Isotope 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 diageneticoverprinting. The Aptian platform carbonate succession from the western Maestrat Basin, in Spain, serves as anideal example for potentially linking these processes, particularly establishing the relationships between isotoperecords (d18O and d13C) and third-order sea-level trends with implications for sequence stratigraphy. Thissuccession is biostratigraphically well constrained and comprises two depositional sequences that were controlledby a major relative sea-level fall and a subsequent rise. These depositional sequences exhibit stratal terminationsand stacking patterns, enabling the establishment of a well-defined sequence stratigraphic framework comprisingfour systems tracts and their key bounding stratigraphic surfaces. The analytical results reveal that both d18Oand d13C values outline distinct temporal trends, which can be correlated with specific third-order stages ofrelative sea-level fluctuations. The transgressive and highstand systems tracts exhibit the most positive d13Cvalues (up to 15%) and the least negative d18O values (up to–1.8%). This range, similar to values of carbonatesin equilibrium with Cretaceous seawater, d13C and d18O values from 12% to 15% and from–2% to–5%,respectively, likely reflects the marine influence on the isotope values during the stage of high relative sea level. Incontrast, the forced regressive and lowstand intervals exhibit less positive d13C values (reaching 10.5%) andsignificantly more negative d18O values (around–6.1%), interpreted as the influence of soil-derived organicmatter and meteoric waters, respectively, during stages of lower relative sea level. Furthermore, the sedimentarysuccession records a decreasing trend of d13C and d18O values towards the sequence boundary, which marks thelowest point of relative sea level. This study underscores the potential of carbonate platforms to recordgeochemical signals that can be directly correlated with different third-order stages of relative sea-levelvariations. In addition, this framework allows for linking and predicting the potential fluid–rock interactionprocesses in each systems tract. The proposed approach could offer a predictive framework for characterizingcarbonate reservoirs and other carbonate platform successions with poorly defined sequence-stratigraphicframeworks elsewhere.