MIDDLE-HOLOCENE AND LATE-HOLOCENE DUNE REACTIVATION THE NEBRASKA SANDHILLS, USA

A combination of optical dating of aeolian sand, and radiocarbon datin g of palaeosols and interdune sediments, has allowed some refinements to the chronostratigraphy of the Nebraska Sand Hills. Our results dire ctly confirm previous inferences that multiple phases of aeolian depos ition, including megabarchan dune reactivation, occurred within the Sa nd Hills during the middle and late Holocene. Optical dates on stratif ied aeolian (barchan dune) sediments from two localities provide ages ranging from c. 6 ka to <300 years ago. Weakly developed palaeosols in tercalated within the dune sediments and other forms of post-depositio nal modification indicate periods of relative stability and reduced du ne mobilization. Optical dates on the upper 12-15 m of two dune exposu res indicate repeated, possibly regionally correlative, reactivations which occurred between c. 400-500 and 200-300 years ago. Comparison of late-Holocene aeolian activity with local and regional palaeoclimatic proxy records suggests that dune reactivation may have occurred frequ ently, possibly in response to periods of extended (>20 yr) drought.