A 7000-year record of floods and ecological feedbacks in Weeks Bay, Alabama, USA

Climate change, sea-level rise, and human activities present major concerns for coastal environments. Paleoenvironmental records allow us to extend the instrumented record and study recent environmental impacts in a long-term context with natural pre-industrial conditions. Here, we investigate grain size, stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes, elemental composition, and diatom abundance in sediments to construct a 7000-year paleoenvironmental history of Weeks Bay, Alabama, a NOAA National Estuarine Research Reserve. Four major floods of the Fish River since 1986 CE are independently identified in the Weeks Bay sediment record, validating the bay setting as an archive of flood events. Thirty-four flood events were identified over the last 5000 years, with two periods of intense flood activity coinciding with the Medieval Climate Anomaly and the Little Ice Age, indicating association of relatively short-term climate events and enhanced storm activity. Further, multiple paleoenvironmental proxies indicate marine conditions during formation of the bay ~6600 calendar years Before Present (cal yr BP) and a brackish transition as the estuary became restricted ~2000 cal yr BP. High total organic carbon/nitrogen values indicate nitrogen limitation in Weeks Bay. Increase in organic content, diatoms, and redox- and nutrient-associated elemental proxies over the last 300 years, with dramatic increase in algal abundance since the 1980s, strongly suggest that human activities (i.e. land clearing, agriculture) increased ecological feedbacks in the bay. Comparing past and present environmental conditions of coastal estuaries advances our understanding of estuarine response to climate change and sea level, floods, and human activities, which is important for environmental management and wetland conservation policy.