Pedogenic carbonate crusts (calcretes) in karstic landscapes as archives for paleoenvironmental reconstructions – A case study from Yucatan Peninsula, Mexico

Abstract In the northeastern margin of the Yucatan Peninsula, which is located in southeastern Mexico, three levels of compact carbonate crusts (calcretes) interlayered with indurated coastal calcareous sediments (calcarenites) are discernible. Since both the calcretes and calcarenites form a continuous carbonate sequence, a combined methodology (macro- and micromorphology, geochemistry and mineralogy) is used to differentiate them. Additionally, the calcrete layers are dated using the Uranium-Thorium method in order to establish a chronological framework. The carbonate differentiation and chronology are then used to establish the paleoenvironmental conditions that prevailed during calcrete formation as well as to reconstruct the coastal landscape evolution and its relation to Pleistocene sea-level changes during Marine Isotope Stages (MIS) 6 and 5. Samples of calcrete layers and calcarenites were taken from two sections in the Cancun-Tulum corridor. The lowest level of calcrete underlies a sequence of calcarenites, and the two younger levels cap the sequence. In calcretes, the micromorphology provides evidence of the presence of a relict soil with subangular blocky microstructure, root traces, and needle-fiber calcite (associated with soil-fungi interactions). In contrast, the micromorphology of the calcarenites shows ooids and bioclasts, typical of a coastal environment. Geochemically, calcretes are depleted in δ13C, compared to calcarenite, with values ranging between −2‰ and −10‰, and they have a lower proportion of rare earth elements. Calcrete mineralogy consists mainly of calcite over aragonite with a low proportion of Mg-calcite. U/Th dating indicates that the oldest calcrete was formed during the Penultimate Glacial Maximum (PGM), in MIS 6, and the two younger calcretes correspond to the Last Interglacial Period, in MIS 5c. The calcarenite formation occurred earlier, also during MIS 5. Soil formation, strong karstification and soil erosion have occurred on younger calcretes. This study suggests that these processes developed between MIS 4 to MIS 1.