image: The stratigraphic records from the Wuke carbonate section in South China show two negative carbonate cerium isotope excursions associated with the two extinction pulses. Neodymium and lithium isotope records track accompanying changes in continental weathering.
Credit: ©Science China Press
About 443 million years ago, the Late Ordovician Mass Extinction eliminated approximately 85 percent of marine species in the first of the Phanerozoic 'Big Five' mass extinctions in less than 1 million years. The crisis unfolded in two pulses under contrasting climates: the first accompanied rapid cooling and expanding continental glaciers, whereas the second occurred during deglaciation and warming. Beyond the direct ecological effects of changing temperature, how climate shifts may have indirectly stressed marine ecosystems by altering their environments remains poorly understood.
A research team led by scientists from the Institute of Geology and Geophysics, Chinese Academy of Sciences, and Chengdu University of Technology investigated how rapid climate change was transmitted from land to the shallow seas where much of Ordovician benthic life lived. The team analyzed paired stable cerium and neodymium isotope records in Upper Ordovician carbonates from the Wuke section on the western Yangtze platform in South China. This succession formed in shallow water above the storm wave base and preserves fossil records of both extinction pulses and the short interval between them.
Cerium isotopes provide information about regional marine oxygenation because cerium cycling is closely linked to manganese oxides. In oxygenated water, manganese oxides preferentially remove lighter cerium isotopes. When oxygen declines and manganese-oxide burial weakens, seawater and carbonate cerium isotope values can shift downward. Because cerium has a short residence time in seawater, the carbonate record is interpreted as a sensitive indicator of local-to-basinal, rather than global, redox change.
The Wuke carbonate outcrop contains two pronounced negative cerium isotope excursions, one at the onset of the first extinction pulse and another during the second. These shifts indicate that low-oxygen waters twice expanded into shallow carbonate-platform habitats above the storm wave base. A semi-quantitative cerium-manganese mass-balance model estimates a change from more oxygenated conditions of about 80 micromoles per liter to near-anoxic conditions below 10 micromoles per liter. The absolute values remain model-dependent, but the timing and direction of the oxygen changes are robust. A brief reoxygenation interval between the two events broadly coincided with expansion of the cold-adapted Hirnantia fauna.
The findings add shallow-water oxygen loss to the environmental stresses associated with the first extinction pulse. Rapid cooling, falling sea level and habitat contraction remain important, but deoxygenation reaching biologically rich carbonate platforms may have intensified their ecological effects.
Stable neodymium isotopes, considered alongside an existing lithium isotope record from the same section, trace a parallel reorganization of continental weathering. Warm late Katian conditions favored relatively congruent weathering, in which primary minerals dissolved more completely and fewer secondary minerals formed. Early Hirnantian glaciation shifted the system toward more incongruent weathering, with greater secondary-mineral formation. Deglacial warming then promoted a return toward congruent weathering. These transitions coincided with the two extinction pulses and the intervening oxygenation episode.
The team used biogeochemical and Earth-system models to test how plausible changes in weathering-derived phosphorus supply could affect carbon cycling and marine oxygen. The simulations were designed as sensitivity experiments rather than direct reconstructions of ancient phosphorus fluxes. They show that pulsed nutrient delivery can enhance primary productivity, organic-carbon burial and oxygen consumption. Closely spaced pulses can also preserve the effects of earlier disturbances and amplify later carbon-cycle and climate responses. Additional simulations indicate that low-latitude shallow seas are especially vulnerable to oxygen loss when nutrient availability increases.
Together, the isotope records and models support a cross-sphere mechanism linking climate, continental weathering, nutrient delivery and marine redox change. Rapid climate oscillations may therefore do more than expose organisms to changing temperature and sea level: by repeatedly reorganizing land-to-ocean material transfer, they can destabilize shallow-marine oxygenation and amplify ecological crises. This framework helps explain how two extinction pulses that occurred under contrasting climates produced similar stress in shallow marine ecosystems.
The study was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, and the project Theory of Hydrocarbon Enrichment under Multi-Spheric Interactions of the Earth.