Geochemistry of oceanic anoxic events

2010/03/01 by Hugh C. Jenkyns · 29 citations
Earth and Planetary Sciences · Environmental Science · #Paleontology and Stratigraphy of Fossils #Geochemistry and Elemental Analysis #Methane Hydrates and Related Phenomena

paper · doi:10.1029/2009gc002788

openalex publication_date 2010/03/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/04

Abstract

Oceanic anoxic events (OAEs) record profound changes in the climatic and paleoceanographic state of the planet and represent major disturbances in the global carbon cycle. OAEs that manifestly caused major chemical change in the Mesozoic Ocean include those of the early Toarcian (Posidonienschiefer event, T‐OAE, ∼183 Ma), early Aptian (Selli event, OAE 1a, ∼120 Ma), early Albian (Paquier event, OAE 1b, ∼111 Ma), and Cenomanian–Turonian (Bonarelli event, C/T OAE, OAE 2, ∼93 Ma). Currently available data suggest that the major forcing function behind OAEs was an abrupt rise in temperature, induced by rapid influx of CO 2 into the atmosphere from volcanogenic and/or methanogenic sources. Global warming was accompanied by an accelerated hydrological cycle, increased continental weathering, enhanced nutrient discharge to oceans and lakes, intensified upwelling, and an increase in organic productivity. An increase in continental weathering is typically recorded by transient increases in the seawater values of 87 Sr/ 86 Sr and 187 Os/ 188 Os ratios acting against, in the case of the Cenomanian‐Turonian and early Aptian OAEs, a longer‐term trend to less radiogenic values. This latter trend indicates that hydrothermally and volcanically sourced nutrients may also have stimulated local increases in organic productivity. Increased flux of organic matter favored intense oxygen demand in the water column, as well as increased rates of marine and lacustrine carbon burial. Particularly in those restricted oceans and seaways where density stratification was favored by paleogeography and significant fluvial input, conditions could readily evolve from poorly oxygenated to anoxic and ultimately euxinic (i.e., sulfidic), this latter state being geochemically the most significant. The progressive evolution in redox conditions through phases of denitrification/anammox, through to sulfate reduction accompanied by water column precipitation of pyrite framboids, resulted in fractionation of many isotope systems (e.g., N, S, Fe, Mo, and U) and mobilization and incorporation of certain trace elements into carbonates (Mn), sulfides, and organic matter. Sequestration of CO 2 in organic‐rich black shales and by reaction with silicate rocks exposed on continents would ultimately restore climatic equilibrium but at the expense of massive chemical change in the oceans and over time scales of tens to hundreds of thousands of years.

Citations

• Oceanic anoxic events and plankton evolution: Biotic response to tectonic forcing during the mid‐Cretaceous
• Mo–total organic carbon covariation in modern anoxic marine environments: Implications for analysis of paleoredox and paleohydrographic conditions
• Organic matter composition and distribution through the Aptian–Albian lacustrine sequences of the Araripe Basin, northeastern Brazil
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• Timing and paleoceanography of oceanic dysoxia/anoxia in the late Barremian to early Aptian (Early Cretaceous)
• The early Toarcian and Cenomanian-Turonian anoxic events in Europe: comparisons and contrasts
• Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals
• Stratigraphy, paleoceanography, and evolution of Cretaceous Pacific guyots; relics from a greenhouse Earth
• The early Toarcian (Jurassic) anoxic event; stratigraphic, sedimentary and geochemical evidence
• Stable-Isotope Analyses of Belemnite Rostra from the Whitby Mudstone Fm., England: Surface Water Conditions during Deposition of a Marine Black Shale

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• Foraminiferal assemblages, extinctions and appearances associated with the Early Toarcian Oceanic Anoxic Event in the Llanbedr (Mochras Farm) Borehole, Cardigan Bay Basin, United Kingdom
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• Persistent global marine euxinia in the early Silurian
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• Abrupt global-ocean anoxia during the Late Ordovician–early Silurian detected using uranium isotopes of marine carbonates
• Global deep ocean oxygenation by enhanced ventilation in the Southern Ocean under long‐term global warming
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• Abrupt global-ocean anoxia during the Late Ordovician–early Silurian detected using uranium isotopes of marine carbonates
• Astronomical forcing and chronology of the early Toarcian (Early Jurassic) oceanic anoxic event in Yorkshire, UK