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Monday, April 12, 2021
Aaron Satkoski used the UW-Madison Department of Geoscience's mass spectrometer to measure isotopes in samples collected from South Africa.

Aaron Satkoski used the UW-Madison Department of Geoscience's mass spectrometer to measure isotopes in samples collected from South Africa.

Chemical composition of oceans helps scientists understand ancient life

New research on the chemical composition of the ocean has shown that, 3.26 billion years ago, the continents were actually above water. This pieces together several other studies into a cohesive, big-picture idea of how the world once looked, according to Aaron Satkoski and his team of researchers who studied the chemical composition of erosion in the ocean back in 2013 in the Barite Valley, near Barberton, South Africa.

Satkoski, lead researcher and scientist in the geoscience department at the University of Wisconsin-Madison, and his team formulated the research idea of chemical composition from erosion based on past research showing the potential that the continents were above sea level and weathering into the oceans 3.26 billion years ago, but no studies previously existed to support this theory.

“There were no really good studies that supported [continents were above water and eroding into the ocean]. People have done work previously and they said nope, the chemical composition in the oceans is entirely controlled by mid-ocean ridges,” Satkoski said.

The chemical composition of the oceans is controlled by continental erosion and fluids at the mid ocean ridges.

“Evidence was starting to suggest that the continents were above sea level and were eroding and those erosion products should make their way into the ocean, and that should also affect the chemistry,” Satkoski said, adding that several areas of geology suggested that ocean chemistry should be affected by continental run-off, like we know to be true today.

Satkoski’s study involved analyzing the isotope levels of strontium in barite between the erosion from weathering of continents and deposits on mid-ocean ridges from hydrothermal circulation, which is the process of hot water circuiting through Earth’s crust.

When barite forms in seawater, it records the chemistry of the water it is formed in, preserving certain elements, like strontium, to test the chemical composition of the water from the time it was formed.

The isotope levels were an important part to this study because they helped understand where the isotopes came from, continents or hydrothermal circulation.

“The continents will have a very specific isotope ratio, and the mid-ocean ridges will have a very specific isotope ratio,” Satkoski said, adding that strontium from land has a higher isotope level than strontium found on mid-ocean ridges.

By measuring the mixture of the isotopes, which is now seawater, Satkoski’s team of scientists analyzed where exactly the strontium isotopes came from.

“What it turned out to be was the isotope composition was a mixture between hydrothermal and continents. It was more shifted towards the continents than ever previously before,” Satkoski explained.

These results proved that the continents were above water and eroding because the isotope levels in the strontium showed that a larger portion than previously thought came from continental weathering.

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This concept of continental weathering ties together previous studies about the Archean time period, stretching 4 to 2.5 billion years ago, when there was an increase in carbon dioxide that resulted in acid rain. Consequently, the acid rain caused erosion of continents into the ocean, leading to a high level of continental weathering isotopes in the barite samples during that time period.

The acid rain of the Archean created high erosion, which lead to increased phosphorus levels, a limiting nutrient for life, off the coast of the continents, much like we have today, suggesting that marine life would have lived in shallow marine environments, where nutrients, such as phosphorus, were plentiful.

Satkoski’s study helps to give a holistic reconstruction of what the environment might have looked like 3.3 billion years ago, as his research helps to piece other research together into a cohesive picture of a 3.26 billion year-old Earth.

“[Our study] actually went out and showed yes, this was the ocean chemistry and now all these pieces that people have put out there now all come together in one complete story: the solid earth with uplifted and erosion of crust, high carbon dioxide concentrations, life at this time needing those limiting nutrients, and life needs a place to live,” said Satkoski, adding that they are still unsure if life needed sunlight 3.26 billion years ago.

This holistic understanding of the evolution of life and our planet will help us to better understand other planets, such as Mars, creating more testable hypotheses of similar planets that got their foothold at the same time as Earth. According to Satkoski, understanding our world helps us to better understand other worlds. 

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