An Examination of Historical Inorganic Sedimentation and Organic Matter Accumulation in Several Marsh Types within the Mobile Bay and Mobile-Tensaw River Delta Region

Mass accumulation rates (MAR; g cm(-2) y(-1)), linear sedimentation rates (LSR; cm y(-1)), and core geochronology derived from excess lead-210 (Pb-210) profiles and inventories measured in six sediment cores collected from marsh sites from the Mobile-Tensaw River Delta and Mobile Bay region record the importance of both continuous and event-driven inorganic sedimentation over the last 120 years. MAR in freshwater marshes varied considerably between sites and through time (0.24 and 1.31 g cm(-2) y(-1)). The highest MARs occurred in the 1950s and 1960s and correspond to record discharge events along the Mobile and Tensaw Rivers. In comparison, MAR at salt marsh sites increased almost threefold over the last 120 years (0.05 to 0.18 g cm(-2) y(-1) or 0.23 to 0.48 cm y(-1)). From 1880 to 1960, organic accumulation remained fairly constant (similar to 20%), while intermittent pulses of high inorganic sedimentation were observed following 1960. The pulses in inorganic sedimentation coincide with several major hurricanes (e.g., Hurricanes Camille, Fredric, Georges, and Ivan). The nearly threefold increase in MAR in salt marshes during the last 120 years would thus appear to be partially dependent on inorganic sedimentation from storm events. This study shows that while hurricanes, floods, and other natural hazards are well-known threats to human infrastructure and coastal ecosystems, these events also transport sediment to marshes that help abate other pressures such as sea-level rise (SLR) and subsidence.