ESTUARINE SEDIMENTATION, 210PB AND AGE MODELS IN A CHANGING WORLD – THE RIGHT FIT MAKES ALL THE DIFFERENCE

The supply of sediment to an estuarine system varies in response to forces that alter source environments (e.g., storms, sea-level rise, land-use). Determining changes in the rate of sedimentation within undisturbed portions of the estuarine environment can be an effective method for determining changing sediment flux within the system. Excess ²¹⁰Pb is a powerful and effective tool for determining changes in the rate of sedimentation over time, but use of different age models can result in drastically different representations of the changing sedimentation rate.

In 2010, a 465-cm core was taken within the Cape Lookout Bight (CLB), NC estuarine settling basin, with excess ²¹⁰Pb analysis conducted at a 1-cm interval for the entirety of the core. Initial rates of sedimentation were determined through use of the constant initial concentration (CIC) sedimentation rate model. Sedimentation rates determined through use of the CIC model correlated well with established age horizons within the core, but indicated an abrupt increase in the rate of sedimentation ~1983-1985, with no corresponding lithologic change. The periodic flux (PF) age model, which allows for known age horizons to be included, was also applied to the excess ²¹⁰Pb profile. Using this model, ages were determined at 1-cm intervals down core, allowing for sedimentation rates to be determined on a nearly monthly basis. The PF model did not display an abrupt change in the rate of sedimentation that the CIC model did, but instead indicated abrupt shifts and cyclic trends in the rate of sedimentation that appear to correlate with isolated storms and seasonal-to-decadal trends.

Although both models are useful, the CIC model, which assumes a constant rate of sedimentation over long periods, may miss isolated events or cyclic trends in the data, resulting in misinterpretations of the changing sediment flux within the system. Choosing a more robust age model, like the PF model, may be more appropriate for high-resolution datasets like the one obtained from CLB because it allows for determination of changes in sedimentation rates and, by proxy, changes in sediment flux on time scales ranging from individual events to seasonal and decadal trends.