Paleosalinity (or palaeosalinity) is the salinity of the global ocean or of an ocean basin at a point in geological history.

Importance

From Bjerrum plots, it is found that a decrease in the salinity of an aqueous fluid will act to increase the value of the carbon dioxide-carbonate system equilibrium constants, (pK*). This means that the relative proportion of

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...

with respect to

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

is higher in more saline fluids, e.g.

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...

, than in fresher waters. Of crucial importance for

paleoclimatology Paleoclimatology (British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the reconstruction of ancient climate is important to ...

is the observation that an increase in

salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...

will thus reduce the

solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...

of carbon dioxide in the oceans. Since there is thought to have been a 120 m depression in

sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardis ...

at the

last glacial maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eu ...

due to the extensive formation of

ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at La ...

s (which are solely freshwater), this represents a significant fractionation towards saltier seas during glacial periods. Correspondingly, this will cause a net outgassing of carbon dioxide into the atmosphere because of its reduced solubility, acting to increase atmospheric carbon dioxide by 6.5

‰ Per mille (from Latin , "in each thousand") is an expression that means parts per thousand. Other recognised spellings include per mil, per mill, permil, permill, or permille. The associated sign is written , which looks like a percent ...

. This is thought to partly offset the net decrease of 80-100‰ observed during glacial periods.

Stratification

In addition, it is thought that extensive salinity stratification can lead to a reduction in the meridional overturning circulation (MOC) through the slowing of thermohaline circulation. Increased stratification means that there is effectively a barrier to subduction of parcels of water; isopycnals effectively do not outcrop at the surface and are parallel to the surface. The ocean, in this case, can be described as "less ventilated", and this has been implicated in the slowing down of the MOC.

Measuring paleosalinity

There may exist proxies for salinity, but to date the main way that salinity has been measured has been by directly measuring

chlorinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

in pore fluids. Adkins et al. (2002) used pore fluid chlorinity in ODP cores, with the paleo-depth estimated from nearby coral horizons. Chlorinity was measured rather than pure salinity because the major ions in seawater are not constant with depth in the sediment column; for example, sulfate reduction and cation-clay interactions can change overall salinity, whereas chlorinity is not heavily affected.

Paleosalinity during the Last Glacial Maximum

Adkins' study found that global salinity increased with a global sea level drop of 120 m. Analyzing ¹⁸O data they also found that deep waters were within error of the freezing point, with oceanic waters exhibiting a greater degree of homogeneity in temperatures. In contrast, variations in salinity were much greater than they are today. Modern day salinities are all within 0.5 psu of the global average salinity of 34.7 psu, whereas salinities during the

(LGM) ranged from 35.8 psu in the North Atlantic to 37.1 in the Southern Ocean. There are some notable differences in the hydrography at the LGM and present day. Today the North Atlantic Deep Water (NADW) is observed to be more saline than Antarctic Bottom Water (AABW), whereas at the last glacial maximum it was observed that the AABW was in fact more saline; a complete reversal. Today the NADW is more salty because of the

Gulf Stream The Gulf Stream, together with its northern extension the North Atlantic Drift, is a warm and swift Atlantic ocean current that originates in the Gulf of Mexico and flows through the Straits of Florida and up the eastern coastline of the Unit ...

; this could thus indicate a reduction of flow through the

Florida Strait The Straits of Florida, Florida Straits, or Florida Strait ( es, Estrecho de Florida) is a strait located south-southeast of the North American mainland, generally accepted to be between the Gulf of Mexico and the Atlantic Ocean, and between th ...

s due to lowered sea level. Another observation is that the Southern Ocean was vastly more salty at the LGM than today. This is particularly intriguing given the assumed importance of the

Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-smal ...

in oceanic dynamical regulation of ice ages. The extreme value of 37.1 psu is assumed to be a consequence of an increased degree of

sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's o ...

formation and export. This would account for the increased salinity, but would also account for the lack of oxygen isotopic fractionation;

brine Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for ...

rejection without oxygen isotopic fractionation is thought to be highly characteristic of sea ice formation.

The increased role of salinity

The presence of waters near the freezing point alters the balance of the relative effects of contrasts in salinity and temperature on sea water density. This is described in the equation, :

\frac=\alpha\Delta T-\beta\Delta S

where

\alpha

is the

thermal expansion coefficient Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...

and

\beta

is the haline contraction coefficient. In particular, the ratio

\frac

is crucial. Using the observed temperatures and salinities, in the modern ocean,

\frac

is about 10 whilst at the LGM

\frac

it is estimated to have been closer to 25. The modern thermohaline circulation is thus more controlled by density contrasts due to thermal differences, whereas during the LGM the oceans were more than twice as sensitive to differences in salinity rather than temperature. In this way, the thermohaline circulation can be considered to have been less "thermo" and more "haline".

References

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External links

History of salinity.
Chemical oceanography Aquatic ecology Oceanography Coastal geography