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Fact check: What caused the Paleocene-Eocene Thermal Maximum event?
Executive Summary
The Paleocene–Eocene Thermal Maximum (PETM) was driven by a massive, relatively rapid injection of carbon into the ocean–atmosphere system that caused global warming and a negative carbon isotope excursion; scientists remain divided over the dominant source, with recent work favoring volcanic/thermogenic carbon but older hypotheses emphasizing methane hydrate release. Multiple lines of evidence constrain the timing, magnitude, and isotopic composition of the carbon pulse and show that both thermogenic CO2 associated with magmatism and destabilization of sedimentary organic reservoirs (including methane) are viable contributors, with different studies quantifying different proportions and rates [1] [2] [3]. The balance of current literature increasingly points to sill-related thermogenic CO2 preceding and accompanying more isotopically light releases, but key uncertainties in rate, total mass, and proximal triggers persist [2] [1] [4].
1. Why researchers argue volcanoes and sills did the heavy lifting
Recent studies emphasize large volcanic and thermogenic carbon sources tied to the North Atlantic Igneous Province and sill intrusions that heated organic-rich sediments. Geochemical fingerprints in carbon isotopes, modeling of required δ13C values, and physical plausibility led authors to conclude a very large and somewhat heavier carbon source than pure methane would imply; a 2017 Nature study found evidence for a “very large release of mostly volcanic carbon,” shifting interpretations away from exclusively hydrate-driven scenarios [1]. More recent work modeled millennial-timescale thermogenic CO2 release preceding the PETM and linked it to sill intrusions that could generate sustained CO2 fluxes and also liberate biogenic methane as a secondary feedback, reconciling heavier volcanic signatures with later lighter pulses [2]. These studies present a multi-stage view: intrusive magmatism drives initial CO2 release, triggering feedbacks that release additional lighter carbon.
2. Why methane hydrates and sedimentary carbon remain in the picture
A longstanding hypothesis invokes rapid dissociation of marine methane hydrates, which would deliver very 13C-depleted carbon and help explain the sharp negative carbon isotope excursion. Early analyses and models showed that massive methane release could account for the observed isotopic shift and abrupt warming, and regional studies argued that tectonic uplift or warming could destabilize hydrate reservoirs [3] [5]. Reviews of hydrate formation and stability underscore that geological systems can store and rapidly release large methane inventories under the right conditions, so hydrates remain a physically plausible contributor [6]. The hydrate hypothesis explains the most extreme isotopic excursions with a single very light source, whereas volcanic/thermogenic scenarios require a mixture of heavier and lighter pulses to reproduce the isotope record.
3. What the quantitative constraints tell us about scale and tempo
Numerical studies and model–data comparisons place strong constraints on the minimum carbon mass and rates needed to reproduce ocean acidification and CaCO3 dissolution patterns during the PETM. Intermediate-complexity models find that thousands of petagrams of carbon (for example, ~6800 Pg C with δ13C ≈ −22‰ in one study) are needed as a lower bound to match seafloor carbonate dissolution, pushing against what pure hydrate reservoirs could supply unless combined with other sources [4]. Rate estimates vary: some reconstructions infer peak carbon addition rates in the range ~0.3–1.7 Pg C yr−1 and total isotopic excursions of about 4‰, consistent with a substantial but not instantaneous release over millennial timescales [7]. These quantitative constraints favor mixed-source scenarios or very large volcanic/thermogenic pulses when reproducing both isotope and sedimentary responses.
4. How spatial climate fingerprints and sensitivity complicate the cause-and-effect story
Paleoclimate reconstructions show PETM temperature and precipitation changes that are spatially heterogeneous and broadly similar to high-end future warming patterns, and data assimilation efforts using proxy data indicate higher equilibrium climate sensitivity under very high greenhouse gas concentrations [8]. These climate responses feed back on carbon reservoirs: warmer oceans and altered hydrology can amplify the release of biogenic methane and destabilize methane hydrates, producing cascading effects. Thus the climatic fingerprints do not uniquely identify the carbon source; instead they show that once a large carbon pulse occurred—whatever the trigger—Earth system feedbacks likely magnified and prolonged warming, making trigger vs. feedback distinctions critical but difficult to isolate using climate fingerprints alone [8] [2].
5. Synthesis: converging evidence and remaining uncertainties
Taken together, the literature paints a multi-stage, mixed-source scenario as the best current explanation: intrusive magmatism and sill emplacement associated with the North Atlantic Igneous Province likely produced substantial thermogenic CO2 that initiated warming, and this initial warming and/or direct heating of sediments provoked additional releases of isotopically light carbon (methane and/or thermogenic hydrocarbons), producing the observed isotope excursion and sedimentary responses [1] [2] [3]. Key unresolved questions remain about the exact partitioning between volcanic, thermogenic, and hydrate-derived carbon, the peak rates and durations of pulses, and local vs. global variability in release and impact. Resolving these requires integrated high-resolution stratigraphy, targeted isotopic constraints, and coupled carbon–climate modeling that respects the quantitative bounds established by carbonate dissolution and isotope excursions [4] [7].