Chert Rocks Provide Clues About Earth’s Early Climate

Researchers examined 550-million-year-old chert rocks from southeastern China.
These samples record that after the deposition of mud sediments, the rocks crystallize hundreds of meters below the Earth’s surface, capturing the temperatures of deep ocean waters.

Billions of years ago, the oceans were not as warm as often assumed their water temperatures were much lower.
This is the conclusion of a research team from the University of Göttingen and the German Research Centre for Geography in Potsdam.
The scientists analyzed chert a hard, thin sedimentary rock formed from seawater and remains of silica-secreting organisms.

Using these “time capsules,” the team demonstrated that oxygen isotope ratios are determined by the cooling of the solid Earth and are less dependent on sea water temperatures.
The findings were published in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS).
Questions arise about how ancient chert rocks dating from 3.85 to 2.5 billion years ago became enriched in the lighter oxygen isotope (oxygen-16) and what information these precious time capsules record about Earth’s history.

To investigate this long-standing mystery, the team examined approximately 550-million-year-old chert samples from southeastern China.
These samples show that after mud sediments were deposited, the chert crystallized hundreds of meters below the Earth’s surface, recording deep ocean water temperatures rather than the temperature of the overlying ocean waters.
This finding prompted the idea that oxygen isotope ratios could depend on heat flow from the Earth’s interior a completely new perspective on an old puzzle.

“Our calculations show that when heat flow is higher, the proportion of oxygen-16 increases because recrystallization occurs at higher temperatures, enriching the seawater with oxygen-16,” explained Dr. Michael Tatzel from the German Research Centre for Geography at the University of Göttingen.
This resolves the mystery of why there is a large portion of lighter oxygen isotopes in ancient chert, as heat flow on the early Earth was roughly twice the modern values.
“It is clear that chert rocks do not provide an exact record of past sea water temperatures.

However, our findings suggest that we need to interpret oxygen isotopes in chert in a completely new way,” said Tatzel.
“I believe this research will open the door to exciting new developments in the coming years, as our understanding of heat flow effects will allow more accurate reconstruction of seawater temperatures in deep geological time.
Additionally, we will be able to decipher the thermal structure and tectonic history of ancient sedimentary basins,” added Patrick Frings from the German Research Centre for Geography in Potsdam, a co-author of the study.

The calculated impact of heat flow on oxygen isotopes in chert also implies that the oldest sedimentary rocks of this type, which are isotopically light, indicate a temperate to warm climate on the early Earth.
Thus, it is unlikely that ocean waters were as warm as previously estimated, providing a key insight into the development of life on Earth.