Reconstruction of paleostorms and paleoenvironment using geochemical proxies archived in the sediments of two coastal lakes in northwest Florida

Late Holocene paleoclimate records from coastal regions are important for understanding long-term variability of hurricane activity. Here we present a nearly 4000-year record of severe storm landfalls and environmental changes based on organic geochemical proxies (OGPs) preserved in sediment cores from two coastal lakes in northwest Florida. Our analysis shows that there are significant variations in δ13C, δ15N, C%, N% and C/N with depth, reflecting changes in lake environment, which in turn affected the processes delivering water and sediment to the lake as well as biological productivity within the lake. Isotopic signatures of modern organic materials in the lakes and their surrounding areas show that the major sources of sedimentary organic matters in the lakes are aquatic and terrestrial C3 vegetation. C4 grasses do not contribute significantly to the sedimentary organic matters in the lake, although they can be found in the mostly forested watershed. Thus, the positive C and N isotopic shifts, concurrent with negative shifts in C/N ratios, most likely indicate shifts to a marine-like environment in coastal lakes following the influx of marine water and nutrients and marine biota associated with major storm events. Some of these isotopic shifts observed in the sediment cores correspond to visible sand layers presumably representing overwash deposits associated with severe storm events. Radiocarbon dating of bulk sediment organic matters, wood fragments and shells indicates that the sediment in these cores was deposited over the last 3000–4000 years. Based on our age model and OGP interpretation, Eastern Lake data suggest that the recurrence interval of severe storms (i.e., large enough to cause seawater flooding of the lakes) is approximately 84 years over the last 2900 years, whereas Western lake data suggest an average recurrence interval of 86 years in the past 3900 years. Organic geochemical proxies in coastal lake sediment are indicators of lake environment. Main sources of organic matter in coastal lakes of NW Florida are C3 plants. Concurrent positive δ13C and δ15N shifts and negative C/N shift indicate an overwash event. A minimum of 45 large storms hit the northwest coast of Florida in the last 3871 years. Landfalling tropical systems large enough to cause overwash occurred every 84–86 years in the last 3–4 millennia.