Oceans acid-base chemistry

As scientists have learnt more about the way chemical constituents of the Earth s surface operate, it has become clear that it is insufficient to consider only individual environmental reservoirs. These reservoirs do not exist in isolation— there are large and continuous flows of chemicals between them. Furthermore, the outflow of material from one reservoir may have little effect on it, but can have a very large impact on the receiving reservoir. For example, the natural flow of reduced sulphur gas from the oceans to the atmosphere has essentially no impact on the chemistry of seawater, and yet has a major role in the acid-base chemistry of the atmosphere, as well as affecting the amount of cloud cover (Section 7.3). 

There are two major carbon cycles on Earth. The two cycles operate in parallel. One cycle is slow and abiotic. Its effects are observed on multimillion-year timescales and are dictated by tectonics and weathering (Berner, 1990). In this cycle, CO2 is released from the mantle to the atmosphere and oceans via vulcanism and seafloor spreading, and removed from the atmosphere and ocean primarily by reaction with silicates to form carbonates in the latter reservoir. Most of the carbonates are subsequently subducted into the mantle, where they are heated, and their carbon is released as CO2 to the atmosphere and ocean, to carry out the cycle again. The chemistry of this cycle is dependent on acid-base reactions, and would operate whether or not there was life on the planet (Kasting et al., 1988). This slow carbon cycle is a critical determinate of the concentration of CO2 in Earth s atmosphere and oceans on timescales of tens and hundreds of milhons of years (Kasting, 1993). 

To understand the chemistry that underlies coral reef formation and other processes in the ocean and in aqueous systems such as living cells, we must understand the concepts of aqueous equilibria. In this chapter, we teike a step towcud understanding such complex solutions by looking first at further apphcations of acid-base equilibria. The idea is to consider not only solutions in which there is a single solute but... 

The chemistry of the carbonic acid system in seawater has been one of the more intensely studied areas of carbonate geochemistry. This is because a very precise and detailed knowledge of this system is necessary to understand carbon dioxide cycling and the deposition of carbonate sediments in the marine environment. A major concept applicable to problems dealing with the behavior of carbonic acid and carbonate minerals in seawater is the idea of a constant ionic medium. This concept is based on the observation that the salt in seawater has almost constant composition, i.e., the ratios of the major ions are the same from place to place in the ocean (Marcet s principle). Possible exceptions can include seawater in evaporative lagoons, pores of marine sediments, and near river mouths. Consequently, the major ion composition of seawater can generally be determined from its salinity. It has been possible, therefore, to develop equations in which the influence of seawater composition on carbonate equilibria is described simply in terms of salinity. 

The models therefore reflect our level of knowledge of die soil solution and its interaction with soil solids. Since these models have the potential to predict the composition of natural waters (groundwater, lakes and streams, oceans as well as the soil solution), soil fertility, the effects of fertilizers and soil amendments, the effects of acid rain, and the attenuation and release of pollutants in soils, this important area of research should be actively pursued. The accuracy of the models, however, is still based on our understanding of the soil s chemistry and cannot be more accurate than that. 

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