QUANTIFYING SEDIMENT THICKNESS AND PRE- AND POST-SETTLEMENT SEDIMENTATION RATES FOR HIGH PLAINS PLAYA WETLANDS UTILIZING 210PB AND 14C DATING TECHNIQUES

Grasslands are some of the most heavily modified ecosystems in the world. On the High Plains of the central United States, a grassland region with a prolonged history of intensive agricultural land use, rates of conversion to cropland have been increasing. The High Plains landscape is a vast expanse of highly productive soils and subdued topography, allowing for easy conversion to cropland.

Scattered across the High Plains are >90,000 depressional playa wetlands. Playas are critical resources for this semi-arid region, serving as foci for surface water storage, wetland habitat, groundwater recharge, and biodiversity. Increased sediment accumulation is the primary impairment to playas. Playas are particularly susceptible to degradation as a result of cultivation because they are located at the lowest point within internally drained watersheds and accumulate sediment generated by run-off from the entire watershed.

To examine the impacts of conversion of playas and their watersheds from grassland to cropland, seven playas on the High Plains of western Kansas were examined. Five playas were cultivated with cropland-dominated watersheds and two playas were grassed with grassland-dominated watersheds. Playas were examined to quantify the amount of anthropogenically accelerated sediment accumulated within playas and calculate sedimentation rates for pre- and post-settlement periods. Thickness of accumulated sediment was measured in the field and soil cores were collected from each playa. Four cores were analyzed for lead-210 to establish post-settlement sedimentation rates. Long-term, pre-settlement sedimentation rates were calculated by radiocarbon dating of buried soils within cores.

Playa stratigraphic records span the entire Holocene, and pre-settlement sedimentation rates were highest during the early Holocene, at ~2 – 4 cm/century. During the middle and late Holocene, sedimentation rates were ~1 cm/century. Lead-210 data indicate that post-settlement sedimentation rates averaged ~6.5 – 10 cm/century for cropland-dominated playas, while sedimentation rates for grassland playas were comparable to middle and late Holocene rates. Conversion of playas and watersheds to cropland has greatly accelerated sediment accumulation within playas, impairing critical playa ecosystem functions.