A research team from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (NIGPAS), along with collaborators, has found that long-term orbital variations occurring over million-year timescales may have served as the "pacemaker" for Earth's ancient oxygenation pulses. Their findings were recently published in Geophysical Research Letters.
The Cambrian Explosion stands as one of the milestones in Earth's evolutionary history, a period when nearly all modern animal phyla emerged rapidly. Fossil and geochemical evidence show that the diversification of early Cambrian animals unfolded in multiple evolutionary pulses, paired with synchronous fluctuations in seawater inorganic carbon and sulfate sulfur isotopes. These variations are widely interpreted as markers of periodic oxygenation events in the atmosphere and shallow oceans. However, the driving force behind these rhythmic oxygen pulses has long remained unclear.
To fill this knowledge gap, the team launched a study in 2019 on well-preserved early Cambrian carbonate successions from the southeastern Siberian Platform. Their analysis revealed that between approximately 524 and 514 million years ago (early Cambrian), marine animal diversity fluctuated periodically every two-three million years, which aligned with shifts in seawater carbon and sulfur isotopes.
The team proposed that cyclic changes in the global burial of organic carbon and pyrite drove periodic variations in oxygen levels in the atmosphere and shallow marine environments, in turn shaping the evolutionary dynamics of early marine animals.
Building on this work, the team's latest research further suggests that these million-year-scale environmental oscillations were likely triggered by long-period orbital variations. Changes in Earth's orbital configuration altered the distribution of solar radiation across latitudes, leading to periodic climate shifts. These climatic changes, the team notes, probably regulated the intensity of continental weathering and the flux of key nutrients, such as phosphorus, into the oceans. The cyclic input of these nutrients would have boosted marine photosynthesis and increased organic carbon burial, ultimately driving periodic rises in atmospheric and oceanic oxygen levels.
To test this hypothesis, the researchers conducted spectral analyses of previously published early Cambrian carbon-sulfur isotope records. The results uncovered long-period cycles of 1.2, 2.6, and 4.5 million years, matching the frequencies of known long-term orbital cycles.
By integrating orbitally driven climate forcing into the SCION Earth system box model for the first time, the researchers successfully replicated the observed synchronous periodic variations in seawater carbon and sulfur isotopes. This outcome confirms the plausibility of an orbitally forced oxygenation mechanism.
Additionally, model sensitivity experiments showed that the low sulfate concentration in early Cambrian oceans made the Earth system unusually unstable, amplifying the response of coupled carbon-sulfur-oxygen biogeochemical cycles to orbitally modulated nutrient inputs.
This finding offers new insights into the timing of the Cambrian Explosion and provides a broader framework for understanding long-term carbon, sulfur, and oxygen cycles in other geological eras.
The study received support from the National Key Research and Development Program of China, the National Natural Science Foundation of China, and other funding sources.
Carbonate carbon and carbonate-associated sulfate sulfur isotopes. (Image by NIGPAS)
Orbitally induced climate approximation in the SCION model. (Image by NIGPAS)
