North-Central Section - 39th Annual Meeting (May 19–20, 2005)

Paper No. 10
Presentation Time: 11:20 AM


HAJ Jr, A.E.1, BETTIS, A.2, DORALE, Jeffrey A.1 and CARPENTER, Scott J.3, (1)Geoscience, Univ of Iowa, 121 Trowbridge Hall, Iowa City, IA 52242, (2)Geoscience, Univ of Iowa, Trowbridge Hall, North Capitol St, Iowa City, IA 52242, (3)Geoscience, Univ of Iowa, 121 Trowbridge Hall, Iowa City, IA 52242-1379,

Continuing stratigraphic and geochemical investigation of Bear Creek, a fourth order tributary to the Sac River along the western margin of the Ozark Plateau, southwestern Missouri, offers a detailed picture of landscape response to Holocene climate and vegetation changes in the southern portion of the prairie-forest ecotone. The chronology, lithology, and distribution of valley fill sequences reconstructed from outcrops and coring transects provide a framework for stable carbon isotope (d13C values) that yield a history of vegetation changes within the valley. Episodes of aggradation, stability, and erosion similar to those previously documented in larger valleys in the western Ozarks are present within the Bear Creek valley fill. A record of Holocene storm activity recorded in the speleothem record of Crevice Cave, located on the eastern margin of the Ozark Plateau, shows a shift in cave flood frequency around 4500 years BP to a state of more frequent and intense flooding (Lepley, 2004). Leply found the timing of the shift similar to a shift in state of El Niño events to one of increased frequency and intensity. At Bear Creek a period of increased channel activity and removal of floodplain sediment at approximately 4400 years BP corresponds with the increase in storm activity as recorded at Crevice Cave. Floodplain aggradation occurred from 4400 years BP until the late Holocene. During this period the vegetation record (soil d13C values) suggests a landscape of lowland forests and upland prairie. In the late Holocene, approximately 1400 years BP, channel activity increased and forests emerged from the lowlands to dominate the uplands. The climate record at Crevice Cave indicates that major changes in the Bear Creek stream system are driven by increased precipitation and vegetation change brought about by a shift in the climate state to one where El Niño events are more frequent and intense.