Isotopic record of Aptian third-order sea-level trends in platform margin carbonates: implications for sequence stratigraphic analysis

In ancient carbonate systems, establishing relationships among sea-level fluctuations, carbonate factory productivity and carbonate geochemistry is challenging due to complex depositional and diagenetic overprinting. The Aptian platform carbonate succession from the western Maestrat Basin, in Spain, serves as an ideal example for potentially linking these processes, particularly establishing the relationships between isotopic records (δ18O and δ13C) and third-order sea-level trends with implications for sequence stratigraphy. This succession is biostratigraphically well-constrained and comprises two depositional sequences that were controlled by a major relative sea-level fall and a subsequent rise. These depositional sequences exhibit stratal terminations and stacking patterns, enabling the establishment of a well-defined sequence stratigraphic framework comprising four systems tracts and their key bounding stratigraphic surfaces. The analytical results reveal that both δ18O and δ13C values outline di stinct temporal trends, which can be correlated with specific third-order stages of relative sea-level fluctuations. The transgressive and highstand systems tracts exhibit the most positive δ13C values (up to +5‰) and the least negative δ18O values (up to -1.8‰). This range, similar to values of carbonates in equilibrium with Cretaceous seawater, δ13C and δ18O values from +2‰ to +5‰ and from -2‰ to -5‰, respectively, likely reflects the marine influence on the isotope values during the stage of high relative sea level. In contrast, the forced regressive and lowstand intervals exhibit less positive δ13C values (reaching +0.5‰) and significantly more negative δ18O values (around -6.1‰), interpreted as the influence of soil-derived organic matter and meteoric waters, respectively, during stages of lower relative sea level. Furthermore, the sedimentary succession records a decreasing trend of δ13C and δ18O values towards the sequence boundary, which marks the lowest point of relative sea level. This study und erscores the potential of carbonate platforms to record geochemical signals that can be directly correlated with different third-order stages of relative sea-level variations. In addition, this framework allows for linking and predicting the potential fluid-rock interaction processes in each systems tract. The proposed approach could offer a predictive framework for characterizing carbonate reservoirs and other carbonate platform successions with poorly defined sequence-stratigraphic frameworks elsewhere.