Early cements in ammonites as seawater UPb and 87Sr/86Sr archives to constrain oceanic anoxic events in Earth's history

Accurate dating of oceanic anoxic events (OAEs) is essential for paleoceanographic and paleoclimatic reconstructions of Earth's history, contributing to our knowledge of major shifts in global carbon cycling. Although OAEs have been widely studied, direct age constraints on the biostratigraphy associated with these events are limited. Attempts to establish absolute ages of OAEs have relied on U Pb zircon dating of ash layers and Re Os geochronology in organic-rich sediments. However, U Pb zircon ages are inherently limited by the spatially-restricted character of ash layers in carbonate successions, whereas Re Os systematics are affected by variability in initial Os values (e.g., bioturbation or inherited organic sources), collectively hindering robust global paleoenvironmental correlations and reliable reconstructions of seawater 87 Sr/ 86 Sr records from marine carbonates. Here, we demonstrate the feasibility of using early marine fibrous cements from ammonite chambers, retrieved from marine black shales of the OAE-3 as absolute radiometric archives to reconstruct minimum depositional ages by U Pb dating via Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The U Pb age of early ammonite cements from OAE-3 sediments is estimated at 86.65 ± 2.35 Ma and validated against 87 Sr/ 86 Sr isotopic ratios (i.e., 0.707406 ± 0.000161) using Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS), confirming the local biostratigraphic framework of the OAE-3 record of northeastern South America. Our results demonstrate that early fibrous calcite cements preserved within ammonite chambers can serve as reliable U Pb geochronometers and archives for the Sr isotopic composition of seawater, providing an integrated geochronological, petrographic and biostratigraphic tool for refining our understanding of Cretaceous oceanic anoxic events.