GEOCHRONOLOGY OF LATE-HOLOCENE MANGROVE ISLAND SEDIMENTATION IN FLORIDA BAY

Mangrove islands began forming in Florida Bay after Holocene sea-level rise flooded the shallow Miami Limestone floored basin between the southern tip of the Florida peninsula and the Florida Keys approximately 4,500 years ago. Various models have been proposed to explain the locations and processes of island formation, including subsurface topographic controls, upward growth of shallow sediment shoals, intertidal mudbank precursors, among others. Because the Florida Bay islands represent a range of lithologies, from end members comprising dominantly mangrove peat (e.g., Pigeon Key) to carbonate mud (e.g., Cotton Key), it is likely that no single model is universally applicable. In this study we explore the relationship between sedimentation rates and lithologies in sediment cores from Pigeon Key and Cotton Key using amino acid geochronology and radiocarbon age estimates in order to assess their response to late-Holocene sea-level rise, which may have implications for their sustainability under future sea-level rise projections.

Uncompacted thickness of sediments recovered in cores collected at Cotton Key and Pigeon Key were 193 and 170 cm, respectively. The Cotton Key core penetrated to bedrock, while depth to bedrock at the Pigeon Key core site was 350 cm. A radiocarbon age of 2580 ± 40 ¹⁴C yr B.P. was determined on a large wood fragment within a peat lens at 165 cm depth at Cotton Key, and 970 ± 40 ¹⁴C yr B.P. on peat at 110 cm depth in the Pigeon Key core. D/L aspartic acid values measured in shells of Cerithidea sp. suggest a reduction in sedimentation rates beginning at about 90 cm depth in Cotton Key and 130 cm in Pigeon Key, which may correspond to decrease in the rate of late-Holocene sea-level rise around 1500 years ago. A reversal in D/L aspartic and glutamic acid values accompanied by an abrupt lithological change at 25cm depth in the Pigeon Key core suggests erosion and redeposition of older sediments onto the surface, probably due to a significant storm event. Amino acid D/L values in shells from the upper Cotton Key core continue to decrease linearly toward the surface, indicating that erosional events on islands are localized within Florida Bay. Overall, geochronological and lithological data reflect island aggradation and occasional erosion accompanying gradual sea-level rise during the last three millennia.