Terrestrial reservoirs

In summary, there is a clear mineralogical control on the partitioning of the Mg isotopes among carbonates and waters and, apparently, a weak dependence on temperature at low T. An important question is the extent to which these measured values for carbonates and waters reflect isotopic equilibrium. Hints to the answer come from comparing 5 Mg to 5 Mg, as shown in the section on terrestrial reservoirs. 

Measurements of terrestrial Mg isotope ratios on a plot of A Mg vs. 5 Mg are all within the region bounded by the equilibrium and kinetic mass fractionation laws given expected uncertainties (Fig. 5). Apparently, all of the terrestrial reservoirs represented by the data thus far are related to the primitive chondrite/mantle reservoir by relatively simple fractionation histories. Adherence of the data to the regions accessible by simple mass fractionation processes in Figure 5 (the shaded regions in Fig. 3) is testimony to the veracity of the fractionation laws since there is no reason to suspect that Mg could be affected by any processes other than purely mass-dependent fractionation on Earth. 

Newsom HE (1995) Composition of the solar system, planets, meteorites, and major terrestrial reservoirs. In Global Earth Physics. A handbook of physical constants. Ahrens TJ (ed) American Geophysical Union, Washington, 159-189... 

The crust is the largest carbon reservoir in the crustal-ocean-atmosphere factory (8 x 10 Pg C including the sediments). Most of this carbon is in the form of inorganic minerals, predominantly limestone, with the rest being organic matter, predominantly contained in shale and secondarily in fossil fuel deposits (coal, oil, and natural gas). The oceanic reservoir (4 X lO" Pg C) and the terrestrial reservoir (2 to 3 x 10 Pg C) are both far smaller than the crustal reservoir. The smallest reservoir is found in the atmospheric, primarily as CO2 (preindustrial 6 x 10 Pg C, now 8 x 10 Pg C and rising). The flux estimates in Figure 25.1 have been constrained by an assumption that the preindustrial atmospheric and oceanic reservoirs were in steady state over intermediate time scales (millennia). 

Another method uses the decay of cosmogenic isotopes that are produced in the atmosphere and then incorporated into terrestrial reservoirs. Examples of this approach include standard 14C and l0Bc dating. 

Zartman R. E. and Haines S. M. (1988) The plumbotectonic model for Pb isotopic systematics among major terrestrial reservoirs—a case for bi-directional transport. Geochim. Cosmochim. Acta 52, 1327—1339. 

Noble gases have been at the forefront of studies defining volatile fluxes between the mantle and other terrestrial reservoirs. This stems from the fact that in the case of He there is no question or ambiguity regarding its origin the mantle He... 

The Fe/Ni value of the core (16.5) is well constrained by the limited variation in chondritic meteorites (17.5 0.5) and the mantle ratio (32), as well as the mass fraction of these elements in the two terrestrial reservoirs. The total content of... 

There are various terrestrial reservoirs that have distinct volatile characteristics. Data from midocean ridge basalts (MORBs) characterize the underlying convecting upper mantle, and are described here without any assumptions about the depth of this reservoir. Other mantle reservoirs are sampled by ocean island basalts (OIBs) and may represent a significant fraction of the mantle (Chapter 2.06). Note that significant krypton isotopic variations due to radiogenic additions are neither expected nor observed, and there are no isotopic fractionation observed between any terrestrial noble gas reservoirs. Therefore, no constraints on mantle degassing can be obtained from krypton, and so krypton is not discussed further. Comparison between terrestrial and solar system krypton is discussed in Chapter 4.12. 

Figure 1.5 extends this notion to the geochemical level which shows an estimate of the influence of the global biosphere carbon and oxygen cycles on the fluxes of major elements through the terrestrial reservoirs, and includes the effects of both primary and secondary biogeochemical processes. 

World sulfur reserves. The earth s crust contains about 0.6% S, where it occurs as elemental S (brimstone) in deposits associated with gypsum and calcite combined S in metal sulfide ores and mineral sulfates as a contaminant in natural gas and crude oils as pyritic and organic compounds in coal and as organic compounds in tar sands (Tisdale and Nelson, 1966). The elemental form commonly occurs near active or extinct volcanoes, or in association with hot mineral spings. Estimates by Holser and Kaplan (1966) of the terrestrial reservoirs of S suggest that about 50% of crustal S is present in relatively mobile reservoirs such as sea water, evaporites, and sediments. The chief deposits of S in the form of brimstone and pyrites are in Western European countries, particularly in France, Spain, Poland, Japan, Russia, U.S.A., Canada, and Mexico. World production of S in the form of brimstone and pyrites was approximately 41 Tg in 1973 other sources accounted for about 8 Tg, making a total of 49 Tg (Anon, 1973). Byproduct S from sour-gas, fossil fuel combustion, and other sources now accounts for over 50% of S used by western countries, as shown in Fig. 9.1. This percentage may increase as pollution abatement measures increase the removal of SO2 from fossil fuel, particularly in the U.S.A. Atmospheric S, returned to the earth in rainwater, is also a very important source of S for plants. 

He, Ne, Ar, C and N between cosmochemical potential precursors (PSN and chondrites) and terrestrial reservoirs (the atmosphere and the mantle source of MORB) is given in Figure 2. Volatile abundances are normalized to Ne and the Sun, which, in this figure, results in a flat pattern for the solar abundance. Neon is used for normalization as its isotopic (non-radiogenic) composition in the mantle is clearly different from that of the atmosphere (see below). 

Atmospheric xenon is also isotopically unique among Xe components in the Solar System (with the possible exception of Martian atmospheric Xe, e.g. Swindle 1995) as it is fractionated by 3% per atomic mass unit (a.m.u.). In addition, atmospheric xenon is depleted relative to the chondritic or solar patterns the Kr/Xe ratio of air is 25 times the mean ratio of chondrites (Fig. 2). The depletion of xenon is not consistent with its isotopic fractionation because, in the first case, the heavy element (Xe) is depleted relative to the lightest one (Kr) and, in the second case, the light isotopes (e.g. Xe) are depleted relative to the heavy ones (e.g. Xe). The elemental depletion of Xe could be due to xenon trapping in a terrestrial reservoir, but possibilities exclude ice or sediments, which cannot explain the 25-fold depletion (Podosek et al. 1981 Ber-natowicz et al. 1985). Preferential subduction of Xe trapped in sediments could account qualitatively for it, but is not yet documented. 

I 1.3.3 Partitioning anthropogenic CO2 among the ocean, atmosphere, and terrestrial reservoirs... 

The biotic activity of organisms is accompanied by the fractionating of the isotopic composition of carbon dioxide. Vladimir Vernadsky (1926) predicted this effect before the first experimental evidence became available. It is known that the carbon of terrestrial reservoirs consists of two stable isotopes and supplemented with small quantities of C radioactive species (T i/2 = 5730 years). For more details on this fractionation see Chapter 2, Box 6. 

---