GEOLOGICAL AND BIOLOGICAL STORM INDICATORS ALONG ASSATEAGUE ISLAND, MARYLAND

Assateague Island is an undeveloped, wave-dominated barrier, which offers an excellent opportunity to examine storm signatures in both the geological and biological media: depositional storm ridges, buried erosional scarps, and dendrochronological records. The deposition and reworking of one of the large recurved storm ridges enclosing Green Run Bay is constrained by optical and radiocarbon dates, as well as historical documents to have occurred during the 1720s to 1760s. High-resolution (250 and 500 MHz) ground- penetrating radar (GPR) images revealed prominent subsurface truncations, which are interpreted as buried erosional scarps. The apparent dip angles of truncating reflections reach up to 20°, in contrast to gentler angles of 1-5° for accretionary beach strata. The younger of the two mapped paleo-scarps, which pre-date the historic storm ridge, displays a sharp coarsening of mean grain size that coincides with the depth of the scarp on the GPR profile and is consistent with the higher energy setting during a storm. In addition to geological evidence, dendrochronological records were examined for occurrences of abrupt thinning of tree rings as a proxy for intense environmental stress. We tested the hypothesis that storms add a stress on trees by the act of pruning branches and leaves resulting in an attenuation of photosynthesis, and impact of salt water via saltspray caused by excessive winds. The annual ring of a tree is completed at the end of the growth season; therefore, a hurricane impact (July-November) will be manifested by a delayed effect in the tree record (narrow ring in a post-storm year), whereas extra-tropical storms (December-April) should be reflected by a narrow ring during the year of the storm. In fifteen tree core samples collected along northern and southern parts of the island, annual ring widths were expressed as reciprocals scaled to the narrowest ring, so that a sharp increase represents an acute environmental stress. Tree samples that exceed 50 years in age (n=7) display a prominent 1962 peak that coincides with the Ash Wednesday extra-tropical storm of record. Other signatures of intense storms (hurricanes of 1938, 1954, and 1992) are expressed more variably across the study area, possibly due to the tree’s proximity to the ocean and elevation (dune vs. swale).