| Abstract | Geological evidence on the northern Great Plains, between 50 and 51° N latitude, reveals former dune fields, used today primarily as cattle rangeland, were essentially desert-like barchanoid and
transverse dunes with unvegetated interdune sand sheets during the latter part of the 1700s. These dunes migrated across dry lacustrine and low-lying basins, and inter-dune areas consisted of bare sand sheets. Our results combine data from LiDAR
(Light Detection and Ranging) imagery with optically-dated dune sands to document the transition of barchanoid to parabolic dunes between AD 1810 and 1880. We interpret residual dune ridges (RDRs) observed in this study to record the transition of
barchanoid to parabolic dunes, based on observations of RDRs in other dunes setting, and numerical models of barchanoid-to-parabolic dune transformation. Reconstruction of dune field morphology for the last few hundred years is based on 32 optical
ages from buried stratigraphic sections, residual dune ridges, stabilized parabolic dunes and blowouts at six field sites. We also reconstruct a simplified history of dune mobility, based on generalized geomorphic-system responses following
perturbation, and historically-documented and modelled Canadian prairie dune field stabilization rates in last century. We then compare our reconstruction of dune field type and change in dune mobility to proxy records of climatic variability in
southwestern Canada for the last few hundred years. We interpret the dune fields to have had reduced vegetation cover and lower groundwater tables prior to the 1800s. Residual dune ridges, formed by increased soil moisture and vegetation cover,
recorded the transition from barchanoid to parabolic dunes over a period of about 70 years. Widespread stabilization of parabolic dunes occurred between AD 1880 and 1910, with stabilization of blowouts and a few larger active dunes continuing to
today. Barchanoid dune fields existed at least since the mid-1740s, in concert with cooler, dry, windy conditions. The relatively long period of stabilization, in comparison to dune stabilization rates in other parts of the Great Plains, may be
attributed to shorter growing seasons on the Canadian prairies. Cooler climatic conditions, interspersed with episodes of severe drought allowed sand transport to out-compete with stabilizing vegetation. In addition, shorter growing seasons and
continued strong winds may have assisted in perpetuating dune activity. These cooler but dry and windy conditions were related to the climate persisting during the Little Ice Age on the Canadian prairies. These findings explain the apparent
paradoxical observations that sand dunes have been stabilizing under historical warming. Projected warmer climatic conditions in the region for the coming century, though having the potential to increase dune activity, are less likely to bring about
a return to barchan dunes, than would a return to cooler but dry climatic conditions of preceding centuries. Nevertheless, the presently observed landscape stability of southern Canadian prairies may be viewed as residing within a narrow window,
where change of a few degrees warmer or cooler can have a significant effect when coupled with reductions in moisture availability. This study also demonstrates that interpretations of dune responses to shorter-term episodes of drought and
"mega-droughts" must be viewed in the context of longer-term dune system responses to changing climatic and ecological conditions. |