Paper No. 11
Presentation Time: 11:15 AM
AN 8000 YEAR RECORD OF RELATIVE SEA-LEVEL RISE IN FLORIDA, USA
HAWKES, Andrea D., Geography and Geology, University of North Carolina Wilmington, Wilmington, NC 02543, KEMP, Andrew C., Department of Earth and Ocean Sciences, Tufts University, Medford, MA 02155, DONNELLY, J., Geology & Geophysics Department, Woods Hole Oceanographic Institution, MS #22, 360 Woods Hole Rd, Woods Hole, MA 02543, HORTON, B.P., Department of Marine and Coastal Science, Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901 and PELTIER, W. Richard, Physics, University of Toronto, 60 St George Street, Toronto, ON M5S 1A7, Canada, ahawkes@whoi.edu
Field-based constraints on the geometry of the Laurentide Ice Sheet’s collapsing forebulge are from reconstructions of relative sea level on millennial timescales. An absence of valid sea-level index points in Georgia and Florida prohibits validation of Earth-Ice models in this region. Additionally, sea-level data from these southeast Atlantic locations could provide regional constraint on ice-equivalent meltwater input and gravitational fingerprinting. We present a new, 8000 year reconstruction of relative sea level in northern Florida developed from basal salt-marsh peat sediment. Preserved agglutinated benthic foraminiferal assemblages are used to estimate the elevation of former sea level, while age estimates came from radiocarbon-dated plant macrofossils. The use of foraminifera as a sea-level indicator is underpinned by understanding the relationship between the foraminifera and sea level. This approach produced 25 valid sea-level index points. This data confirms a 5.2 m upward trend of Holocene relative sea level in the region with no evidence of a high stand. Sea-level rises rapidly in the early and mid-Holocene in excess of c. 0.8 mm/yr. The rate of sea-level rise slows in the late Holocene to approximately 0.3 mm/yr which we attribute to the continuing collapse of the Laurentide forebuldge. In comparison to neighboring regions this rate is in agreement with the pattern of land subsidence cause by forbulge collapse.
A detailed 2000 year record of sea-level change was developed nearby from Nassau River Florida to provide appropriate geologic context for modern rates of sea-level rise. Consistent downcore foraminiferal assemblages within brackish marsh sediment indicate that the site maintained its tidal elevation for the past 2000 yrs. An age depth model was developed for the core results from radiocarbon dating, 210Pb and 137Cs. Following adjustments made for the contribution of glacio-isostatic subsidence to reveal climate-related sea level variability, the proxy dataset reproduces trends recorded by reliable nearby tide gauges at Fernandina Beach, FL and Fort Pulaski, GA. The relative sea-level record indicates that modern rates of rise were initiated in the latest part of the 19th century and are greater than any persistent trend in the last 4000 years.
