Relict nebkhas (pimple mounds) record prolonged late Holocene drought in the forested region of south-central United States
Christopher L. Seifert, Randel Tom Cox, Steven L. Forman, Tom L. Foti, Thad A. Wasklewicz, and Andrew T. McColgan
Summarized by Isaac Pope
Introduction: Even before their first geologic description in the nineteenth century, the Mima Mounds of the Puget Lowland have captivated the human mind. The elliptical dome shapes of the millions of regularly spaced mounds have defied explanation (figure 1), yet these mounds appear to be a globe phenomenon. Across prairies on North America and other continents, mounds resembling the Puget Lowland Mima Mounds (termed “Mima-like mounds”) have incited geologists to propose a host of forces from earthquakes to flooding rivers and even rodents as explanations for these enigmatic mounds (Johnson and Burnham, 2012; Tucker, 2015). Because of the diverse settings in which these mounds are found, some researchers have suggested a single cause of all Mima-like mounds is unlikely, opining that instead a variety of forces may have been the cause. Amid this debate, six scientists have proposed that the Mima-like mounds of south-central United States are the remnants of a geologically recent drought.
The Data: Locally known as pimple mounds, the Mima-like mounds of south-central United States are found on flat benches near rivers and streams across Arkansas, Oklahoma, and nearby states. While some may be currently or historically forested, most mounded areas appear to have originally been prairies or open areas within forests. The mounds are underlain by bedrock or a subsoil pan relatively impervious to water seepage, which may be one reason for the general lack of forests in mounded areas.
The Methods: The research team cored three prairies in Arkansas and Oklahoma to analyze the grain size and potential age of the mound material, collecting six cores of each sampled mound and one core in the low area adjacent to the mound. Four of the mound cores were collected on the North, South, East, and West axes of the mound and a fifth was taken in the center. Taken in a specialized metal pipe, the sixth was collected near the central core for luminescence dating, a method of dating the extent of time since a silicate mineral such as quartz has been buried by observing its reaction with light. Some of the mounds in one of the prairies were also measured using laser-scanner surveying technology to evaluate the asymmetry of the mounds.
The Results: Often steepest on their northwestern slope, the mounds were found to be composed primarily of silt and sand, being coarsest towards the northwest. Luminescence dating indicated that the mound sediment had been deposited within the past several thousand years during the Holocene with age increasing with depth.
Implications: The asymmetry of mounds both in their shape and composition suggest that they are nebkhas or coppice dunes, which commonly form in sub-arid areas as shrubs capture sand and silt in windy conditions. The origin of these Mima-like mounds as relict nebkhas supports extended droughts in south-central United States through middle and late Holocene, which was dominated by winds trending towards east or southeast.
A Broader Perspective: The identification of these Mima-like mounds (“pimple mounds”) as nebkhas provides insight not only into the local paleoclimate but also the potential origin of some Mima-like mounds. The mounds of south-central United States are one of the only local datasets spanning the middle and late Holocene, thereby recording information on the duration and extent of past droughts in the region. It is also possible that other Mima-like mounds, such as the classic mounds of the Puget Lowland, are also nebkhas, although more recent research indicates that a wind-based model for mound formation in the Puget Lowland is unlikely due to the extensive cobbles and boulders among the mounds (Pope et al., 2020). Further study of the nebkhas of south-central United States may continue to reveal information for solving one of the most baffling mysteries of geology
Citation: Seifert, C.L., Cox, R.T, Forman, S.L., Foti, T.L., Wasklewicz, T.A., and McColgan, A.T., 2009, Relict nebkhas (pimple mounds) record prolonged late Holocene drought in the forested region of south-central United States: Quaternary Research, vol. 71, p. 329-339, doi: 10.1016/j.yqres.2009.01.006.
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