LATE QUATERNARY PALEOCEANOGRAPHY OF THE FRENCH-GUIANA CONTINENTAL-SHELF - CLAY-MINERAL EVIDENCE

Marine muds deposited on the French Guiana coast mostly originate in t he Amazon. Recent sediments are composed of (a) illite (33 %) and chlo rite (13 %) of Andean mountain origin; and (b) kaolinite (28 %) and sm ectite (26 %), principally from the Amazonian lowlands but also from t he Guiana Shield. In the coastal mud prism, high-resolution seismic pr ofiles, together with sedimentological, micropaleontological and soil engineering studies, supplemented by C-14 dates, permitted a stratigra phic interpretation of eight cores. Three episodes have been determine d on the basis of clay-mineral variations related to Amazonian lowland , Andean and Guiana shield sources. The older episodes 3 (3000-1700 y BP) and 2 (1700-1000 y BP) are predominantly characterized by lowland- derived clays (smectite = 43 %, kaolinite = 26 %) from Amazonian and l ocal sources. The most important event is a reduction of the Andean so urce, due to successive dry phases which occurred in western Amazonia about 2200 and 1200 y BP, confirming that regional decreases in rainfa ll, water discharge and erosion were associated with climatic fluctuat ions. It is suggested that these dry phases (within the last 3000 year s) are the consequence of several protracted (10-100 years) periods, d uring which atmospheric conditions mimicked the present-day ''El Nino Southern Oscillation'' (ENSO) phenomenon. As a result, particulate flu x from the ocean decreased, leading to reduced silting of the Guiana c oast, In the past 1000 years (episode 1), illite and chlorite have inc reased, indicating a stronger Andean contribution to the Amazonian con tinental flux that reaches the ocean. This is the first demonstration that the nature of the coastal marine muds of northern South America r eflects climatic changes in the Amazon basin.