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
