Cosmic cycles that shifted Earth’s oxygen may have triggered life’s greatest expansion

Earth’s orbit doesn’t move in a perfectly steady rhythm, and a new study argues that these slow, celestial cycles helped trigger ancient pulses of oxygen.

Those pulses appeared roughly every two to three million years. They may have set the tempo for one of the most important biological expansions in history: the rise of complex animal life.

Researchers at the Chinese Academy of Sciences (CAS) linked these long-term orbital cycles to swings in climate, nutrient delivery, and ocean oxygenation. Their work focuses on the early Cambrian period, when animals first diversified widely across Earth’s seas.

Ancient oceans felt orbital pulses

Before this study, scientists already knew that the Cambrian Explosion, which was a rapid surge in animal diversity in the oceans, came in several distinct bursts. Each burst added new body plans and ways of living to shallow seafloor habitats.

Geochemical records from Siberia showed that these biodiversity highs lined up with swings in carbon and sulfur chemistry that pointed to repeated oxygen pulses.

One influential record tied extreme oxygen perturbations to alternating booms and busts in early Cambrian animal groups.

Earlier work couldn’t explain what maintained such regular multimillion-year swings in oxygen levels. Zhang and colleagues set out to test whether the answer might lie in very slow changes in Earth’s orbit around the Sun.

Isotopes reveal ancient oxygen cycles

To follow ancient oxygen, the team measured isotopes, versions of the same element with different masses, in layers of Cambrian carbonate rock from Siberia.

Ratios of carbon and sulfur isotopes in rocks show where organic matter and pyrite were buried, and how oxygen remained in seawater and air.

They applied spectral analysis, a mathematical method for uncovering repeating cycles in noisy data, to isotope records from 525 to 512 million years ago.

The analysis revealed cycles about 2.6 million years long, along with weaker rhythms near 1.2 and 4.5 million years that match known orbital variations.

The study used a computer model simulating Earth’s climate and biogeochemical cycles, including interactions among life, rocks, water, and gases.

The model tests whether orbital forcing altered nutrient delivery enough to reproduce the observed swings in carbon and sulfur isotopes.

Sunlight shifts reshape oceans

Long orbital cycles – slow changes in Earth’s tilt and orbit shape that alter how sunlight is spread across latitudes – are studied for ice ages.

Zhang’s team explored what would happen if similar million-year cycles operated during the early Cambrian. That interval saw a warm climate with high carbon dioxide.

In their model, changes in incoming solar energy – often called insolation – shifted temperatures most strongly at high latitudes. That was where large landmasses sat.

Warmer periods at high latitudes sped up continental weathering, the breakdown of rocks on land that releases nutrients like phosphorus into rivers and oceans.

Extra nutrients fueled marine photosynthesis, the process where organisms use light to build organic matter. That process pulls carbon into sediments on shallow continental shelves.

That burial left more oxygen in the atmosphere and surface ocean, creating oxygenation pulses that matched the isotope cycles and bursts of animal evolution.

Small reservoirs, big impacts

A key twist is the low concentration of sulfate – a dissolved form of sulfur that helps buffer Earth’s chemistry – in early Cambrian seawater.

Ancient estimates suggest sulfate levels were far lower than today, making the entire reservoir small enough to shift quickly.

In the model runs, a small sulfate reservoir made the linked carbon, sulfur, and oxygen cycles respond strongly to orbitally driven nutrient pulses.

Similar changes in the sulfate reservoir during the Devonian showed how instability reshapes redox, the balance between oxidized and reduced chemical states.

Oxygen cycles expand habitats

Atmospheric oxygen likely stayed at low to moderate levels through much of the early Paleozoic, rather than jumping straight to modern values.

Under those conditions, modest extra oxygen from orbital pacing could expand seafloor areas. It could also stabilize communities that otherwise lived close to their physiological limits.

Recent work on early Cambrian marine ecosystems links rising oxygen and nutrient availability to complex food webs and the spread of burrowing animals.

The orbital pacing study adds a missing rhythm, showing how slow cycles in Earth’s orbit could synchronize climate, chemistry, and evolution in that interval.

The study is published in Geophysical Research Letters.

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