Evolution of progradation/erosion along the French Guiana mangrove coast: a comparison of mapped shorelines since the 18th century with Holocene data

The problems in mapping the mangrove shoreline of French Guiana are reviewed. The present variability of the coastline due to the shifting mudbanks with Avicennia mangroves on their shoreface-attached upper fringe is extended to the historical period two and a half centuries ago, by means of ancient maps and nautical charts. In selected sites (around the Sinnamary River and the Mana River estuaries, Cayenne Peninsula) the earliest reported shoreline (before 1765), compared with other 18th and 19th century maps, is relatively stable, demonstrating that erosion repeatedly counterbalanced progradation on a centennial time scale. This data contrasts with the 20th century initiation of a sedimentary regime that results in very high local progradation rates. An examination of the entire Holocene sediment record in preserved in French Guiana, including palynological evidence, suggests that most of the Holocene progradation may have occurred by different accretion processes than have formed the historical and recent mudbanks settled by Avicennia mangals.