EVENT SEDIMENTATION IN EMERGENT AND SALT MARSHES AROUND MOBILE-TENSAW RIVER DELTA AND MOBILE BAY REGION

Hurricanes, floods and other natural hazards threaten human infrastructure and communities as well as natural landscapes around them. However, these natural processes may also transport sediment to coastal marshes that help abate pressures associated with sea-level rise (SLR). In Louisiana, both marsh degradation and sedimentation associated with storms have been documented in great detail; however for the rest of the Gulf of Mexico, these topics are not understood as well. For example, the Mobile-Tensaw River Delta (MTRD) and adjacent Mobile Bay contain a variety of non-tidal to tidal, hardwood and marsh wetlands. This area has been subjected to a number of major storms, most of which occurred after 1950s (e.g. Hurricane Camile-1969, Hurricane Fredrick-1979 and Hurricane Ivan-2004). To quantify marsh response to storm-event deposition and the inclusion of inorganic sediment in marsh ecosystems, push-cores were collected from marshes around MTRD and Mobile Bay. Marsh-accumulation rates and core geochronology were computed using a constant flux model applied to excess lead-210 (²¹⁰Pb) measured in the sediments. Freshwater marsh accumulation rates were highly variable (0.24 and 1.31 g cm^-2 y^-1) with a general decrease over the last 100-y. The highest accumulation rates were observed in the 1950s and probably reflect an influx of sediment during the 1955 flood event that affected this region. In comparison, salt marshes along the bay experienced a general increase in accumulation rates over the last 120-y (0.05 to 0.18 g cm^-2 y^-1 or 0.23 to 0.48 cm y^-1). Between 1880 and 1960 (a relatively quiescent period), organic accumulation appeared to remain fairly constant (~20%); however between 1960 and present day, intermittent pulses of higher inorganic sediment supply were observed. Specific periods include the late 1960s, late 1970s to early 1980s and early 2000s; which corresponds to several major hurricanes (e.g. hurricanes Camile, Fredrick, and Ivan). The nearly three-fold increase in sediment deposition in salt marshes during the last 120-y would thus appear to be partially dependant on inorganic sediment supply from storm events. Based on this data set, marshes along the MTRD and Mobile Bay are accreting at rates sufficient to keep pace with local SLR (~0.4 cm y^-1).