Reading the climate signals hidden in lateritic iron duricrusts

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 (sub)tropical climates. 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. 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.

In this contribution we present how paleoclimatic information can be extracted from lateritic iron duricrusts by combining geochemical, mineralogical and geochronological methods. Our coupled data from the northeastern Guiana shield reveal the existence of important weathering events during the Oligocene and Middle Miocene related to ferruginous lateritic weathering conditions representative of contrasted tropical monsoonal climate. A significant change towards bauxitic weathering conditions, characteristic of more humid tropical monsoonal climate, happened during the Late Neogene as indicated by an increase in Th concentration, Al-substitution in goethite and precipitation of gibbsite.