Carbon crustal abundance

The real values of CO2 content in the early atmosphere are very uncertain. One way to calculate this value is to estimate the crustal abundance of carbon. This element is mainly stored in carbonate rocks and modern assessments give the value of lO" Tg. This is enough to produce an atmospheric pressure of 60 bars if it had been presented as gaseous CO2. Even if only one-third of this amount was present in the atmosphere at the moment of accretion, the pressure would be about 20 bars. How long this dense CO2 atmosphere would have lasted depends on the rate of rock silicate transformation to the carbonates. This rate, in turn, would have depended on the surface temperature on the early Earth and on the amount of continental area exposed to weathering. 

Despite the term traditionally applied to this group of elements, rare earths, their crustal abundance is not particularly low. Cerium ranks around 25 in the listing of all the naturally occurring elements, its abundance being similar to that of Ni or Cu [1]. Even the least abimdant lanthanoid elements, Tb, Tm, and Lu, are more abundant than Ag [2]. Because of their geo-chemical characteristics, however, the rare earth-containing minerals consist of mixtures of the elements with relatively low concentration of them [3]. Accordingly, the number of their exploitable deposits, mainly consisting of phosphates and fluoro-carbonates, is rather small [1,3]. 

Taylor and McLennan (1981) suggested that while lanthanide patterns in finegrained sedimentary rocks were parallel to upper crustal abundances, they probably overestimated the absolute abundances by about 20%. Mass balance calculations involving averages of the various sedimentary rock types (shales, sandstones, carbonates, evaporites) substantiate this adjustment (Taylor and McLennan 1985). 

In the geosphere, strontium is ranked fifteenth in the order of the elemental crustal abundance at 450 ppm. Its presence is greater than that of carbon or chlorine, and slightly less than that of sulfur or fluorine. Of the trace elements, it is ranked fifth [7). Strontium is mainly found in Scotland and the United States, where calcareous rocks with natural apatites containing up to 73000 ppm of strontium are located [8]. 

Sodium, 22 700 ppm (2.27%) is the seventh most abundant element in crustal rocks and the fifth most abundant metal, after Al, Fe, Ca and Mg. Potassium (18 400 ppm) is the next most abundant element after sodium. Vast deposits of both Na and K salts occur in relatively pure form on all continents as a result of evaporation of ancient seas, and this process still continues today in the Great Salt Lake (Utah), the Dead Sea and elsewhere. Sodium occurs as rock-salt (NaCl) and as the carbonate (trona), nitrate (saltpetre), sulfate (mirabilite), borate (borax, kemite), etc. Potassium occurs principally as the simple chloride (sylvite), as the double chloride KCl.MgCl2.6H2O (camallite) and the anhydrous sulfate K2Mg2(S04)3 (langbeinite). There are also unlimited supplies of NaCl in natural brines and oceanic waters ( 30kgm ). Thus, it has been calculated that rock-salt equivalent to the NaCl in the oceans of the world would occupy... 

In addition to its presence as the free element in the atmosphere and dissolved in surface waters, oxygen occurs in combined form both as water, and a constituent of most rocks, minerals, and soils. The estimated abundance of oxygen in the crustal rocks of the earth is 455 000 ppm (i.e. 45.5% by weight) see silicates, p. 347 aluminosilicates, p. 347 carbonates, p. 109 phosphates, p. 475, etc. 

Occurrence. In order of abundance in the earth s crustal rocks, it is the third within the transition elements (after Fe and Ti) and the 12th in the general order of all the elements. It occurs in several minerals such as primary deposits of silicates and as secondary deposits (commercially more important) of oxides and carbonates as pyrolusite, Mn02, hausmannite, Mn304, rhodochrosite, MnC03, etc. Large amounts of manganese are present in the deep sea nodules located over certain areas of the ocean floor. 

CO2 is the second most abundant gas species in magmatic systems. In a survey of CO2 emanations from tectonically active areas worldwide, Barnes et al. (1978) attributed 8 C-values between -8 and -4%c to a mantle source. This is, however, problematic, because average crustal and mantle isotope compositions are more or less identical and surflcial processes that can modify the carbon isotope composition are numerous. A more promising approach may be to analyze the C-content of CO2 collected directly from magmas at high temperatures. 

Estimates of the abundance of zinc in the sun, in meteorites, in the Earth s core and crust, and in the oceans are very difficult to make, but its abundance in the Earth s crustal rocks and soils is of the order of 100 ppm, about 1000 times as abundant as its congeners cadmium and mercury. All three elements are Chalcophiles so that, in the reducing atmosphere that prevailed when the earth s crust solidified, they were deposited in the sulfide phase giving rise to the sulfide ores, their most important source. Eater, as weathering took place, zinc became soluble only to be precipitated as the carbonate, silicate, or phosphate. 

Crustal carbon. In contrast to the noble gases, carbon near the planetary surface is concentrated in crustal rocks, and is largely divided between carbonates, with an isotopic composition of = Q%c and sedimentary organic carbon with = -25%c. While Hoefs (1969) estimated a total of 2.6 X 10 g of C, with 66% in sedimentary rocks, Hunt (1972) reassessed the budget using sedimentary rock abundance data from Ronov and Yaroshevsky (1969) and carbon concentration data compilations to obtain a higher value of 9 X 10 g. Ronov and Yaroshevsky (1976) updated their earlier work with new additional data to obtain a... 

The practical miner does not, of course, select an average crustal rock as a source of metal. Instead he seeks those rocks in which the desired metal has been concentrated by some natural process and in which the metal occurs in a desired mineral form such as an oxide or a carbonate. Nevertheless, because most of the minerals he seeks are widespread in common rocks, the miner can look forward confidently to a long future with respect to geochemically abundant metals. Because many of the present ore deposits grade slowly into common rocks, as the extreme local enrichments he now... 

Fig. 13. Four important reservoirs of CO2 are shown as functions of time for the models in Figure 12. High heat flow is denoted by continuous lines, low heat flow by dashed lines. Here we have chosen models in which the crustal reservoirs are initially constant in time i.e. we have started from the equilibrium reservoirs. In particular, the equilibrium continental reservoirs are small and so these models begin with very little continental carbonate. The high heat-flow models chiun the reservoirs fast enough that if we do not start at equilibrium values, the model quickly evolves to them, but in the low heat-flow models circulation is slow enough that the arbitrary initial conditions are remembered well into Archaean time. In general, the effect of abundant Hadean impact ejecta is to remove CO2 from the continents and oceans and put it into the mantle.

Iron is the most abundant transition element in the Earth s crust and, in general, in all life forms. An outline of the distribution of iron in the Earth s crust is shown in Table 1.2. As can be seen, approximately one-third of the Earth s mass is estimated to be iron. Of course, only the Earth s crust is relevant for life forms, but even there it is the most abundant transition element. Its concentration is relatively high in most crustal rocks (lowest in limestone, which is more or less pure calcium carbonate). In the oceans, which constitute 70 percent of the Earth s surface, the concentration of iron is low but increases with depth, since this iron exists as suspended particulate matter rather than as a soluble species. Iron is a limiting factor in plankton growth, and the rich... 

In Earth s crustal composition, sulfur ranks thirteenth in abundance, with an estimated concentration of 0.05 percent. Sulfur exists in elemental form, as metallic sulfides, as sulfates, and, when combined with carbon and nitrogen, in organic forms. Most of the world s sulfur resource is located in North America. It is distributed, in descending order according to share of that resource, as follows the United States and Canada have 26 percent and 22 percent, respectively, followed by Russia (11 %), Saudi Arabia (5%), Japan (5%), Poland (4%), Germany (4%), and France (2%) the remaining 21 percent is distributed in other countries. 

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