Seawater steady-state

Silicic acid (H4Si04) is a necessary nutrient for diatoms, who build their shells from opal (Si02 H20). Whether silicic acid becomes limiting for diatoms in seawater depends on the availability of Si relative to N and P. Estimates of diatom uptake of Si relative to P range from 16 1 to 23 1. Dugdale and Wilkerson (1998) and Dunne et al. (1999) have shown that much of the variability in new production in the equatorial Pacific may be tied to variability in diatom production. Diatom control is most important at times of very high nutrient concentrations and during non-steady-state times, perhaps because more iron is available at those times. 

Therefore, it is likely that the steady state is maintained with regard to As concentration in seawater. 

The uptake and retention of technetium from seawater by young lobsters have been demonstrated by Swift [52], The accumulation of 95raTe was rapid and whole body concentration factors of over 2,000 were observed in some lobsters. The kinetics of the uptake and loss of 95mTc were studied in the crab Pachygrapus marmoratus [53]. The steady-state concentration factor was estimated to be 18. 

The evolution of the profiles of the isotope ratio is shown in Figure 8-12, which plots the profiles at various times in the calculation. Early in the calculation, isotope ratios at shallow depths have been driven more negative by the release of isotopically light respiration carbon, but little change has occurred at greater depths. As the evolution proceeds, the ratios at shallow depths become more positive as the result of the dissolution and diffusion of heavier carbon from both above and below. In the final steady state, after some 15,000 years, the isotope ratio is nearly constant at about -0.6 per mil at depths below 100 centimeters, rising rapidly to the seawater value, +2 per mil in the top 100 centimeters. The final values reflect a balance between the release of isotopically light carbon by respiration and the release of isotopically heavy carbon by dissolution, with the additional influence of the diffusion of isotopically heavy seawater carbon. 

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.. 

Equation 8.4 predicts that aerobic respiration should release dissolved inorganic nitrogen and phosphorus into seawater in the same ratio that is present in plankton, i.e., 16 1. As shown in Figure 8.3, a plot of nitrate versus phosphate for seawater taken from all depths through all the ocean basins has a slope close to 16 1. Why do both plankton and seawater have an N-to-P ratio of 16 1 Does the ratio in seawater determine the ratio in the plankton or vice versa Current thinking is that the N-to-P ratio of seawater reflects a quasi steady state that has been established and stabilized by the collective impacts of several biological processes controlled by marine plankton. 

For example, the average atom of potassium spends 10 million years dissolved in the ocean before becoming incorporated into the sediments. (Potassium is in steady state, so its oceanic residence time can be computed by dividing its input rate into the total amount in seawater.) This is plenty of time for ocean mixing, which occius on time scales of a thousand years, to homogenize out any horizontal or vertical concentration gradients. 

Marine chemists have taken increasingly more sophisticated approaches towards modeling seawater composition. The goal of these models is to understand the biogeochemical controls on seawater composition well enough that the effects of future perturbations can be predicted. As described next, the first modeling efforts were based on a series of reactions that were assumed to reach equilibrium the next efforts took a steady-state approach as the composition of seawater was thought to have been relatively constant over time. 

If the chemical composition of seawater has remained constant over time, a steady-state balance should exist in which the total supply rate of a particular ion is matched by its... 

The kinetic interpretation of the chemistry of oceanic waters (kinetics of inputs of primary constituents interactions between biologic and mixing cycles) leads to the development of steady state models, in which the relatively constant chemistry of seawater in the recent past (i.e., Phanerozoic cf. Rubey, 1951) represents a condition of kinetic equilibrium among the dominant processes. In a system at... 

Primary outputs are produced essentially by sedimentation and (to a much lower extent) by emissions in the atmosphere. The steady state models proposed for seawater are essentially of two types box models and tube models. In box models, oceans are visualized as neighboring interconnected boxes. Mass transfer between these boxes depends on the mean residence time in each box. The difference between mean residence times in two neighboring boxes determines the rate of flux of matter from one to the other. The box model is particularly efficient when the time of residence is derived through the chronological properties of first-order decay reactions in radiogenic isotopes. For instance, figure 8.39 shows the box model of Broecker et al. (1961), based on The ratio, normal-... 

Figure 8.39 Box model for steady state chemistry of seawater. Numbers in boxes ...

The kinetics of the oxidation of Cr(III) and Cu(I) have been discussed before. Cr(VI) is reduced by dissolved organic matter, the slow re-oxidation resulting in a large enough ti for an existence of Cr(III). Also the existence of Cu(I) in seawater is a steady state between the reduction- and back-oxidation reactions. The lifetime is dependent on pH, PC>2, complexing ligands and redox intermediates such as H2O2 (Moffet and Zika, 1983). 

Although americium (Am) exists in seawater exclusively in the trivalent oxidation state, its profiles in Fig. 12.4 contrast sharply with those of the trivalent lanthanides. Assessments of Nd isotopic ratios in seawater (e.g. Bertram and Elderfield, 1993) indicate that more than 1000 years are required for attainment of steady-state distributions of lanthanides and chemically similar elements in seawater. On such a basis it is expected that, in spite of substantial chemical similarities to the lanthanides, 241Am, a relatively short-lived isotope (half-life 470 years) with variable and recent anthropogenic inputs, will not exhibit profiles similar to those of the lanthanides. 

If DMS concentrations at the surface of the ocean are presumed to be at steady state, production must balance loss. The fate of DMS is thought to be evasion across the sea surface into the marine atmospheric boundary layer. However, since rates of DMS production are unknown, it is impossible to compare production with flux to the atmosphere, which is relatively well constrained. An alternative sink for DMS in seawater is microbial consumption. The ability of bacteria to metabolize DMS in anaerobic environments is well documented (32-341. Data for aerobic metabolism of DMS are fewer (there are at present none for marine bacteria), but Sivela and Sundman (25) and de Bont et al. (25) have described non-marine aerobic bacteria which utilize DMS as their sole source of carbon. It is likely that bacterial turnover of DMS plays a major role in the DMS cycle in seawater. 

Various fluxes and processes considered in the following discussion are represented schematically in Figure 9.20, adapted from Wollast and Mackenzie (1983). Steady-state conditions require that the mass balance for each element is fulfilled for the entire system and also separately for the water and sedimentary columns. To write these mass balances, we must consider the global rate of the reactions occurring in the two subsystems. Therefore, we define R and D as the annual amount of a given element transferred from the solid phase to the aqueous phase or vice versa. The fluxes are taken as positive if there is a net input to seawater, and negative if there is a net output. To maintain the concentration of an element constant in seawater, the net flux resulting from Rj + + Lj + Pj in... 

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