HOLOCENE SEA-LEVEL CHANGE IN SOUTHERN NEW ENGLAND AND ITS INFLUENCE ON THE BARRIER DYNAMICS OF MARTHA'S VINEYARD, MA

Spring sapping valleys offshore of Southern Martha’s Vineyard provided topographic lows that preserved Holocene transgressive sequences during the post-glacial sea-level rise (SLR). VIbracores taken offshore in a paleovalley extending from Edgartown Great Pond capture this sequence and provide sedimentary evidence of the changing coastal environments over the past 8.0 ka. The sequence recovered in 11 cores indicates a transition from basal glacial outwash to low energy embayment facies, followed by back-barrier and finally shore-face deposits. Washover deposits comprised of coarser-grained material indicate close proximity to a paleo-barrier. Carbonate continuous-flow radiocarbon dates associated with washover sediments roughly constrain the timing on barrier positions through time. The dates, along with cross-shore core distances, provide an approximate time-averaged barrier retreat rate of 47 cm/yr from ~8-6 ka, compared to the historic barrier retreat rate of 173cm/yr. Continuous-flow radiocarbon samples also provide new Holocene SLR data for Southern New England and, along with previously published data, produce a curve with at least two different slopes. Before 4.0 ka, the average SLR rate was 3.2 mm/yr, decreasing by half (1.67 mm/yr) and from 4.0 to 0.5 ka. The late-Holocene portion of the curve will be confined by basal-peat radiocarbon dates (pending) from Westport River, MA. Samples submitted for dating taken from 50-400 cm below mean high water are expected to produce ages ranging from 500-3000 YBP. Ratios of time-averaged barrier retreat to SLR show an increase in barrier movement per unit SLR from pre-6 ka to the present by more than a factor of three.