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International audience; Thick regoliths developed under tropical climate, namely, laterites, resulting from long-term and pronounced geochemical and mineralogical rearrangement of the parent rock in response to environmental changes. Little information is available on the timing of laterite and bauxite formations, especially on the chronology of the main weathering episodes responsible for lateritic cover formation on the Guiana shield. For this purpose, we focused on both lateritic and bauxitic duricrusts developed over the Paleoproterozoic Greenstone Belt in the Brownsberg, Suriname. The duricrust samples have a relatively simple mineralogy (i.e., goethite, gibbsite, hematite, and kaolinite) but reveal, when observed at a microscopic scale, a complex history of formation with multiple episodes of dissolution/reprecipitation. The (U-Th)/He dating of 179 Fe-oxides subsamples shows that duricrusts sampled at the top of the Brownsberg plateau have ages ranging from

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.

Tropical weathering leads to the formation of deep weathering profiles called laterites. The Guiana shield has been tectonically stable and in tropical latitudes supposedly since the Cretaceous, allowing the formation of very deep (> 100 m) and old (tens of Ma) lateritic profiles. In this contribution we present a coupled data set of kaolinite electron paramagnetic resonance (EPR) ages and (U-Th)/He ages of supergene hematite and goethite from a deep (> 120 m) weathering profile exposed in an open pit gold mine in the northern Brazilian state of Amapá. Coupling of the two methods which target different components of the lateritic profile allows us to reconstruct the formation and evolution of the presented profile throughout the entire Cenozoic. While (U-Th)/He ages on supergene hematite and goethite from the ferruginous duricrusts on the top of the profile record three discrete weathering events during the Late Cretaceous/Early Paleocene, Oligocene and Middle Miocene, kaolinite ages allow an insight into the vertical evolution of the profile. The oldest kaolinites, which are very well-ordered, are preserved in the saprolitic part of the profile whereas the kaolinites in the top of the profile show very recent (< 5Ma) rejuvenation. Disordered kaolinite captured in the lateritic duricrust was protected from this rejuvenation.

The Miocene Climatic Optimum (MCO) represents a global warm period from approximately 17.0 to 14.7 Ma, which interrupted a period of long-term Cenozoic cooling. Continental paleoclimate records for the MCO vary in quality and global coverage, and due to the unique formation conditions for bauxites, they may serve as an excellent constraint of local climate during this period. In order to elucidate if the MCO promoted the establishment of a wet and warm climate allowing the formation of bauxites in southeastern mid- latitude Europe, we studied a section of previously undated massive karst bauxite (Crveni Klanac, CK) in central Dalmatia, Croatia, hosted in Upper Cretaceous limestones and overlain by Miocene Sinj Basin lacustrine deposits. Integrated mineralogical, morphological, and geochemical analyses indicate the predominant mineral phases of the homogenous bauxite matrix are authigenic, subhedral to euhedral kaolinite, and gibbsite. The in situ mineralization as a consequence of pedogenic processes indicates that the CK bauxites formed autochthonously. In situ U-Pb zircon ages of the lower, middle and upper stratigraphy of the CK bauxite are similar, dominated by Miocene and Oligocene ages, indicating they share similar protolith(s). Subsequent high-precision chemical abrasion-isotope dilution-thermal ionization mass spectrometry analyses indicate a maximum depositional age for bauxite parent material of 16.9576 ± 0.021 Ma. This maximum age of autochthonous bauxitization coincides with the onset of the MCO, and additional stratigraphic geochronological constraints limit the duration of bauxitization to less than approximately 700 ka. In order for insitu bauxitization to have occurred in southeastern parts of the mid-latitude continental European areas, paleoclimatic and paleoenvironmental conditions must have had mean annual temperature greater than 17–22°C and mean annual precipitation of more than 1100–1200 mm (literature data). Therefore, the CK autochtonous bauxites provide an independent climatic constraint, and serve as a novel example of using bauxite deposits for high precision paleoclimate constraints.