LINEAR DUNES AS CLIMATE INDICATORS IN THE NEBRASKA SAND HILLS

The Nebraska Sand Hills is the largest dune field in North America and is composed of several types of dunes, none of which are currently active. Linear dunes occupy a large area of the dune field and are unique in their response to changes in wind regime. Within the Sand Hills, the linear dunes may have been destabilized in the past even while other dunes remained inactive. In order for future activation of the dunes to occur, removal of the current vegetation would be necessary, as a sediment source and strong winds are already present. The precise conditions necessary for reactivation are unknown. Many reactivations have occurred in the Nebraska Sand Hills within the last 15,000 years. The most recent reactivation occurred 650 to 950 years ago during the Medieval Warm Period.

Linear dunes in the southeastern part of the Sand Hills are 12 to 15 meters high and several kilometers long. These dunes are built upon older cores of dunes that are thousands of years old. Linear dunes to the northwest are smaller (about 9 meters tall and a few hundred meters long) and are located on the sides of megadunes. Linear dunes throughout the Sand Hills are oriented ESE to WNW, with a younging direction to the southeast, as determined by optically stimulated luminescence dating. The internal structures of linear dunes have been studied by examining blowouts throughout the Sand Hills. The bedding is dominated by wind ripple laminations. Paleosols have been defined and dated throughout the dunes and represent periods of stability. Silt and clay-rich lamellae are well developed beneath the modern soil and paleosols and commonly cross-cut primary sedimentary structures. Tracks and burrows that are preserved within the dunes reflect the cohesive nature of the sand. Bison tracks are preserved as depressed laminations. Mammal burrows are very abundant in the blowouts, and are extremely variable in size and orientation. The burrows of pocket gophers, badgers, and other small mammals have been identified. The burrows occur both normal and oblique to wind ripple laminations. Many burrows have been actively backfilled, although meniscate structure is rare.