FLOODPLAIN LAKE SEDIMENTS AS ARCHIVES FOR ENVIRONMENTAL CHANGE

S.E. Arkansas experienced two 100-year flood layers in March and April 2008. In order to characterize the recent flood layer deposited in oxbow lakes in order to have an analogue layer for past events we recovered short (80 cm) surface sediment cores from oxbow lakes at the lower White River. In addition, water samples from different depth and measured the temperature, oxygen and pH profiles were taken. A PCA shows that the more frequently flooded lakes have a higher alkalinity and conductivity but the flood frequency is unrelated to inorganic nutrient concentration. We found that lakes closer to the river have higher Zn concentrations. In order to identify the flood layers in the short cores, the sediment cores were split, documented by a high resolution digital camera and the magnetic susceptibility was measured with a laser point sensor at the LRC. Zn, derived from XRF scanning, shows possible synchronous fluctuations in both cores. The chronology is in progress and the first results are encouraging. The bottom age of the 3 m long core from Maddox Bay Slough dates back to AD 1280 ± 10. According to a flood probability map from Ducks Unlimited, the 3 m core comes from a less frequently flooded area than the 6 m long core. We expect a higher sedimentation rate at Horseshoe Lake at Russell (location of 6 m long core). Zinc and diatom concentrations from both oxbow lake sediment cores are strongly negatively correlated. Samples with higher Zn concentrations had higher abundance of the diatom taxon Aulacoseira granulata known for its planktonic life form. Zinc is likely a useful indicator for past flood events as it occurs in elevated concentrations in the watershed of the White River, AR and shows high variability and possible cyclicity. Brad Hubeny will measure together with John King the paleomagnetic stratigraphy of two cores, which will contribute to further identification the flood layers. Samples for grain size analysis are in progress at the LRC. Peter Appleby (Liverpool, UK) is working on the ¹²⁰Pb dating. First results indicate that these sediments might be used as archives for environmental change and worthwhile to explore further.