A growing body of research suggests that the marine environments of south Florida provide a critical link between the tropical and high-latitude Atlantic. Changes in the characteristics of water masses off south Florida may therefore have important implications for our understanding of climatic and oceanographic variability over a broad spatial scale; however, the sources of variability within this oceanic corridor remain poorly understood. Measurements of ΔR, the local offset of the radiocarbon reservoir age, from shallow-water marine environments can serve as a powerful tracer of water-mass sources that can be used to reconstruct variability in local-to regional-scale oceanography and hydrology. We combined radiocarbon and U-series measurements of Holocene-aged corals from the shallow-water environments of the Florida Keys reef tract (FKRT) with robust statistical modeling to quantify the millennial-scale variability in ΔR at locations with (“nearshore”) and without (“open ocean”) substantial terrestrial influence. Our reconstructions demonstrate that there was significant spatial and temporal variability in ΔR on the FKRT during the Holocene. Whereas ΔR was similar throughout the region after ∼4000 years ago, nearshore ΔR was significantly higher than in the open ocean during the middle Holocene. We suggest that the elevated nearshore ΔR from ∼8000 to 5000 years ago was most likely the result of greater groundwater influence associated with lower sea level at this time. In the open ocean, which would have been isolated from the influence of groundwater, ΔR was lowest ∼7000 years ago, and was highest ∼3000 years ago. We evaluated our open-ocean model of ΔR variability against records of local-to regional-scale oceanography and conclude that local upwelling was not a significant driver of open-ocean radiocarbon variability in this region. Instead, the millennial-scale trends in open-ocean ΔR were more likely a result of broader-scale changes in western Atlantic circulation associated with an increase in the supply of equatorial South Atlantic water to the Caribbean and shifts in the character of South Atlantic waters resulting from variation in the intensity of upwelling off the southwest coast of Africa. Because accurate estimates of ΔR are critical to precise calibrations of radiocarbon dates from marine samples, we also developed models of nearshore and open-ocean ΔR versus conventional 14C ages that can be used for regional radiocarbon calibrations for the Holocene. Our study provides new insights into the patterns and drivers of oceanographic and hydrologic variability in the Straits of Florida and highlights the value of the paleoceanographic records from south Florida to our understanding of Holocene changes in climate and ocean circulation throughout the Atlantic.
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We thank the following researchers whose work in the Florida Keys provided many of the core records used in this study: Eugene Shinn, Harold Hudson, Robert Halley, Barbara Lidz, Daniel Robbin, Christopher Reich, T. Don Hickey, H. Gray Multer, J. Edward Hoffmeister, Marguerite Toscano, and Joyce Lundberg. These previously collected cores sampled from the USGS Core Archive ( http://olga.er.usgs.gov/coreviewer/ ) were collected under permits from Dry Tortugas National Park (formerly Fort Jefferson National Monument), Biscayne National Park (formerly Biscayne National Monument), the Florida Keys National Marine Sanctuary (FKNMS), and the Florida State Department of Natural Resources. New cores from the Upper and Middle Keys were collected under permits FKNMS-2013-097-A1 and FKNMS-2015-054, repectively by L.T.T., Anestasios Stathakopoulos, B.J. Reynolds, Hunter Wilcox, Jennifer Morrison, and Marc Blouin of the USGS. We thank Jack McGeehin of the USGS Radiocarbon Dating Laboratory for facilitating the radiocarbon analysis for this study. XRD analyses of the corals were conducted as part of USGS studies led by Chris Reich and T. Don Hickey. Jennifer Flannery of the USGS assisted with SEM-imaging of the samples. Daniel R. Muhs and Nicole S. Khan of the USGS provided helpful commentary on early drafts of the manuscript. This study was supported by a Mendenhall Research Fellowship awarded to L.T.T. by the United States Geological Survey (USGS) Coastal and Marine Geology Program and the Natural Hazards Mission Area, and by the Climate and Land Use Research and Development Program of USGS . Erica Ashe was supported by U.S. National Science Foundation [grant numbers ARC-1203415 , OCE-1458904 ] and National Oceanic and Atmospheric Administration [grant number NA11OAR431010 ]. All data used in this study are published and freely available for download in USGS data release at https://doi.org/10.5066/F7P8492Q . Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the United States Government.
- Western Atlantic