| Other
paleo proxy records
Tree rings are not the only environmental proxy that can be
used to examine past streamflow and climate for the Colorado
River basin and the southwest U.S. Fossil clams, packrat middens,
pollen in lake and bog sediments, and changes in lake chemistry
in lake sediments all record information about the past. These
proxies can provide longer records than with the tree-ring data,
but the information has coarser time-resolution than tree rings.
(Most of these proxies are based on radiocarbon dating, which
has a dating precision of decades to centuries, compared to
the annual precision of tree-ring data.) Instead, these proxies
record slower shifts in precipitation or temperature regimes,
complementing the high-resolution information in the tree rings.
Clams
Oxygen isotopes in clam shells in the Colorado River delta
in the Gulf of California closely match the salinity level of
the delta estuary water, and salinity level is very closely
tied to the amount of streamflow. Thus, after calibrating the
isotope levels in modern clams with measured salinity and gaged
streamflow (much like calibrating tree ring records), the isotope
levels in long-dead clams can in principle be used to estimate
past streamflow (Dettman
et al. 2004). A clam-based streamflow reconstruction of
the Colorado River at its mouth is currently being developed,
and it will be interesting to compare these results to the tree-ring
reconstruction.
Packrat
Middens
Packrats (genus Neotema) live throughout the interior
western US. They bring large quantities of nearby plant material
into their dens, inadvertently creating excellent records of
the composition of the vegetation. The urine of the packrats
dries and cements the plant material together, preserving these
middens for thousands of years. Many packrat middens have been
collected in the Colorado River basin (see map here).
In a recent study, middens sampled in the Gunnison Basin in
western Colorado indicated changes in vegetation over the past
4000 years, including evidence of warmer (and possibly drier)
conditions from about AD 500-1050, and cooler (and possibly
wetter) conditions from about 1350-1800 (Emslie
et al. 2005).
Pollen
Sediments in lake bottoms and bogs serve as natural repositories
for pollen grains that are released annually from nearby plants.
Cores taken from lake bottoms are sampled at intervals along
the core. The pollen present at each interval provides a "snapshot"
of the species composition of local vegetation. Changes in the
pollen record over time (for example, from domination by pine
pollen to sagebrush pollen) can be taken to represent changes
in climate, in the example, from wetter to drier conditions.
As mentioned previously, the pollen record cannot be dated as
precisely as tree rings. Nonetheless, pollen records can help
confirm the general timing of major climatic shifts in a region.
Lake
sediments
Lakes are a repository of sediments from their surrounding
environments, and sediment cores taken from lakes can record
environmental (including climatic) changes. Lake sediments can
provide a number of different environmental proxies, including
pollen, described above. Another useful proxy is the changes
in isotopes of oxygen and carbon within carbonate-rich sediments.
Oxygen isotopes can reflect the varying composition and sources
of water coming into the lake, thus indicating precipitation
variability and other climatic features. Work is currently underway
to sample and analyze the sediment records from several lakes
in western Colorado, with the goal of extracting long (up to
10,000 years) proxy climate records to compare with nearby tree-ring
records. The tree-ring records can help define the nature of
the climate information in the lake chemistry. The lake sediment
records could be used to extend our knowledge about hydrologic
variability in the upper Colorado River basin thousands of years
into the past.
On to...A Final Word
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