Carbonates, terrestrial distribution

The content of the material in a carbon reservoir is a measure of that reservoir s direct or indirect exchange rate with the atmosphere, although variations in solar also create variations in atmospheric content activity (Stuiver and Quay, 1980, 1981). Geologically important reservoirs (i.e., carbonate rocks and fossil carbon) contain no radiocarbon because the turnover times of these reservoirs are much longer than the isotope s half-life. The distribution of is used in studies of ocean circulation, soil sciences, and studies of the terrestrial biosphere. 

Until recently, the distribution of carbon among the different terrestrial spheres was stable. When humans began burning fossil fuels, however, such burning transferred carbon into the atmosphere as CO2. This has become a rapidly changing feature of the overall carbon cycle. Over the last quarter century, the atmospheric concentration of CO2 has grown by more than 10%. 

Considerable geographic variability exists in the distribution of the source rocks contributing salts to river and groundwaters. As shown in Table 21.3, most of the evaporites, which are the dominant natural source of Na and Cl in river water, lie in marginal and endorheic (internal) seas. Some of these subsurfece evaporite deposits dissolve into groundwaters, which eventually carry Na and Cl into the ocean. Carbonates are the prevalent rock type between 15°N and 65°N. Precambrian-age crustal rocks and meta-morphic minerals predominate between 25°S and 15°N and north of 55°N. Shales and sandstones represent on average 16% of the terrestrial surfece lithology. 

Some component of the terrestrial POM must be extremely nonreactive to enable a higher burial efficiency as compared to autochthonous POM. A possible candidate for this nonreactive terrestrial POM is black carbon. This material is a carbon-rich residue produced by biomass burning and fossil fuel combustion. Some black carbon also appears to be derived from graphite weathered from rocks. It is widely distributed in marine sediments and possibly carried to the open ocean via aeolian transport. 

Schoeninger MJ, DeNiro MJ (1984) Nitrogen and carbon isotopic composition of bone collagen from marine and terrestrial animals, Geochim Cosmochim Acta 48 625-639 Scholten SO (1991) The distribution of nitrogen isotopes in sediments, PhD Thesis University of Utrecht... 

Lipids are important biomarkers because they carry a strong carbon-number predominance that is inherited from their biosynthesis (e.g. buildup from acetate), and their homolog distribution can reflect biogenic origin (i.e. marine vs. terrestrial origin) They are generally derived from... 

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. 

The distribution and abundance of n-alkanols (fatty alcohols) can be used to distinguish between terrestrial plant and algal/bacterial inputs to aquatic systems (Cranwell, 1982). The epicuticular waxes of vascular plants have been shown to have n-alkanols with an even number of carbon atoms (typically C22-C30) (Eglinton and Hamilton, 1967 Rieley et al., 1991). One of the most abundant n-alkanols in higher plants is 1 -octacosanol [CH3(CH2)26CH20H]. Alkanols are synthesized by enzymatic reduction of fatty acids, as shown in the following example ... 

An alternative to the terrestrial synthesis of the nucleobases is to invoke interstellar chemistry. Martins has shown, using an analysis of the isotopic abundance of 13C, that a sample of the 4.6 billion year old Murchison meteorite which fell in Australia in 1969 contains traces of uracil and a pyrimidine derivative, xanthine. Samples of soil that surrounded the meteor when it was retrieved were also analyzed. They gave completely different results for uracil, consistent with its expected terrestrial origin, and xanthine was undetectable [48], The isotopic distributions of carbon clearly ruled out terrestrial contamination as a source of the organic compounds present in the meteorite. At 0°C and neutral pH cytosine slowly decomposes to uracil and guanine decomposes to xanthine so both compounds could be the decomposition products of DNA or RNA nucleobases. They must have either travelled with the meteorite from its extraterrestrial origin or been formed from components present in the meteorite and others encountered on its journey to Earth. Either way, delivery of nucleobases to a prebiotic Earth could plausibly have been undertaken by meteors. The conditions that formed the bases need not have been those of an early Earth at all but of a far more hostile environment elsewhere in the Solar System. That environment may have been conducive to the production of individual bases but they may never have been able to form stable DNA or RNA polymers this development may have required the less extreme conditions prevalent on Earth. 

Sea water contains a much lower concentration of dissolved organic matter than river water. More than half of this dissolved organic load is of a humic nature. These dissolved organic acids tend to flocculate as the salinity increases (10). Hair and Bassett (11) have observed an increase in the particulate humic acid load of an estuary as one approaches the sea. Although no studies of the distribution of humic materials throughout an estuarine system have been performed, it would appear that estuaries and their sediments in particular, act as a major sink for the dissolved and particulate humic materials. Nissenbaum and Kaplan (12) have observed that terrestrial humic materials are not deposited at great distances from shore in the marine system. A study of the flux of particulate carbon through the Chesapeake Bay comes to a similar conclusion (13). 

It appears likely that the application of stable isotope techniques to the study of paleonutrition will continue. Carbon isotopes have been shown to be useful to trace the introductions of tropical Ck crops into temperate regions where C3 plants predominate. Other recent studies with carbon isotopes have demonstrated that seafoods are more enriched in than terrestrial foods, and that the proportions of marine vs land-derived foods can be estimated in prehistoric coastal people ( ). Elements other than carbon may demonstrate isotopic distributions that will provide useful dietary information. For example, ratios of... 

Distributions in the solar system. More data on volatiles throughout the solar system are clearly required to confidently describe the volatile acquisition history of the terrestrial planets in the proper context. There are several unknown values for the solar composition, including the nitrogen-and carbon-isotope compositions. The compositions of comets from different orbital distances are needed to assess the extent of radial transport of volatiles late in accretion history. In addition, the causes of carbon- and nitrogen-isotope variations in chondrites must be better understood. While it is clear that the Earth cannot be constructed simply by mixing of different meteorite classes, it is not yet possible to unambiguously extrapolate to the volatile compositions of protoplanetary materials. 

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