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).
At the subcontinental scale, relationships between modern climate and pollen were used to reconstruct the climate of the past from fossil pollen. Past distribution of fossil pollen have been used to generate estimates of past climate conditions in eastern North America (Webb et al. 1998.) One such map of eastern North America (shown to the right) indicates regional increases and decreases in annual precipitation 6,000 years ago, relative to today's conditions. The southeastern United States was up to 20% wetter than it is today, but much drier conditions prevailed from the Great Lakes to the Northeast, and from central Canada south to Louisiana. Accompanying these decreases in precipitation was an increase of 1 to 2 degree centigrade in mean July temperature in central North America.
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 status (level or depth) data in central North America. Their analysis, showing drying to the north and wetter or no change in the southwest, used physical, chemical, and biological data from sediment cores or exposed sections for 35 lake basins. The figure to the right shows the difference in lake levels or depth between 6000 years before present and present with + indicating wetter, - indicating drier than present, and o indicating no change from present.
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