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paleoenvironmental picture and for comparing dif erent types of records
from dif erent locations. Researchers can also use age information to cal-
culate how quickly climate changed in the past. h ese past rates of change
provide a context for climate changes occurring today.
Establishing a chronology for climate records is challenging, but paleocli-
matologists have devised a number of techniques. h e ideal method provides
annual time resolution, like the annual growth rings that trees produce,
which can be counted to provide highly accurate ages. Similarly, corals,
mollusks, and other organisms typically secrete annual growth bands when
forming their hard parts. h e deposition of sediments in lakes or coastal
ocean settings during certain seasons can also yield annual time resolution in
the form of annual layers. As discussed above, the varves of the Santa Barbara
Basin provide a good example because these layers are deposited annually, so
can be counted.
More typically, though, climate records contained in sediments from the
sea l oor, lakes, estuaries, and marshes do not contain annual varves. Most
of these sedimentary sequences require the use of radiometric dating, which
is based on known rates at which certain naturally occurring radioactive
isotopes decay. h e familiar radiocarbon (carbon-14, also known as C-14)
method is the most common approach used for samples younger than 50,000
years old. Radioactive carbon is produced in the upper atmosphere when
cosmic rays directed at the earth interact with nitrogen, transforming nitro-
gen-14 into carbon-14. Radiocarbon, just like the more abundant (and stable)
forms of carbon (carbon-12 and carbon-13), combines with oxygen to form
carbon dioxide and is eventually taken up by plants during photosynthesis.
Carbon-14 also dissolves into the surface waters of the ocean, where organ-
isms use it to build their calcium carbonate shells. Once these organisms
die and become incorporated into the sediment, the radiocarbon begins to
decay back to nitrogen. We know the rate of decay, so we can use the initial
concentration, rate of decay, and amount of radiocarbon measured in our
sample to give us the age of the sample.
For sediments older than about 50,000 years, scientists can use other
radiometric methods to date other components of the sediment, such as
mineral grains from volcanic ash that were produced during ancient volca-
nic eruptions. h e ash is commonly deposited in distinct, thin layers within
the sediment. Another approach uses the chemical “i ngerprint” of an ash
layer, allowing it to be traced to a specii c volcanic eruption that has been
independently dated. Volcanic activity has been a common occurrence in the
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