Geoscience Reference
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climate but because water managers, starting in 1913, diverted the Owens
River south through the Los Angeles Aqueduct to Southern California. By
1924, the lake was completely desiccated, leaving in its place an enormous salt
l at composed of clay, silt, and evaporite minerals—the salts that precipitate
from the briny waters during evaporation. Beneath this surface, however,
lie thousands of years of subsurface sediment layers documenting the lake's
response to climate change.
Using the methods he developed for Pyramid Lake, Benson found that
Owens Lake had also diminished during the mid-Holocene—between
7,700 and 3,200 years ago. h is i nding was based partly on oxygen isotope
measurements, showing an increase in oxygen-18 in carbonate sediments,
and on an assessment of the carbonate mineral types in the lake sediments.
Sedimentary carbonates precipitating in lakes generally have two origins:
either the carbonate is formed organically during the formation of the shells
of organisms that live in the water, or it is formed inorganically, as carbonate
precipitates out of the water through chemical reactions. Benson's research
at Owens Lake showed that the amount of carbonate in the sediments that
formed inorganically (the total inorganic carbon, or TIC), compared with
the carbonate that formed organically, was signii cantly higher during this
mid-Holocene period. A higher level of TIC usually indicates that the pre-
vailing climate conditions were drier, resulting in more evaporation of the
lake surface, thereby leaving a greater concentration of dissolved salts in the
lake waters that precipitate as carbonate minerals. Moreover, the time period
between 6,480 and 3,930 years ago was devoid of sediments, and Benson
hypothesized that this “ hiatus” in sediment deposition resulted from desicca-
tion of the lake during that period, suggesting an extremely prolonged period
of drought during the mid-Holocene.
Another large and ancient lake, Tulare, of ers its own evidence of a mid-
Holocene drought. Located in the southern end of California's Central
Valley, Tulare Lake was once the largest freshwater lake west of the Great
Lakes until its source of freshwater was diverted for agriculture in the early
twentieth century. As recently as 1879, the surface area of the lake was 690
square miles. However, lake sediments and ancient shorelines show that
Tulare Lake completely evaporated during the mid-Holocene, again suggest-
ing an extremely dry climate during that period. Researchers have identii ed
imprints of buried mud cracks that developed in the lake bottom as the water
evaporated away, and they have studied the fossil remains of land plants that
grew on the newly exposed lake bottom, all dating to 5,500 years ago. h
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