Oeanic Anoxic Events: 50 Years On

Over the past five decades, Oceanic Anoxic Events (OAEs) have evolved from recognition of supra-regional intervals of broadly coeval black shale into globally recognized benchmarks of Phanerozoic Earth history. The recovery of Mesozoic black shales at sites drilled in all oceans coupled with outcrop investigations has demonstrated that many OAEs record the spread of anoxic–euxinic facies across a wide range of environments.This recognition transformed OAEs into first-order tools for long-distance stratigraphic correlation and for reconstructing global paleoenvironmental change. It is important to realize that some Mesozoic global events, such as the Carnian Pluvial Episode and the Triassic–Jurassic Extinction Event are similar to OAEs but the vagaries of geologcal nomenclature and the lack of a record from existing ocean basins means they have not been labelled as such. Research over the last 50 years has reframed OAEs as some of the best-documented examples of coupled climate–ocean–biosphere perturbations in the geological record. Their origins are now linked to large-scale disturbances in the carbon cycle resulting in an accelerated hydrological cycle, hyperthermal coditions, enhanced nutrient delivery from the continents, expanded marine productivity, widespread oxygen depletion, and intensified organic-carbon burial. Although the principal drivers and feedbacks are increasingly understood, major challenges remain. Such challenges include constraining the true spatial extent and heterogeneity of marine anoxia–euxinia, resolving event tempo and internal dynamics, quantifying feedback mechanisms, and translating insights from ancient events to the ongoing global changes. Central to this progress has been the development of integrated stratigraphy. Distinctive stable carbon-isotope excursions, calibrated with biostratigraphy and magnetostratigraphy and complemented by ancillary chemostratigraphic proxies (e.g., Sr and Os isotopes and mercury), enable OAEs to function as high-resolution temporal markers linking marine and terrestrial records. These integrated frameworks underpin refined Jurassic and Cretaceous timescales, facilitate correlation among heterogeneous sedimentary successions, and allow improved estimates of rates/magnitudes of environmental change. As some of the most powerful stratigraphic markers of the Mesozoic, OAEs anchor our understanding of past climate–carbon-cycle perturbations and offer an essential reference frame for evaluating modern oceanic deoxygenation. However, further advances require tighter chronological control, improved inter-basin correlation, and the integration of proxy data with numerical models. We stress the importance of distinguishing between lithological expressions of anoxia–euxinia (e.g., organic-rich black shales) and geochemical anomalies (e.g., isotope and trace-element excursions), which are related but not synonymous. We advocate a coupled yet independent classification of lithostratigraphic and chemostratigraphic markers to clarify definitions, enhance comparability among case studies, and strengthen the interpretive framework for future investigations. To this end, we propose a dual classification that couples OAEs to CIAs (carbon-isotope anomaly), While oceanic anoxia has, by definition, occurred only in marine settings, isotopic anomalies are detected on a global scale in both marine and continental successions. Furthermore, in many cases, it has been shown that anoxic sediments were deposited diachronously, whereas chemostratigraphic anomalies may be synchronous, particularly in the case of geochemical species with long residence times in the oceans.