zones(OMZs)) are now expanding slowly.^[25]This expansion is happening as a consequence of human causedclimate change.^[26]^[27]The resulting decrease in oxygen content of the oceans poses a threat tomarine life,as well as to people who depend on marine life for nutrition or livelihood.^[28]^[29]^[30]A decrease in ocean oxygen levels affects howproductive the oceanis, hownutrientsandcarbon move around,and howmarine habitatsfunction.^[31]^[32]

As theoceans become warmerthis increases the loss of oxygen in the oceans. This is because the warmer temperatures increaseocean stratification.The reason for this lies in the multiple connections between density and solubility effects that result from warming.^[33]^[34]As a side effect, the availability of nutrients for marine life is reduced, therefore adding further stress tomarine organisms.

The rising temperatures in the oceans also 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 O₂content over the next hundred years. The decline of oxygen is projected to continue for a thousand years or more.^[38]

History

HMS Challenger (1858)

Early inquiries into marine chemistry usually concerned the origin ofsalinityin the ocean, including work byRobert Boyle.Modern chemical oceanography began as a field with the 1872–1876Challengerexpedition,which made the first systematic measurements of ocean chemistry.

Tools

Chemical oceanographers collect and measure chemicals in seawater, using the standard toolset ofanalytical chemistryas well as instruments likepH meters,electrical conductivity meters,fluorometers,and dissolved CO₂ meters. Most data are collected through shipboard measurements and from autonomousfloats or buoys,butremote sensingis used as well. On an oceanographicresearch vessel,aCTDis used to measureelectrical conductivity,temperature,andpressure,and is often mounted on arosetteofNansen bottlesto collect seawater for analysis. Sediments are commonly studied with abox coreror asediment trap,and older sediments may be recovered byscientific drilling.