ESPM 220c Homework 2: Hydrological Cycle
Name:__________________
1. The isotopic composition of precipitation in the Mojave Desert of southern California
and Nevada shows a consistent decrease in delta values with increasing elevation
(Friedman et al. 1992. Stable isotope composition of waters in southeastern California. 1.
Modern precipitation. Journal of Geophysical Research). Though several explanations
might be applicable, one likely source of the variation (based on observation) is that there
is an increasing amount of evaporation of precipitation, before it hits the ground, with
decreasing elevation.
Delta D (per mil)
FRIEDMAN ET AL 1992
-90
y = -60.724 - 0.010262x R= 0.68288
-85
-80
Delta D (per mil)
-75
-70
-65
-60
-55
-50
-500
0
500
1000
1500
2000
2500
3000
elevation (m)
In the 1980’s, I worked with colleagues on the soil isotopic behavior along an elevation
transect north of Las Vegas. Below are the site names (dominant vegetation) and
geographical information.
a. Calculate the 18O value of the precipitation at each site using the relationship that
Friedman et al found for the Mojave: D = (18O•6.5)-9.7.
b. Is the slope the same as the Meteoric Water Line? If not, what is the common
explanation given the different slope?
c. Assuming that the precipitation evaporates into a dry atmosphere, calculate the
fraction of water (using O isotopes) that must have evaporated to produce the
precipitation at each site, assuming the highest elevation has an f =1.
2. Using the water and temperature information, calculate the 18O value of soil
carbonate that would form at each site. Does this O-bearing compound in equilibrium
with water actually show the same O isotope trend with elevation (and temperature) as
the rainfall? WHY (plot data to visualize trends).
3. We measured the C and O isotope composition of soil CO2 and found a consistent
trend in the 18O of the with elevation that followed the trend of precipitation. A result of
several years of work revealed that soil CO2 at any depth is nearly (but not quite) in
isotopic equilibrium with the water at that depth (the non-equilibrium is due to the fact
CO2 is constantly diffusing upward, and must equilibrate with the new water it
encounters). For the data you have, calculate the 18O value of soil CO2 in isotopic
equilibrium with the precipitation at the site.
KYLE CANYON DATA
SITE
Creosote
Blackbrush
Pinyon-Juniper
Fir-Pine
PRECIPITATION
CARBONATECO
ELEV (M)MAT ( C)MAP (mm)D (o/oo) 18O (o/oo)18O (o/oo)18
840
17.9
160-69.34408
1400
14.1
326 -75.0908
1750
11.6
436 -78.6825
2150
9
549 -82.7873
4.
The measurement of O isotopes in rain (or any water) almost always involves the
equilibration of CO2 with the water in the lab, and the subsequent isotopic analysis of the
isotopic composition of the CO2 (and a back calculation to what the water value is). In
practice, a “small” amount of CO2 is equilibrated with a “large” amount of water – so that
the water value doesn’t change and water dictates CO2 values.
As an example, 10 ml of water is equilibrated with CO2 at a pressure of 1 atm in a
vial that is 30 ml. Assume, for simplicity, all the CO2 is in the gas phase.
a. How many moles of O are in the water?
b. How many moles of O are in the CO2 in the headspace? (see Hints below).
c. The equilibration takes place @ 25 0C. The 18O value of the initial CO2 is + 4
o/oo (SMOW). The 18O value of the water is – 9 o/oo. What is the equilibrium
value of the CO2 ?
d. In an upcoming homework, we will evaluate how FAST this reaction occurs…..
HINTS:
1. Use the Rayleigh distillation model into dry air for question 1. This is reasonable
given the very low relative humidities in the desert.
2. Use/familize yourself with fractionation factor equations either via the book form
(Friedman and O’Neil) or the web site I provide for you. Please use (if you follow
Freidman and O’Neil):
a. CaCO3-water fractionation of Oneil, Clayton and Mayeda (1969)
b. Carbon dioxide-water fractionation of O”Neil and Adami (1969)
3. For question 5, use the exceedingly useful relationship for gas source isotope
geochemists: PV = nRT. The CRC Handbook of Chemistry and Physics (now online) has
one page devoted to this important relationship and the units that can be used (moles,
atm, K here).