Researchers found that the common assumption that the ancient ocean was hot close to the boiling point is incorrect.
In transitional seasons, weather forecasters often report on wave height and water temperature for surfers seeking high waves on one hand, and swimmers preferring low waves and relatively warm water on the other.
This “conflict of interest” between the two communities somewhat mirrors a debate that has been ongoing for decades among scientists regarding the properties of the ancient ocean on Earth.
In short, the question can be formulated as follows: were the waters of the ancient ocean very hot (around 70 degrees Celsius or more), while the isotopic composition of oxygen in the water was similar to what we know today? Or did the ancient ocean have a temperature similar to the modern average (with variations of up to 20 degrees Celsius), alongside a more significant difference in the isotopic composition of oxygen compared to what we know today?
Regarding isotopic composition, this refers to the quantitative ratio between the common isotope of oxygen, oxygen-16, and the heavier, rarer isotope, oxygen-18.
The answer could provide important information about conditions on the early Earth during the first stages of animal evolution, as well as help understand mechanisms of global climate regulation and change.
The distribution of oxygen isotopes in the ooids reflects the isotopic composition of oxygen in the ocean when they were formed. In this sense, they can be seen as “oceanic time capsules.” By collecting ooids from dozens of locations worldwide, from layers formed at different times in Earth’s history (in this study, samples up to two billion years old were collected), the chemical and isotopic picture of the ancient ocean is obtained.
Study their isotopic composition relative to that of the ancient ocean, we needed calibration.” For this, the team synthesized these oxides in the laboratory, experimentally linking the isotopic composition of the oxides to the composition of the water from which they formed.
By combining the calibration results with the composition of the ooids, the researchers decoded the isotopic composition of ancient ocean waters over the last two billion years.
In this way, the scientists showed that the earlier the period, the lower the relative proportion of the heavy oxygen isotope (oxygen-18) compared to its lighter counterpart (oxygen-16).
In other words, over time since the formation of the first oceans, seawater contained more heavy oxygen isotopes.
Although the difference is only 1%–2%, it is enough to understand that the theory that the ancient ocean was hot close to the boiling point with far-reaching implications for early life forms and climate feedback mechanisms, is incorrect.