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carbonate minerals in lake sediments. h ese minerals precipitate in the sur-
face waters and eventually settle to the bottom, where they accumulate over
time, preserving information about the climate. During dry periods, more
water evaporates from the lake surface than is replenished by river inl ow,
lowering the lake level. Benson reasoned that, since the waters evaporating
from the lake are more enriched in oxygen's lighter isotope (oxygen-16),
the remaining lake waters should be enriched in oxygen's heavier isotope
(ox yg en-18). h e enriched lake water is then incorporated into the carbonate
minerals precipitating in the lake. h erefore, carbonate minerals that precipi-
tate in the lake during dry spells should have a relatively high amount of the
heavier oxygen isotope. Conversely, during wetter periods, rain and runof
enter the lake (freshwater that is enriched in the lighter oxygen-16 isotope)
is greater than evaporation of the lake's surface; the lake waters and carbon-
ates forming in them are more enriched in oxygen-16 than they are during
droughts. h us, these isotopes in precipitated carbonates provide Benson
and other researchers with an archive of past lake chemistry and a record of
climate change in the region.
Benson uncovered this long climate history at Pyramid Lake by coring
sediments from its bottom. At er bringing these cores back to his laboratory
in Denver, he separated the carbonate minerals from the lake sediments and
dated the cores, i nding that the sediments spanned the past 6,500 years. h is
record revealed that, between 6,500 and 3,800 years ago (the mid-Holocene
period), the oxygen isotopes in the lake water were enriched in the heavier
isotope, oxygen-18, relative to the lighter isotope, oxygen-16, by an amount
indicating that the climate during that time was about 30 percent drier than
today, and warmer by 3-5°C (5.5-9°F).
disappearing lakes
Benson and colleagues also studied lake sediments from other lakes in the
Great Basin, including Owens Lake, to assess the impacts of past climate
changes in the Owens Valley east of the Sierra Nevada in California. Like
many Great Basin lakes, Owens Lake is a closed basin, receiving inl ow only
from the Owens River, with no outlet. For many thousands of years, this
large lake, twelve miles long and eight miles wide, persisted in the Owens
Va l le y (s e e i gure 18), its size l uctuating in response to changes in the amount
of precipitation and evaporation. h
e lake is now dry—not because of a drier
