RECENT BARRIER BEACH RETREAT IN GEORGIA - DATING EXHUMED SALT MARSHESBY ASPARTIC-ACID RACEMIZATION AND POST-BOMB RADIOCARBON

On St. Catherines Island, Georgia, the barrier beach is retreating rap idly landward, burying living salt marsh as it moves and later exhumin g it on the ocean side. Two methods were used to reconstruct the histo ry of overwash events, by dating the time of death of salt marsh musse ls (Geukensia demissa) and grasses (Spartina alterniflora) killed by o verwash sands (and now exhumed on the ocean side of the barrier beach) : aspartic acid (Asp) racemization and post-bomb radiocarbon analysis. The former method involves the conversion of L-Asp to D-Asp in shell proteins and provides good resolution for dating the last few centurie s. Radiocarbon analysis offers high-resolution dating subsequent to th e thermonuclear bomb tests of the late 1950s. However, for a number of reasons, it is of little use for resolving ages within the preceding three centuries. It is suggested that one problem with radiocarbon dat ing is that, in intertidal mollusk shells, some atmospheric carbon is incorporated, complicating correction for the marine reservoir age. De termination of the rate of Asp racemization in Geukensia shells on St. Catherines Island was made by comparison of D/L Asp values with the a ge of one sample determined by post-bomb radiocarbon to be AD 1980 (+/ -1 yr). This rate was used to determine ages from D/L Asp values at th ree relict marsh sites on the ocean side of the island. In the souther n and middle sites, living marsh was destroyed by overwash around 1980 -82, whereas at the northernmost site, the marsh was destroyed in the late 1930s or early 1940s. The rate of retreat on the ocean side of th e barrier beach at the northernmost site was determined by study of ae rial photos from 1945 and 1990. The retreat rate during this period av eraged 3.8 m/yr. The photos also indicate that the landward margin of the overwash fans here have been stationary over this period, so the b arrier beach has narrowed considerably at this site. A reduction in th e sediment supply to the area combined with the lack of supply of sand s from the relict marsh muds eroding along the shoreface probably caus ed erosion to outpace landward extension of the barrier beach by overw ash. Evidence of the earlier dynamics of the salt marsh is exposed in the relict marshes eroding away on the ocean side of the barrier islan d. These contain older layers of marsh, buried by muds. At the middle site, accretion of muds buried these portions of the salt marsh ca. 16 and 26 yr before the site was covered by an overwash fan from the bar rier island, whereas at the southerly site, the earlier marsh was buri ed by muds some 160 yr before the overwash event. Aspartic acid racemi zation has excellent potential for the analysis of coastal dynamics ov er the last few centuries. It provides a means of comparing the geolog ical record with historical records and also provides information on c oastal behavior before written records.