QUANTIFYING THE ANNUAL CARBON BUDGET FROM A RAPIDLY ERODING COASTAL FRESHWATER WETLAND USING FIELD AND MODEL DATA

Coastal freshwater wetlands are important components of the global carbon cycle and are generally presumed to be carbon sinks. Along Great Lakes shorelines, shoreline erosion is potentially transitioning these landscapes from carbon sinks to sources through the loss of wetland area and export of stored carbon. Previous saltmarsh carbon budget models include export of carbon through erosion and reduction in carbon burial through overwash (Theuerkauf et al., 2015; Theuerkauf and Rodriguez, 2017), but no such model exists for freshwater coastal wetlands, such as those along the Great Lakes. Here, we develop a model of carbon storage in freshwater wetlands that includes shoreline erosion and overwash. We validate the model with observational data collected from a Great Lakes coastal freshwater wetland at Illinois Beach State Park (IBSP), which is located along the southwestern shore of Lake Michigan. The model generates an annual carbon budget by subtracting net carbon export from net carbon storage. Inputs for net carbon storage include the width of the wetland, age of the marsh material, and carbon inventory. Net carbon export is evaluated using the shoreline change rate and the carbon inventory. To gather the inputs for the model, eleven sediment cores were collected across two shore-normal transects. The carbon inventory was quantified through elemental analysis on samples from the sediment cores, and wetland age was determined through C-14 dating of basal marsh material. Wetland width, shoreline erosion rates, and washover frequency and landward extent were measured through GIS analysis of historical aerial photographs dating back to the 1930s as well as from RTK-GPS topographic survey data collected over three months in the fall of 2017. These observational data will be used to evaluate whether the wetlands at IBSP are currently functioning as carbon sinks or sources. Results from the model will be used to explore the historical carbon budget at IBSP, the role of fluctuating Great Lakes water levels in dictating wetland carbon storage, and the future of carbon storage in these wetlands.