Geochronology of Carbonates to Constrain Paleohydrology of the Bear River Range, and CO2-charged Systems of SE Utah
2005 Research Initiation Award Report
Investigators
James Evans—Department of Geology, Utah State University
Summary
Understanding hydrologic flow regimes in areas where water is scarce and
vulnerable is paramount in sustaining culinary water sources. By characterizing
the relationship between elevation and stable isotopic composition of
precipitation in the Wasatch and Bear River Ranges of Utah we gain insight into
the hydrology of the Wasatch Mountains and provide a framework for further
studies that will be useful in protecting a precious natural resource. Stable
isotope analyses are useful for separating the sources of recharge to springs
and streams, as precipitation tends to fractionate with elevation.
Over the course of the 2005-2006 winter season we sampled new snow from 5 sites
within the Middle Bowl drainage in the northern Wasatch Range. The Middle Bowl
drainage is the site of Snowbasin Ski Resort and located approximately 15 km
east of Ogden. The area is characterized by gently sloping (<35°) terrain
and leeward of prevailing winds. Five collection sites were chosen for
sampling, 2 of which have snow stakes that allow for accurate snow depth
readings and 1 site that has an automatic snow depth recorder, thermometer,
barometer and anemometer. The 5 collection sites make up a transect extending
approximately 2.2 km measured map distance from 2176m to 2654m (AMSL). A total
of 11 storms were sampled throughout the winter combining for a total of 55 new
snow samples. We define “new snow” as snow sampled concurrently with
precipitation where rates of precipitation were at least 2.5 cm per hour for at
least one hour before sampling. No more than 3 cm of surface snow was
collected. These criteria ensured that snow no older than 1 hour was sampled;
this maintained a basis for comparing storm events. Samples were analyzed for
δ18O using the method of CO2 equilibration. A Micromass isotope ratio mass
spectrometer with continuous-flow helium carrier gas was used to determine
δ18O/16O. All data are reported with respect to SMOW. The 2-sigma error is not worse
than 0.16 per mil for the dataset.
We find that the variation in δ18O values amongst storms is great. Mean 18O
values for individual storms varied from -13.32 per mil to -25.35 per mil. Mean
18O values for each storm were calculated by averaging the samples taken from
each collection site within a storm. The lapse rate, defined as the change in
δ18O per increase in elevation, was calculated based on a linear relationship
identified by Poage and Chamberlain. The lapse rate calculated in this study is
0.1 per mil/100m. The lapse rate we have calculated differs from Poage and
Chamberlain’s lapse rate of -0.31 per mil/100m for North America. What is
interesting about this discrepancy is the sign change between the two studies.
Our lapse rate has enrichment of δ18O with increasing elevation whereas Poage
and Chamberlain’s study has enrichment of δ18O with decreasing elevation. We
interpret this difference to be due to the different nature of the collection
sites, in which our study area lays on the lee side of prevailing storm tracks.
Thus orographically driven lapse rates may be overprinted by more dominant
continental effects.
We also performed a synoptic snow isotope transect in the spring 2006 from the
Bear River Range. Snow cores were acquired over a ~ 500 m elevation range
across a 7 km transect. The fractionation of with elevation was seen, but
several complexities in the isotopic signatures were noted. Metamorphism of the
snow was clearly seen, and isotopic signatures near the snow melt line were
observed.
Contact Information
James Evans
jpevans‹at›cc.usu.edu