Sedimentation in Submerged Sinkholes in Fillman’s Creek, A Microtidal Estuary in Western Florida, USA

Sediment archives in sinkholes and blueholes in both Cenozoic and Mesozoic carbonate platforms preserve important records of paleoenvironmental change. However, there is still a poor understanding of detailed sedimentary mechanisms operating in these systems in western Florida. Fillman’s Creek is a microtidal estuary in western Florida that contains two sub-tidal sinkholes, with an urbanized watershed around its periphery. Given the close proximity (~500 m) of the two sinkholes, Huck’s Hole (HCK) and Sawyer’s Sink (SYR), one would expect coherent sedimentation patterns at each site (e.g., texture, structure, and inputs). However, the two sinkholes have different sedimentation patterns and sedimentation rates, which indicate different processes likely control sedimentation in these two sites. The different sedimentary constituents at each site dictated that different methods of textural analysis were used for each site (e.g., laser particle size analysis, classic loss on ignition). Chronological control was established with radiocarbon dating and ^137Cs activity in the sediment cores. Organic matter particles and lenticular and horizontal sand deposits dominate the stratigraphy in HCK, and a 6 m sediment core spans 1942 CE to present. These sediment patterns indicate (i) a high sedimentation rate, (ii) the preferential trapping of mangrove-derived particles in the downstream sinkhole site, and (iii) that multiple hydrodynamic processes are driving sand deposition into HCK (e.g., tides and storms). In contrast, sedimentation in SYR is dominated by fine-grained siliclastic and organic sediment particles that are interrupted with visually-distinct coarse-grained horizons. Problematically, suitable age control for the successions from Sawyer’s Basin is hampered by a lack of terrestrial plant macrofossils, but available radiocarbon ages (on bivalves) and 137Cs data do suggest a change in sedimentation rate in Sawyer’s Basin sometime in the last several hundred years. A lack of suitable age control and low sedimentation rates precludes a confident association of coarse-grained horizons to possible hurricane layers in Sawyer’s Basin. Given the significant sedimentary differences observed in the sinkholes from the same microtidal estuary, these results indicate the complexity of sedimentation in sinkholes on Florida’s western coast