Eve Eisemann '13
Spring sapping paleo-valleys offshore of Southern Martha’s Vineyard provide topographic lows that preserved Holocene transgressive sequences during the post-glacial sea-level rise (SLR). Vibracores taken offshore in a paleo-valley 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.
Ilyanassa obsoleta, the Eastern Mudsnail, is commonly recovered from offshore and onshore cores in Massachusetts and all along the Eastern seaboard. Its habitat is limited to low energy, intertidal areas. Radiocarbon dating of specimens found in cores taken in the back-barrier marshes of Cape May, New Jersey reveals that well-preserved shells can provide accurate sea-level data when recovered in low-energy back-barrier sediments. Fragmented shells were also found to correlate with storm events, and indicate high-energy layers.
Carbonate continuous-flow radiocarbon dates associated with washover sediments roughly constrain the timing on paleobarrier positions. 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 173 cm/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 almost half (1.7 mm/yr) from 4.0 to 0.5 ka. The late-Holocene portion of the curve is confined by new basal-peat radiocarbon dates from Westport River, MA. Samples submitted for dating taken from 1-4 m below mean high water produced ages ranging from 1000-3000 YBP respectively.
Tide gauge data indicate that the pre-6 ka SLR rates are similar to the modern rates; however, 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. The more extreme current rate of barrier movement indicates that other factors, such as sediment supply, barrier stability or a change in SLR rate, play an important role in the barrier dynamics of Southern Martha’s Vineyard.