Sediment exchange between Amazon mudbanks and shore-fringing mangroves in French Guiana

Abstract

Time-series remote sensing data, sediment cores, and water column profiles of fluid properties and suspended sediment concentration were utilized to examine the mechanisms and volumes of sediment exchange between the mangrove fringe and the coastal zone in French Guiana. Remote sensing estimates of sediment yield from mangrove shoreline erosion by wave attack during the ∼30-year periods when offshore Amazon mudbanks are not adjacent to a 30–40 km long section of the shoreline are 6–23 million tons year −1. 7Be, 137Cs, and 210Pb inventories in sediment cores from inner (<5 m water depth) mudbank areas suggest the mangrove erosion occurs primarily during the January–April period of high wave energy, and is responsible for the bulk of sediment deposition on the leading edge of the mudbank. Remote sensing estimates of shoreline accretion associated with onshore flux of sediment from a mudbank are also large (8–26 million tons year −1). Radiotracer-generated strata age relationships indicate that the lower intertidal zone of the leading (depositional) edge of the mudbank is a relict surface, suggesting the offshore mudbank is decoupled from the shoreline. Water column profiles and radiochemical signatures point to fluid mud suspensions (10–400 g l −1) as the primary mechanism for delivering sediment across the relict intertidal zone to create the accretion at the shoreline. If the estimates of sediment volume involved in shoreline accretion and erosion in the studied area are extrapolated along the entire 1400-km-long Guianas coastline possessing offshore mudbanks, the annual exchange of sediment to and from the mangrove fringe is at least equivalent to the average input of new sediment from the Amazon.