Chemistry in the Marine Environment
112
Chemistry in the Marine Environment
112Paperback
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Overview
Product Details
| ISBN-13: | 9780854042609 |
|---|---|
| Publisher: | RSC |
| Publication date: | 05/10/2000 |
| Series: | Issues in Environmental Science and Technology , #13 |
| Pages: | 112 |
| Product dimensions: | 7.45(w) x 9.70(h) x (d) |
About the Author
Read an Excerpt
Chemistry in the Marine Environment
By R. E. Hester, R. M. Harrison
The Royal Society of Chemistry
Copyright © 2000 The Royal Society of ChemistryAll rights reserved.
ISBN: 978-1-84755-045-3
CHAPTER 1
Introduction and Overview
WILLIAM L. MILLER
1 Introduction
Why does Chemistry in the Marine Environment deserve separate treatment within the Issues in Environmental Science and Technology series? Is it not true that chemical principles are universal and chemistry in the oceans must therefore simply abide by these well-known laws? What is special about marine chemistry and chemical oceanography?
The long answer to those questions would probably include a discourse on complex system dynamics, carefully balanced biogeochemical cycles, and perhaps throw in a bit about global warming, ozone holes, and marine resources for relevance. The short answer is that marine chemistry does follow fundamental chemical laws. The application of these laws to the ocean, however, can severely test the chemist's ability to interpret their validity. The reason for this relates to three things: (1) the ocean is a complex mixture of salts, (2) it contains living organisms and their assorted byproducts, and (3) it covers 75% of the surface of the Earth to an average depth of almost 4000 metres. Consequently, for the overwhelming majority of aquatic chemical reactions taking place on this planet, chemists are left with the challenge of describing the chemical conditions in a high ionic strength solution that contains an unidentified, modified mixture of organic material. Moreover, considering its tremendous size, how can we reasonably extrapolate from a single water sample to the whole of the oceans with any confidence?
The following brief introduction to this issue will attempt to provide a backdrop for examining some marine chemical reactions and distributions in the context of chemical and physical fundamentals. The detailed discussions contained in the chapters that follow this one will provide examples of just how well (or poorly) we can interpret specific chemical oceanographic processes within the basic framework of marine chemistry.
2 The Complex Medium Called Seawater
For all of the millions of years following the cooling of planet Earth, liquid water has flowed from land to the sea. Beginning with the first raindrop that fell on rock, water has been, and continues to be, transformed into planetary bath water as it passes over and through the Earth's crust. Rivers and groundwater, although referred to as 'fresh', contain a milieu of ions that reflect the solubility of the material with which they come into contact during their trip to the sea. On a much grander scale even than the flow of ions and material to the ocean, there is an enormous equilibration continually in progress between the water in the ocean and the rock and sediment that represents its container. Both the low-temperature chemical exchanges that occur in the dark, high-pressure expanses of the abyssal plains and the high-temperature reactions occurring within the dynamic volcanic ridge systems contribute controlling factors to the ultimate composition of seawater.
After all those many years, the blend of dissolved materials we call seawater has largely settled into an inorganic composition that has remained unchanged for thousands of years prior to now. Ultimately, while Na+ and Cl- are the most concentrated dissolved components in the ocean, seawater is much more complex than a solution of table salt. In fact, if one works hard enough, every element in the periodic table can be measured as a dissolved component in seawater. In addition to this mix of inorganic ions, there is a continual flux of organic molecules cycling through organisms into the ocean on timescales much shorter than those applicable to salts. Any rigorous chemical calculation must address both.
Salinity and Ionic Strength
The saltiness of the ocean is defined in terms of salinity. In theory, this term is meant to represent the total number of grams of dissolved inorganic ions present in a kilogram of seawater. In practice, salinity is determined by measuring the conductivity of a sample and by calibration through empirical relationships to the International Association of Physical Sciences of the Ocean (IAPSO) Standard Sea Water. With this approach, salinity can be measured with a precision of at least 0.001 parts per thousand. This is fortunate, considering that 75% of all of the water in the ocean falls neatly between a salinity of 34 and 35. Obviously, these high-precision measurements are required to observe the small salinity variations in the ocean.
So, why concern ourselves with such a precise measurement of salinity? One physical consequence of salinity variations is their critical role in driving large-scale circulation in the ocean through density gradients. As for chemical consequences, salinity is directly related to ionic concentration and the consequent electrostatic interactions between dissolved constituents in solution. As salinity increases, so does ionic strength. Because the thermodynamic constants relating to any given reaction in solution are defined in terms of chemical activity (not chemical concentration), high ionic strength solutions such as seawater can result in chemical equilibria that are very different from that defined with thermodynamic constants at infinite dilution. This is especially true of seawater, which contains substantial concentrations of CO32-, SO42-, Mg2+, and Ca2+. These doubly charged ions create stronger electrostatic interactions than the singly charged ions found in a simple NaCl solution.
Changes in activity coefficients (and hence the relationship between concentration and chemical activity) due to the increased electrostatic interaction between ions in solution can be nicely modeled with well-known theoretical approaches such as the Debye–Hückel equation:
log γi = - Azi2 [square root of (1)] (1)
where γ is the activity coefficient of ion i, A is its characteristic constant, z is its charge, and I is the ionic strength of the solution. Unfortunately, this equation is only valid at ionic strength values less than about 0.01 molal. Seawater is typically much higher, around 0.7 molal. Inclusion of additional terms in this basic equation (i.e. the extended Debye–Hückel, the Davies equation) can extend the utility of this approach to higher ionic strength and works fine within an ion pairing model for a number of the major and minor ions. Ultimately, however, this approach is limited by a lack of experimental data on the exceedingly large number of possible ion pairs in seawater.
Another approach in the modeling of activity coefficient variations in seawater attempts to take into account all interactions between all species. The Pitzer equations present a general construct to calculate activity coefficients for both charged and uncharged species in solution and form the foundation of the specific interaction model. This complex set of equations, covered thoroughly elsewhere, is a formidable tool in the calculation of chemical activity for both charged and uncharged solutes in seawater. Both the ion pairing and the specific interaction models (or a combination of the two) provide valuable information about speciation of both major and trace components in seawater.
Often chemical research in the ocean focuses so intently on specific problems with higher public profiles or greater perceived societal relevance that the fundamental importance of physicochemical models is overlooked. But make no mistake; the inorganic speciation of salts in seawater represents the stage on which all other chemistry in the ocean is played out. These comprehensive inorganic models provide the setting for the specific topics in the following chapters. While these models represent significant advances in the understanding of marine chemistry, seawater, however, is such a complex mixture that on occasion even sophisticated models fail to accurately describe observations in the real ocean. In these cases, the marine chemist is left with empirical descriptions as the best predictive tool. Sometimes this situation arises owing to processes such as photochemistry or biochemical redox reactions that push systems away from equilibrium. Other times it results from the presence of unknown and/or uncharacterized compounds. Many of these latter compounds are of biological origin.
Biological Contributions
In sharp contrast to the cool precision of the electrostatic equations used to describe the inorganic interactions discussed above, the study of organic chemistry in the ocean does not enjoy such a clear approach to the evaluation of organic compounds in seawater. There is a boundless variety of both terrestrial and marine organisms that contribute organic compounds to the sea. While their initial contributions may be recognized as familiar biochemicals, much of this material is quickly transformed by microbial and chemical reactions into a suite of complex macromolecules with only a slight resemblance to their precursors. Consequently, the starting point for evaluation of a general approach for organic chemistry in the ocean is a situation where more than half of the dissolved organic carbon (DOC) is contained in molecules and condensates that are not structurally characterized; a mixture usually referred to as humic substances (HS). In other words, for many of the organic reactions in the ocean, we simply do not know the reactants.
Humic substances in the ocean are thought to be long lived and relatively unavailable for biological consumption. They are found at all depths and their average age in the deep sea is estimated in the thousands of years. This suggests that they are resilient enough to survive multiple complete trips through the entire ocean system. The chromophoric (or coloured) dissolved organic matter (CDOM), which absorbs most of the biologically damaging, high-energy ultraviolet radiation (UVR) entering the ocean, is composed largely of HS. Consequently, HS, through its light gathering role in the ocean, protects organisms from lethal genetic damage and provides the primary photon absorption that drives photochemistry in the ocean. Since UVR-driven degradation of CDOM (and HS) both oxidizes DOC directly to volatile gases (primarily CO2 and CO) and creates new substrate for biological degradation, the degree to which HS is exposed to sunlight may ultimately determine its lifetime in the ocean. Since DOC represents the largest organic carbon pool reactive enough to respond to climate change on timescales relevant to human activity, its sources and sinks represent an important aspect in understanding the relation between ocean chemistry and climate change.
The presence of HS in seawater does more than provide a carbon source for microbes and alter the UV optical properties in the ocean. It can also affect the chemical speciation and distribution of trace elements in seawater. Residual reactive sites within the highly polymerized mixture (i.e. carboxylic and phenolic acids, alcohols, and amino groups) can provide binding sites for trace compounds. The chemical speciation of Cu in seawater is a good example of a potentially toxic metal that has a distribution closely linked to that of HS and DOC. A very large percentage of Cu is complexed to organic compounds in seawater and consequently rendered non-toxic to most organisms since the free ion form of Cu is usually required for accumulation. One study of Cu in a sewage outfall area within Narragansett Bay, RI, USA shows this effect dramatically. As expected, the highest total Cu concentrations were found in this most impacted area of the estuary. Exactly coincident with high Cu concentrations, the researchers found the lowest Cu toxicity due to high DOC concentrations and increased complexation. Even though specific organic ligands could not be identified, it was clear that the presence of undefined organic compounds had turned a potentially lethal Cu solution into a refuge from toxicity.
The compounds that are identifiable in the sea represent a vast array of biochemicals attributable to the life and death of marine plants and animals. They are generally grouped into six classes based on structural similarities: hydrocarbons, carbohydrates, lipids, fatty acids, amino acids, and nucleic acids. Because they represent compounds that can be quantified and understood for their chemical properties and known role in biological systems, a great deal of information has been accumulated over the years about these groups and the specific compounds found within them.
While each individual organic compound may exist in exceedingly low concentrations, its presence in solution can be quite important. Organic carbon leaking into solution from the death of organisms can serve as a potential food source for a community of decomposers. Other compounds are intentionally excreted into solution, potentially affecting both biological and chemical surroundings. Certain of these compounds found in marine organisms are unique in their ability to elicit a particular biological or chemical effect. Some biochemicals may serve to attract mates or repel predators and others have the ability to sequester specific required nutrients, in particular, essential trace metals. An excellent example of the ability of small concentrations of biochemicals to significantly impact marine chemistry can be seen in a recent examination of iron speciation in the ocean.
Given the slightly alkaline pH of seawater, and relatively high stability constants for Fe(III) complexes with hydroxide in seawater, it has long been believed that the hydrolysis of Fe(III) represents the main speciation for iron in the ocean. The low solubility of Fe(OH)3 keeps total iron concentrations in the nanomolar range. Consequently, calculations of iron speciation based on known thermodynamic relationships have been extremely difficult to confirm experimentally at natural concentrations. In recent years, the use of ultraclean techniques with electrochemical titrations has turned the idea of a seawater iron speciation dominated by inorganic chemistry on its ear. Working on seawater samples from many locations, several groups have shown the presence of a natural organic ligand (also at nanomolar concentrations) that specifically binds to Fe(III). In fact, this ligand possesses conditional stability constants for association with the ferric ion that are so high (KL ≈ 1020 M-1 that it completely dominates the speciation of iron in the ocean. Calculations that include this ligand predict that essentially all of the iron in the ocean is organically complexed. In view of the fact that Fe is an essential nutrient and can limit primary productivity in the ocean, the chemistry associated with this Fe ligand represents quite a global impact for such a seemingly insignificant concentration of a very specific organic compound; a compound that was only discovered as a dissolved constituent in seawater within the last 10 years.
3 Spatial Scales and the Potential for Change
As mentioned in the introduction to this chapter, the ocean is enormous. One compilation that includes all of the oceans and adjacent seas puts the volume of seawater on the planet at 1.37 x 109 km3 covering 3.61 x 108 km2. The Atlantic, Pacific, and Indian oceans alone contain about 320 million km 3 (or 3 x 1020 litres) of seawater. Consequently, when we consider a ubiquitous chemical reaction in seawater, no matter how insignificant it may seem to our ordinary scale of thinking, its extrapolation to such huge proportions can result in the reaction taking on global significance. Conversely, chemical modifications that create a considerable local impact may be of no consequence when considered in the context of the whole ocean. The sheer size of the ocean forces a unique approach when applying chemical principles to the sea.
Separation of the Elements
Because the ocean spreads continuously almost from pole to pole, there is a large degree of difference in the heating of surface waters owing to varying solar radiation. This causes variations in both temperature (obviously) and salinity (from differential evaporation:precipitation ratios). These variations in heat and salt drive a great thermohaline circulation pattern in the ocean that witnesses cold, salty water sinking in the north Atlantic and in Antarctica's Weddell Sea, flowing darkly through the ocean depths, and surfacing again in the North Pacific; a journey lasting approximately 1000 years. This deep, dense water flows beneath the less dense surface waters and results in a permanent pycnocline (density gradient) at about 1000 metres; a global barrier to efficient mixing between the surface and deep oceans. The notable exceptions to this stable situation are in areas of the ocean with active upwelling driven by surface currents. On a large scale, the ocean is separated into two volumes of water, largely isolated from one another owing to differences in salinity and temperature. As mentioned above, both of these variables will produce changes in fundamental equilibrium and kinetic constants and we can expect different chemistry in the two layers.
Another layering that occurs within the 1000 metre surface ocean is the distinction between seawater receiving solar irradiation (the photic zone) and the dark water below. The sun provides heat, UVR, and photosynthetically active radiation (PAR) to the upper reaches of the ocean. Heat will produce seasonal pycnoclines that are much shallower than the permanent 1000 metre boundary. Winter storms limit the timescale for seasonal pycnoclines by remixing the top 1000 metres on roughly a yearly basis. Ultraviolet radiation does not penetrate deeply into the ocean and limits photochemical reactions to the near surface (metres to tens of metres depending on the concentration of CDOM). The visible wavelengths that drive photosynthesis penetrate deeper than UVR but are still generally restricted to the upper hundred metres.
(Continues...)
Excerpted from Chemistry in the Marine Environment by R. E. Hester, R. M. Harrison. Copyright © 2000 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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