MICROMORPHOLOGICAL, STABLE ISOTOPE, NMR, CRYSTALLOGRAPHIC, AND GEOMICROBIAL ANALYSIS OF QUATERNARY CALCRETES, PUERTO RICO

Pedogenic calcretes are commonly used as proxy records for arid and semi-arid paleoclimates, however they have been documented in areas with precipitation up to 1400 mm/year. To improve understanding of the impacts of climate on calcrete formation, this study examines the micromorphology, stable isotopes, petrophysical properties, and geomicrobiology of calcretes formed under tropical semiarid (736-1120 mm/year) and tropical humid (1422-1650 mm/year) climates of Puerto Rico. Calcretes forming under semiarid conditions reveal complex Alpha (non-biogenic) and Beta (biogenic) microstructures, whereas calcrete forming under humid conditions display simpler laminated Beta structures. The arid profiles cements consist of micrite, and microspar, whereas the humid profiles cements consist almost exclusively of microspar and pseudopsar. NMR measurements reveal that arid calcretes have lower porosity relative to the host sediment, which is inferred to result from the extensive cementation and recrystallization. Humid calcretes have greater porosity relative to the host sediment, reflecting either incomplete cementation or possibly occasional dissolution. Humid calcretes contain branching microorganisms within the cements and fabrics, whereas arid calcretes do not, reflecting the prominent role organisms have on humid calcrete formation. Integrated calcrete micromorphology, geophysics, and preliminary geomicrobiology have revealed distinct criteria for distinguishing between semiarid and humid calcretes. Ongoing work on calcrete stable isotope signatures, crystallography, and geomicrobiology to be reported during presentation.