VERTICAL ACCRETION IN MARSHES WITH VARYING RATES OF SEA LEVEL RISE

Abstract

Marsh systems are often considered sites of major sediment accumulation, where vertical accretion rates are equal to or greater than sea level rise. However, the degree of apparent sea level rise (land subsidence plus change in eustatic sea level) along the coast of the United States is variable, ranging from over 10 mm y-1 decline along the coast of southeastern Alaska to almost 10 mm y-1 rise along the northeastern Maine and Louisiana coasts. This variability in sea level rise accounts for some of the discrepancy in vertical accretion rates in marshes. We reviewed 15 areas dated with Lead-210 or Cesium-137, however, and at least four are clearly not keeping pace with sea level rise. Among these are Blackwater marsh in Chesapeake Bay, and the backmarsh areas of Barataria Bay, Four-league Bay and Lake Calcasieu in Louisiana. There is surprisingly strong correlation (r = 0.83) between mean tidal range and accretionary balance for 15 marshes where we obtained data. Based on overall sedimentary processes, tidal wetlands can be classified into at least six major categories: emerging coastal, submerging coastal, estuarine, submerged upland, floating, and tidal freshwater marshes. Acceleration of present rates of sea level rise could cause major alterations in each of these six types and reduce their carbon export potential to surrounding coastal waters.