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 (δ¹³C, δ¹⁸O, δ³⁴S_CAS and ∆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 δ³⁴S 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 ∆₄₇-derived temperatures of the concretions range between 29 to 55°C, indicating significantly warmer temperatures than are measured from the host sediments, which average 24°C. This indicates that carbonate concretion ∆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.