Storm Signatures along Sandy Coasts
|Erosional berm scarp fronted by heavy-mineral concentrations, Madaket, Nantucket Island, Massachusetts|
|Heavy-mineral accumulation at the base of a partially healed dune scarp, Popham Beach, Maine|
Coastal barriers and wetlands have long been used as sedimentary archives of climatic and oceanographic changes over the past 2,000-6,000 years. My recent work has focused on an integrated onshore-offshore approach of looking at coastal sedimentary sequences as archives of coastal morphological and climatic links. Over the past several years, I have pursued high-resolution geophysical and sedimentological analysis of sedimentary sequences of coastal lakes and ponds to document their environmental history since the mid-Holocene climatic optimum. Recent findings highlight regional patterns in wetland expansion punctuated by storm deposition. The elevated depositional surfaces and lack of tidal sediment fluxes, make freshwater backbarrier settings ideal for capturing the record of the high-magnitude events (powerful storms and possible tsunamis). A new dataset from Maine point to a massive overwash event ca. 2,300 years ago and research is ongoing. As part of reconstructing the direct impacts of storms on sandy beaches, I have been actively involved in identifying and mapping a variety of geological signatures related to erosion and retreat, such as buried berm/dune scarps and regional erosional unconformities. In some areas, these features complement the backbarrier indicators of storm activity. More often, however, these barrier-stratigraphic signatures provide the only records of coastal change in areas where coastal wetlands are absent or where barrier width and height precludes overwash. In the past, identification and dating of erosional features in clastic coastal deposits has been a challenging task. With the increasing applications of ground-penetrating radar (GPR), continuous high-resuolution records of barrier subsurface can be obtained relativly quickly. The identification of erosional unconformities is facilitated in geophysical records and sediment cores as event horizons are often enriched in coarser fraction, bioclastic materials, or heavy minerals, such as magnetite, ilmenite, garnet, etc. (see photo). Recent developments in optically stimulated luminescence (OSL) dating technique offer great opportunities for obtaining ages on clastic sediments, thus expanding the chronological control to coastal sequences commonly devoid of organics. Barrier sands associated with buried erosional scarps in northern Massachusetts and central Maine have been optically dated to 1,800 and 1,600 years, respectively, thus providing for the first time a barrier-derived chronology of large-magnitude erosional events in New England over the past 2,000 years. In addition to research along the U.S. East Coast, I am pursuing field research and development of new methodologies aimed at reconstructing the long-term history of coastal erosion by cyclones and tsunamis at a number of sites in the Southern Hemisphere.