Spatial geochemistry of Upper Jurassic marine carbonates (Iberian subplate)

Chemostratigraphy applied to ancient marine carbonates is commonly based on one-dimensional (stratigraphic) sections or core data. As demonstrated from modern oceans, this approach underestimates the spatial complexity of physico-chemical seawater properties. Here, a several-hundred-kilometer long transect consisting of seven Upper Jurassic sections from settings ranging from (proximal) neritic middle shelf to the epioceanic (distal) fringe across the southern and eastern palaeomargins of the Iberian sub-plate reveals variability in sedimentologic, stratigraphic, and geochemical records. The comparison of isotopic and elemental data from different carbonate materials (matrix micrite, carbonate cements in veinlets, belemnite rostra, and ammonite shells) reveals differential diagenetic pathways. Microfacies, cathodoluminescence and geochemical data retrieved from biostratigraphically well constrained sections reveal that epioceanic matrix micrite geochemical data provide valuable proxies for palaeo-seawater properties. Our data are reviewed in the context of published Late Jurassic records. The outcome shows a higher level of complexity including the potential admixture of marine, continental, and diagenetic geochemical signals in the epicontinental record. The stratigraphic trend in carbon isotopes of epioceanic sections agrees upon that of Upper Jurassic reference sections from the northern Tethyan margins, while oxygen isotope ratios are relatively 18O-enriched. Palaeo-seawater temperatures across the transect investigated were estimated using δ18O as tentative proxy for interpreting distance from shore, differences in water masses, relative depth variations, and potential local forcing factors. Palaeoenvironmental conditions evaluated through the combined record of isotope (δ18O), elemental (Mn), and skeletal content contrast with relative fluctuations in sea level. Site-specific changes in palaeoceanographic parameters such as water depth, seawater temperature, salinity and upwelling are considered in comparison to examples from ancient and modern oceans.