HOLOCENE HISTORY OF LACUSTRINE AND MARSH SEDIMENTS IN A DUNE-BLOCKED DRAINAGE, SOUTHWESTERN NEBRASKA SAND HILLS, USA

As many as 2500 interdune lakes lie within the Nebraska Sand Hills, a 50 000 km stabilized sand sea. The few published data on cores from th ese lakes indicate they are typically underlain by less than two m of Holocene lacustrine sediments. However, three lakes in the southwester n Sand Hills, Swan, Blue, and Crescent, contain anomalously thick mars h (peat) and lacustrine (gyttja) sediments. Swan Lake basin contains a s much as 8 m of peat, which was deposited between about 9000 and 3300 years ago. This pear is conformably overlain by as much as 10.5 m of gyttja. The sediment record in Blue lake, which is 3 km downgradient f rom Swan lake, dates back to only about 6000 years ago. Less than two m of peat, which was deposited from 6000 to 5000 years ago, is overlai n by 12 m of gyttja deposited in the last 4300 years. Crescent Lake ba sin, one km downgradient from Blue Lake, has a similar sediment histor y except for a lack of known peat deposits. Recently, a 8-km long segm ent of a paleovalley was documented running beneath the three lakes an d connecting to the head of Blue Creek Valley. Blockage of this paleov alley by dune sand during two arid intervals, one shortly before 10 50 0 yr BP and one in the mid-Holocene, has resulted in a 25 m rise in th e regional water table. This made possible the deposition of organic-r ich sediment in all three lakes. Although these lakes, especially Swan , would seem ideal places to look for a nearly complete record of Holo cene climatic fluctuations, the paleoclimatic record is confounded by the effect dune dams have on the water table. In Swan Lake, the abrupt conversion from marsh to lacustrine deposition 3300 years ago does no t simply record the change to a wetter regional climate; it reflects t he complex local hydrologic changes surrounding the emplacement and se aling of dune dams, as well as regional climate.