engleski

The link between Central Dalmatian karst bauxites (Croatia) and Miocene Climatic Optimum

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.

Karst bauxite ; CA-ID-TIMS zircon geochronology ; Maximum depositional age ; Kaolinite and gibbsite ; Miocene Climatic Optimum ; Central Dalmatia

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