Linking subaerial exposure surfaces to Early Pennsylvanian climatic changes — Insights from the Valdorria carbonate platform, northern Spain

During the Late Paleozoic, subaerial exposures at the top of cyclothems were important markers of sea-level lowstands linked to glacioeustasy. Although subaerial exposure horizons carry valuable information on atmospheric conditions, they still tend to be underutilized in climate reconstructions. This study focuses on subaerial exposures in the isolated Pennsylvanian Valdorria carbonate platform situated in the Cantabrian Mountains, Spain. Previous studies have reported three major subaerial exposure horizons exhibiting distinct calcrete and/or paleokarst profiles, which were identified to have developed over a 2-My time interval. This research applies various analytical techniques, including scanning electron microscopy and X-ray diffraction, combined with major and trace element and 87Sr/86Sr isotope mass spectroscopy to further characterize the horizons. The results shed light on paleoclimatic conditions along the active margins of southeastern Laurentia–northern Gondwana at paleoequatorial latitude (ca. 5-15º south latitude) during the Late Paleozoic. As a result of this integrated approach, the oldest emersion surface displays a thick calcrete profile characterized by the presence of illite-smectite and lack of kaolinite, and a minor karstic system indicating a semi-arid climate. The second emersion surface exhibits a thin calcrete profile that displays a distinct red matrix with similar clay mineralogy to that of the older profile and lacks evidence of karstification, suggesting a shift to an arid climate. Finally, the youngest emersion surface at the platform-top exhibits an extensive karstic system with abundant kaolinite throughout, which indicates that the platform was exposed during a hot and wet climate prior to burial of the carbonate succession. Geochemistry of the subaerial exposure horizons combined with presence of diagnostic trace minerals indicate that siliciclastic sediment was derived from an intermediate volcanic arc source through aeolian transport. These results demonstrate that combining a variety of analytical techniques is a powerful tool for the study of paleosols beyond the investigated deposystem. The high-resolution record of climate variations, partly deviating from long-term trends in overall climate evolution, highlights the value of using an integrated methodology to better constrain changes documented in underexplored regions, such as the western Paleotethyan realm during the Late Paleozoic.