MICROMORPHOLOGICAL, STABLE ISOTOPE, MINERALOGICAL, GEOMICROBIAL, AND NMR ANALYSIS OF QUATERNARY CALCRETE FORMATION, PUERTO RICO

Abstract

Pedogenic calcretes are widely used as proxy records for arid and semiarid paleoclimates, however they have been documented in tropical areas with precipitation up to 1400 mm/year. To improve our interpretation of environment of deposition (EOD) using calcretes, this study analyses calcrete forming under humid and semiarid conditions using sedimentological, petrographic, mineralogic, isotopic, geomicrobial and geophysical techniques. Results show humid zone calcretes form patchy deposits comprised of Beta (biogenic) microfabrics of LMC whereas semiarid zone calcretes form extensive deposits of Alpha (nonbiogenic) and Beta textures of LMC. Despite precipitation differences, the ranges and variability of δ18O and δ13C values is nearly identical between humid and semiarid zone calcretes. The similar δ18O values are interpreted to be the result of long resident times on the humid coast. The similar δ13C values indicate similar contributions of dissolved carbon from the bedrock and the oxidation of organic matter. Strong positive covariance of δ18O and δ13C values suggests that precipitation amount, evaporation, evapotranspiration, and vegetation density interact to control isotopic values. Calcretes forming under humid and semiarid conditions exhibit dense cell clusters, cell holes, EPS surfaces, fungal hyphae and microbially induced carbonate textures, indicating that calcrete formation is influenced by metabolically medicated precipitation, and biotically induced nucleation. Geophysical analyses reveal that calcrete formation does not significantly affect primary porosity under humid conditions but decreases primary porosity under semiarid conditions potentially forming small sized caps, emphasizing the impact of climate on porosity. By integrating such analyses, improved EOD interpretations can be made, providing important insights into paleoenvironmental changes and the subsequent affects on reservoir quality.