Oceans chemical residence times

Chemically reactive elements should have a short residence time in seawater and a low concentration. A positive correlation exists between the mean ocean residence time and the mean oceanic concentration however, the scatter is too great for the plot to be used for predictive purposes. Whitfield and Turner (1979) and Whitfield (1979) have shown that a more important correlation exists between residence time and a measure of the partitioning of the elements between the ocean and crustal rocks. The rationale behind this approach is that the oceanic concentrations have been roughly constant, while the elements in crustal rocks have cycled through the oceans. This partitioning of the elements may reflect the long-term chemical controls. The relationship can be summarized by an equation of the form... 

The geochemical fate of most reactive substances (trace metals, pollutants) is controlled by the reaction of solutes with solid surfaces. Simple chemical models for the residence time of reactive elements in oceans, lakes, sediment, and soil systems are based on the partitioning of chemical species between the aqueous solution and the particle surface. The rates of processes involved in precipitation (heterogeneous nucleation, crystal growth) and dissolution of mineral phases, of importance in the weathering of rocks, in the formation of soils, and sediment diagenesis, are critically dependent on surface species and their structural identity. 

The steady-state concentration of a chemical with an oceanic residence time much longer than that of water can be predicted if it is assumed that its removal rate is directly proportional to its abundance in seawater, i.e.. 

Residence time The average amount of time that a chemical species spends in the ocean or sediment, assuming the species is at a steady state. 

Fig. 22 Schematic comparison between the residence times tR of waters in the hydrosphere (ocean to rainfall), the dissolution of various minerals in unsaturated solutions at pH 5 (quartz to gypsum) and the half-life tm of chemical processes (recristallization to solid-solid reactions) (data after Langmuir 1997, Drever 1997)...

Carbon dioxide is chemically stable and has an average residence time in the atmosphere of about four years before it enters either the oceans or terrestrial ecosystems. 

Since there are about 100 mol of CO2 above each square meter (1 atm = KXX) g air cm and the average molecular weight of air is 29 g thus there are 100 g/29 g = 34 mol air molecules above 1 cm of which 0.03% are CO2), a residence time of tqoi = years is obtained. Similar residence times have been established on die basis of carbon-14 data. Thus, under these circumstances, the chemical enhancement of the exchange rate does not appear to be very significant. Thus the diflusion of CO2 is more important than the chemical cycle of HCO and CO3 for the ocean-atmosphere CO2 exchange. 

An Equilibrium Model for the Sea Abstracting from the complexity of nature, an idealized counterpart of the oxic ocean (atmosphere, water, sediment) may be visualized. Oxygen obviously is the atmospheric oxidant that is most influential in regulating (with its redox partner, water) the redox level of oxic water. It is more abundant—within the time span of its atmospheric residence time—in the atmosphere than in the other accessible exchange reservoirs. It is chemically and biologically reactive its redox processes (photosynthesis... 

Obviously the sea is an open, dynamic system with variable inputs and outputs of mass and energy for which the state of equilibrium is a construct. As we have seen, the concept of free energy, however, is no less important in dynamic systems. In considering equilibria and kinetics in ocean systems, it is useful to recall that different time scales need to be identified for the various processes. When a particular reaction of a phase or species has—within the time scale of consideration—a negligible rate, it is permissive to define a metastable equilibrium state. Similarly, in a flow system the time-invariant condition of a well-mixed volume approaches chemical equilibrium when the residence time is sufficiently large relative to the appropriate time scale of the reaction. 

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