7 NATURAL ABUNDANCE OF THE STABLE ISOTOPES OF C, O AND H This chapter is concerned with the natural concentrations of the stable isotopes of hydrogen, carbon and oxygen, with particular attention paid to those compounds relevant in the hydrological cycle. For each isotope separately, we discuss the natural fractionation effects, internationally agreed definitions, standards, and reference materials, and variations in the natural abundances. In order to help the reader to appreciate isotopic abundance values as they occur in nature, Fig.7.1 shows some actual isotope ratios and fractionations in a choice of equilibrium systems. Surveys of some practical data of all isotopes concerned are given in the respective Sections. 2R = 0.00014354 18R = 0.00198567 H2O (vapour) H2O (vapour) 2 18 v/l = 78.4‰ v/l = 9.74‰ o o H2O (liquid) at 20 C H2O (liquid) at 20 C 2R = 0.00015575 18R = 0.00200520 13R = 0.0111421 18R = 0.00200520 o CO2 (gas) H2O (liquid) at 20 C 13 18 g/b = 8.46‰ s/l = +30.42‰ o HCO3 (bicarbonate) at 20 C CaCO3 (solid calcite) 13R = 0.0112372 18R = 0.00206620 Fig.7.1 Examples of isotope ratios of compounds in isotopic equilibrium and the respective isotope fractionations, as defined in Sect.3.3. As an example may serve the calculation of 13 13 13 g/b as g/b 1 with g/b = 0.0111421/0.0112372 = 0.99154. 89 Chapter 7 7.1 STABLE CARBON ISOTOPES Table 7.1 The stable and radioactive isotopes of carbon: practical data for the natural abundance, properties, analytical techniques and standards. Further details are given in Sect.7.1 and 8.1, and in Chapters 10 and 11. MS = mass spectrometry, PGC = proportional gas counting, LSS = liquid scintillation spectrometry, AMS = accelerator mass spectrometry 12C 13C 14C stability stable stable radioactive natural abundance 0.989 0.011 < 1012 natural specific activity < 0.25 Bq/gC decay mode / daughter / 14N half-life (T1/2) 5730 a decay constant () 1.21P10 4/a = 1/8267 a 1 max. energy 156 keV abundance range 30‰ 0 to 1012 in hydrological cycle reported as 13 or 13C 14A, 14a, 14, or 14 in ‰ dpm/gC, Bq/gC, %, or ‰ instrument MS PGC, LSS, AMS analytical medium CO2 CO2, C2H2, CH4, C6H6, graphite usual standard deviation 0.03‰ 1‰ to 1% at natural level international standard VPDB Oxalic acid: Ox1, Ox2 with absolute value 0.0112372 13.56 dpm/gC 7.1.1 THE NATURAL ABUNDANCE The chemical element carbon has two stable isotopes, 12C and 13C. Their abundance is about 98.9% and 1.1%, so that the 13C/12C ratio is about 0.011 (Nier, 1950). As a result of several fractionation processes, kinetic as well as equilibrium, the isotope ratio shows a natural variation of almost 100‰. 90 Stable Isotopes of Hydrogen, Carbon and Oxygen atmospheric CO2 marine HCO3 marine carbonate marine plants marine plankton land plants C4 type land plants C3 type groundwater HCO3 fresh-water carbonate wood peat coal oil natural gas bacterial CH4 animal bone diamond 80 60 40 20 0 +20 13 VPDB (‰) Fig.7.2 General view of 13C/12C variations in natural compounds. The ranges are indicative for the materials shown. Fig.7.2 presents a broad survey of natural abundances of various compounds, at the low-13C end bacterial methane (marsh-gas), at the high end the bicarbonate fraction of groundwater under special conditions. In the carbonic acid system variations up to 30‰ are normally observed. Wider variations occur in systems in which carbon oxidation or reduction reactions take place, such as the CO2 (carbonate) CH4 (methane) or the CO2 (CH2O)x (carbohydrate) systems. 7.1.2 CARBON ISOTOPE FRACTIONATIONS It will later be shown that the presence of dissolved inorganic carbon (DIC) in sea-, ground-, and surface water presents the possibility of studying gas-water exchange processes and of measuring water transport rates in oceans and in the ground. In connection with studying these phenomena, the stable and radioactive isotopes of carbon and their interactions pay an important contribution, often together with the water chemistry. 91 Chapter 7 fractionation 13 (‰) 4 s/b CaCO3 HCO3 0 c/b 2 CO3 -4 g/b CO2 g a/b -8 CO2 aq -12 0 10 20 30 40 temperature t (LC) Fig.7.3 Temperature-dependent equilibrium isotope fractionation for carbon isotopes of gaseous CO2 (g), dissolved CO2 (a), dissolved carbonate ions (c), and solid carbonate (s) with respect to dissolved HCO3 (b). The actual data and equations are given in Table 7.2. In nature equilibrium carbon isotope effects occur specifically between the phases CO2 H2O H2CO3 CaCO3. Values for the fractionations involved only depend on temperature and are obtained from laboratory experiments. A survey is presented in Fig.7.3 and Table 7.2. The kinetic fractionation of specific interest is that during carbon dioxide assimilation, i.e. the CO2 uptake by plants. The relatively large fractionation (up to about 18‰) is comparable to the effect observed during absorption of CO2 by an alkaline solution. A quantitative estimate shows that the isotope effect as a result of diffusion of CO2 through air can not explain the fractionation (Sect.3.5). The resulting value of 13 is 0.9956, so that only 4.4‰ of the total assimilation fractionation in favour of 12C can be explained by the diffusion. The remaining 13.6‰, therefore, has to be found in the surface of the liquid phase and in the subsequent biochemical process. 92 Stable Isotopes of Hydrogen, Carbon and Oxygen Table 7.2 Carbon isotope fractionation in the equilibrium system CO2HCO3CO3CaCO3; 13 y/x represents the fractionation of compound y relative to compound x. Values for intermediate temperatures may be calculated by linear interpolation (see also Fig.7.3). T = t (LC) + 273.15 K 2 g = gaseous CO2 , a = dissolved CO2 , b = dissolved HCO3 , c = dissolved CO3 ions, s = solid calcite. 13 1 13 2 13 3 13 4 13 5 13 6 t g/b ) a/g ) a/b ) c/b ) s/b ) s/g ) L ( C) (‰) (‰) (‰) (‰) (‰) (‰) 0 10.83 1.18 12.00 0.65 0.39 +10.55 5 10.20 1.16 11.35 0.60 0.11 +10.19 10 9.60 1.13 10 72 0.54 + 0.15 + 9.85 15 9.02 1.11 10.12 0.49 + 0.41 + 9.52 20 8.46 1.09 9.54 0.44 + 0.66 + 9.20 25 7.92 1.06 8.97 0.39 + 0.91 + 8.86 30 7.39 1.04 8.42 0.34 + 1.14 + 8.60 35 6.88 1.02 7.90 0.29 + 1.37 + 8.31 40 6.39 1.00 7.39 0.25 + 1.59 + 8.03 1 13 ) Mook et al., 1974 : g/b = 9483/T + 23.89‰ (7.1) 2 13 ) Vogel et al., 1970 : a/g = 373/T + 0.19‰ (7.2) 3 1 2 13 ) From ) and ) : a/b = 9866/T + 24.12‰ (7.3) 4 1 13 ) Thode et al., 1965 and ) : c/b = 867/T + 2.52‰ (7.4) 5) Our evaluation of the original data from Rubinson & Clayton, 1969 13 and Emrich et al., 1970 : s/b = 4232/T + 15.10‰ (7.5) 6 1 5 13 ) From ) and ) : s/g = + 5380/T 9.15‰ (7.6) Another kinetic process occurring in the soil is the bacterial decomposition of organic matter to form methane (CH4). Here the largest fractionation amounts to about 55‰. In this process 13 CO2 is simultaneously produced with a fractionation of +25‰, resulting in a value of about 0‰. A special problem is the kinetic fractionation during uptake and release of CO2 by seawater. This fractionation is included in calculations on global 13C modelling. First we have to emphasise that the difference in isotopic composition between, for instance, gaseous CO2 and the dissolved inorganic carbon content of water can not be addressed by isotope fractionation between CO2 and DIC. 93 Chapter 7 13 13 (‰) rel. to airCO2 VPDB (‰) pH=8.2 +10 20 LC equilibrium kinetic +2 oceanic HCO +8 3 CO 2 3 DIC 0 +6 2 +4 13 13 13 13 J b/g c/g /g (sea air) 4 +2 6 atmospheric 0 CO2 13 J CO2 8 CO2(aq) (air sea) 2 13 a-g 10 CO taken up released 4 2 by the sea 12 Fig.7.4 Fractionation for the isotopic equilibrium between CO2(gas)dissolved 2 CO2(aq)dissolved bicarbonate (HCO3 )dissolved carbonate (CO3 ) (Mook, 1986). The right-hand scale indicates approximate natural 13 values with respect to VPDB. The right-hand part shows kinetic fractionations during uptake and release of atmospheric oceanic CO2, respectively. DIC is the total dissolved inorganic carbon content (CT). Isotope fractionation is the phenomenon that, due to an isotope exchange process, a difference between the isotopic composition of two compounds occurs, while seawater carbon consists of 3 fractions, i.e. dissolved CO2 (H2CO3 is hardly present), dissolved HCO3 , and dissolved 2 13 CO3 . All these fractions are fractionated isotopically with respect to each other. The R value of DIC is: 13 13 2 13 [CO2aq] R CO +[HCO3 ] R HCO +[CO3 ] R CO 13 2aq 3 3 R DIC = 2 [CO2aq] +[HCO3 ] +[CO3 ] or in terms of the respective fractionations: 13 13 13 a a / b b c c / b 13 R DIC R b (7.7) CT 94 Stable Isotopes of Hydrogen, Carbon and Oxygen or in terms of values: 13 13 13 13 13 a a / b b c c / b 13 a a / b c c / b DIC b (7.8) CT CT where the brackets indicate the respective concentrations, which are also denoted by a (acid), b (bicarbonate), and c (carbonate ions), so that a + b + c = CT.