What was done
The authors say they "used ¹³C:¹²C ratio (δ¹³C) and ¹⁴C dating to evaluate relationships and changes in warm (C4) versus cool season (C3) plant signatures with age of soil organic carbon" across "three soil temperature regimes and three soil moisture regimes within the historic grasslands [of the] US Great Plains and Western Corn Belt" for the entire period of the current interglacial or Holocene.

What was learned
Follett et al. report that their data "indicate a change from C4 plants to increasing C3 plant dominance (as a surrogate of cooler temperature) at ~1,500 yr B.P."  More specifically, they say that "the yr B.P. when δ¹³C was least negative was 1560," and that "δ¹³C was more negative before or after that time."

What it means
The authors appear troubled by the fact that the timing of the cold period they discovered does not mesh very well (to say the least!) with that of the Little Ice Age (650-150 yr B.P.).  But why should it?  The date they obtained for the time of its maximum expression (~1,500 yr B.P.) puts it right in the middle of the Dark Ages Cold Period, as clearly delineated by the δ¹⁸O data of McDermott et al. (2001), whose work also indicates that the cold of this multi-century period of inclement weather that preceded the Medieval Warm Period was more extreme than the cold of the Little Ice Age that followed it.  In addition, it meshes extremely well with the results of the close to 30 other reports of this cold phase of earth's millennial-scale cycle of climate that we have reviewed and archived in the Dark Ages Cold Period section of our Subject Index.

The ultimate significance of all of these findings, of course, is that they demonstrate the reality of the natural millennial-scale oscillation of earth's climate, which all on its own, without any help from a similar oscillation of atmospheric CO2 concentration, produced the nearly identical warmings of the Roman Warm Period, the Medieval Warm Period and, most recently, the Modern Warm Period.

References
McDermott, F., Mattey, D.P. and Hawkesworth, C.  2001.  Centennial-scale Holocene climate variability revealed by a high-resolution speleothem δ¹⁸O record from SW Ireland.  Science 294: 1328-1331.