Pimple Mound OSL Ages: Late Holocene Great Plains Drought Chronology Extended to the South Central United States

Pimple mounds (a.k.a. prairie mounds) are ~1 m-high, ~ 20 m-diameter hillocks that are found in groups on river and marine terraces in the south central United States. We previously reported that these mounds have statistically significant excesses of coarse sediment on their northwest flanks and concluded that this textural asymmetry reveals pimple mounds to be nebkhas (coppice dunes), eolian deposition around shrub wind-breaks.

Using optically stimulated luminescence, we dated multiple levels of four pimple mounds from the Arkansas River Valley, Ozark Plateau, and Gulf Coastal Plain. All yield ages between 2400 and 700 yr B.P., the upper part of mounds from the Arkansas Valley and Gulf Coastal Plain yield ages between 1100 and 700 yr B.P., and the lower part of one Arkansas Valley mound yields a mid-Holocene age (~6800 yr B.P.).

These late Holocene ages correlate with well-documented periods of eolian activity and droughts on the southern Great Plains, including the Medieval Climate Anomaly (ca. 1100 to 700 yr B.P.). We suggest that eolian deflation and deposition giving rise to pimple mounds was the consequence of forest loss during extended droughts. Although pimple mound distribution is closely related to sub-soil pans that may have exacerbated local drought severity, the recognition of young relict nebkhas in the central U.S. is important in understanding the potential for extended drought. These mounds reflect the landscape response to decadal-scale, potentially continent-wide megadroughts for the south central U.S. in the late Holocene. The spatial persistence of pimple mounds across the south central U.S. further underscores the severity and duration of late Holocene droughts, which were significantly greater than historic climate variability.