Abstract:
A study of sedimentary rocks in South Africa from about 2.3 billion to 2.25 billion years ago provides a new explanation for how oxygen rose and fell in Earth's early atmosphere. Researchers analyzed drill core samples from the Transvaal Supergroup in South Africa and found that the phosphorus cycle, redox state, and sulfate changes in the ocean are intertwined with each other. Combined with biogeochemical models, they believe that these feedbacks may lead to repeated dramatic changes in atmospheric oxygen content during the Great Oxidation Event, and at the same time push the earth to gradually shift to a persistent oxygen-containing atmospheric environment.
The first significant increase in oxygen in the Earth's atmosphere occurred about 2.43 billion to 2.06 billion years ago, often called the "Great Oxygenation Period." The emergence of oxygen does not mean that the atmosphere was immediately stable thereafter: this stage also experienced dramatic climate changes and multiple ice ages, and the chemical environment of the ocean and the nutrients available to life also changed. Phosphorus is a key element required for the growth of life, but the total phosphorus detected in rocks does not equal the phosphorus that ancient organisms could use.
The research team used a phosphorus form classification method to gradually dissolve different minerals in rocks, distinguishing components such as iron-bound, diagenetic, organic-bound, and crystallized phosphorus mainly derived from detritus. Based on this, they can determine which phosphorus may have been bioavailable in the ancient ocean. Samples show that enhanced weathering after the end of the ice age may have brought more phosphorus into the ocean, promoting biological productivity; subsequently, changes in ocean oxygen status and sulfate supply affected the decomposition of organic matter in sediments and the recycling of phosphorus. Phosphorus reenters the seawater, potentially providing conditions for more organisms to grow.
This process in turn affects atmospheric oxygen. As biological productivity increases, more organic carbon may be deposited and buried on the seafloor; if this carbon does not re-consume oxygen during decomposition, there will be a net accumulation of more oxygen. Based on this, the research team proposed that a feedback chain is formed between weathering input, phosphorus recycling, ocean redox state and sulfate, but the feedback does not maintain oxygen smoothly at a fixed level, but may cause significant fluctuations on the scale of tens of millions of years. The paper's model results support this interpretation, but the ancient atmospheric changes are still reconstructed based on rock geochemical records and models rather than direct measurements.
The research was published in "Nature Communications". The authors point out that this mechanism helps explain how oxygen, nutrients and early life co-evolved, and also suggests that oxygen is not the only condition that affects the emergence of complex life. The research may also provide a historical reference for understanding modern ocean anoxia: if seawater oxygen levels decline, phosphorus availability and ocean productivity may also change. However, feedback relationships in paleogeological time cannot be directly equated to specific predictions of modern ecosystems.
Comments