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....