Pteropods dissolution

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. 

The rate of dissolution of synthetic and deep sea biogenic (pteropods) aragonite in seawater have also been determined in the laboratory by the pH-stat method (57). The results of the experiments to determine the change in the rate of dissolution as a function of undersaturation are presented in Figure 16. The pteropods were found to dissolve at. only about 3% the rate, per unit surface area, of the synthetic aragonite. The results also indicate a change in the empirical reaction order from 2.92 to 7.37 at = 0,44. The rate equations for pteropod dissolution are ... 

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. 

Berner R.A. (1977) Sedimentation and dissolution of pteropods in the Ocean. In The Fate of Fossil Fuel CO2 in the Oceans (eds. N.R. Anderson and A. Malahoff), pp. 243-260. Plenum Press, New York. 

Morse J.W., deKanel J. and Harris J. (1979) Dissolution kinetics of calcium carbonate in seawater. VII The dissolution kinetics of synthetic aragonite and pteropod tests. Amer. J. Sci. 279, 482-502. 

Figure 16. Log of the dissolution rate vs. the log of (1 — Cl) for synthetic aragonite and pteropods (after Ref. 57)...

Figure 17. Log of the dissolution rate vs. total carbonate ion concentration for synthetic aragonite, pteropods, calcitic Pacific Ocean sediment, and foraminifera in the 125-500 iim size fraction. (A) indicates ihe aragonite equilibrium total carbonate ion concentration at 25°C, 1 atm (26). (C) indicates the calcite equilibrium total carbonate ion concentration at 25°C, 1 atm (25).

Figure 18. Ratio of measured rates of dissolution per gram to the initial dissolution for pteropods and calcite Pacific Ocean sediment (57)...

Morse, J.W., de Kanel, J., and Harris, K. The dissolution kinetics of calcium carbonate in seawater VII. The dissolution kinetics of synthetic aragonite and pteropods, Amer. Jour. Sci. (in press). 

There are several studies that have been successful in determining the dissolution rate at conditions near seawater saturation. Acker et al. (1987) was able to employ very precise determinations of pH to measure the rate of dissolution of a single pteropod shell at different pressures from 15 atm to 300 atm. Because his measurements were at different pressures and is a function of pressure, he was able to determine whether the rate constant is indeed a function of K p. He found that Equation (9) fit his data better than (10), suggesting that the constant is not pressure dependent and the former is a more accurate universal rate law. An exponent oin= 1.9 was obtained for this surface-controlled dissolution reaction and a partial molal volume. Ay, of —39 cm mol (very close to the mean of the values determined in laboratory experiments for calcite) best fit the data. 

Figure 5 Results of in situ dissolution experiments. Peterson (1966) re-weighed polished calcite spheres after a 250 d deployment on a mooring in the North Pacific. Honjo and Erez (1978) observed the weight loss for calcitic samples (coccoliths, foraminifera and reagent calcite) and an aragonitic sample (pteropods) held at depth for a period of 79 d. While Peterson hung his spheres directly in seawater, the Honjo-Erez samples were held in containers through which water was pumped. The results suggest that the calcite saturation horizon lies at 4,800 200 m in the North Atlantic and at about 3,800 200 m in the North Pacific. For aragonite, which is 1.4 times more soluble than calcite, the saturation horizon in the North Atlantic is estimated to be in the range 3,400 200 m.

The most important carbonate-secreting organisms in the oceans are foraminifera, cocco-lithophorides, and pteropods. The carbonate tests vary in size, appearance, crystal form (calcite or aragonite), and magnesium content. The solubility depends on the depth (pressure), temperature, and concentration of CO2 besides the crystal form. For example, the pteropods which secrete shells of aragonite undergo dissolution at shallower depths than the coccolithophorides which secrete calcite shells. 

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