Limestone carbon isotopes

C-values of limestones vary mostly within a band of 0 3%c since at least 3.5 Ga (Veizer and Hoefs 1976). The longer term C-isotope trend for carbonates has be punctuated by sudden shifts over short time intervals named carbon isotope events , which are considered to represent characteristic features, and have been used as time markers for stratigraphic correlations. 

It has an I80 / 160 ratio of 2005.20 ( 0.45) x 106 (Baertschi, 1976). In many studies involving carbonates, PDB is the standard relative to which values are reported. The PDB standard is based on the oxygen isotope ratio of a Cretaceous belemnite from the Peedee formation in South Carolina. Because it was composed of calcite, many limestones have isotopic ratios close to it. Values ("X") reported relative to the two standards are related by equation 3.16. 

This latter statement is supported by carbon isotope data on limestone samples collected from the Pleistocene carbonate sequences of Barbados Island in... 

The use of the stable isotope as a tracer in biological research has become increasingly common as evidenced by recent bibliographies (1,2), The effective use of this isotope has been established in the field of nutrition, where it has been applied in human clinical studies (3,4), in food science research and in ecological studies of animal food habits (6,7) Many nutrition studies are carried out at natural abundance levels of C Because these levels are low and because differences in the content of natural materials are small, stable carbon isotope ratios (13q/1 ) expressed in relative terms as 6 values A value represents the per mil (parts per thousand) deviation of the content of the sample from the international PDB limestone standard, the value of which has been set arbitrarily to 0 /oo Thus, a value of -27 0 %o would mean that the sample contained 27 parts per thousand less than the PDB standard Although the PDB standard no... 

Bickle M. J., Chapman H. J., Ferry J. M., Rumble D., Ill, and Fallick A. E. (1997) Fluid flow and diffusion in the Waterville Limestone, south-central Maine constraints from strontium, oxygen, and carbon isotope profiles. J. Petrol. 38, 1489-1512. 

Scholle P. and Arthur M. A. (1980) Carbon isotope fluctuations in Cretaceous pelagic limestones potential stratigraphic and petroleum exploration tool. Am. Assoc. Petrol. Geol. Bull. 64, 67-87. 

The carbon isotopic composition of marine limestones is generally interpreted to reflect the rate of organic carbon burial (Forg, molyr ), although a number of other factors are actually involved and may be as significant for particular events or trends. The time rate of change of the of the ocean, can be expressed as (e.g., Kump and Arthur, 1999)... 

Whole-rock samples of mudrock from both shallowly and deeply buried rocks have similar carbon isotopic ratios, but distinctly different oxygen isotopic ratios (Fig. 8). 5 C values for most samples are from -2 to +l%o, values typical of marine biogenic carbonate or detrital clasts of marine limestone. One anomalously light sample (-8%o) in the Antog-nola Formation, and other samples with 5 C values less than -2%o, contain some authigenic carbon derived from organic material. 

Fig. 5. Stable isotopic compositions of saddle dolomite. Note the tendency toward negative carbon isotope values from siliciclastic settings. Data from Friedman (1989) could not be included because the author did not report 6 - C values, and data from Woronick Land (1985) are not included because of inadequate data presentation. Data from Wojcik et al. (1992) on saddle dolomite and ankerite are plotted in the limestone and sandstone categories, rather than the mixed category (see Table 1).

Baker A.J., FallickA.E. (1989) Evidence from Lewisian limestones for isotopically heavy carbon in two-thousand-million-year-old water. Nature 337, 352—4. 

Most of the samples collected over this interval and from certain other regions represent only non-transformed micrite carbonates. XRD analyses and microscopic observations only reveal very poorly crystallized limestones, the carbon isotope composition of which corresponds to that of carbonate rocks of marine origin. Even here,... 

PeeDee Belemnite (PDB) Oxygen and carbon isotope ratios are measured as relative deviations (5 0 per mil 5 C per mil) from a laboratory standard value. The standard normally employed for the analysis of carbonates is a PDB limestone (belemnite shell from the Cretaceous Peedee Formation of South Carolina). 

Oceans play a significant role in the exchange of CO2 between the atmosphere and biosphere. Carbon isotope exchange between CO2 in the air and HCO3 in the oceans results in C enrichment of the latter. CO2 in air is well mixed globally and currently has a <5 C value of - 84 V-PDB compared to marine limestones at 04 V-PDB. Freshwater carbonates have much more variable and lower values due to the oxidation of C depleted organic matter and subsequent formation of HCOj. 

In a similar vein, mean seawater temperatures can be estimated from the ratio of 0 to 0 in limestone. The latter rock is composed of calcium carbonate, laid down from shells of countless small sea creatures as they die and fall to the bottom of the ocean. The ratio of the oxygen isotopes locked up as carbon dioxide varies with the temperature of sea water. Any organisms building shells will fix the ratio in the calcium carbonate of their shells. As the limestone deposits form, the layers represent a chronological description of the mean sea temperature. To assess mean sea temperatures from thousands or millions of years ago, it is necessary only to measure accurately the ratio and use a precalibrated graph that relates temperatures to isotope ratios in sea water. 

Meyers, W. J. and K. C. Lohmann, 1985, Isotope geochemistry of regional extensive calcite cement zones and marine components in Mississippian limestones, New Mexico. In N. Schneidermann and P. M. Harris (eds.), Carbonate Cements. SEPM Special Publication 36,223-239. 

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