Geoscience Reference
In-Depth Information
American West over past millennia, so ash layers are widely used in dating
lake sediments.
age r esolution
h e precision with which a researcher can resolve time within a paleoclimate
record, or “age resolution,” determines the level of detail that can be recon-
structed. Tree-ring records with annual resolution allow researchers to derive
long histories of events that have a recurrence rate of less than a decade, such
as El Niño events. At the other end of the spectrum, slowly accumulating
sediments in the deep ocean may provide information only about climate
shit s that occur over centuries or millennia. One of the primary factors in
age resolution is the rate of growth (of a shell, for example) or accumulation
(of s e d i ment).
Sediments deposited in anoxic, partially enclosed coastal basins, such as
the Santa Barbara Basin, and certain lakes accumulate at a relatively high
rate of several centimeters per century. In these sediments, researchers can
sample the varves and achieve annual time resolution. In the deeper parts of
the oceans, far from land, sediments accumulate at a much slower rate—on
average one centimeter per thousand years. h ese deeper ocean environments
are usually higher in oxygen, and therefore some burrowing and churning
by bottom-dwelling organisms searching for food means that the sediments
will have been stirred, or “bioturbated,” blurring the record and averaging
conditions over centuries or longer. In deep ocean sediment, time resolution
can be limited to thousands, or even tens of thousands, of years.
h e pollen record contained in lake cores provides a sequential history
of vegetation. However, because pollen accumulates very slowly, less than a
quarter of a teaspoon of pollen collected for analysis may represent several
decades of accumulation. Hence, the age resolution for pollen studies is nor-
mally no better than decades to centuries.
Time resolution also depends on the accuracy of dating methods.
Sediments dated using radiocarbon or other radiometric techniques have
age uncertainties of thirty years to upward of hundreds of years, depending
on the age and environment of deposition. In some cases, age measurements
can be done only at irregular intervals, in which case ages for most of the
sediment core must be interpolated from just a few layers that are suitable for
dating. Examples of such sporadic or opportunistic age measurements would
