Geoscience Reference
In-Depth Information
Additional evidence for increased wetness during the Little Ice Age
has been found downstream of the Sierra Nevada, in the Central Valley.
Tulare Lake, i lled with increased river inl ows and l oodwaters, reached a
high stand during the Little Ice Age. San Francisco Bay became fresher as
river inl ows increased. Vegetation records from the marsh sediment cores
surrounding San Francisco Bay reveal a shit to a dominance of freshwater
species, rel ecting greater inl ows from rivers swollen with heavy rainfall in
the watershed.
climate swings, wildfires, and droughts
Many of the paleoclimate records show increased variability during the Little
Ice Age, with a generally wetter and cooler climate punctuated by episodes
of drought—sometimes severe. Droughts have been detected in tree rings
from the Sierra Nevada, salinity levels in San Francisco Bay, and lake levels
of Mono, Pyramid, and Owens lakes. At Mono Lake, for instance, Larry
Benson and his colleagues examined sediment cores using the oxygen isoto-
pic ratios of the sedimentary carbonates to determine changes in the lake's
surface level. As discussed in chapters 5 and 7, when the inl ow of water enter-
ing a closed-basin lake exceeds the rate of evaporation, the lake expands and
the ratio of oxygen-18 to oxygen-16 is lower. When evaporation is the domi-
nant process, however, the lake shrinks, and the lighter oxygen-16 water mol-
ecules evaporate more readily, leaving behind more oxygen-18 molecules in
the lake. Benson and his colleagues reconstructed the relative changes in lake
level based on the oxygen isotopic ratios in carbonates, and they have shown
that the water level in Mono Lake dropped i ve times over the past 300 years,
implying severe droughts. h ese dry periods lasted for several years, causing
a signii cant drop in lake levels, centered on the dates AD 1710, 1770, 1820,
1850, and 1930.
h e San Francisco Bay records also contain evidence of these periodic
swings in climate seen in the Sierra during the Little Ice Age. Oxygen iso-
tope measurements of sediments from the northern part of the estuary have
enabled the authors to reconstruct the bay's salinity (as discussed in chapters
5, 8, and 9). h e results reveal multiple l uctuations in salinity over the cen-
turies, exhibiting 55-, 90-, and 200-year cycles.
In the American Southwest, climate during the Little Ice Age appears to
have been highly variable. Tree-ring studies from the Colorado Plateau show
