Dissolution water column

The iron cycle shown in Fig. 10.14 illustrates some redox processes typically observed in soils, sediments and waters, especially at oxic-anoxic boundaries. The cycle includes the reductive dissolution of iron(lll) hydr)oxides by organic ligands, which may also be photocatalyzed in surface waters, and the oxidation of Fe(II) by oxygen, which is catalyzed by surfaces. The oxidation of Fe(II) to Fe(III)(hydr)-oxides is accompanied by the binding of reactive compounds (heavy metals, phosphate, or organic compounds) to the surface, and the reduction of the ferric (hydr) oxides is accompanied by the release of these substances into the water column. 

This rate law suggests that dissolution should be relatively slow compared to the time that detrital PIC takes to settle to the seafloor. But recent observations indicate that a significant amoimt of dissolution occurs high in the water column, even in saturated waters. Dissolution imder saturated conditions is thought to be a consequence of PIC exposure to metabolic CO2 in acidic microenvironments such as found in zooplankton guts and feces and within aggregations of marine snow. 

The rain rate of BSi is dependent on (1) the rate of its production by marine organisms, (2) shell dissolution rates, and (3) the time required for a shell to reach the seafloor. High rates of production by siliceous plankton ensure a large supply of opal to the water column. The fraction reaching the seafloor is largest when transit times are shortest. Thus, shells that sink fastest will be preferentially preserved and a greater fraction of the particulate silica flux reaches sediments that lie in shallow waters. 

In Illustrative Example 19.4, the dissolution of a non-aqueous-phase liquid (NAPL) into groundwater was discussed. Here we consider a similar (although somewhat hypothetical) case. Assume that a mixture of chlorinated solvents totally covers the flat bottom of a small pond (maximum depth zmax = 4 m, surface area Asurface = 104 m2) forming a dense non-aqueous-phase liquid (DNAPL). The DNAPL is contaminated by benzene which dissolves into the water column and is vertically transported by turbulent diffusion. The pond is horizontally well mixed. The vertical turbulent diffusion coefficient is , = 0.1 cm2s l and approximately constant over the whole water column. 

Deposition during the mixed period (up to day 165) was calculated from a mass balance on water-column Si and the Si P ratio in sediment trap material, because sediment traps overestimate the net particle deposition flux in a mixing water column (19). Our calculations assumed that losses of dissolved reactive Si resulting from diatom uptake that are not accounted for by increases in particulate biogenic Si are caused by Si deposition. The estimate of mixed-period P deposition was conservative because we assumed that nondiatom particulate P was removed at a rate similar to diatom P. We also assumed that loss of P in traps resulting from diagenesis-dissolution was negligible. The use of short collection periods (2-3 weeks) and a poison should minimize loss. 

The time-sequence pattern of P deposition to the sediment surface differed markedly from that observed with silicon (25). Biogenic silicon underwent relatively little dissolution in the water column as compared with phosphorus, which resulted in a bell-shaped deposition pattern (Figure 11). Rapid regeneration of P during sedimentation resulted in a slower net settling rate for P than for biogenic silicon. 

Figure 2.14. The log of dissolution rate in percent per day versus the log of (1-fi). A = whole Indian Ocean sediment dissolved in deep-sea sediment pore water B = whole Pacific Ocean sediment dissolved in Atlantic Ocean deep seawater C = whole Atlantic Ocean sediment dissolved in Long Island Sound seawater (Morse and Berner, 1972) D = > 62 pm size fraction of the Indian Ocean sediment dissolved in Atlantic Ocean deep seawater, E = the 125 to 500 pm size fraction of Pacific Ocean sediment dissolved in Atlantic Ocean deep seawater F = 150 to 500 pm Foraminifera dissolved in the Pacific Ocean water column. (After Morse, 1978.)...

Sinkin Biogei Materi Biologic Formation of, CaC03 and Org-C 9 lie 1 al l > Dissolution and 0 Decomposition in Water Column... 

Because of their small size, individual coccoliths should sink slowly (Lerman et al., 1974) and, consequently, spend long periods of time (on the order of 100 years) in the water column. This long residence time should lead to dissolution of the coccoliths in the undersaturated part of the water column. The origin of coccolith ooze on portions of the seafloor overlain by undersaturated waters, therefore, is difficult to explain by arguments based on settling rates of individual particles (e.g., Bramlette, 1958, 1961 Honjo, 1975, 1976 Roth et al., 1975). Schneidermann (1977) has summarized the major factors considered to be important in accumulation of coccoliths in sediments overlain by undersaturated waters. They are ... 

More recent studies by Byrne et al. (1984) and Betzer et al. (1984, 1986) have contributed substantially to our understanding of aragonite sedimentation in the pelagic environment. In their work, short term deployments of large sediment traps were utilized to minimize the problem of dissolution of material within the trap. Byrne et al. (1984) carried out an elegant examination of pteropod dissolution in the water column in the North Pacific. Considerable variation of the expected extent of dissolution for different species was observed. Some of the pteropods. 

Betzer et al. (1984, 1986) studied the sedimentation of pteropods and foraminifera in the North Pacific. Their sediment trap results confirmed that considerable dissolution of pteropods was taking place in the water column. They calculated that approximately 90% of the aragonite flux was remineralized in the upper 2.2 km of the water column. Dissolution was estimated to be almost enough to balance the alkalinity budget for the intermediate water maximum of the Pacific Ocean. It should be noted that the depth for total dissolution in the water column is considerably deeper than the aragonite compensation depth. This is probably due to the short residence time of pteropods in the water column because of their rapid rates of sinking. 

It is also important to keep in mind that the relation between the saturation state of seawater and carbonate dissolution kinetics is not a simple first order dependency. Instead it is an exponential of about third to fourth order (e.g., Berner and Morse, 1974). Thus dissolution rates are very sensitive to saturation state. This type of behavior has not only been demonstrated in the laboratory (see Chapter 2), but also has been observed in numerous in situ experiments in which carbonate materials and tests have been suspended in the oceanic water column. The depth at which a rapid increase in dissolution rate with increasing water depth is observed usually has been referred to as the chemical or hydrographic lysocline. In some areas of the ocean it is close to coincident with the FL (e.g., Morse and Berner, 1972). 

Figure 9.20 must be balanced by precipitation or dissolution reactions in the water column ... 

The process of removal of calcium by marine organisms in the water column is well known. Production of calcium carbonate by water column biological processes may be estimated from primary productivity and from the mean chemical composition of plankton. After death of the organisms and removal of the organic protective layer, the skeletons may undergo dissolution if they encounter water undersaturated with respect to their mineral composition. Active dissolution of calcium carbonate occurs mainly near the sediment-water interface in deep waters that are undersaturated with respect to both calcite and aragonite (see Chapter 4). Thus, calcium is regenerated from calcareous skeletons and, finally, only a small fraction of the initial production of these materials accumulates in sediments. An... 

Byrne R.H., Acker J.G., Betzer P.R., Feely R.A. and Cates M.H. (1984) Water column dissolution of aragonite in the Pacific Ocean. Nature 312,321-326. 

Honjo S. (1975) Dissolution of suspended coccoliths in the deep-sea water column and sedimentation of coccolith ooze. In Dissolution of Deep-Sea Carbonates (eds. W. Sliter, A.W.H. Be and W.H. Berger) pp. 115-128. Cushman Found. Foraminiferal Res., Spec. Publ. No. 13. 

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