Unraveling climatic changes from intraprofile variation in oxygen and hydrogen isotopic composition of goethite and kaolinite in laterites: An integrated study from Yaou, French Guiana

An integrated study of O and H isotopes in the lateritic profile of Yaou, F rench Guiana, was undertaken to investigate the usefulness of stable isotop es as tracers of climatic changes in continental environments. The studied profile is composed of a 27 m thick saprolite, mostly developed in the past under wet-and-dry tropical climate in association with a duricrust, overla in by a 3 m thick yellow latosol formed more recently under present equator ial hot and humid climate. delta(18)O-delta D values determined for weather ing goethite (pseudomorphs after pyrite) and kaolinite (microcrystalline cl ay groundmass) throughout the 30 m deep profile reflect formation temperatu res consistent with present (25 degrees C) and realistic past climatic temp eratures (20 degrees C-30 degrees C), indicating that weathering minerals f ormed in isotopic equilibrium with their genetic environment and were not s ubjected to significant isotope exchange after formation. A distinct shift downward (2 parts per thousand for delta(18)0, 15 parts per thousand for de lta D) from low to high delta(18)O-delta D values occurs around 20 m depth in the saprolite. It is interpreted as recording the change from the past t ropical to the present equatorial climate. Goethite and kaolinite in the 5- 10 m thick saprolite interval immediately above the active basement weather ing front are in isotopic equilibrium with modern water and must have forme d under present equatorial-humid conditions. In contrast, goethite and kaol inite found higher up in the saprolite and in the duricrust formed in the p ast under tropical wet and dry climate from waters distinctly depleted in O -18 and D relative to modem water. The marked depletion of paleo-meteoric w ater at Yaou most likely reflects a more contrasted or "monsoonal" characte r of the ancient tropical climate. The present study shows that ancient wea thering minerals in lateritic profiles preserve their delta(18)O-delta D va lues and carry a time signal. The time signal is best expressed in minerals formed rapidly at the weathering front and not subjected to post-formation al remobilization, such as the goethite pseudomorphs after pyrite occurring at Yaou. Groundmass kaolinite is more susceptible to partial remobilizatio n, through successive dissolution-precipitation reactions, which may oblite rate the paleoclimatic signal. Unraveling the climatic record carried by we athering minerals in old soil systems is greatly enhanced by studying both oxygen and hydrogen isotopic compositions. Copyright (C) 1999 Elsevier Scie nce Ltd.