A paleosol-based proxy: pedogenic goethite pisoliths

In the continental stratigraphic record, paleosols are unique archives allowing for the reconstruction of ancient surface processes, paleo-ecosystems, and local- to global-scale paleoclimate conditions. Geochemical proxies derived from paleosol chemistry are powerful indicators of chemical weathering and various environmental parameters (rainfall, temperature, acidity) and provide a wealth of information to understand Earth's climatic history, but these paleosol archives also have their own limitations. Many paleosol-based proxies are applied to a limited number of paleosol profiles, as determined by outcrop exposure and quality, and most of them are not well-constrained on landscape or basin-scales, which are often the scales of interest for paleoclimate and paleoenvironmental reconstruction. Paleosol proxies including e.g., traditional stable and clumped isotopes in soil carbonates or organic matter, phytoliths, mineralogical indicators such as the goethite/hematite ratio have great potential to improve our understanding of how different terrestrial biogeochemical processes are linked in landscape scales, and how their different preservation biases could impact the interpretation of the paleosol proxy records. Here, we present geochemical data from pedogenic goethite pisoliths and nodules from Plio-Pleistocene red paleosols of the Carpathian Basin, East-Central Europe. Pisoliths and nodules are generally dominated by Fe oxides and oxihydroxides. Inorganic geochemistry and mineralogical data of these goethite pisoliths can provide a context for the depositional environment, climate, age, and origin of the red clays. Size, morphology, abundance, and location of goethite (hematite) nodules within a paleosol profile can yield valuable information on pedogenic processes and environmental conditions during soil development. Goethite is the main mineral in pisoliths and nodules beside quartz, hematite muscovite, and kaolinite. The Fe-hydroxides and Fe-oxides dominate in the samples, which is typical of aerobic environments. The goethite content varies from 55% to 82%, and the hematite from 5% to 12%. The hematite/goethite ratio is very low (0.062–0.160) in the samples showing highly weathered source sediment (paleosol). Clay minerals, such as kaolinite, indicate subtropical weathering of the parent material. The major element composition demonstrates that the soluble constituents (CaO, MgO, Na₂O, and K₂O) are leached, while the insoluble constituents (SiO₂, Al₂O₃, and Fe₂O₃) are accumulated as oxides/hydroxides during surface and sub-surface weathering processes. A concentric laminae fabric was not observed in any of the examined samples, which indicates the development of pisoliths above the water table in the unsaturated zone. The analyzed pisoliths and nodules have no protolith relics and contain less than 10–15% detrital grains, showing homogeneous structures with mottled and syneresis fabrics. According to our previous research on these paleosols, the mean annual paleoprecipitation was 1100–1400 mm and the mean annual paleotemperature was 13–15°C. The studied red paleosols are weathered in a tropical/subtropical climate. In such conditions, fersiallisation and ferruginisation are the most important processes during the diagenesis of the pisoliths and nodules.

This research was funded by National Research, Development and Innovation Office, grant number NKFI K120213.