Diagenetic reddening of Early Eocene paleosols on King George Island, Antarctica

Abstract The objective of this study was to determine if the Early Eocene paleosols on King George Island, Maritime Antarctica, have acquired their reddish color during the paleopedogenesis, by burial diagenesis and/or by heating of a covering lava flow. We used micromorphology, diffuse reflectance spectroscopy, X-ray diffraction, and mineral magnetic properties to identify the iron oxides in these paleosols. These are weakly/moderately developed paleosols formed on a basaltic tephra under a cool humid paleoclimate, therefore, prone to ferrihydrite and goethite rather than hematite formation. However, the iron oxide assembly was dominated by maghemite and hematite. Nevertheless, the large grain size and high crystallinity of hematites suggested they are rather diagenetic than pedogenic. The clustering, content, and distribution of hematites (finely dispersed on the groundmass; coating/replacing primary minerals and as mottles and/or nodules) rather than their crystal size were responsible for the color differences. These properties were likely the result of past pedogenesis processes (forming originally ferrihydrite) linked with paleodrainage conditions. Heating by the covering lava flow affected the magnetic properties of two profiles but without affecting their color. Taken together, all these results suggest that the burial reddening took place by dehydration and transformation of ferrihydrite to hematite. Our findings highlight the importance of detailed mineralogical analysis to identify iron oxides present in reddish paleosols because identification based solely on morphology (e.g. yellowish color, gradual horizon transitions) can lead to misinterpretation of paleoenvironmental conditions.