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Fractionation of oxygen isotopes between mammalian bone-phosphate and environmental drinking water
0016.7037/84/$3.00
Grochmnco CI Cusmwhuno AcIu Vol. 48. pp. 1689-1693 0 Pergamon Press Ltd. 1984. Pnnted in U.S.A.
LE’lTER
+
00
TO THE EDITOR
Fractionation of oxygen isotopes between mammalian environmental drinking water
bone-phosphate
and
BOAZ Luz*, YEHOSHUA KOLODNY* and MICHAL HOROWITZ* *Department of Geology, The Hebrew University of Jerusalem, Israel *Department of Physiology, The Hebrew University of Jerusalem, Israel (Received April 26, 1984; accepted in revised_bm July 23, 1984) A&&act-The b”O of mammalian bone-phosphate varies linearly with 6’*0 of environmental water, but is not in isotopic equilibrium with that water. This situation is explained by a model of 6180 in body water
in which the important fluxes of exchangeableoxygen through the body are taken into account. Fractionation of oxygen isotopes between body and environmental drinking water is dependent on the rates of drinking and respiration. Isotopic fractionation can he estimated from physiological data and the estimates correlate very well with observed fractionation. Species whose water consumption is large relatively to its energy expenditure is sensitive to isotopic ratio changes in environmental water.
LONGINELLI ( 1984) demonstrated convincingly the potential usefulness of 6’*0 in phosphate of mammal bones in paleohydrology. The crux of his approach is that mammalian bone phosphate is formed at constant body temperature (about 37”(Z), and acquires an isotopic composition (6,) which is independent on environmental temperature. The bone isotopic composition differs by a constant value from b”O of body water (I&), and correlates linearly with 6”O of environmental drinking water (6,). LONGINELLI( 1984) does not offer a quantitative explanation for the observed relationships of II&,, 6, and 6,. Our purpose is to present some additional results which support LGNGINELLI’S( 1984) conclusions, and further formulate a model of biogenic fractionation of oxygen isotopes between drinking water and mammalian bone phosphate. This model can serve in assessment of isotopic data of fossil bones, and can help in the use of such data for the estimate of past isotopic composition of environmental water. Figure 1 displays 8, values of our recent analyses of bones and teeth and their respective bbW.The analytical procedure followed that of KOLODNY et al. (1983), and the analytical precision is about +O.% (1 a). 6, was estimated from published data on precipitation (GAT, 1980), (except for samples from Israel and Finland where tap water was analysed by us). The error in the 6, estimates is about 1k in low latitudes, and about 2% in high latitudes. Relative to the large isotopic range of the data these errors are small, and 6, varies linearly with 6,. Regression on data of human teeth (excluding dog and muskox) yields the following equation 6& = .786, + 22.7 (Fig. 1) and correlation coefficient r = .97. The data points representing dog and muskox fall close to the human teeth regression line. This is probably just coincidental, since bN-
(1984) demonstrated different slopes for different animals. Analysis of the relationship between 6, and 6, requires some knowledge of the temperature dependent fractionation of oxygen between water and phosphate. No experiments were ever carried out on the isotopic equilibration of inorganic phosphate with water. However, LONGINELLIand NUTI (1973a,b) and KOLODNY et al. ( 1983) obtained similar paleotemperature equations for both phosphate in marine carbonate shells and fish bones. It is therefore likely that this equation represents conditions close to equilibrium. Solving the phosphate paleotemperature equation (I = 1 I 1.4 - 4.3(6, - 6,)) for 6, at 37”C, isothermal points plot on a straight line with unity slope and 17.3 intercept (Fig. 1). Thus 6, of teeth and bones is clearly out of isotopic equilibrium with drinking water of the mammal (see also LONGINELLI,1984). Such observation is hardly surprising. It is to be expected that the oxygen in bone phosphate be in equilibrium with body water rather than with drinking water. Furthermore, the fact that the slope of the plot of 6, VS. 6, is not unity, suggests that the explanation to any divergence of 6, - 6, from equilibrium is not due to simple fractionation of drinking water and body water. In order to understand b, vs. 6, relationships we measured 6”O of laboratory rats’ body-water (6,) and 6, of their bones. bbWwas measured on breath CO*, which is close (with slight “0 depletion of about 0.4%0) to isotopic equilibrium with body water, and at 37°C is about 38% enriched in “0 (PPLUG ef al., 1979; BRENNINKMEIJERet al., 1983). CO* was collected by placing the rats for 5 minutes in a closed chamber. Air enriched in breath CO2 was transferred to a preevacuated vessel. CO1 was purified according to PFLUG et al. (1979), and its 6”O measured on a Micromass GINELLI
1689
I690
B. Luz, Y. Kolodny and M. Horowitz
l
MUSKOX
FIG. 1. Plotof 6, (PO in phosphate, 760SMOW) of human teeth and other mammal bones vs. 6, (6% in average precipitation, %oSMOW). The vertical error bars indicate the standard deviation in sites where more than one specimen was analyzed. The horizontal error bars indicate the uncertainty in estimating 6, from pub&bed data (GAT, 1980). The regession line was computed for human data only (exctuding dog and muskox).
602C mass spectrometer. The average & of 10 subjects thus analysed was -2.6 + 0.4%. Three of those rats were sacrificed and b, of their bone phosphate was measured. It averaged 15.8 -+ 0.3% which is close to equilibrium at body temperature (14.7% for 6, = -2.6%~). The key question is then, what factors control the isotopic composition of body water. This question has been addressed from a different point of view by Lifson and his coworkers (LIFSON and MCCLINTOCK, 1966), in studying rates of turn-over of water and CO, in animals. Below we follow their approach with the significant difference of applying it at low “0 enrichment levels, to a paieoclimatoiogical rather than physiological end. Our aim is to arrive at bbwvs. 6, relationships. The oxygen isotopic composition ofbody water depends on the total oxygen flux through the body. In addition to the obvious input and output of water oxygen, metabolic CO* undergoes rapid isotopic exchange with body water due to enzymatic catalysis of carbonic anhydrase (LIFSONet al., 1949). Thus oxygen in CO1 has to be taken into account in any isotopic mass balance. Oxygen enten the body in three principal forms: water, atmospheric oxygen, and oxywn bound in organic food compounds (Fig. 2). (a) Water is taken up as drinking water, water contained in solid foods, and small amounts as vapor. Neglecting the influx of water in the form of vapor, the total influx of water (F, in Fig. 2) is divided into two fractions. Fraction
(y) entering the body as liquid (isotopic composition 6,) and fraction (1 - y) as water contained in foods. The latter has a complicated relationship with the isotopic composition of meteoric water, and in certain cases is enriched in ‘80 by up to 15%~(GONFIANTINI et al., 1965; FARRISand STRAIN, 1978). We let CX, represent the isotopic fractionation between meteoric water (with composition 6,) and water contained in foods. (b) Atmospheric 0, (flux F2 in Fig. 2) has a nearly constant isotopic composition of 23%0 VS.
Food-O
F3
===+SF
FIG. 2. Oxygen isotopic compositions and fluxes through a mammal body.
1691
0 isotope Fractionation SMOW (DOLE et al., 1954). Fractionation may take place when O2 is taken up by the organism. For example, oxygen enters the blood to form oxyhemoglobin (OHb). There may be a fractionation between O2 and OHb. Assuming such fractionation has magnitude (~2, the isotopic composition of oxygen entering as gas is expressed as (23 + 1000az - 1000). (c) Oxygen bound in organicfiod compounds (flux F, in Fig. 2) has isotopic composition which depends in some way on 6, of the environmental water (BRICOUT,1979). We designate by drthe isotopic composition of oxygen combined in food, and by a3 the possible fractionation occurring when food-oxygen is converted to CO2 and metabolic water. Oxygen leaves the body as wafer (urine, sweat, exhaled vapor etc.) and as C02. The flux of water output is greater than water input (F,) due to production of metabolic water (produced by oxydation of food hydrogen) (flux F4). A fraction (x) of this output is in liquid form with composition Sbwr and another fraction (1 - x) is water vapor with composition (6, - 8%~)(equal or close to 37°C equilibrium fractionation between water and its vapor which is about 1.008, FRIEDMANand O’NEIL, 1977). Oxygen in respiratory CO2 leaves (flux F,) with composition (&,, + 38) (PFLUG et al., 1979). Equation (1) is written for isotopic mass balance in mammals at steady state.
F,y& + F,(l - y)(6, + 1000~1, - 1000) + Fz(23 + 1000az - 1000) + F3(br+ lOOOa>- 1000) = x(F, + F&,,+ + ( 1 - x)(F, + F&&w - 8) + F&,
+ 38)
(1)
As mentioned above a,, at least in certain cases, is significantly different than unity. It will have to be considered in future studies, especially with regard to animals consuming large amounts of leaves. However, in animals which are fed dry food (as those discussed below) its influence on the isotopic composition of water is insignificant (fraction y is very small). The fractionation coefficients az and (Y~are unknown and cannot be discussed further. For the sake of simplicity we assume that they equal one. We thus write Eqn. (1) in a simplified form.
F,6, + 23F2 + b,-F3= x(F, + F&,, + (1 - x)0=, + F4X6bw- 8) + Fs(&, + 38) by solving for &, we obtain I& =
FI 6, F, + F., + F5
+ 8( I - x)(F, + F,) + 23F2 - 38F5 + F& . F, + F4 + F5
0
0
0 I
fin 275
-
--Q
-0-g OA 1
20
6,= S, -(6,-6,) e-“’
/
97
-50I.
26.
DAYS It)
41, FROM
6 I.
a11
THE BEWNNING
(3)
Thus on a i&wvs. 6, plot, a slope of less than unity is to be expected. The physical meaning of that slope (F,/(F, + F4 + Fs)) expresses the ratio between the amount of drinking water intake and the intensity of metabolism. This slope is independent of the isotopic composition of any input or output. Moreover, the slope can be estimated solely from physiological data. Holding all other variables constant the intercept of Eqn. (3) increases with an increase of the fraction of water output as vapor. A lower intercept is expected if there is a transition to fat metabolism, so that F3 X =036. tvz=28
6ew i 25
(2)
to 1 WTtiE
a
12 IS
14 1
01,
16
18 I
EXPERIMENT
FIG.3. Attainment of isotopic steady state in body-water oftwo groups of rats. t,,* designates the biological half times for replacement of oxygen of body water. &,#‘O of body water, 9k SMOW; &,-initial &; d=--steady state I&. Vertical error bars indicate standard deviation where all five rats were measured.
1602
B.Luz,Y. Kolodny and M. Horowitz
b,,=O%
-5&-
’ 0
6,+024
i-L______L__~
5
10
15
20
2s
39
35
90
45 %v
RG. 4.Plot of &. (6”O of body water. 46pSMOW) of five groups of rats in isotopic steady state with drinking water of composition 6, (PO in water 96eSMOW). Error bars as in Fig. 3. decreases (relatively low oxygen content of lipids) as does & (BRICOUT, 1979). In summary, the slope of our line will vary from species to species, depending on the amount of drinking, energy expenditure and the form of water taken in. The line’s intercept is both species dependent and influenced by dietary and environmental factors. In order to test the above model and estimate the slope of Eqn. (3X a controlled experiment was carried out in which the isotopic composition of the drinking water was modified, while attempting to hold all other variables constant. Groups of five rats were placed at normal conditions and diet (Amrcd pellets No. 935, “Ambar”) and were given water of different d, (water was labeled with H2’*0, “Yeda” Rehovoth). By monitoring &b with time it is demonstrated that steady state was reached after two weeks (Fig. 3). Steady state
Aversge *SD
11.55 io.07
Humana ,. 2. 3. 4.
85.2 75.5 71.9 93.3
99.9 82.3 82. i 101.9
34.9 16.1 ?3.‘ 26.2
li.hi li. 77 il. 68 0.7,
u.59
values of & and 6, of five groups are plotted in Fig. 4. S,, correlates very well with 6, fr = 0.991, as was the case with 5, of bones (Fig. 1). Again the regression line has a slope considerably less than unity. Very similar experimental results to both Figs. 3 and 4 are repotted by LONGINELLIand PEREITI PA~ALINO (1980). By substituting rates of water and CO2 input and output (Table 1) i@the slope term of Eqn. (31, we can test its applicability for predicting actual 15, vs. dh,, or CL vs. 6, relationships. We use published values for these variables in rats which were obtained by physiological measurements (LEEand LIFSON,1960), and obtain an average slope of 0.55 zt 0.07. This result is in good agreement with our experimental result of 0.59 (Fig. 4). Average physiological data on human subjects (SCHOELLER~~~VANSANTEN, 1982)yielda slope of 0.70 t 0.06. It compares well with the average slope of human subjects of 0.67 t 0.9 (measured slope Fig. 1: 0.78 (6, vs. 6,); LONGINELLI'S (1984) measured slopes: 0.6 (S, vs. 8,) and 0.64 (8, vs. 6,)). When selecting species suitable for paleoclimatological studies, our mode1 can be used to identify those most sensitive to variations in 6,. An optimal species is one the dbrvvalues of which plot with a large slope vs. 6,. In other words it is a mammal that drinks much and has low metabolic activity. Thus desert animals that conserve water should be avoided. Similarly, active marine mammals that also conserve water (DE-AS et al., 1971) should be used with caution. Arknowledgmenrs-We thank Drs. S. E. Calvert, I. Gedalia, I. R. KapIan, 0. Kouvo, D. 3. Oftner, B. Peters,and S. S&hat for contribution of samples. WCalso thank J. Erez for careful reading of the manuscript and hctpful suggestions. A. Longin&Ii and M. J. DeNirn made very useful comments which helped us impmvc the manuscript. H. P. Schwartz contributed samples, reviewed the manuscript and in early discussions with us helped to formulate the model. I. Caster was very
efficient in her typing. This research has been supported by
1693
0 isotope Fractionation a grant from the Israel Academy of Science and Humanities to B. Luz and Y. Kolodny. REFERENCES BRENNINKMEIJER C. A. M., KRAFT P. and MONK W. G. (1983) Oxygen isotope fractionation between CO2 and HrO. Hot. Geosci. 1, 18 I - 190. BRICOUTJ. (1979) Natural Abundance Levels of *H and “0 in Plant Organic Matter. In Stable Isotopes: Proceedings of the Third International Conference (eds. E. R. KLEIN and P. D. KLEIN), pp. 215-222. Academic Press. DEPOCASF., HART J. S. and FISHERH. D. (197 I) Sea water drinking and water flux in starved and in fed harbor seals. Phocu vitulina. Can. J. Physiol. PhQrmQCOl. 49, 53-62. DOLE G. A., LANE D. P. R. and ZANKELIESD. A. (1954) Isotopic composition of atmospheric oxygen and nitrogen. Geochim. Cosmochim. Acta 6, 65-78.
FARRIS F. and STRAIN B. R. (1978) The effects of waterstress on leaf H2”0 enrichment. Rad. and Environm. Biophys. IS, 167-202.
FRIEDMAN1. and O’NEIL J. R. (1977) Compilation of stable isotope fractionation factors of geochemical interest. USGS Pro/ Paper 44OKK, 1-12. GAT J. ( 1980) The isotopes of hydrogen and oxygen in precipitation. In Handbook of Environmenlal Isotope Geechemistrv (eds. P. FRITZ and J. Ch. FONTES). ,_Vol. I. Chao. 1, pp. 21-48. Elsevier. GONFIANTINIG., GRATZIUS. and TONGIORGIE. (1965) Oxygen isotopic composition of water in leaves. In Isotopes and Radiation in Soil Plant Nutrition Studies. 405-410.
IAEA, Vienna. KOLODNY Y., Luz B. and NAVON
0. (1983) Oxygen isotope variations in phosphate of biogenic apatites. 1. Fish bone
apatite-rechecking the rules of the game. Earth Planet Sci. Lett. 64, 405-416. LEE J. S. and LIFSONN. (1960) Measurement of total energy and material balance in rats by means of doubly labeled water. Amer. J. Physiol. 199, 238-242. LIFSON N., GoRD~~~ G. B., VIS~CHER M. B. and NIER A. 0. C. (1949) The fate of utilized molecular oxygen and the source of the oxygen of respiratory carbon dioxide, studied with the aid of heavy oxygen. J. Eiol. Chem. 180, 803-8
11.
LIFSONN. and MCCLINTOCKR. (1966) Theory of use of the turnover rates of body water for measuring energy and material balance. J. Theoret. Viol. 12, 46-74. LQNGINELLIA. (1984) Oxygen isotopes in mammal bone phosphate: A new tool for paleohydrological and paleoclimatological research? Geochim. Cosmochim. AC~Q 48, 385-390. LONGINELLI A. and NUTI S. (1973a) Revised phosphate-water isotopic temperature scale. Earth Planet. Sci. Left. 19, 373376. LONGINELLIA. and NUTI S. (1973b) Oxygen isotope measurements of phosphate from fish teeth and bones. Eurrh Planet. Sci. Lett. 20, 337-340.
LONGINELLIA. and PERETTI PADALINOA. (I 980) Oxygen isotopic composition of water from mammal blood: first results. Mass. Spectr. in Bioch. Med. and Envir. Res. 1, 135-139. F’FLUG K. P., SCHUSTERK. D., PICHOTKAJ. P. and FORSTEL H. ( 1979) Fractionation effects of oxygen isotopes in mammals. In Stable Isotopes: Proceedings of the Third International Conference (eds. E. R. KLEIN and P. D. KLEIN), DD. 553-561. Academic Press. S&OELLER D. A. and VANSANTENE. (1982) Measurements of energy expenditure in humans by doubly labeled water method. J. Appl. Physiol. 53, 955-959.
Grochmnco CI Cusmwhuno AcIu Vol. 48. pp. 1689-1693 0 Pergamon Press Ltd. 1984. Pnnted in U.S.A.
LE’lTER
+
00
TO THE EDITOR
Fractionation of oxygen isotopes between mammalian environmental drinking water
bone-phosphate
and
BOAZ Luz*, YEHOSHUA KOLODNY* and MICHAL HOROWITZ* *Department of Geology, The Hebrew University of Jerusalem, Israel *Department of Physiology, The Hebrew University of Jerusalem, Israel (Received April 26, 1984; accepted in revised_bm July 23, 1984) A&&act-The b”O of mammalian bone-phosphate varies linearly with 6’*0 of environmental water, but is not in isotopic equilibrium with that water. This situation is explained by a model of 6180 in body water
in which the important fluxes of exchangeableoxygen through the body are taken into account. Fractionation of oxygen isotopes between body and environmental drinking water is dependent on the rates of drinking and respiration. Isotopic fractionation can he estimated from physiological data and the estimates correlate very well with observed fractionation. Species whose water consumption is large relatively to its energy expenditure is sensitive to isotopic ratio changes in environmental water.
LONGINELLI ( 1984) demonstrated convincingly the potential usefulness of 6’*0 in phosphate of mammal bones in paleohydrology. The crux of his approach is that mammalian bone phosphate is formed at constant body temperature (about 37”(Z), and acquires an isotopic composition (6,) which is independent on environmental temperature. The bone isotopic composition differs by a constant value from b”O of body water (I&), and correlates linearly with 6”O of environmental drinking water (6,). LONGINELLI( 1984) does not offer a quantitative explanation for the observed relationships of II&,, 6, and 6,. Our purpose is to present some additional results which support LGNGINELLI’S( 1984) conclusions, and further formulate a model of biogenic fractionation of oxygen isotopes between drinking water and mammalian bone phosphate. This model can serve in assessment of isotopic data of fossil bones, and can help in the use of such data for the estimate of past isotopic composition of environmental water. Figure 1 displays 8, values of our recent analyses of bones and teeth and their respective bbW.The analytical procedure followed that of KOLODNY et al. (1983), and the analytical precision is about +O.% (1 a). 6, was estimated from published data on precipitation (GAT, 1980), (except for samples from Israel and Finland where tap water was analysed by us). The error in the 6, estimates is about 1k in low latitudes, and about 2% in high latitudes. Relative to the large isotopic range of the data these errors are small, and 6, varies linearly with 6,. Regression on data of human teeth (excluding dog and muskox) yields the following equation 6& = .786, + 22.7 (Fig. 1) and correlation coefficient r = .97. The data points representing dog and muskox fall close to the human teeth regression line. This is probably just coincidental, since bN-
(1984) demonstrated different slopes for different animals. Analysis of the relationship between 6, and 6, requires some knowledge of the temperature dependent fractionation of oxygen between water and phosphate. No experiments were ever carried out on the isotopic equilibration of inorganic phosphate with water. However, LONGINELLIand NUTI (1973a,b) and KOLODNY et al. ( 1983) obtained similar paleotemperature equations for both phosphate in marine carbonate shells and fish bones. It is therefore likely that this equation represents conditions close to equilibrium. Solving the phosphate paleotemperature equation (I = 1 I 1.4 - 4.3(6, - 6,)) for 6, at 37”C, isothermal points plot on a straight line with unity slope and 17.3 intercept (Fig. 1). Thus 6, of teeth and bones is clearly out of isotopic equilibrium with drinking water of the mammal (see also LONGINELLI,1984). Such observation is hardly surprising. It is to be expected that the oxygen in bone phosphate be in equilibrium with body water rather than with drinking water. Furthermore, the fact that the slope of the plot of 6, VS. 6, is not unity, suggests that the explanation to any divergence of 6, - 6, from equilibrium is not due to simple fractionation of drinking water and body water. In order to understand b, vs. 6, relationships we measured 6”O of laboratory rats’ body-water (6,) and 6, of their bones. bbWwas measured on breath CO*, which is close (with slight “0 depletion of about 0.4%0) to isotopic equilibrium with body water, and at 37°C is about 38% enriched in “0 (PPLUG ef al., 1979; BRENNINKMEIJERet al., 1983). CO* was collected by placing the rats for 5 minutes in a closed chamber. Air enriched in breath CO2 was transferred to a preevacuated vessel. CO1 was purified according to PFLUG et al. (1979), and its 6”O measured on a Micromass GINELLI
1689
I690
B. Luz, Y. Kolodny and M. Horowitz
l
MUSKOX
FIG. 1. Plotof 6, (PO in phosphate, 760SMOW) of human teeth and other mammal bones vs. 6, (6% in average precipitation, %oSMOW). The vertical error bars indicate the standard deviation in sites where more than one specimen was analyzed. The horizontal error bars indicate the uncertainty in estimating 6, from pub&bed data (GAT, 1980). The regession line was computed for human data only (exctuding dog and muskox).
602C mass spectrometer. The average & of 10 subjects thus analysed was -2.6 + 0.4%. Three of those rats were sacrificed and b, of their bone phosphate was measured. It averaged 15.8 -+ 0.3% which is close to equilibrium at body temperature (14.7% for 6, = -2.6%~). The key question is then, what factors control the isotopic composition of body water. This question has been addressed from a different point of view by Lifson and his coworkers (LIFSON and MCCLINTOCK, 1966), in studying rates of turn-over of water and CO, in animals. Below we follow their approach with the significant difference of applying it at low “0 enrichment levels, to a paieoclimatoiogical rather than physiological end. Our aim is to arrive at bbwvs. 6, relationships. The oxygen isotopic composition ofbody water depends on the total oxygen flux through the body. In addition to the obvious input and output of water oxygen, metabolic CO* undergoes rapid isotopic exchange with body water due to enzymatic catalysis of carbonic anhydrase (LIFSONet al., 1949). Thus oxygen in CO1 has to be taken into account in any isotopic mass balance. Oxygen enten the body in three principal forms: water, atmospheric oxygen, and oxywn bound in organic food compounds (Fig. 2). (a) Water is taken up as drinking water, water contained in solid foods, and small amounts as vapor. Neglecting the influx of water in the form of vapor, the total influx of water (F, in Fig. 2) is divided into two fractions. Fraction
(y) entering the body as liquid (isotopic composition 6,) and fraction (1 - y) as water contained in foods. The latter has a complicated relationship with the isotopic composition of meteoric water, and in certain cases is enriched in ‘80 by up to 15%~(GONFIANTINI et al., 1965; FARRISand STRAIN, 1978). We let CX, represent the isotopic fractionation between meteoric water (with composition 6,) and water contained in foods. (b) Atmospheric 0, (flux F2 in Fig. 2) has a nearly constant isotopic composition of 23%0 VS.
Food-O
F3
===+SF
FIG. 2. Oxygen isotopic compositions and fluxes through a mammal body.
1691
0 isotope Fractionation SMOW (DOLE et al., 1954). Fractionation may take place when O2 is taken up by the organism. For example, oxygen enters the blood to form oxyhemoglobin (OHb). There may be a fractionation between O2 and OHb. Assuming such fractionation has magnitude (~2, the isotopic composition of oxygen entering as gas is expressed as (23 + 1000az - 1000). (c) Oxygen bound in organicfiod compounds (flux F, in Fig. 2) has isotopic composition which depends in some way on 6, of the environmental water (BRICOUT,1979). We designate by drthe isotopic composition of oxygen combined in food, and by a3 the possible fractionation occurring when food-oxygen is converted to CO2 and metabolic water. Oxygen leaves the body as wafer (urine, sweat, exhaled vapor etc.) and as C02. The flux of water output is greater than water input (F,) due to production of metabolic water (produced by oxydation of food hydrogen) (flux F4). A fraction (x) of this output is in liquid form with composition Sbwr and another fraction (1 - x) is water vapor with composition (6, - 8%~)(equal or close to 37°C equilibrium fractionation between water and its vapor which is about 1.008, FRIEDMANand O’NEIL, 1977). Oxygen in respiratory CO2 leaves (flux F,) with composition (&,, + 38) (PFLUG et al., 1979). Equation (1) is written for isotopic mass balance in mammals at steady state.
F,y& + F,(l - y)(6, + 1000~1, - 1000) + Fz(23 + 1000az - 1000) + F3(br+ lOOOa>- 1000) = x(F, + F&,,+ + ( 1 - x)(F, + F&&w - 8) + F&,
+ 38)
(1)
As mentioned above a,, at least in certain cases, is significantly different than unity. It will have to be considered in future studies, especially with regard to animals consuming large amounts of leaves. However, in animals which are fed dry food (as those discussed below) its influence on the isotopic composition of water is insignificant (fraction y is very small). The fractionation coefficients az and (Y~are unknown and cannot be discussed further. For the sake of simplicity we assume that they equal one. We thus write Eqn. (1) in a simplified form.
F,6, + 23F2 + b,-F3= x(F, + F&,, + (1 - x)0=, + F4X6bw- 8) + Fs(&, + 38) by solving for &, we obtain I& =
FI 6, F, + F., + F5
+ 8( I - x)(F, + F,) + 23F2 - 38F5 + F& . F, + F4 + F5
0
0
0 I
fin 275
-
--Q
-0-g OA 1
20
6,= S, -(6,-6,) e-“’
/
97
-50I.
26.
DAYS It)
41, FROM
6 I.
a11
THE BEWNNING
(3)
Thus on a i&wvs. 6, plot, a slope of less than unity is to be expected. The physical meaning of that slope (F,/(F, + F4 + Fs)) expresses the ratio between the amount of drinking water intake and the intensity of metabolism. This slope is independent of the isotopic composition of any input or output. Moreover, the slope can be estimated solely from physiological data. Holding all other variables constant the intercept of Eqn. (3) increases with an increase of the fraction of water output as vapor. A lower intercept is expected if there is a transition to fat metabolism, so that F3 X =036. tvz=28
6ew i 25
(2)
to 1 WTtiE
a
12 IS
14 1
01,
16
18 I
EXPERIMENT
FIG.3. Attainment of isotopic steady state in body-water oftwo groups of rats. t,,* designates the biological half times for replacement of oxygen of body water. &,#‘O of body water, 9k SMOW; &,-initial &; d=--steady state I&. Vertical error bars indicate standard deviation where all five rats were measured.
1602
B.Luz,Y. Kolodny and M. Horowitz
b,,=O%
-5&-
’ 0
6,+024
i-L______L__~
5
10
15
20
2s
39
35
90
45 %v
RG. 4.Plot of &. (6”O of body water. 46pSMOW) of five groups of rats in isotopic steady state with drinking water of composition 6, (PO in water 96eSMOW). Error bars as in Fig. 3. decreases (relatively low oxygen content of lipids) as does & (BRICOUT, 1979). In summary, the slope of our line will vary from species to species, depending on the amount of drinking, energy expenditure and the form of water taken in. The line’s intercept is both species dependent and influenced by dietary and environmental factors. In order to test the above model and estimate the slope of Eqn. (3X a controlled experiment was carried out in which the isotopic composition of the drinking water was modified, while attempting to hold all other variables constant. Groups of five rats were placed at normal conditions and diet (Amrcd pellets No. 935, “Ambar”) and were given water of different d, (water was labeled with H2’*0, “Yeda” Rehovoth). By monitoring &b with time it is demonstrated that steady state was reached after two weeks (Fig. 3). Steady state
Aversge *SD
11.55 io.07
Humana ,. 2. 3. 4.
85.2 75.5 71.9 93.3
99.9 82.3 82. i 101.9
34.9 16.1 ?3.‘ 26.2
li.hi li. 77 il. 68 0.7,
u.59
values of & and 6, of five groups are plotted in Fig. 4. S,, correlates very well with 6, fr = 0.991, as was the case with 5, of bones (Fig. 1). Again the regression line has a slope considerably less than unity. Very similar experimental results to both Figs. 3 and 4 are repotted by LONGINELLIand PEREITI PA~ALINO (1980). By substituting rates of water and CO2 input and output (Table 1) i@the slope term of Eqn. (31, we can test its applicability for predicting actual 15, vs. dh,, or CL vs. 6, relationships. We use published values for these variables in rats which were obtained by physiological measurements (LEEand LIFSON,1960), and obtain an average slope of 0.55 zt 0.07. This result is in good agreement with our experimental result of 0.59 (Fig. 4). Average physiological data on human subjects (SCHOELLER~~~VANSANTEN, 1982)yielda slope of 0.70 t 0.06. It compares well with the average slope of human subjects of 0.67 t 0.9 (measured slope Fig. 1: 0.78 (6, vs. 6,); LONGINELLI'S (1984) measured slopes: 0.6 (S, vs. 8,) and 0.64 (8, vs. 6,)). When selecting species suitable for paleoclimatological studies, our mode1 can be used to identify those most sensitive to variations in 6,. An optimal species is one the dbrvvalues of which plot with a large slope vs. 6,. In other words it is a mammal that drinks much and has low metabolic activity. Thus desert animals that conserve water should be avoided. Similarly, active marine mammals that also conserve water (DE-AS et al., 1971) should be used with caution. Arknowledgmenrs-We thank Drs. S. E. Calvert, I. Gedalia, I. R. KapIan, 0. Kouvo, D. 3. Oftner, B. Peters,and S. S&hat for contribution of samples. WCalso thank J. Erez for careful reading of the manuscript and hctpful suggestions. A. Longin&Ii and M. J. DeNirn made very useful comments which helped us impmvc the manuscript. H. P. Schwartz contributed samples, reviewed the manuscript and in early discussions with us helped to formulate the model. I. Caster was very
efficient in her typing. This research has been supported by
1693
0 isotope Fractionation a grant from the Israel Academy of Science and Humanities to B. Luz and Y. Kolodny. REFERENCES BRENNINKMEIJER C. A. M., KRAFT P. and MONK W. G. (1983) Oxygen isotope fractionation between CO2 and HrO. Hot. Geosci. 1, 18 I - 190. BRICOUTJ. (1979) Natural Abundance Levels of *H and “0 in Plant Organic Matter. In Stable Isotopes: Proceedings of the Third International Conference (eds. E. R. KLEIN and P. D. KLEIN), pp. 215-222. Academic Press. DEPOCASF., HART J. S. and FISHERH. D. (197 I) Sea water drinking and water flux in starved and in fed harbor seals. Phocu vitulina. Can. J. Physiol. PhQrmQCOl. 49, 53-62. DOLE G. A., LANE D. P. R. and ZANKELIESD. A. (1954) Isotopic composition of atmospheric oxygen and nitrogen. Geochim. Cosmochim. Acta 6, 65-78.
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