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 earth
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.[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]
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 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.[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 impacts
Marine pollution
Marine pollution occurs when substances used or spread by humans, such as industrial, agricultural and residentialwaste, 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.
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 acidification
Ocean acidification is the ongoing decrease in the pH value of the Earth's oceans, caused by the uptake of carbon dioxide (CO2) from the atmosphere.[15][16] The main cause of ocean acidification is human burning of fossil fuels. As the amount of carbon dioxide in the atmosphere increases, the amount of carbon dioxide absorbed by the ocean also increases. This leads to a series of chemical reactions in the seawater which has a negative spillover on the ocean and species living below water.[17] When carbon dioxide dissolves into seawater, it forms carbonic acid (H2CO3). Some of the 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,[18] representing an increase of almost 30% in H+ 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 H+ ion concentration).[19][20]
The ocean's pH value as of 2020 was 8.1, meaning it is currently lightly basic (the pH being higher than 7).[17] Ocean acidification will result in a shift towards a lower pH value, meaning the water will become less basic and therefore more acidic.[16] Ocean acidification can lead to 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.
Ocean acidification impacts many species, especially organisms like oysters and corals. It is one of several effects of climate change on oceans. Ocean acidification can lead to 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 cannot reproduce under high saturated acidotic waters.
Marine chemistry on other planets and their moons
A planetary scientist using data from the Cassini spacecraft has been researching the marine chemistry of Saturn's moonEnceladus using geochemical models to look at changes through time.[21] 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".[21][22]
^DOE (1994). "5"(PDF). In A.G. Dickson; C. Goyet (eds.). Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water. 2. ORNL/CDIAC-74.
^Coble, Paula G. (2007). "Marine Optical Biogeochemistry: The Chemistry of Ocean Color". Chemical Reviews. 107 (2): 402–418. doi:10.1021/cr050350+. PMID17256912.
^ abRies, Justin B. (2004). "Effect of ambient Mg/Ca ratio on Mg fractionation in calcareous marine invertebrates: A record of the oceanic Mg/Ca ratio over the Phanerozoic". Geology. 32 (11): 981. Bibcode:2004Geo....32..981R. doi:10.1130/G20851.1. ISSN0091-7613.
^Charles Sheppard, ed. (2019). World seas : an Environmental Evaluation. Vol. III, Ecological Issues and Environmental Impacts (Second ed.). London, United Kingdom. ISBN978-0-12-805204-4. OCLC1052566532.
^ ab"Ocean Acidification". www.whoi.edu/. Retrieved 2021-09-13. According to the Intergovernmental Panel on Climate Change (IPCC), economic and population scenarios predict that atmospheric CO2 levels could reach 500 ppm by 2050 and 800 ppm or more by the end of the century. This will [reduce] the pH an estimated 0.3 to 0.4 units by 2100, a 150 percent increase in acidity over preindustrial times.Cite error: The named reference "Ocean acidification not acid" was defined multiple times with different content (see the help page).
