READING THE CLIMATE SIGNALS HIDDEN IN LATERITIC IRON DURICRUSTS (Invited Presentation)

Laterites are deep weathering profiles which develop under tropical and subtropical climatic conditions. Their geochemical and mineralogical composition is strongly influenced by the climatic condition under which they form, whereas the composition of the parental material plays a subordinate role. Due to their long-term stability throughout the intertropical zone, they are archives of past climates. It has been proposed that laterites formed episodically throughout the Earth’s history in periods with very favorable climate, but little is known about the duration, frequency and spatial extent of such weathering events. As different climatic signals are potentially superposed in old and constantly exposed laterites, disentangling of the paleoclimatic information preserved in these archives is a complex task. Since laterites are mainly composed of clay minerals, iron (oxyhydr)oxides and aluminum (oxy)hydroxides, most conventional geochronological methods cannot be applied to those formations. However, hematite and goethite, which are the main components of the ferruginous duricrust that is present in most lateritic profiles, can be dated using the (U-Th)/He method allowing important geochronological constraints.

This contribution presents how paleoclimatic information can be extracted from lateritic iron duricrusts using a new approach which couples (U-Th)/He geochronology of supergene hematite and goethite with high resolution mineralogical and geochemical methods as well as bulk mineralogy and geochemistry. This combination allows us a deepened insight into weathering processes and conditions by placing mineralogical and geochemical processes into a temporal framework.

Our data from the Guiana shield, located in the northern part of South America, reveal the existence of several important weathering events during the Cenozoic. Comparison of results from different sites indicate a spatial and temporal variability of the weathering conditions in between these events and show how different (paleo)climates impact the obtained iron (oxyhydr)oxide age distributions.