THE EVOLUTION OF AN OXBOW LAKE IN THE ANCESTRAL FLOODPLAIN OF THE MISSISSIPPI RIVER: CHANGES IN SEDIMENT ACCUMULATION RATES INFERRED FROM 210PB, 137CS, AND THE 14C ACTIVITY OF BULK SEDIMENT FRACTIONS

Sky Lake is an oxbow lake in northwest Mississippi that is thought to be an ancient meander loop of the Mississippi River. The lake bottom consists of approximately 2.5 m of silt and clay that sits above coarse sands with an intermediate transition zone several centimeters in thickness. Four sedimentation regimes have been identified in the lake sediments. Variable ¹⁴C activities within the transition zone suggest a period of intermittent scour and redeposition when the channel was first abandoned between 4100 and 5300 years BP. Above the transition zone, changes in the ¹⁴C activity with depth become highly linear and can be used to calculate sediment accumulation rates. A sedimentation rate of 0.7 mm/yr lasted for approximately 650 years after abandonment followed by a reduction to 0.2 mm/yr that lasted several thousand years. The reduction in rate is consistent with migration of the river channel away from the oxbow and loss of sediment from frequent flooding events. ¹⁴C, ²¹⁰Pb and ¹³⁷Cs analyses of the most recent sediments document a dramatic increase over the last century to nearly 10 mm/yr, concurrent with clearing of surrounding land for agricultural use.

Carbon-14 activities for this study were obtained from bulk sediment passed through a 250 μm sieve. Samples were combusted at low temperature (400°C) to avoid release of any carbon incorporated into clay mineral structures at the time of weathering. Bulk sediment fractions are not typically used in lake chronology studies because the fine-grained organic matter represents a mixture of inwashed material of variable age, and aquatic organisms that die and settle are subject to reservoir effects caused by incorporation of carbon from sources such as coal or calcite. If the various contributions of carbon remain relatively constant over an interval of time, however, the change in ¹⁴C activity with depth may yield accurate rates of sediment accumulation even if the activity at a specific depth yields an erroneous absolute age. Highly linear plots of the natural log of ¹⁴C-activity with respect to sediment depth suggest that such conditions have prevailed at Sky Lake, allowing accurate estimates of rates of deposition and the timing of transitions.