Terrestrial carbon values

Recently, Swart et al. 67 found carbon (probably present as elemental carbon) highly enriched in C-13 in Murchison and Allende. The 8 values up to + 1100 per mil correspond to 12C/13C = 42 compared to between 88 and 93 for terrestrial carbon. This exotic carbon is associated as the carrier phase with several noble gases, also characterised by anomalous isotopic distribution. 

Estimates of the size of the global soil organic carbon (SOC) pool have ranged between 700 Pg (Bolin, 1970) and 2946 Pg (Bohn, 1976), with a value of around 1500 Gt now generally accepted as the most appropriate (Table 1). This value is considered to be between one-half (e.g., Townsend et ah, 1995) and two-thirds (e.g., Trumbore et ai, 1996) of the total terrestrial carbon pool. 

Sano and Williams (1996) calculated present-day volcanic carbon flux from subduction zones to be 3.1 x 10 mol/year based on He and C isotopes and C02/ He ratios of volcanic gases and fumaroles in circum-Pacific volcanic regions. Williams et al. (1992) and Brantley and Koepenich (1995) reported that the global CO2 flux by subaerial volcanoes is (0.5-2.0) x lO mol/m.y. and (2-3) x 10 mol/m.y. (maximum value), respectively. Le Guern (1982) has compiled several measurements from terrestrial individual volcanoes to derive a CO2 flux of ca. 2 x 10 mol/m.y. Le Cloarec and Marty (1991) and Marty and Jambon (1987) estimated a volcanic gas carbon flux of 3.3 X 10 mol/m.y. based on C/S ratio of volcanic gas and sulfur flux. Gerlach (1991) estimated about 1.8 x 10 mol/m.y. based on an extrapolation of measured flux. Thus, from previous estimates it is considered that the volcanic gas carbon flux from subduction zones is similar to or lower than that of hydrothermal solution from back-arc basins. 

PCO2 values were estimated by 8 C method by Cerling (1984, 1991, 1992a,b) who used 8 C of carbonates in terrestrial soil as an indicator of PcOi- However, these data on Cenozoic age are scarce and scattered. 

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. 

Figure 5. A plot of A Mg vs. 5 Mg for terrestrial Mg materials. Within best estimates of uncertainties (cross) all of the data lie in the region bounded by equilibrium and kinetic mass fractionation laws. Waters, carbonates, and organic Mg (chlorophyll) have higher A Mg values than mantle and crustal Mg reservoirs represented by mantle pyroxene, loess, and continental basalts. The difference in A Mg values is attributable to episodes of kinetic mass fractionation.

Figure 7. A Mg vs. 5 Mg plot of calcite speleothems and their drip waters from the Soreq cave site, Israel (data from Galy et al. 2002) compared with seawater. The horizontal trend of the data suggests that Mg in carbonates is related to aqueous Mg by equilibrium fractionation processes. Results of a three-isotope regression, shown on the figure and in Table 3, confirm that the (3 value defined by the data is similar to the predicted equilibrium value of 0.521 and distinct from kinetic values. The positive A Mg characteristic of the speleothem carbonates is apparently inherited from the waters. The positive A Mg values of the waters appear to be produced by kinetic fractionation relative to primitive terrestrial Mg reservoirs (the origin).

Note The calculation of relative molecular mass, Mr, of organic molecules exceeding 2000 u is significantly influenced by the basis it is performed on. Both the atomic weights of the constituent elements and the natural variations in isotopic abundance contribute to the differences between monoisotopic- and relative atomic mass-based values. In addition, they tend to characteristically differ between major classes of biomolecules. This is primarily because of molar carbon content, e.g., the difference between polypeptides and nucleic acids is about 4 u at Mr = 25,000 u. Considering terrestrial sources alone, variations in the isotopic abundance of carbon lead to differences of about 10-25 ppm in Mr which is significant with respect to mass measurement accuracy in the region up to several 10 u. [41]... 

The carbon isotopic composition of marine humic compounds is heavy 8 C values ranging from -20%o to -23%o Lighter carbon isotopic composition (8 C = —25 to -28%o) is characteristic of terrestrially derived humus... 

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