INTERPRETING THE SEDIMENT RECORD IN A SHALLOW, ESTUARINE ENVIRONMENT – LAKE PONTCHARTRAIN, LA

Lake Pontchartrain is a shallow (~3.7 m), estuarine environment typical of many of the estuaries surrounding the Gulf of Mexico. The estuary is dominated by physical mixing due to seasonal winds from the southeast and northeast. The majority of the sediment input to the estuary is from infrequent openings of the Bonnet Carre spillway, which divert the turbid waters of the Mississippi River during periods of high flow. The volume of sediment and water that is released to Lake Pontchartrain during these events dwarfs the input from any other sources.

At the field site (30 18.800 N, 90 16.831 W) in the western portion of Lake Pontchartrain, we have constructed an instrumented platform that continuously acquires meteorlogic and hydrologic data. From this data, a wind-driven resuspension model was created to predict resuspension events based on wind speed and wind direction. Rates of deposition, mixing, and burial were determined using short-lived radionuclide (Be-7, Pb-210, Cs-137) data collected from sediment cores taken nearby. Cs-137, which is a tracer produced by above-ground atomic weapons testing, was used to constrain sedimentation rates since ~1963, and Pb-210 (t_1/2 = 22 y), which is part of the U-238 decay series, was used to determine sedimentation rates during the past 120 years. The combination of Pb-210 and Cs-137 data indicates a sedimentation rate of  0.11 cm y^-1.

The down-core distribution of Be-7 (t_1/2 = 53 d ), which is formed cosmogenically, is controlled by deposition, resuspension, and mixing over a period of months. The Be-7 data and x-radiography indicate that there is continuous mixing to an interval of ~ 3 to 5 cm depth. Frequently, a shell hash layer is present below the depth of mixing, which may act as a barrier to physical mixing/resuspension. This shell hash layer may have reduced the depth of physical mixing in the past.

Like so many estuaries, Lake Pontchartrain is a dynamic environment, where sediment input and physical mixing are intermittent. This affects our understanding and interpretation of sedimentary processes occurring in this environment. Derivations of the less complex models used to determine depth of mixing and sedimentation rates were constructed to incorporate the temporal variability identified in the sediment profiles.