Chemistry of the Marine Environment

Chemistry of the marine environment

Introduction

The chemistry of sea water can be considered in the context of climate change: the ocean is a fully open system (exchange of both matter and energy, exchanges with the atmosphere, rivers, and the continental crust, biological activities, hydrothermal vents). For example, an increase in temperature is just a net effect of the chemical changes, such as the injection of compounds like CO₂ into the marine environment. Any change or perturbation will be reflected in the ocean.

97% of the water on our planet is sea water, with most of the rest being frozen. The volume consists of 1 billion cubic kilometers of salty water; the rest is ice. Sea ice is related to the circulation on our planet.

Marine Chemistry describes the chemical reactions and processes happening in the water both qualitatively and quantitatively. Every equation can predict quantitatively the future behavior of the system (generalization is the main purpose). The main topics are the distribution and dynamics of elements, isotopes, atoms (like noble gases), and molecules.

Most research was carried out in the last two centuries; it is a modern science. Before recent research, work on chemistry was mainly an observational activity. Boyle can be considered the father of oceanography (we still, nowadays, don’t know how to measure the salinity from the point of view of a metrologist). Chemistry of the marine environment is correlated to both inorganic and organic chemistry.

Moreover, any process that can be studied has to be considered on both the spatial scale and the temporal scale (they are related to one another). For this reason, we can consider it an interdisciplinary topic. The longer the time required, the wider the area involved, but there are more patterns in the graph.

The response to change in climate takes centuries. Any climate change will influence the ocean, and another steady state will be reached.

Variables

• Range of depth: 0–10/11 km
• Range of pressures: from 1 atm to 1000 atm (pressure influences every chemical reaction)
• Range of temperature: from -2°C to 40°C (even temperature influences the reactions significantly)
• Saltiness: from 0 to 41 g/Kg (%). In the ocean, the chemical equilibrium constants are completely different.

For example, salinity: the average salinities of different oceans are quite close to each other. In the Atlantic, the salinity is higher (34.90 vs. 34.62 of the Pacific). Focusing on the drainage basins, we can see that the drainage basin of the Atlantic Ocean is wider than that of the Pacific Ocean. It is also important to consider the interactions between the atmosphere and the land. The distribution of mountain chains regulates the basins.

There are some portions of the planet in which the easterlies transport freshwater from the Atlantic to the Pacific Ocean (0.36 Sv, measuring unit of the flux). This scheme shows why the Atlantic is saltier than the Pacific.

Another example refers to the distribution of different substances, affected by the ocean circulation (the surface circulation, influenced by the winds, can modify the distribution of pollutants in the marine environment). This can also influence the distribution of microplastics, which tend to accumulate in the middle of the surface circulation gyres. This can help us define forecasts.

Distribution of energy on our planet

The movement generates the so-called conveyor belt of currents. We are nowadays forcing our system, and one of the things that influence the amount of energy is the amount of radiation reflecting on the surface, impacting the sea ice.

Arctic Death Spiral shows the amount of ice on the planet in the Arctic. In September, we have the lowest amount of ice (natural oscillation, because it is the end of the summer). The minimum level is continuously decreasing with the passing of the years.

Blue ocean event: possible melting of all ice in the Arctic; we are reaching this event.

By changing the amount of reflection, we are changing the energy budget. There is an increase in the energy trapped in the Arctic Sea, with higher levels in Summer. The Arctic is acquiring energy, which can be translated into an increase of temperature in the Arctic. The starting of the melting of the ice is happening earlier and earlier every new year (reduction of the Julian day).

A change in the atmosphere results in a response from the ocean. In the Arctic, the increase in temperature is significantly higher than in the rest of the world, known as Arctic amplification. Having less ice changes the temperature of the ocean (it increases) and salinity (it also increases), leading to effects on the biota (Atlantification of the Arctic: continuous transformation of the Arctic Ocean into a system similar to the Atlantic Ocean). This causes the biological invasions of Atlantic species into the Arctic sea.

The Keeling curve, named after the researcher studying it, shows the increase of atmospheric CO₂ (intensive observation of the same variable over time, revolutionary in the chemistry field). The CO₂ also influences the ocean carbon cycle: pumps of CO₂ into the deeper part of the ocean.

Touristic activities can pollute the Arctic but are also promoted by the decrease of the sea ice itself. When we consider the ocean, we have to consider the choices we make regarding interactions with the sea. Routes around the Arctic are an example of this.

Research Methods

The easiest way to study the ocean is to use research vessels: we can use instruments like the Rosette to collect samples for the measurements of different parameters (they permit the definitions of models). Once the samples are collected directly in the field, the research must continue in a laboratory to measure the chemical-physical properties of the environment. In a laboratory, we can also simulate environmental conditions in a controlled system (in the field, environmental conditions can be extremely variable and cannot be controlled). Some conditions can interfere with the collection of samples, like waves when a Rosette is used. By using laboratories, we can regulate the conditions and use controlled systems. Field and laboratory activities are equally important.

Water properties

Dipole momentum is the consequence of the difference in electronegativity between O and H (one of the greatest differences, 3.44 vs. 2.20). The structure of the water molecule is related to the hybridization of the orbitals: O → 1s² 2s² 2p⁴. The 2s² and 2p⁴ hybridize to obtain sp³. The final angle obtained is 104.5° with a tetrahedral geometry, due to the repulsion between two negative dipoles. Two positive charges on H and two negative charges in the orbitals of O.

Whenever there is a disparity of charges, we consider the dipole momentum as the vectorial product between the distance of charges and the amount of the electronic charges involved. The electrons will be shifted towards O so that they are not in the middle, forming the dipole (which is a vector). In the 3D model of the molecule, we have to consider the sum of the moment dipole of all the links and the lone pairs of electrons. Each water molecule can be involved in four hydrogen bonds (the energy of a hydrogen bond is around 20-40 kJ mol^-1).

The diagram of boiling point: water deviates significantly compared to other elements of the same group as O regarding its properties. This happens because of the difference in electronegativity of the elements (Te and Se are less electronegative). The second reason is the angle of the structure: in other elements, there are no hybridizations, so the role pairs of electrons do not generate the dipole in these compounds, whereas in water, the lone pairs are important for this reason.

This is because of the period of O: it is in the second period, so it has only two different shells. In the case of tellurium, instead, the P orbitals are distant from the center and not concentrated in just one direction but are more uniformly distributed.

It takes a lot of energy to break up the hydrogen bonds. At the end, we have these properties:

• High boiling point (100°C at P=1 atm)
• High melting point (0°C at P=1 atm)
• The highest latent heat of evaporation
• The highest latent heat of melting (just lower than ammonia)
• The highest specific heat (1 cal g^-1 °C^-1, just lower than ammonia)
• Pure water reaches its maximum density at 4°C
• The solid phase (ice) is less dense than the liquid phase (pure water)
• The highest dielectric constant compared to other liquids

Cluster model of water: it is common to imagine water as a mixture of free molecules linked with hydrogen bonds. In reality, most of the water is not free, but part of the molecules are in an ice-like structure, meaning they are not free but are linked together as in ice. Hydrogen bonds are so-called cooperative bonds: whenever we obtain an H bond, we increase the probability of having another H bond. The result is the cluster of water. This is evident when we melt ice: only 50% of the H bonds are broken up, the remaining stay in an ice-like structure.

It is important because of:

• Chemical relationship with density. When temperature drops, the cluster increase (more molecules in a smaller volume with an increase in density, but an ice-like structure decreases the density, so the two phenomena shape the curvature of density with temperature. At 4 degrees, we have the maximum density; below 4 degrees, the cluster starts to compete with other molecules.
• Capability of water to trap hydrophobic species: ice-like structure is responsible for the capability to trap hydrophobic compounds. We obtain clathrates→ a common name for special gas hydrates that include methane. Methane is trapped into the cluster, not solubilization. Two conditions required:
• Lower temperature
• Higher pressure (no pressure will cause higher fugacity of the gas from the cluster; high pressure increases the probability of an ice-like structure).

The water that is able to do it is the one in the deepest part of the ocean, in a condition where the organic material can reach the sediments, gets reduced, and methane is obtained. The pressure must be high, so even the depth must be high or the temperature must be very low. Graph of phase boundary → (every depth has a temperature associated that permits the maintenance of the clathrate structure).

Methane that is produced is then trapped in a clathrate. The amount of clathrate worldwide is around 500 to 2500 Gt of carbon (5–25 times the known proved commercial reserves of natural gases). Any change that will change the temperature may cause the release of methanes from the clathrates (both in the deep ocean and on the surface in the permafrost).

The water properties also can influence the capability of the water to hydrate ions and solubilize salts, because of the high dielectric constant. This happens because salts and ions in water don’t remain free in water but go under full hydration, due to the presence of a hydration shell. The hydration shell is an ordered structure. The orientation of water molecules in the shell depends on the charge of the ion in the center of the shell. The electrostatic force of the ions is dispersed because of the water molecules, so the capability of two opposite ions to attract each other will be completely reduced.

Hydration shells are different for each ion (e.g., H⁺ is completely hydrated, it doesn’t exist alone). The ion promotes its own shell with its spreadness. The property that follows the hydration shell is electrostriction: it influences the volume of the water due to the striction given by the electrostatic attraction into the hydration shell.

In the cluster model, we have to add the ions. Free water molecules are not completely free because we have an ion. The ion attracts water molecules: for this reason, they are not completely free; they are ordered because of the presence of the ion. The space occupied is less than the one occupied without the ion. The electrostatic attraction implies a reduction of the volume (electrostriction). It was tested thanks to an experiment. The result was that the volume measured (observed) was different from the one theoretically calculated: it was smaller.

The decrease in the volume is not proportional to the amount of salt in terms of mass. It depends on the 3D structure of the hydration shell (force of the ion and water molecules surrounding it). We can obtain a diagram. NaCl has a lower effect than MgCl and MgSO₄: what is changing is the kind of ion generated. MgCl and MgSO₄ are divalent and double divalent salts. The volume constriction is higher when the ions obtained have more than one charge: divalent ions cause a higher volume constriction than monovalent ions.

It is the basis of the changes in the equilibrium constant along the water column. (Change of pressure causes a change in the packing of water). Limit case: the attraction induced by the charges of the NaCl crystal with respect to the water molecules is so high that theoretically, the density of water near the crystals can jump to a value that is 2 or 3 times the normal density of water.

(The hydration shell of a simple salt is obtained thanks to the mass and the molecular weight of the salt, so moles. From moles, the number of ions released are known. The amount of volume that should be there into the solution: by the difference, we have the amount of volume that is involved in every hydration shell from which we can compute the radius of the hydration shell).

When a salt is added, we observe:

• The density is increased: The greater the amount of solute, the greater the effect. As a result of the salt content in seawater, the density of seawater increases continuously with decreasing temperature, and there is no density maximum like for freshwater.
• The freezing point is depressed: Salts lower the temperature at which water freezes because dissolved salts inhibit the tendency of water molecules to form direct bonds with other water molecules.
• The boiling point is elevated: The salts have the effect of making the water molecules cluster harder to pull apart and evaporate. (Boiling point is dependent on the vapor tension of water).
• The conductivity is increased: The transport of electrons causing an electric current to flow is enhanced by the strong electrolyte nature of salts. Important for salinity definition and measurements.

The temperature of maximum density is also changed because we are packing molecules in the hydration shell. You can see it in a graph. The two curves intersect at a salinity of around 24.628%. If we consider the global average, we can see that the surface of the ocean doesn’t reach the temperature of maximum density (this affects the mixing).

Another important thing is that in seawater, some ion complexes may be found because of the high [ ]. The probability that two hydration shells interfere with each other is quite high, so complexes may be formed. Free ions can attract each other, and new compounds may be formed. In some cases, they can also share their hydration shells. The third case does not consider the fusion of two shells, but they move along together. These cases are typical when the salinity is high.

Water molecules are not equal to themselves every time: atoms that constitute water are not always the same. We may have normal O and normal H, but we can also have stable isotopes. They change the vapor pressure or the capability of water to pass from one phase to another. For H, we have deuterium and two isotopes for ¹⁷O and ¹⁸O.

Stable isotopes

First of all, we have to look for the standard: the 1st standard was distilled water by Potomac River (so from freshwater, it was easier to collect), called NBS-1. It was used to determine the amount of stable isotopes in seawater. Then, by adding the measurements of samples coming from different oceans, the measurements were averaged to obtain the Standard Mean Ocean Water (SMOW). It is measured as relative to the distilled water standard (NBS-1).

The second step was to recreate the standard in the laboratory. The International Atomic Agency prepared a huge amount of distilled water with the same chemical ratio found averaging the different oceans. This is called Vienna-SMOW (VSMOW). The knowledge of stable isotopes is important because the molecular weight of elements changes the molecular weight of water, thus increasing the energy that water requires to evaporate.

This causes the fractionation process. The water that evaporates more easily from the ocean is the one that contains ¹⁶O because it requires less energy. The movement of vapor permits the formation of clouds. The rain will contain a higher amount of heavy O than lighter O, but the amount of heavy oxygen will be reduced moving from the equator to the poles. They can give us information regarding the climate of the planet.

R is the ratio between D/H or ¹⁸O/¹⁶O in the sample/VSMOW. Delta represents the change in statistics with respect to the average ratio. Graph with different sites: linear relationship showing that at lower temperatures, we have higher depletion of ¹⁸O. Using these data, we can track down the mean annual temperature of a location (historical climate records).

Models

Two: equilibrium and steady state (concentrations are away from the equilibrium but the concentrations of chemicals are stationary, do not change over time). A model is necessary because it permits the study of a small part of a complex system.