RECONSTRUCTION OF PALEOSTORMS AND PALEOENVIRONMENT USING GEOCHEMICAL PROXIES IN SEDIMENT CORES FROM 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 storms and environmental changes based on organic geochemical proxies (OGPs) preserved in sediment cores from two coastal dune lakes in northwest Florida. The OGPs show that there are significant variations in δ¹³C, δ¹⁵N, C%, N% and C/N with depth, reflecting changes in lake environment that affected the processes delivering water and sediment to the lake as well as biological productivity within the lake. Analysis of modern organic materials in the lakes and their surrounding areas shows that the major sources of sedimentary organic matters in the lake 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 3-4 thousand years. Assuming the radiocarbon dates are reliable, Eastern Lake data suggest that the recurrence interval of severe storms (i.e., large enough to cause seawater flooding of the lakes) is approximately 83 years over the last 2900 years, whereas Western lake data suggest an average recurrence interval of 87 years in the past 3900 years.