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Even Longer Records
Evidence of Changes in Aridity in the Upper Mississippi Basin
Dean, W.E., T.S. Ahlbrandt, R.Y. Anderson, and J.P. Bradbury
Complete Scientific Reference
Elk Lake Sediment Data and
Data Description from the WDC Paleoclimatology archive.
Summary:
The distribution of the vegetation present in Minnesota today reflects both
north-south and east-west climate gradients (figure to the right),
and identifies this as an important region for studies of past drought and
climate variability. Lakes sediments from Elk Lake, in the headwaters region
of the Upper Mississippi Basin (northeastern Minnesota), record variations in
climate and other environmental factors over time scales of years to millennia.
Elk Lake sediments are layered in varves, which make it possible to reconstruct
information on climate variability with annual resolution. Sediment cores taken
from the lake bottom have been used to generate a regional record of aridity
based on the chemical, sedimentological, and biological characteristics of
the sediments. Changes in varve thickness as well as the abundances of
wind-blown quartz-rich silt and clay (quartz values increase under dry
conditions), or the sodium from plagioclase feldspar (under dry conditions,
retained in soil until washed or blown into lakes) are indicative of
regionally wet and dry conditions. Increases in the remains of the diatom
Aulacoseira indicate windy conditions, commonly accompanying increased aridity.
The Elk Lake record, spanning almost 11,000 years, shows that conditions were
warmer, drier, and windier between 8000 and 6000 year ago as inferred from
increases in varve thickness, sage pollen, quartz abundance, sodium levels,
and the diatom Aulacoseira (figure to the right). Similar changes
in the sediment composition between 4800 and 4300 years ago are indicative of
another interval of dry conditions. The subsequent decrease in varve thickness,
sage pollen, quartz abundance, sodium levels, and Aulacoseira, reflects an
overall trend to moister conditions from 4000 to 1000 years ago. High amplitude
changes in sediment composition over the last 1500 years are also interpreted to
reflect a number of shorter but equally severe intervals of dry conditions
(see Dean, 1997 for details on this work).
For more details on Elk Lake paleoenvironmental studies,
click here.
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