Calcium carbonate accumulation rates

An estimate of the calcium carbonate accumulation rate based on pilot unit experience is shown as the dashed line in Figure 7. This concept for calcium carbonate control is to be demonstrated in the 250 T/D pilot plant during operations on a subbituminous coal. 

Continental shelves and slopes comprise approximately 10 percent of the Earth s surface, and contain over half the sediments in the ocean (Heezen and Tharp, 1965 Gregor, 1985). Recent estimates of marine carbonate burial rates (e.g., Hay and Southam, 1977 Sundquist, 1985) indicate that between about 35 to 70 percent of Holocene carbonate deposition has taken place on continental shelves. In spite of their importance for carbonate accumulation and the global CO2 cycle, relatively few studies have been made on the chemical controls of calcium carbonate accumulation in these sediments, and most of these studies have been confined to near-shore environments. 

In order to understand the chemistry of calcium carbonate accumulation in the deep oceans, the sources of calcium carbonate, its distribution in recent pelagic sediments, the saturation state of seawater overlying deep-ocean sediments with respect to calcite and aragonite, and the relation between saturation state and dissolution rate must be known. These aspects of calcium carbonate chemistry are examined in this paper. 

In the subsoils of arid and semiarid soils, Ca commonly precipitates as cakite (CaCC>3) rather than being leached away. It is found as indurated layers (caliche and other local names) in many arid soils and as more diffuse CaC03 in Aridisols and Mollisols. Precipitation of CaCCTj in soils is affected by the rates of soil water movement, CO2 production by roots and microbes, CO2 diffusion to the atmosphere, and water loss by soil evaporation and plant transpiration. CaCC>3 layers are also derived from upward movement and evaporation of Ca-rich waters. Calcium carbonate accumulations can amount to as much as 90% of the mass of affected soil horizons. Gypsum precipitates in some arid soils, despite being about 10 x as water soluble as Ca carbonate. 

Figure 3 The net flux of CO2 between coastal zone waters and the atmosphere due to organic metabolism and calcium carbonate accumulation in coastal marine sediments, under three scenarios of changing thermohaline circulation rate compared to a business-as-usual scenario, in units of 10 molCyr ...

It should be kept in mind that, in spite of these major variations in the CO2-carbonic acid system, virtually all surface seawater is supersaturated with respect to calcite and aragonite. However, variations in the composition of surface waters can have a major influence on the depth at which deep seawater becomes undersaturated with respect to these minerals. The CO2 content of the water is the primary factor controlling its initial saturation state. The productivity and temperature of surface seawater also play major roles, in determining the types and amounts of biogenic carbonates that are produced. Later it will be shown that there is a definite relation between the saturation state of deep seawater, the rain rate of biogenic material and the accumulation of calcium carbonate in deep sea sediments. 

The high calcium content of the younger coals has led to the formation and deposition of calcium carbonate in the liquefaction reactor in the form of wall scale and oolites which were first observed in German operations (10). These deposits form as calcium salts of humic acids in the coal decompose under liquefaction conditions. The deposits continue to grow with time and could lead to unwanted solids accumulation in the reactor itself as well as fouling of downstream equipment (11). Data shown in Figure 7 indicate the accumulation rate of the calcium carbonate in the liquefaction reactor for different coals under typical EDS conditions as well as two methods for controlling the solids build-up. 

For weakly acidic systems (pH 5-6) in which the accumulation of hydrobromic acid is prevented by buffering agents such as calcium carbonate or benzoic acid salts, more information is available. Isbell and Pigman have made an extensive study of such systems, including a thorough consideration of the effect of the concentration of total bromine, free bromine, hypobromous acid and bromide ion on the velocity of the reaction. The results very definitely showed a direct correlation between free bromine concentration and the velocity of the oxidation. No such correlation could be found with hypobromous acid. The results are shown in Tables VII and VIII. The velocity constants were determined for a- and for 8-D-glucose. In the table for /S-D-glucose, in experiments 2 and 5, the hypobromous acid concentration varied 1 10 but the reaction rate varied 1 3. The variations in free bromine concentration follow the variations in the reaction rate constants and the kf values are based on the assumption that free bromine is the oxidant. The concentration of the oxidant (a in equation 31) is therefore the concentration of free bromine. 

The atmospheric concentration of CO2 rose from 280 ppm in 1800 to 370 ppm in 2000, mainly due to the consumption of fossil fuels. This increase in CO2 concentration is expected to have various environmental and ecological effects. For example, doubling of the CO2 concentration in the atmosphere reduces the rate of calcium carbonate deposition in coral reefs by 30-40%.Most of this rise has occurred over the last few decades, and, unless action is taken, the projected growth over the 21st century could lead to a doubling or tripling of the preindustrial level of C02. Unlike for SO2 emission, the total accumulation of CO2 matters rather than the rate of CO2 emission. Oceanic uptake of CO2 can compensate for some emissions, but this uptake will collapse once CO2... 

Although calcium carbonate formation in soil is a result of high evapotranspiration rates relative to precipitation rates, the term evaporite is usually restricted to compounds more soluble than CaCC>3. Where drainage water from surrounding soils accumulates and where the amount of percolated water is small compared to the amount of water evaporated, soluble salts tend to accumulate. This subject is dealt with in more detail in Chapter 11. The present section is restricted to the extreme case of natural salt flats and play as (former and intermittent lake beds). 

While the seafloor depths of the lysocline and CCD can be readily identified from sedimentary criteria, this information is of limited use without realistic knowledge of the rates at which calcium carbonate is lost from the sediments to dissolution. In practice, it is much easier to determine carbonate accumulation in the deep sea than it is to estimate carbonate loss. Yet the latter information is clearly needed in order to close sediment budgets and to reconstruct changes in the carbonate system. 

Atlantic Ocean Nodule abundance in the Atlantic Ocean appears to be more limited than in the Pacific or Indian Oceans, probably as a result of its relatively high sedimentation rates. Another feature which inhibits nodule abundance in the Atlantic is that much of the seafloor is above the calcium carbonate compensation depth (CCD). The areas of the Atlantic where nodules do occur in appreciable amounts are those where sedimentation is inhibited. The deep water basins on either side of the Mid-Atlantic Ridge which are below the CCD and which accumulate only limited sediment contain nodules in reasonable abundance, particularly in the western Atlantic. Similarly, there is a widespread occurrence of nodules and encrustations in the Drake Passage-Scotia Sea area probably due to the strong bottom currents under the Circum-Antarctic current inhibiting sediment deposition in this region. Abundant nodule deposits on the Blake Plateau can also be related to high bottom currents. 

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