Isotopic studies of Yucca Mountain soil fluids and carbonate pedogenesis Page: 4 of 8
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RADIOACTIVE WASTE MANAGEMENT
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Figure 2. Carbon-13 content of soil gas carbon dioxide
from vertical profiles at two sites in the Yucca Mountain
region, March-September 1993. The PWT site (open
squares), near the head of Pagany Wash, is the highest soil
gas sampling site on Yucca Mountain. The USE site near
Beatty (filled circles) is located in the Amargosa Desert
southwest of Yucca Mountain.
tote usually occurs at about the depth (30 to 50 cm) where
plant roots appear to be concentrated.
2) Most sites show a seasonal progression from isotopi-
cally light (low S13C) in the spring to isotopically heavy in
late summer. Soil gas CO2 concentrations are likewise
highest during the spring. The 813C minimum occurs earlier
in the year at the more arid low elevation sites than at high
elevation sites. These seasonal patterns reflect higher rates of
soil respiration during spring, when soils are moist and
warm. Summertime desiccation also increases soil porosity
and allows faster escape of respired CO,, further lowering
CO, concentrations and increasing downward penetration
of isotopically heavier atmospheric CO,.
3) Moister, high elevation soils have higher CO2 con-
centrations and lower 813C, consistent with large contribu-
tions from respired CO2, and isotopically light CO2 derived
from C-3 plants. Soil gas CO2 from the highest sites (PWT,
on North Yucca Mountain, Yucca Crest near UZ-6, and RM.
1 and PIA on Rainier Mesa) contain about 10%o lower 813C
than the low elevation sites (such as USE, near Beatty).
At least as interesting as these trends are the large isoto.
pic variations observed at individual sites. This is especially
pronounced within the stream-terrace alluvium sampled at
Fortymile Wash, where seasonal isotopic variations at a
single depth sometimes span nearly the entire isotopic range
observed at all sites, for all depths and seasons. Shallow holes
drilled into tuffs on Yucca Crest also illustrate this variability
(Fig. 3). Factors controlling the range of 513C at a particular
site are currently under investigation.
B. Isotopic Mixtures.
Soil CO derives from two main sources: atmospheric
CO and Cd2 respired by plant roots and soil organisms.
Each has a characteristic carbon isotopic composition.
Atmospheric CO2 has a S 3C of about -8.5 %o, down some.
what from the pre-industrial value of about -6.5 %o which
prevailed during the time most existing recent soil car-
bonates precipitated. Respired CO2 resembles surrounding
vegetation, which ranges from about -27 to -12 %, depend-
ing on the local mix of C-3 and C-4 plants, respectively.
This in turn correlates with altitude and other factors.
Carbonate-rich dusts derived largely from Paleozoic lime-
stones and playa sediments supply much of the calcium in
soil carbonates, but their contributions to carbon are minor
compared to atmospheric and respired CO 2.
Carbonates precipitated in isotopic e uilibrium with
soil CO2 should be about 8%o increased in S 3C, depending
to some extent on temperature. Based on the data presented
above, this defines a contemporary equilibrium range of
around -14 to +00/oo. The lower values apply to high eleva-
tion sites and deeper than 50 cm in the soil column, and the
heavier values apply where atmospheric CO2 contributes
most of the carbon to the mixture. Atmospheric CO was
about 20/oo heavier before the industrial revolution, anf con-
sidering the uncertainties in environmental temperatures at
the time of soil calcification, and in the associated equilibri-
um isotope fractionation factors, it is reasonable to extend
the range for equilibrium pedogenic carbonates to about
+30/o. Most carbonates from the Yucca Mountain region fall
within this range"3'5, and e uilibrium models shed consid-
erable light on their genesis
Carbonate 513C values as high as +50/ are occasionally
observed however, and are hard to fit into this scheme.
Conversely, careful sampling of . stinctive carbonate mor-
photypes at trenches 14, 14c, 5a. nd at Busted Butte has
revealed the existence of anomalously light carbonate
"pellets", ooids, and layers near the bottom of the B soil
horizon3. Although it is not clear --hat the isotopic composi-
tions of soil gases were at the :.ne of precipitation, such
carbonates appear to be lighter than would be expected from
equilibrium precipitation from contemporary soil CO2' and
stretch the credibility of arguments based on isotopically
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McConnaughey, T.A.; Whelan, J.F.; Wickland, K.P. & Moscati, R.J. Isotopic studies of Yucca Mountain soil fluids and carbonate pedogenesis, article, December 31, 1994; Denver, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc619823/m1/4/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.