A remote sensing/geochronological study of coastal morphology associated with episodic Amazon sediment supply

Abstract

A combination of remotely sensed change detection surveys and geochronology (e.g., ²³⁴Th, ⁷Be, ²¹⁰Pb, and ¹³⁷Cs) of sediment cores in three separate areas in Brazil and French Guiana are used to develop an understanding of how coastal morphology in northeastern South America is associated with episodic changes in alongshore Amazon sediment dispersal. A variety of sediment cores collected from intertidal and shallow sub-tidal environments "ground truth" the remote sensing data by determining sediment inventories (e.g., origin signatures) and accumulation rates. This thesis helps establish that the Cabo Cassipore mudcape in Brazil is the location for the initial spawning of a growing, downdrift-migrating series of mudbanks. As these mudbanks shield the shoreline, rapid deposition and deep physical mixing yields thick layers of uniform ²¹⁰Pb and ²³⁴Th activity. Frequent resuspension scavenges the isotopes from the water column resulting in inshore and offshore inventories much higher than predicted values. The introduction of older, low activity sediment from the coastal mangrove fringe, as indicated from the seasonal introduction of ¹³⁷Cs and ⁷Be, causes radiochemical activities to vary alongshore seasonally. Inventories of ²³⁴Th in the surface sediments increase westward toward the leading edge of the mudbank from 30 to 100 times the predicted inventory, and inshore activities are reduced by at least 50% from offshore sediments as seen in ²¹⁰Pb and ²³⁴Th core profiles. Comparison of remote sensing data gives a time-series picture of mudbank/mudcape evolution over the past 50 years and a quantitative area and volume change in shorelines. Increasing mudbank sediment volume leads to increasing sediment exchange between the shoreline and the coastal zone. Remote sensing analysis has established an equilibrium between the amount of sediment accreted in front of a mudbank and that eroded from adjacent interbanks. The result is no long-term addition or loss of the coastal plain. The tremendous seasonal release and storage of sediments from the mangrove fringe (7-20% of the bank volume annually) suggests a change in perception of mudbank migration from previous studies suggesting they behaved as a bedform series to that of a "leapfrog" process of cycling between the mudbank and shoreline.