Calcareous organisms

As we have seen in Chapter 6, the crystal growth has to be preceded by nucleation (Fig. 6.7). In fresh waters this nucleation occurs heterogeneously on particle surfaces. In seawater nucleation occurs primarily upon the templates of calcareous organisms. 

Watson LL, Hutcheon ID, Epstein S, Stolper EM (1994) Water on Mars clues from deu-terium/hydrogen and water contents of hydrous phases in SNC meteorites. Science 265 86-90 Weber IN, Raup DM (1966a) Eractionation of the stable isotopes of carbon and oxygen in marine calcareous organisms-the Echinoidea. 1. Variation of and content within individuals. Geochim Cosmochim Acta 30 681-703... 

Weber IN, Raup DM (1966b) Eractionation of the stable isotopes of carbon and oxygen in marine calcareous organisms-the Echinoidea. 11. Environmental and genetic factors. Geochim Cosmochim Acta 30 705-736... 

Although only a few examples were discussed, these examples are rather characteristic for skeletal growth patterns in calcareous organism. We could demonstrate that carbonate deposition is in many ways a clock-controlled biological rhythm. 

Figure 5.7. Isotopic composition of various groups of calcareous organisms. (After Swart, 1983.)...

Agegian C.R. and Mackenzie F.T. (1989) Calcareous organisms and sediment mineralogy on a mid-depth bank in the Hawaiian Archipelago. Pacific Sci. 43, 56-66. 

Accurate comparison of results requires knowledge of reaction site density per unit surface area. Calcite materials used for kinetic study have included natural marbles, limestones, hydro-thermal crystals of Iceland spar, tests of calcareous organisms and laboratory and commercial precipitates. Surface areas, estimated by BET methods and graphical methods (based on particle size distribution) range from about 0.005 to 2 m g . There are apparent discrepancies between graphical and BET surface areas and the question is raised as to which type of surface area estimate is most representative of the reacting surface area. 

CO2 is normally present in sea water in adequate amounts (Pruder and Bolton, 1979 Raven, 1994). Nevertheless, the anthropogenic rise in CO2 may increase primary production (Riebesell et al., 1993 Chen and Durbin, 1994 Hein and Sand-Jensen, 1997 Wolf-Gladrow et al., 1999 Qiu and Gao, 2002 for discussion also Engel et al., 2005). The acidification of water will enhance the CO2 availability, but reduce the CO, concentration because of the pH-dependent equilibrium between the different carbon species, which is disadvantageous for calcareous organisms. As cyanobacteria lose competitiveness in acid water, other algal groups may benefit. 

Adsorption of Mo to hydrous Fe and A1 oxides is pH-dependent (Reisenauer, Tabikh, and Stout, 1962), and the rate of adsorption is highest at acidic pH. It decreases with increases in pH from 4.45 to 7.5. Compared with acid soils, alkaline soils are high in soluble or available Mo (Davis, 1956). As in any other soil, in alkaline soil the availability of Mo is also influenced by several other factors. The relative Mo content of the parent rock, the process of soil evolution, and the physicochemical attributes of the soil (pH, calcareousness, organic-matter content, cation-exchange capacity, texture, moisture, relative concentrations of other mineral elements) all influence the availability of Mo. 

Processes (2.115) and (2.116) describe the solid-liquid equilibrium at the oceanic bottom (sediment-seawater interface) with suspended matter in seawater (calcareous organisms). Moreover, similarly the processes of heterogeneous nucleation and droplet formation from CCN as well as in general the solid-water interfacial process are described (Fig. 2.93). Hence, seawater is an excellent solvent for acidic gases such as SO2 (used for flue-gas desulfurization at some coastal site power stations) and atmospheric CO2. The ratio between atmospheric and oceanic (water dissolved) CO2 is described by the Henry equilibrium (2.117 see also Chapter 4.3.2) ... 

As mentioned already, the CO2 cycle has one major problem in the atmosphere -there is no direct chemical sink. In nature, CO2 can only be assimilated by plants (biological sink) through conversion into hydrocarbons (Chapter 2.2.2.3) and stored in calcareous organisms, partly buried in sediments but almost completely turned back into CO2 by respiration hence, CO2 partitions between the biosphere and atmosphere. With respect to time periods being of interest for humankind (from decades to hundreds of years) this natural biogeoehemical recycling can be regarded to be closed or, in other words, the net flux is zero ... 

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