Ocean salt has created a life-friendly Earth – Increased salinity in the oceans could warm and stabilize the early climate

The Earth’s oceans have been the bearers of life in several ways: Only their increased salinity could bring carbon dioxide levels and the climate of the early Earth to the population, as the simulations suggest. Accordingly, saltier water absorbed less CO2 and later froze, which in turn encouraged a warmer climate and compensated for the much fainter radiation from the young sun at the time, according to scientists.

In fact, the Earth should have been rather cold and less habitable in its early days. This is because the young sun radiated 20 to 25 percent less light and heat about four billion years ago. So the young country should actually be too cold for liquid water and life. Instead, their climate was mild and the sea covered the entire planet.

Does the ocean solve the weak paradox of the young sun?

How was that possible? This contradiction, also known as the young weak sun paradox, has not been clearly elucidated. Although some hypotheses suggest that the lack of radiation may have been offset by increased concentrations of greenhouse gases, such as methane or carbon dioxide in the primordial atmosphere, this has not yet been clearly demonstrated.

Stephanie Olson of Purdue University in Indiana and her colleagues could now find another explanation for the paradox. They studied whether and how the salinity of the oceans affects the Earth’s climate. It is already known that the increased content of dissolved salts inhibits the absorption of gases into the water – the saltier ocean absorbs less CO2 or methane and thus increases their content in the air. “In addition, higher salinity lowers the freezing point of water and thus prevents the formation of sea ice,” the scientists explain.

Prehistoric land in three variants

But it is not yet clear how salty the primordial sea was. “But we have every reason to believe that the salinity of the oceans has changed over the course of Earth’s history,” the team writes. On the one hand, evaporation, hydrothermal vents, but also weathering and other geochemical processes can change the salinity of seawater. On the other hand, dissolved sodium and chloride ions remain in ocean water for only about 80 to 98 million years on average, and therefore must be added again and again through such processes.

For their study, Olson and her colleagues reconstructed three variants of the primordial Earth, which was still largely covered by water, in a combined ocean-atmosphere model. These differed only in the salinity of seawater, which was 2, 3.5 and 5 percent lower, the same and higher than today. All three models received 20 percent less sunlight than today, and the atmosphere was dominated by CO2 and methane.

ice cover
Early coverage of sea ice on Earth at various levels of atmospheric CO2 and the original salinity of the oceans. © olson et al./ Geophysical Research Letters, CC-by-nc-nd 4.0

More heat and less ice

Result: Even a slightly higher salinity of the primordial ocean would have a positive effect on the development of the early Earth’s climate. “The increased salinity of the ocean has led to warming, especially at high latitudes, and a reduction in sea ice cover,” the team said. In the scenario with the highest salt content, global temperatures were almost one degree higher and in the far north even almost twelve degrees higher than in the primordial sea with less salt. The area of ​​sea ice was about 71 percent smaller.

With the same CO2 content and the same solar radiation, the early Earth, with today’s ocean salinity of 3.5 percent, would be almost completely glaciated and retain only the free water belt at the equator. “But if you increase the salinity to five percent, the model results in a warm climate with good surface temperatures of a good 20 degrees and only seasonal ice at the poles,” says Olson and her team.

In addition, the saltier ocean lowers the CO2 threshold at which the planet falls into a “snowball” state of the global ice age. “The threshold at which the Earth suddenly tilts between different climatic conditions depends on salinity,” scientists say.

“Earth’s salt” as a key ingredient

According to the research team, the prehistoric ocean may have played a more important role in the early Earth’s climate than previously thought. “Our results raise the exciting possibility that the salty primordial ocean may have at least partially offset the weaker luminosity of the young Sun,” writes Olson and colleagues. “Then salt would be an essential component for the habitability of the early Earth.”

It is still unclear whether the prehistoric ocean was really saltier than it is today. According to researchers, however, this makes relatively likely large occurrences of prehistoric saline sediments. According to some studies, the salt bound in them could be enough for the salinity of the Precambrian seas to withstand about five percent. It was only during the later geological history that the salinity of the seas gradually decreased to today’s value due to geochemical processes. (Geophysical Research Letters, 2022; doi: 10.1029 / 2021GL095748)

Quelle: Geophysical research letters

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