What was done
Sediment cores were extracted from Pickerel Lake, South Dakota, USA in the 1960s and again in 1995 and analyzed for magnetic susceptibility, percent organic matter and percent calcium carbonate.  The results of these analyses were then interpreted in terms of environmental and climatic change.

What was learned
Over the past 2000 years, there have been recurring incidences of major drought on the Northern Great Plains of the United States at approximately 400-year intervals.  This cyclic behavior appears to be in synchrony with similar variations in solar irradiance.  The most recent of these droughty periods occurred between 200 and 400 years ago and was contemporaneous with the Maunder Minimum of sunspot activity, which also corresponds with the main cold phase of the Little Ice Age.

What it means
After making the case for "a direct connection between solar irradiance and weather and climate," the authors state "it seems reasonable that the cycles in aridity and eolian activity over the past several thousand years recorded in the sediments of lakes in the northern Great Plains might also have a solar connection."  Their paper is one more piece of evidence that this conclusion is indeed correct.