Biological origin of rhythmites in muddy sediments of French Guiana
Résumé
Sediment deposition along the coasts of French Guiana depends on three different processes: mass deposition when a mudbank reaches the area, seasonal exchange between the mudbank and the shoreline, and the effect of tide cycles. The objectives of this work were to determine whether the three different processes are recorded in the sediments, and if rhythmites are discernible, preserved, and usable for paleoenvironmental studies. The study area was in the Kaw estuary, southeast of Cayenne (French Guiana) at a latitude of about 4 degrees 55' N and longitude about 5 degrees 10' W. Four stations were selected for the collection of push cores, which were then split and described. Redox and water content were measured; grain-size analyses, radioisotopic measurements (Pb-210 geochronology), and X-ray image-analyses were carried out. The SCOPIX X-ray image-processing tool was used for establishing gray-scale profiles. After mass deposition, sediment processes are dominated by reworking of the muddy sediment with a predominance of tidal effects. Erosion and deposition processes are governed by tide cycles and depend on threshold speeds of tidal currents. Microphytobenthos, mainly diatoms, play a major bole in binding the sediment by secretion of extracellular polymeric substances (EPS). They induce the formation of alternating dark and clear laminae. Even if erosion, irregular seasonal sediment supply, and bioturbation do not allow continuous recording of sediment laminae, the cyclic sediment deposits may result in tidal rhythmites, even in the highly size-sorted sediments of French Guiana. When preserved, the rhythmites provide valuable information on fortnightly cycles.