Seawater chemical evolution

To model the chemical effects of evaporation, we construct a reaction path in which H2O is removed from a solution, thereby progressively concentrating the solutes. We also must account in the model for the exchange of gases such as CO2 and O2 between fluid and atmosphere. In this chapter we construct simulations of this sort, modeling the chemical evolution of water from saline alkaline lakes and the reactions that occur as seawater evaporates to desiccation. 

Since the experimental studies of van t Hoff at the turn of the century, geochemists have sought a quantitative basis for describing the chemical evolution of seawater and other complex natural waters, including the minerals that precipitate from them, as they evaporate. The interest has stemmed in large part from a desire to understand the origins of ancient deposits of evaporite minerals, a goal that remains mostly unfulfilled (Hardie, 1991). 

Horita J, Zimmermann H, Holland HD (2002) Chemical evolution of seawater during the Phanerozoic implications from the record of marine evaporates. Geochim Cosmochim Acta 66 3733—3756 Inghram MG, Brown H, Patterson C, Hess DC (1950) The branching ratio of K-40 radioactive decay. Phys Rev 80 916-917... 

The large amount of research that has been completed since the early 1980s has done much to clarify our understanding of the chemical evolution of seawater. In the Precambrian, major advances have centered on the effects of the rise... 

Derry L. A. and Jacobsen S. B. (1990) The chemical evolution of Precambrian seawater evidence from REEs in banded iron formations. Geochim. Cosmochim. Acta 54, 2965-2977. 

Holland, G. and C. J. Ballentine (2006) Seawater subduction controls the heavy noble gas composition of the mantle. Nature 441, 186-191 Holland, G., M. Cassidy and C. I Ballentine (2009) Meteorite Kr in earth s mantle suggests a late accretionary source for the atmosphere. Science 326, 1522-1525 Holland, H. D. (1984) The chemical evolution of the atmosphere and oceans. Princeton Univ. Press, New Jersey (USA), 582 pp. 

Most of the water on Earth s surfece is in the ocean relatively little is present in the atmosphere or on land. Because of its chemical and physical properties, this water has had a great influence on the continuing biogeochemical evolution of our planet. Most notably, water is an excellent solvent. As such, the oceans contain at least a little bit of almost every substance present on this planet. Reaction probability is enhanced if the reactants are in dissolved fitrm as compared with their gaseous or solid phases. Many of the chemical changes that occur in seawater and the sediments are mediated by marine organisms. In some cases, marine organisms have developed unique biosynthetic pathways to help them survive the environmental conditions fitimd only in the oceans. Some of their metabolic products have proven useful to humans as pharmaceuticals, nutraceuticals, food additives, and cosmeceuticals. 

Because sedimentary carbonates represent primarily chemical and biochemical precipitates from seawater, and because they make up 20% of the common sedimentary rock record, these rock types have been particularly good sources of chemical and mineralogical data for interpretation of the secular and cyclic evolution of the Earth s surface environment. This carbonate rock record as a function of geological age is now explored as are age trends in other rock types and sediment properties. With this information as background material, we can then discuss what these relationships tell us about the history of carbonates and the exogenic system throughout geologic dme. 

Mackenzie F.T. (1975) Sedimentary cycling and the evolution of seawater. In Chemical Oceanography, 2nd edition, Volume 1 (eds. J.P. Riley and G. Skirrow), pp. 309-364. Academic Press, London. 

The duration of chemical exchange between seawater and basalt can also be determined by isotopic studies of hydrothermal minerals. A variety of techniques have been used, ranging from direct dating by K/Ar and Rb/Sr isochron techniques to comparisons of the initial strontium isotopic composition of alteration minerals with the isotopic evolution of seawater (Gallahan and Duncan, 1994 Richardson et al., 1980). 

Evolution of thinking about the importance of reactions between seawater and detrital clay minerals has come full circle in the past 35 years. Reverse weathering reactions were hypothesized in very early chemical equilibrium and mass balance (Mackenzie and Garrels, 1966) models of the oceans. Subsequent observations that marine clay minerals generally resemble those weathered from adjacent land and the discovery of hydrothermal circulation put these ideas on the back burner. Recent studies of silicate and aluminum diagenesis, however, have rekindled awareness of this process, and it is back in the minds of geochemists as a potentially important process for closing the marine mass balance of some element (see chapter 2). 

---