Five hundred million years ago during the Cambrian and beyond, the explosive growth of animal life in Earth's oceans is often attributed to the sustained and significant increase in free oxygen (O2) levels in seawater. Some researchers even believe that the oxygen content in the ocean was close to modern levels at that time.
But new research by geologists at the University of Utah and colleagues from other institutions suggests that for a long time after the Cambrian, there were significant fluctuations in O2 levels in the Earth's deep-sea environment.

01. Dynamic Deep Ocean Oxygenation during the Early Middle Paleozoic Era
Researchers reconstructed oxygen levels approximately 485 to 380 million years ago using stable isotope ratios of thallium (Tl) preserved in ancient marine mudstones. This time frame closely followed the rise of Cambrian animals and even intertwined with the later rise of land plants.
The study is titled "Dynamic Deep Ocean Oxygenation during the Early Middle Paleozoic Era". According to Chadlin Ostrander, assistant professor and lead author of the Utah Department of Geology and Geophysics, this research challenges some traditional views on ocean oxygenation.
This doesn't mean that if someone presses the switch, the deep sea will always be filled with oxygen, "Osterland said. Just ten years ago, people believed that the oxygen switch in the deep sea was turned on about 540 million years ago. Our new dataset pushes this time forward by at least 160 million years
To make these findings, Osterland and his colleagues analyzed the stable isotopes of thallium in ancient marine sediments recovered from the Yukon Territory in Canada. There are few processes that can strongly fractionate thallium isotopes, meaning that they are distributed in different proportions.

02. Oxygen in the Deep Sea
The stronger fractionation currently occurs in deep-sea iron manganese deposits. Osterland believes that oxygen must accumulate in deep-sea waters to stabilize these mineral deposits. The thallium isotope ratios in the new study rarely show strong fractionation, which means that these oxygen dependent deep-water deposits are also rare.
We did find some evidence of oxygen accumulation in the deep sea, but this only lasted for a short period of time, "Osterland said. Even in the young stage of our dataset, the ocean seems to have once again fallen into a phase of widespread hypoxia
The research team found that ocean oxygen combines non-stationary or long-term changes. On the contrary, O2 levels exhibit dynamic changes and fluctuate over time. A particularly stable oxygenation period was discovered approximately 405 to 386 million years ago. But even this period seems to have lasted for a short time, ending in the young mudstone samples of the team.
The more we study, the more we find it complex. It's really unstable. Even if our dataset ends, we can't find any stability. We've never found evidence of ocean oxygen levels continuing to rise to near modern levels. This must have happened at some point 380 million years ago, "Osterland said. This is a disappointing aspect of our research results: we still don't know when the deep sea began to be filled with oxygen for a long time
Eric Sperling from Stanford University and Justin Strauss from Dartmouth University are the main collaborators in this study. They led an exploration team to the Yukon region of Canada to search for ancient seafloor sediments that are now exposed in mountainous river channels and use them for research. This is a very special sample set, "Osterland said. It is difficult to find continuous seafloor sediments spanning such a long geological age

03. Deep sea oxygenation
In a 2021 study, Sperling and Strauss described this region and pointed out its potential in revealing the history of Earth after the Cambrian Explosion of Life and the Ordovician Explosion of Biodiversity. At that time, the ocean was filled with strange creatures such as trilobites, tentacle shaped stones, and tiny toothed spines.
Since the rise of oxygen in the atmosphere following the Great Oxidation Event, the Earth's surface oceans have been in an oxygen-containing state for approximately 2.3 billion years. This is because oxygen in the atmosphere can diffuse into the surface ocean.
Transporting oxygen to the deep sea is even more difficult, "Osterland said. This requires cold and dense surface seawater rich in oxygen to sink in polar regions, and the respiration rate of the water is also lower. These processes involve many factors. Therefore, we should not be surprised by the long and complex oxygenation process in the Earth's deep sea
If the deep sea remains partially or intermittently hypoxic during critical biological milestones, the rise of animals may occur under conditions with less O2 enrichment than previously assumed - challenging traditional views on the relationship between oxygen and evolution.
From a biological perspective, this is far after the Cambrian 'Explosion of Life'. At that time, marine organisms were already quite large and capable of performing energy consuming tasks, "Osterland said. During this period, there have been some astonishing changes in evolutionary biology. None of these changes seem to require extensive and sustained deep-sea oxygenation