Intensified lacustrine nitrous oxide emissions linked to global climate warming during the Toarcian Oceanic Anoxic Event

Lacustrine nitrous oxide (N2O) is an important greenhouse gas that contributes to modern climate warming, but its role in past hyperthermal events is poorly understood. Here we report intensified N2O emissions from the Sichuan megalake (>230,000 km2) during the Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE), an event associated with protracted climate warming and environmental perturbations. Sedimentary strata deposited in the megalake during the T-OAE record a positive δ15Nbulk excursion, indicative of enhanced denitrification. This denitrification was coincident with a globally recognized negative carbon-isotope excursion, increased organic-matter deposition, element drawdown, and deoxygenation. The Sichuan megalake was large (covering ∼10% of the present-day global lake surface area), and the interplay of enhanced lacustrine denitrification, deoxygenation, and Cu limitation likely promoted massive N2O emissions (conservatively estimated to be ∼0.9−1.8 Gt) from the megalake alone. Our findings suggest that N2O emissions from expanded lacustrine systems were significant enough to weaken the climate effect of CO2 drawdown through organic-matter burial. This process may have helped sustain global warming during past hyperthermal events.