ABSTRACT
As an example, given that the average concentration of isotopes in seawater is null (obviously this concerns relative composition), the concentration of oxygen 18 can vary from between about -55‰ to 30 ‰. Note that negative values mean that the sample has a lower concentration than the average value of seawater, while the positive values indicate the opposite. For example, if we determine that the value of the relative
concentration of oxygen 18 in a water sample is G 18O (0 ‰) = +5, this means that the analyzed water is 5% richer in oxygen 18 than the SMOW reference value. Very often, we use the linear relation that exists between the relative concentrations of oxygen 18 and deuterium. For rainfall, this relationship carries the name meteoric water line and is expressed as follows for meteoric water around the world
(11.6)
In general, the slope of this line is fairly constant while the ordinate at the origin marking the excess deuterium can exceed the value of 10. This value is reached when water vapor from a seawater source has been significantly enriched by evaporation on the continents or land-locked seas. For example, in the case of the Mediterranean basin, Equation 11.6 is written:
(11.7)
This equation can also be applied locally (for local rain). Again, note that analysis of the relation between oxygen 18 and deuterium makes it possible to identify water that has undergone the evaporation process. Following the various measurements of 18O and D over time allows us to plot a straight line of evaporation that presents a gentler slope than the line for meteoric water, as well as a lower value of the ordinate at
O O sample
O O SMOW ‰( )=
( ) ( )
− ⎡
⎣
⎢ ⎢ ⎢
⎤
⎦
⎥ ⎥ ⎥ ⋅
/
/
H H sample
H H SMOW ‰( )=
( ) ( )
− ⎡
⎣
⎢ ⎢ ⎢
⎤
⎦
⎥ ⎥ ⎥ ⋅
/
/
2 188 10H OG G ˜
2 188 22H OG G ˜
the origin. The intersection of this line with the line of meteoric water allows the possibility of determining the isotopic composition of the water before its evaporation.
