Science and Sulfur Isotopes

Scientific discoveries often arise from the subtle and the overlooked. Volcanic gases interacting with the atmosphere produce specific sulfur isotopes that eventually become part of rock formations. The composition of the atmosphere plays a critical role in determining the final chemical structure of these sulfur isotopes. Consequently, by analyzing sulfur isotopes, researchers can differentiate between oxygen-rich and oxygen-poor (anoxic) atmospheres in the geological record.

At MIT, Genming Luo and his colleagues employed this technology to study rock cores from South Africa, enabling them to pinpoint when the shift from an anoxic atmosphere to an oxygenated one took place. Their research suggests that the Great Oxygenation Event occurred around 2.33 billion years ago, with complete atmospheric oxygenation taking approximately ten million years.

However, this raises a pressing question: If the transition took only ten million years, what happened to the oxygen generated over the preceding billion years?

The balance of atmospheric chemicals, including oxygen, reflects a dynamic interplay between sources and sinks. For instance, when fossil fuels are burned, they release carbon dioxide (CO2) into the atmosphere, acting as a source. In contrast, trees utilize CO2 for photosynthesis, thus serving as a sink. In the early Archean period, blue-green algae were prolific oxygen producers. If this oxygen wasn't accumulating in the atmosphere, some chemical sink must have been removing it.

Volcanic Gases and Earth's Mantle

A 2020 study published in Nature by Shintaro Kadoya and colleagues proposes a potential answer to the mystery of the missing oxygen. Their findings suggest that volcanic gases created a chemical sink responsible for depleting atmospheric oxygen. However, this theory merely scratches the surface of a more complex issue that lies deep within Earth's structure.

The mantle beneath Earth's crust is the origin of magma for volcanic hotspots such as Iceland and Hawaii. Mantle plumes push magma to the surface, leading to volcanic eruptions that release a variety of harmful gases into the atmosphere. Kadoya's research indicates that the mantle's composition during the Archean Eon was distinct from that of today, containing less oxygen. A less-oxidized mantle generates higher volumes of gases that readily react with oxygen. Gases like hydrogen naturally bind with free oxygen, effectively removing it from the atmosphere.

Kadoya theorizes that the oxygenation of the mantle spanned nearly a quarter of Earth's history, marked by a persistent struggle between geological and biological processes. For a billion years, volcanic gases consumed all the oxygen produced by living organisms. Only after the mantle achieved full oxygenation did these oxygen-reactive gases cease to flood the atmosphere, allowing oxygen levels to rise and paving the way for the evolution of oxygen-dependent life forms.

Nevertheless, the mantle oxygenation theory remains just that—a theory. An alternative perspective emphasizes the role of supercontinent formation in understanding the Great Oxygenation Event. This hypothesis links the biological needs of single-celled organisms to nutrient availability, suggesting that this was a critical factor in the protracted timeline of Earth's oxygenation.

Plate Tectonics, Mountain Building, and Nutrients

Today, we grapple with environmental issues stemming from harmful algal blooms, often triggered by excessive pollution. Agricultural runoff and urban waste treatment leaks contribute surplus nutrients to our waterways. Under normal conditions, algae populations naturally regulate themselves; when their growth becomes excessive, they deplete available nutrients, leading to population decline.

However, when nutrients are abundant, algae can proliferate unchecked. This growth creates a feedback loop, where the decay of vast algal populations consumes all available oxygen in the water, resulting in significant die-offs of fish and other aquatic organisms. The toxins released by the algae pose additional dangers, poisoning surrounding marine life.

Conversely, if nutrients are consistently scarce, algae populations remain small, preventing significant oxygen production. This scenario leads to alternating aerobic and anaerobic conditions, contributing to the formation of banded iron formations, as explored in earlier discussions.

The collision of continental plates is one mechanism that generates larger nutrient supplies. Such collisions result in mountain formation, which begins a process of erosion. As mountains weather, they release fresh nutrients into streams and rivers, eventually reaching the ocean. This theory posits that a rapid geological release of nutrients catalyzed the growth of vast oxygen-producing algal populations, ultimately leading to the Great Oxygenation Event.

(For further insights, check out Vanishing Origins available on Wattpad)

Or explore my ongoing articles on Medium, where I chronologically track the evolution of life on Earth: EarthSphere Page — Forgotten Origins.

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Sources:

A billion years of missing oxygen (by WM House; ArcheanWeb)

Formation of supercontinents linked to increases in atmospheric oxygen (by Ian Campbell & Charlotte Allen; Nature Geoscience)

Banded Iron Formation (Source: American Museum of Natural History)

The Great Oxygenation Event — when Earth took its first breath (Source: Scientific Scribbles)

Earth as an Evolving Planetary System (Third Edition), 2016 — Banded Iron Formation (by Kent C. Condie, 2016)