Rudists as paleoenvironmental tools: Multi-proxy records of Late Cretaceous shells from the western Tethys (Cárdenas formation, Mexico)

Rudist bivalves are valuable archives of Late Cretaceous paleoenvironmental conditions, although their geochemical signals may be modified by diagenesis and terrigenous contamination. This study examines the paleoenvironmental significance of well-preserved rudist shells from the Late Maastrichtian of northern Mexico, focusing on specimens recovered from the Cárdenas Formation. Major and trace element concentrations, rare earth element (REE) distributions, and stable isotope compositions were analyzed in selected growth increments of nine rudist specimens representing five taxa. Shell preservation was assessed through a combined approach of petrographic analysis, Energy Dispersive X-ray Spectroscopy (EDS), and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Microstructural observations, together with geochemical screening criteria (Ca > 38 wt%, Si < 1 wt%, Mn < 300 ppm, Fe < 1,000 ppm, Mn/Sr < 0.2), and weak to moderate correlations among Mn-, Fe-, and Al-based proxies, indicate limited diagenetic alteration in specific shell domains. REE patterns exhibit seawater-like signatures with variable redox-sensitive anomalies, suggesting predominantly marine conditions during shell formation and minimal post-depositional overprinting. The REE systematics is characterized by positive La SN , Gd SN and Y SN anomalies, negative Ce SN anomalies, slight enrichment of heavy REE relative to light REE, and superchondritic Y/Ho ratios (>40). Oxygen isotope-derived temperatures indicate warm sea surface conditions (∼28–31 °C) in the western Tethys Ocean, consistent with tropical settings. These findings demonstrate that rudist shells can reliably record marine geochemical conditions when rigorous screening is applied and highlight the complementary value of REE systematics alongside conventional elemental proxies in reconstructing Late Cretaceous paleoenvironments.