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Even Longer Records Hydroclimatic Variability from Pollen, Charcoal, and Lake Level Changes Changes in types of pollen
found in lake sediment layers provide information about hydroclimatic
(i.e., both climatic and hydrologic) variability in central North
America during the mid-Holocene (~ 8,000-6,000 years ago). Pollen
analyses from lakes in the northern Great Plains suggest a mid-Holocene
shift from grasslands to vegetation dominated by weedy annuals,
whereas charcoal evidence indicates persistent decade-to-century
scale drought cycles at this time (Grimm et
al. 1999). The
diagram to the right shows the changes in grass (Poaceae, in
green) and weedy annuals (Ambrosia-type, in red) in the mid-Holocene
for Kettle Lake, North Dakota. When weedy annuals are more prevalent,
relative to grasses, episodic droughts are indicated. This diagram
also shows charcoal found in the lake sediments, which is a sign
of fire. In the Great Plains, higher fire frequency occurred
under wet conditions when more fuel, needed by fire, was produced.
A lack of fire, and a decrease in charcoal, correspond to drier
conditions. The far lefthand columns show changes in minerals
in the sediments which correspond to wetter (aragonite) and drier
(other minerals) conditions. Taken together, the greater relative
amounts of weedy annual pollens, and smaller amounts of charcoal
and aragonite define periods of drought (indicated by horizontal
yellow bars).
Other
hydroclimatic information is provided by lake level and other
geochemical and biological proxies for lakes, such as the amount
of organic or windblown material within sediments, or relative
abundance of shallow versus deep water dwelling organisms. These
proxies provide important hydroclimate information that can be
interpreted in terms of hydrologic extremes (wet/dry, high/low
lake levels or depths). Qin et al.
1998 have
published the most recent continental scale assessment of mid-Holocene
lake
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