Barfield Bay, Marco Island Region, Florida: 7000-Year Record of Holocene Subtidal Deposition

Barfield Bay fills the 3.6km by 2.5km, semi-elliptical, center of a north-south-oriented, 15,000-year-old (OSL) parabolic dune with the primary tidal opening on its southern margin. Three vibracores were taken along the aeolian dune margin and three within Barfield Bay. The Barfield Bay vibracores penetrated a maximum thickness of 7-meters of mollusk-shell-rich, silty, fine-grained, quartz sand of Holocene age. Mastersizer 2000 granulometric analysis of 57 sediment samples produced volume percent histograms with distinct sand- and silt-size modes in 41 (72%) of the 57. However, 16 (28%) have histogram patterns without such a separation. The GRANULO [1] and ROKE [2] grain-size analysis programs identified 163 separate log-normal populations that comprise 6 distinct clusters on a Mean grain-size versus Sorting plot; individual clusters contain log-normal populations from aeolian, modern intertidal, and subtidal environments. Twenty-one samples from the Barfield Bay cores were processed for mollusk-shell identification. The mollusk population is diverse with 63 species (D=0.075, Dmax=0.015) but very uneven (SEQ=0.21). Modern salinity data for the 12 most abundant and widely distributed mollusks indicate a long-term, ambient salinity range of 25-32 psu during the deposition of the sediment-fill. This sedimentary fill is thoroughly bioturbated and contains no mangrove peat material and thus is interpreted to have been deposited in the subtidal environment of a flood-tidal delta that prograded north through the southern opening of Barfield Bay. At a depth of 7-meters MSL there is a terrestrial, allogenic peat (6900±50 Cal years BP) sandwiched between subtidal, shell-rich, quartz-sand. This terrestrial peat records a relative pause in the Holocene transgression. At a depth of 2-meters there is mangrove debris (2410±50 Cal years BP) at the contact between aeolian dune and overlying intertidal deposits. This MSL indicator fits on the RSL curve [3] developed from mangrove material in the nearby Florida Keys but is 2-meters lower than the RSL position indicated by coeval beach ridges 2.5 kilometers seaward of Barfield Bay.