Material exchange between the continental shelf and mangrove-fringed coasts with special reference to the Amazon–Guianas coast

Mangrove-fringed coasts are a major coastal environment in the tropics that exchange enormous quantities of inorganic and organic materials and dissolved ions with the adjacent ocean in unique ways. The 1,600 km long Guiana coast, from the Amazon River mouth to the Orinoco, the worldʹs most extensive example, is exposed to significant oceanic wave and tidal energy but is almost exclusively composed of mud deposits. The huge alluvial discharge of the Amazon advected along the inner continental shelf is reworked into 20–25 shore-attached and mobile mud-banks 10–40 km in length. Frequent and deep resuspension created by the energetic conditions results in fluid mud suspensions (10–400 g 1−1) that dewater upon re-deposition, resulting in extreme shoreline progradation rates (up to hundreds of metres per year) that are equally rapidly mobilized by wave attack. Biological assemblages (mangroves, foraminifera, diatoms, etc.) and biogeochemical conditions are strongly controlled by the intense recycling of sediments on the shoreface. Differences in sedimentary, ecological and biogeochemical processes are striking when this coast is compared with mangrove-fringed deltaic and estuarine coasts elsewhere, including those of west Africa and southeast Asia.