Abstract
Climate change is exacerbating sea level rise (SLR) and potentially increasing the severity of hurricane impacts. Global warming, caused by greenhouse gas emissions trapping heat in the troposphere, is increasing sea surface temperatures (SSTs). Rising SSTs result in the melting of ice sheets and thermal expansion, subsequently driving SLR. Higher rates of SLR lead to inundation of coastal areas and landforms, including barrier islands. Also with rising SSTs, hurricanes may have the ability to rapidly intensify and within the United States, hurricanes are the most devastating natural disaster. Increased SLR, paired with an increase in hurricane activity is of great concern, especially for coastal communities that reside on barrier islands. Barrier islands provide protection to the mainland from hurricanes; however, the effects of climate change are making barrier islands more dynamic and vulnerable, resulting in an increase in exposure of barrier island communities to coastal hazards, like hurricane-induced storm surge for example. This study focuses on Sanibel Island, a large barrier island located off the coast of Southwest Florida. Sanibel is approximately 19 km long and is relatively low-lying (average elevation ~1 m (3 feet) above SL). A large coastal housing community resides on Sanibel Island and the environmental tourism industry is a cornerstone of Sanibel’s existence.The goal of this study was to reconstruct the formational history of a barrier island and analyze the modern-day tempestological record, especially in reference to recent storms, such as Hurricane Ian (2022) and Hurricane Donna (1960). In order to analyze the formation of Sanibel Island, stratigraphic analyses, aided by sedimentological proxies, were used to analyze the paleoenvironmental histories with sediment cores. I note sediment transitions based on several factors including color changes, fossil preservation and occurrences, grain size composition, and organic content, that help form a narrative of environmental/geologic change. Radiometric dating is then used to interpret these successions in the context of time and more specifically reconstruct the barrier island history. These reconstructions are important for understanding past responses to shifts in SLR that may help guide future coastal resiliency planning.
In order to reconstruct hurricane history, paleotempestology methods were used. Paleotempestology is the study of historic/prehistoric overwash and overtop deposits from hurricanes that have been preserved as sediment layers (tempestites) within sediment cores. This field utilizes various geological proxies such as grain size, percent organic, calcium carbonate content, and radiometric dating to distinguish tempestites from background sediments and to further determine individual hurricanes responsible for those deposits. Determination of tempestite frequency within a given period allows for a susceptibility of overwash analysis and a return period calculation as well. These are crucial pieces of information when it comes to preparing coastal communities and coastal mainland areas for future hurricane activity.
This research took place on Sanibel Island, focusing on the back-barrier setting and Angliers Pond, a coastal pond located behind Sanibel’s eastern foredunes. A total of fourteen sediment cores were taken – ten along a parallel transect to the back-barrier and four within Angliers Pond. The results provide evidence for Sanibel forming between 4221 – 2242 ybp (in line with work from Missimer, 1973 and Stapor et al., 1988), which is when an environmental shift occurred from an inner shelf to a mangrove forest or subtidal bay. Findings also confirm the presence of Hurricane Ian and Donna’s tempestites, along with three other occurrences of overwash, within Angliers Pond. Sanibel’s formation prograded from west to east, which corresponds to the prevailing north to south longshore transport trends for the west coast of Florida. In comparing Ian and Donna’s tempestite composition, Ian’s tempestite was found to be finer grained, consisting of fine sands and silts, where Donna’s is composed of sands. This difference in coarseness could be due to several reasons: landfall position, direction of storm movement, water column energy, barrier height changes, and inadequate records of Donna’s footprint due to technological limitations (Donna’s footprint could be larger than the records indicate). This study provides evidence for an increase in vulnerability for Sanibel as SL continues to rise and tail-risk hurricanes continue to affect the Sanibel area.