Internal isotopic variability of Neogene carbonate concretions: Constraining formational growth mechanisms and isotopic disequilibrium

Carbonate concretions collected from the Dominican Republic present a valuable opportunity to evaluate the internal isotopic variations within concretions that have never been exposed to deep burial or structural deformation. Here, three concretions from the Neogene (Late Miocene-Early Pliocene) Cibao Basin are investigated, utilizing a multi-isotope (delta C-13, delta O-18, delta S-34(CAS) and increment (47) values) high-resolution approach, to constrain the microenvironmental conditions associated with multiple stages of concretion growth. Isotopic variability and potential disequilibrium effects, which can influence geological interpretations utilizing concretions, are also considered. The petrographic characteristics and geochemical profiles indicate internal differences relating to concretion growth mechanisms and environmental changes, driven by sea-level fluctuations. The delta S-34 values of carbonate-associated sulphate indicate a closed system environment; however, the overall values are influenced by sulphide oxidation within the sediments, resulting in a complex signal. The increment (47)-derived temperatures of the concretions range between 29 to 55 degrees C, indicating significantly warmer temperatures than are measured from the host sediments, which average 24 degrees C. This indicates that carbonate concretion increment (47) values are in disequilibrium with their environments of formation, likely related to ion diffusion in the pore fluids or isotopic fractionation associated with microbial processes. Here geochemical variations within concretions are utilized to assess the environmental conditions and microbial interactions after sediment deposition. However, for future studies, caution should be taken when using concretions for making environmental assessments as the signals can be influenced by a multitude of processes, even prior to diagenetic alteration.

Automated summary

Carbonate concretions from the Dominican Republic provide an opportunity to study isotopic variations and environmental conditions during concretion growth. A multi-isotope approach was used to evaluate three concretions from the Cibao Basin, revealing differences in growth mechanisms and environmental changes. The study also showed potential disequilibrium effects and cautioned about the influence of multiple processes on concretion signals.