Original isotopic composition of water in precipitation by different methods

Abstract

Stable isotopes of ²H and ¹⁸O in precipitation are different globally and carry all information about water molecules movement in hydrosphere cycles. Isotopic composition is a function of temperature, relative humidity, and speed of evaporation at different latitudes, longitudes, and altitudes. On the basis of this, we observe local meteoric water line measurements in the plot of δ²H versus δ¹⁸O. It will be interesting to know the original isotopic composition (without any modification) in a transition from cloud down to earth in different environmental conditions. This had been done by plotting of slope versus intercept of Local Meteoric Water Line (LMWL) at different altitudes in different years of observations. Intercept of LMWL with Global Meteoric Water Line (GMWL) data taken from the hydrology frame work of Corsica was plotted and it was found that the isotopic composition of water in precipitation by all these methods is same.

Introduction

Stable isotopes of ²H and ¹⁸O are powerful tracers of the water molecules in the hydrosphere and water cycle. Stable isotope ratio between ²H/¹H and ¹⁸O/¹⁶O is expressed by δ²H and δ¹⁸O, where δ is a sample ratio, and δ = (R _sample/R _SMOW − 1) 1000, where R = ²H/¹H or ¹⁸O/¹⁶O is taken standard sea mean ocean water (SMOW now VSMOW). One of the most important contribution had been made by Craig 1961 as a correlation of precipitation on global basis, between δ²H and δ¹⁸O, where δ²H = 8 δ¹⁸O + 10‰. Later, δ²H = 8.13 δ¹⁸O + 10.13‰ has given by Rozanski et al. 1993 and is taken as Global Meteoric Water Line. This gives the global distribution for stable isotopes in fresh precipitation. These are the injected tracers on global basis. Due to environmental conditions especially temperature and humidity, these isotopic composition is modified in different places in the region, and therefore, what we observe is modified local meteoric water line (LMWL). The slope and intercept for these LMWLs do depend on hydrological parameters.

The experimental results of plots on δ²H and δ¹⁸O had been discussed both theoretically and experimentally by various investigators Friedman 1953; Craig 1961; Dansgaard 1964; Yurtsever 1975; Singh and Kumar 2005. All these studies reflect the variation of slope and intercept on δ²H axis on plot of δ²H and δ¹⁸O water in various stages of hydrological cycle. What we measure all the time is LMWL. Knowing LMWL can be found the isotopic composition without modification due to environmental condition, temperature, and humidity. This has been suggested by Singh (2013), and this study has been done from the data of Corsica.

Recently, Geldern et al. 2014 reported the stable isotopic pattern in a climate change and hydrology framework of Corsica. These extensive data, i.e., measurement of δ²H and δ¹⁸O at different altitudes, in different seasons, in lakes, spring, and streams, need further investigation by the method of plotting slopes versus intercepts of LMWL.

The methodology adopted for interpretation of data for local meteoric water line (LMWL)

Global meteoric water line (GMWL) as available is based on precipitation, as well as, on global distribution of stable isotope in precipitation (Rozanski et al. 1993), where

$$\delta^{2} {\text{H }} = 8.13\delta^{18} {\text{O }} + 10.8\permil.$$

(1)

Let us take the water with an isotopic composition of δ¹⁸O = −5, −7, −10, −15, −18, and −20‰. We can find the corresponding value of δ²H to be –29.85, –46.11, –70.50, –111.16, –135.54, and –151.80‰, respectively, from Eq. (1). Let us take the isotopic composition of water for different slopes, for example, 7.4–8.1 [as found in the measurements of local meteoric water line (LMWL)] and calculate the intercepts by the equation:

$$\delta^{2} {\text{H }} = {\text{ slope }}\left( {x{\text{ axis}}} \right) \, \delta^{18} {\text{O }} + {\text{intercept }}\left( {y{\text{ axis}}} \right).$$

(2)

The intercepts thus calculated are given in Table 1, for different isotopic compositions and different slopes. The plots are given in Fig. 1 (Table 2) and have six different lines, i.e., 1–6 are straight lines corresponding to different isotopic compositions of water. Each line corresponds to different isotopic compositions of water on GMWL.

Table 1 Slopes/intercept using relation δ²H = slope δ¹⁸O+ intercept for the water

Fig. 1

Intercept on δ²H axis versus the theoretical values of the slope of δ²H and δ¹⁸O plots for different waters having δ¹⁸O from −5 to −20‰ corresponding values of δ²H, as given by line 1 to line 6

Table 2 Theoretical values of δ¹⁸O and δ²H

If we measure and plot δ²H and δ¹⁸O in different seasons in different regions with different environmental conditions, the average values for each site of the measurement will produce a flatter ellipse. If we plot δ²H versus δ¹⁸O for each site for each season separately, we can obtain different LMWLs in the region. Each plot will give us different slopes and intercepts, and on plotting the slopes versus intercepts, we get a straight line and we can find δ¹⁸O and δ²H, as given in Eq. (2), which is the original isotopic composition of water in precipitation on GMWL. The combined plot giving flatter ellipse-type picture is the combination of many LMWLs. The slope of each LMWLs is very close to each other, but intercept on δ²H is separate and distinguishable as can be seen for different LMWLs.

It looks unusual that slope versus intercept plot is giving isotopic composition of water on GMWL, but if we look to the geometrical consideration, we get the answer. If we draw two lines making angle θ for the length say r, the separation between two lines shall be rθ and provided θ is small. If double the angle says 2θ, separation be r2θ, and if we have 3θ, the separation will be r3θ. The plots of 1θ, 2θ, and 3θ and separation of r1θ, r2θ, and r3θ shall be on a straight line. In this present analysis and observation of GMWL and LMWLs, the change in slope for each site is small; however, intercepts are large. This can also be seen from Table 1, and for slope ranges from 7.4 to 8.1, the intercepts are −3.8–10.2 for the sixth line. This is the basis of interpretation. So far in the literature, we did not find any consideration of intercept, but this analysis of slope versus intercept is a very useful tool to correlate experimental data of the region and get original isotopic composition of water in precipitation. Therefore, slopes and intercepts of LMWLs in a region are useful, as given above.

Experimental data

The island of Corsica situated in the North-Western Mediterranean basin between 41° and 43° latitudes, with a North–South extension of 183 km and a maximum of east–west extension of 83 km, has a moderate-to-rugged regular topology, with a maximum elevation of 2706 m. The annual precipitation is 600 mm/annum. Annual temperature decreases with altitude at the rate of −6 °C per km. At high altitudes, the temperature difference between the coldest and warmest months in mountain is about 20 °C. 166 samples were collected by Geldern et al. 2014 during the years 2003–2009 from rivers, lakes, and streams. The data obtained and reported in a chronological order are on different dates with details of location, elevation (altitude), temperature, latitude, and longitude along with a measured value of δ¹⁸O and δ²H.

Interpretation and discussion

Method I: Geldern et al. 2014 measured values of δ¹⁸O and δ²H for the precipitation at different altitudes. We plotted, as given in Fig. 2 (Table 3), and the measurements of δ¹⁸O and δ²H at different altitudes of 10, 30, 50, 50, 100, and 100 m Above Sea Level (m.a.s.l.) and obtained LMWL to be δ²H = 7.08δ¹⁸O + 4.17‰. The range of altitude for this LMWL is 56.67. Similarly, we plotted δ¹⁸O and δ²H at different altitudes say, from 148 to 269 taking range of altitude to be 206.90 and obtained LMWL to be δ²H = 6.29δ¹⁸O-1.88‰. This has been done for all the different altitudes, as given in Table 4.

Fig. 2

Measured value of δ²H and δ¹⁸O with mean altitude

Table 3 δ²H and δ¹⁸O values at different altitudes with specific dates

Table 4 LMWL at different altitudes

For the LMWL slopes and intercepts as given above, we plotted the slope versus intercept on δ²H axis, as given in Fig. 3 (Table 5). These slopes versus intercepts are straight lines and as per interpretation, Eq. (2), the original isotopic composition on GMWL is as follows:

$$y = 9.0 3x - 6 1. 1 3,{\text{ or}}\; - 6 1. 1 3= - 9.0 3x + y,$$

$${\text{i}}.{\text{e}}.,\;\delta^{ 2} {\text{H}} = - 6 1. 1 3\permil;\delta^{ 1 8} {\text{O}} = - 9.0 3\permil.$$

Fig. 3

Slope versus intercept as obtained at different average heights from the plot of δ¹⁸O and δ²H. Local Meteoric Water Line (LMWL) (Fig. 1a at 1 s) recorded by Geldern et al. 2014 from different average altitudes in the region as per interpretation the original isotopic composition of precipitation as obtained to be δ¹⁸O = −9.03‰ and δ²H = −61.13‰

Table 5 Slope and intercept at different altitudes

Therefore, it can be taken to be original an isotopic composition of water which is modified at different environmental conditions at different altitudes. If this is so, then, we can take δ¹⁸O = −9.03‰, and if we multiply it with the slope of LMWL (as given above) and add to this value of δ²H (as observed value as given above), we can obtain value of δ²H (calculated), as given in Table 6.

Table 6 Calculated δ²H values at different mean altitudes

It is to be noted that all the values are within the statistical error and within the range of −2.00‰, i.e., δ²H = −61.13^‰ (range −63.13 to −59.13‰). δ¹⁸O and δ²H measured at different altitudes are well correlated with each altitude and original isotopic composition of water in cloud formation and rains. We observe modified LMWL due to different environmental conditions and topological conditions of the surface etc. during the transit.

Method II: Geldern et al. 2014 had given measured value of δ²H and δ¹⁸O during different periods in the years 2003–2009. The basic measured value of the data is given for the months of April–May–June for year 2003, May–August–October for year 2004, May–October for the year 2005, May–June–August for the year 2006, May for the year 2007, and June for the year 2008. Plotting of δ¹⁸O versus δ²H has been done to get LMWL lines for each year which is given in Table 7 yearwise.

Table 7 LMWL Line for different years

The slope of these lines and intercept are plotted, as given in Fig. 4 (Table 8), and obtained a straight line; therefore, the isotopic composition of water in precipitation on yearly basis is worked out from this plot to be as per the interpretation by Eq. (2):

Fig. 4

Showing the plot of slope versus intercept as obtained in different years in May/June 2003–2009 from the plot of δ¹⁸O and δ²H as recorded in different dates in these years from Fig. 2a–g by Geldern et al. (2014) as per interpretation for the original isotopic composition as obtained is δ¹⁸O = −9.5‰ and δ²H = −63.66‰

Table 8 Slope and intercept at different altitudes between 2003 and 2009

y = 9.50x − 63.66 or −63.66 = −9.50x + y, i.e., δ¹⁸O = −9.5‰, δ²H = −63.66.

All the values and observations in δ²H and δ¹⁸O are very different for each month and year, as if there is no correlation between them, yet the plot of slope versus intercept is a straight line, which suggests that all the observations are well correlated.

If we multiply δ¹⁸O = −9.5‰ with the experimental slope as obtained and add to it the experimental intercept, as given above, we get the value of δ²H, which is given in Table 9, which can be compared with the observed value, i.e., δ²H = −63.66‰.

Table 9 Calculated δ²H values in different years

Error in these measurements is ±2.0‰ for δ²H range, i.e., −65.66 to −66.66‰. All values are within the range which confirms that the isotopic composition of water in precipitation can be taken to be δ¹⁸O = −9.5‰ and δ²H = −0.63.66‰.

Method III: Geldern et al. 2014 had reported δ²H and δ¹⁸O for water samples from lakes, stream, and springs drawn local water meteoric lines for streams, springs, and lakes and obtained the LMWL for streams, springs, and lakes. The values thus obtained are as follows:For streams δ²H = 7.9(±2)x δ¹⁸O + 10.1(±1.8)‰,For springs δ²H = 7.6(±0.5)x δ¹⁸O + 7.9(±4.1)‰,For lakes δ²H = 5.5(±0.4)x δ¹⁸O − 10.5(±3.7)‰.

Geldern et al. 2014 had defined δp to be the original isotopic composition on the basis of intersection of Local Evaporating Line (LEL) of lakes with Mean Meteoric Water Line (MMWL) for streams and he obtained the value to be δ¹⁸O = −8.6(±0.2)‰ and δ²H = −58(±2)‰. However, plot of slope versus intercept, as given in Fig. 5 (Table 10), resulted in isotopic composition of water from Eq. (2):

Fig. 5

Plot of slope versus intercept of the LMWL for stream, spring, and lake, giving isotopic composition to be δ¹⁸O = −8.87‰ and δ²H = −59.54‰

Table 10 δ¹⁸O and δ²H values at different places

y = 8.87x − 59.54‰ − 59.54 = 8.87x + y, i.e., δ¹⁸O = −8.87‰ δ²H = −59.34‰.

This observed value is the same as obtained by method I and method II earlier.

Conclusion

The isotopic composition by these different methods is from different altitudes in different years and from different surface waters (lake, spring, and stream) as

δ¹⁸O = −9.03‰ δ²H = −61.13‰,

δ¹⁸O = −9.50‰ δ²H = −63.66‰,

δ¹⁸O = −8.87‰ δ²H = −59.34‰.

The error in these measurements is δ¹⁸O = ±0.2‰ and δ²H = ±2.0‰. The mean value of δ¹⁸O and δ²H can be taken to be on GMWL.

It is for the first time, these extensive data have been re-analyzed on the basis of precipitation at different altitudes in different years and from different sources of water. The results obtained are within statistical error, and hence, conclusion can be drawn that the original isotopic composition of water in the formation of clouds and rains had the same origin. It has all happened due to Rayleigh’s fractionation which later on gets modified in transit etc due to different environments, e.g., temperature and kinetic fractionation, etc. Process of modification needs to be worked out to get LMWL at different places and locations and transits from the original isotopic composition of water in precipitation.