Carbon isotopes waters

One method for measuring the temperature of the sea is to measure this ratio. Of course, if you were to do it now, you would take a thermometer and not a mass spectrometer. But how do you determine the temperature of the sea as it was 10,000 years ago The answer lies with tiny sea creatures called diatoms. These have shells made from calcium carbonate, itself derived from carbon dioxide in sea water. As the diatoms die, they fall to the sea floor and build a sediment of calcium carbonate. If a sample is taken from a layer of sediment 10,000 years old, the carbon dioxide can be released by addition of acid. If this carbon dioxide is put into a suitable mass spectrometer, the ratio of carbon isotopes can be measured accurately. From this value and the graph of solubilities of isotopic forms of carbon dioxide with temperature (Figure 46.5), a temperature can be extrapolated. This is the temperature of the sea during the time the diatoms were alive. To conduct such experiments in a significant manner, it is essential that the isotope abundance ratios be measured very accurately. 

The amounts of the standard isotopic species and the tracer isotopic species are represented by X and X for the sample and the reference material. The reference substance is chosen arbitrarily, but is a substance that is homogeneous, available in reasonably large amounts, and measurable using standard analytical techniques for measuring isotopes (generally mass spectrometry). For instance, a sample of ocean water known as Standard Mean Ocean Water (SMOW) is used as a reference for and 0. Calcium carbonate from the Peedee sedimentary formation in North Carolina, USA (PDB) is used for C. More information about using carbon isotopes is presented in Chapter 11. 

Fig. 5. Water-use efficiency (carbon basis) v. average carbon isotope discrimination in the whole plant, r= -0.88. Open symbols represent well-watered plants and closed symbols represent plants that were droughted. Tifton 8 A, Florunner A, VB187 and +, Chico are cultivars of peanut (Arachis hypogaea). (From Flubick etal., 1986).

Farquhar, G.D., Hubick, K.T., Condon, A.G. Richards, R.A. (1988). Carbon isotope fractionation and plant water-use efficiency. In Applications of Stable Isotope Ratios to Ecological Research, ed. P.W. Rundel, J.R. Ehleringer K.A. Nagy, pp.21 0. New York Springer-Verlag. 

Hubick, K.T. Farquhar, G.D. (1987). Carbon isotope discrimination - selecting for water use efficiency. Australian Cotton Grower 8, 66-8. 

Hubick, K.T., Farquhar, G.D. Shorter,R. (1986). Correlation between water-use efficiency and carbon isotope discrimination in diverse peanut (Arachis) germplasm. Australian Journal of Plant Physiology, 13, 803-16. 

Martin, B.J. Thorstenson, Y.R. (1988). Stable carbon isotope composition (6 C), water use efficiency and biomass productivity of Lycopersicon esculen-tum, Lycopersicon pennellii and the Fi hybrid. Plant Physiology 88, 218-23. 

Masle, J. Farquhar, G.D. (1988). Effects of soil strength on the relation of water use efficiency and growth to carbon isotope discrimination in wheat seedlings. Plant Physiology, 6,147-55. 

Shackleton NJ, Imbrie J, Hall MA (1983) Oxygen and carbon isotope record of East Pacific core V19-30 implications for the formation of deep water in the late Pleistocene Nodh Atlantic. Earth Planet Sci... 

Wemer M, Mikolajewicz U, Hoffmann G, Heimann M (2000) Possible changes of in precipitation caused by a meltwater event in the North Atlantic. J Geophys Res 10 10161-10167 Whitehead NE, Ditchbmn RG, Wilhams PW, McCabe WJ (1999) Pa and contamination at zero age a possible hmitation on U/Th series dating of speleothem material. Chem Geol 156 359-366 Wigley TML, Plummer LN, Pearson FJ (1978) Mass transfer and carbon isotope evolntion in natural water systems. Geochim Cosmochim Acta 42 1117-1140... 

These three numerical experiments show how the waters of an evaporating lagoon respond differently to the different seasonal perturbations that might affect them. Some record of these perturbations might, in principle, be preserved in the carbonate sediments precipitated in the lagoon. All three perturbations—productivity, temperature, and evaporation rate— cause seasonal fluctuations in the saturation state of the water and in the rate of carbonate precipitation. Temperature oscillations have little effect on the carbon isotopes. Although seasonally varying evaporation rates affect 14C, they have little effect on 13C. Productivity fluctuations affect both of the carbon isotopes. 

In this chapter I explained how isotope ratios may be calculated from equations that are closely related, but not identical, to the equations for the bulk species. Extra terms arise in the isotope equations because isotopic composition is most conveniently expressed in terms of ratios of concentrations. I illustrated the use of these equations in a calculation of the carbon isotopic composition of atmosphere, surface ocean, and deep ocean and in the response of isotope ratios to the combustion of fossil fuels. As an alternative application, I simulated the response of the carbon system in an evaporating lagoon to seasonal changes in biological productivity, temperature, and evaporation rate. With a simulation like the one presented here it is quite easy to explore the effects of various perturbations. Although not done here, it would be easy also to examine the sensitivity of the results to such parameters as water depth and salinity. 

Rightmire, C. T., Hanshaw, B. B., Relationship between the carbon isotope composition of soil C02 and dissolved carbonate species in qroundwater. Water Resour. Research, 9(4), 958-567 (1973). 

Carbon isotope fractionation by C02 absorption at the air water interface has been measured [61], and the ratio C13/C12 has been found to vary from 9.2 to 6.7 ppt over the temperature range 0° - 30 °C. Thus we may write the enrichment at time t as,... 

Sverjensky DA, Shock EL, Helgeson HC (1997) Prediction of flie thermodynamic properties of aqueous metal complexes to 1000°C and 5 kb. Geochim Cosmochim Acta 61 1359-1412 Tarutani T, Clayton RN, Mayeda TK (1969) The effect of polymorphism and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water. Geochim Cosmochim Acta 33 987-996... 

Bottinga Y (1968) Calculations of fractionation factors for carbon and oxygen isotope exchange in the system calcite-carbon dioxide-water. J Phys Chem 72 800-808 Bullen TD, White AF, Childs CW, Vivit DV, Schulz MS (2001) Demonstration of significant abiotic iron isotope fractionation in nature. Geology 29 699-702... 

Carbonate rocks and foraminifera tests (a sample of mixed species) are consistently lower in 5 Mg than Mg from seawater by several per mil. In addition. Mg in calcite is consistently lower in 5 Mg than Mg in dolomite by approximately 2%o (Fig. 1). These data together with the samples of coeval speleothem calcite and waters show that the heavy isotopes of Mg partition to water relative to carbonate minerals. In this respect the Mg isotopes behave much like the isotopes of Ca (Gussone et al. 2003 Schmitt et al. 2003). There is not yet sufficient data to assess with confidence the temperature dependence of the fractionation of Mg isotopes between carbonates and waters, although Galy et al. (2002) concluded that the evidence so far is that temperature effects are below detection in the range 4-18°C. 

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. 

Moecher DP, Valley JW, Essene EJ, (1994) Exhaction and carbon isotope analysis of COj from scapolite in deep crustal granulites and xenoliths. Geochim Cosmochim Acta 58 959-967 Mojzsis SJ, Harrison TM, Pidgeon RT (2001) Oxygen-isotope evidence from ancient zircons for liquid water at the Earth s surface 4,300 Myr ago. Nature 409 178-181 Muehlenbachs K, Clayton RN (1976) Oxygen isotope composition of the oceanic crust and its bearing on seawater. J Geophys Res 81 4365-4369... 

Deines P, Langmuir D., and Harmon R. S. (1974). Stable carbon isotope ratios and the existence of a gas phase in the evolution of carbonate ground waters. Geochim. Cosmochim. Acta, 38 1147-1164. 

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