FORMATION OF PEDOGENIC CARBONATE CRUSTS (CALCRETES) IN TROPICAL KARST LANDSCAPES AS ARCHIVES FOR PALEOENVIROMENTAL RECONSTRUCTIONS­₋A CASE STUDY FROM YUCATAN PENINSULA, MEXICO

In the NE margin of the Yucatan Peninsula (southeast Mexico), thick compact carbonated crusts (calcretes) interlayered with indurated coast calcareous sediments (calcarenites) are found as a result of the changes in the sea level and climate change. We present the results of morphological (macro and micromorphology), geochemical (stable isotopic composition (δ¹³C and δ¹⁸O), bulk chemical composition, rare earth elements composition (REE), ⁸⁷Sr/⁸⁶Sr ratios) and mineralogical (calcite vs. aragonite contents) properties of thecalcretes and calcarenites, as well as U-series dates, in order to: differentiate primary carbonates (calcarenites) from secondary- pedogenic carbonates (calcretes); establish a chronological frame for the calcrete formation that can be used for regional correlations; reconstruct the paleoenvironmental conditions during calcrete formation; and establish the coast landscape evolution and its relation with Pleistocene sea level changes (MIS 6 and MIS 5). The studied objects are sampled from quarries located in the Cancun-Tulum corridor, where three calcrete levels are found. The lowermost level underlies a sequence of calcarenites. The two younger levels cap the sequence. Micromorphological observations in the calcretes evidence the presence of relicts of soils with a subangular blocky microstructure, root traces, and acicular carbonates habits, which are associated with soil-fungi interactions. In contrast, the calcarenites are characterized by the presence of ooids and bioclasts, typical of a coastal environment. Geochemically calcretes, compared to calcarenite, are depleted in δ¹³C, with values ranging between -2‰ and -10‰, they have a minor proportion of REEs. Mineralogically calcite dominates over aragonite, with a lower proportion of MgO in the calcretes. Uranium-thorium (U/Th)dating indicates the oldest calcrete is formed during the Penultimate Glacial Maximum (MIS 6) while the two younger calcretes correspond to the Las Interglacial Period, in MIS 5c. Although this difference in the period of formation (glacial-interglacial), the three calcretes are developed during sea level drops according to regional models of sea level change.