Petrographic and geochemical analysis of caliche profiles in a Bahamian Pleistocene dune

Two caliche profiles from a Pleistocene carbonate dune on San Salvador Island, Bahamas, were examined by petrographic and geochemical analysis. Profile A is an immature buried caliche profile characterized by caliche pisolites, a friable crust and abundant Cerion. Profile B is a more well-developed caliche profile at the top of the dune which contains abundant pisolites, rhizomorphs, laminated calcrete, a breccia and abundant Cerion. Geochemical changes in caliche profiles relative to the host rock are an increase in Al₂O₃, Fe₂O₃ and total organic carbon, a decrease in Mg and Sr, and a decrease in δ¹³C_carb, δ¹⁸O_carb and δ13C_org. The magnitude of these changes is probably a function of the duration of subaerial exposure and resultant colonization by dune plants and associated microflora. Abundance of calcified filaments and needle-fibre crystals in profile A attests to the importance of microbial processes in the early development of caliche profiles. Biogenic structures are largely destroyed in profile B due to recrystallization, but indirect evidence of biological activity is retained in the form of carbon isotope values.