Ocean salt made the earth friend to life – Rising salinity in the oceans may have warmed and stabilized the initial climate

Earth’s oceans were carriers of life in more ways than one: only their increased salinity could have carried primeval earth’s carbon dioxide levels and climate into the habitable area, as a simulation suggests. As a result, the saltier water absorbed less CO2 and froze later, which in turn promoted a warmer climate and offset the much weaker radiation from the young sun of the time, the researchers report.

In reality, the earth should have been quite cold and uninhabitable in its early days. This is because the young sun emitted 20 to 25 percent less light and heat about four billion years ago. So the young earth really should have been too cold for liquid water and life. Instead, their climate was mild and the seas covered the entire planet.

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

How was this possible? This contradiction, also known as the weak young sun paradox, has not been clearly clarified. Although some hypotheses assume that a higher concentration of greenhouse gases such as methane or carbon dioxide in the early atmosphere could have compensated for the lack of radiation, this has not yet been clearly demonstrated.

Another explanation for the paradox may have been found by Stephanie Olson of Purdue University in Indiana and her colleagues. They studied if and how the salinity of the oceans affects the Earth’s climate. It is already known that a higher content of dissolved salts inhibits the absorption of gases in water: a saltier ocean absorbs less CO2 or methane and therefore increases their content in the air. ‘In addition, higher salinity lowers the freezing point of the water and thus prevents the formation of sea ice,’ explain the researchers.

Primordial land in three variants

But so far it is unclear how salty the primeval seas were. “But we have every reason to believe that ocean salinity has changed over the course of Earth’s history,” the team writes. On the one hand, evaporation, hydrothermal vents, as well as atmospheric agents and other geochemical processes can change the salinity of seawater. On the other hand, dissolved sodium and chloride ions remain in ocean water only for an average of about 80-98 million years and then have to be added again and again through such processes.

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

The first cover of terrestrial sea ice at different levels of atmospheric CO2 and primordial ocean salinities. © olson et al./ Geophysical Research Letters, CC-by-nc-nd 4.0

More heat and less ice

The result: Even a slightly higher salinity of the primeval ocean would have had a positive effect on the climatic development of the primeval earth. “The increase in ocean salinity has led to warming, especially in high latitudes, and reduced sea ice cover,” the team reports. In the higher-salt scenario, global temperatures were nearly a degree higher and in the far north even nearly twelve degrees higher than in the primeval sea with less salt. The sea ice area was approximately 71% smaller.

With the same CO2 content and the same solar radiation, a primeval Earth with today’s ocean salinity of 3.5% would have been almost completely frozen and would have kept only a strip of open water at the equator. “But if you increase the salinity to five percent, the model translates into a warm climate with surface temperatures of as much as 20 degrees and only seasonal ice at the poles,” say Olson and his team.

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

The “salt of the earth” as a key ingredient

According to the research team, the primeval ocean could have played a more important role in Earth’s early climate than previously thought. “Our results raise the exciting possibility that a saline primeval ocean may have compensated at least in part for the younger Sun’s dimmer luminosity,” write Olson and his colleagues. “Then the salt would have been an essential ingredient for the livability of the primordial earth”.

It is still unclear whether the primeval ocean was actually saltier than it is today. According to the researchers, however, the extensive presence of primordial saline sediments makes this quite likely. According to some studies, the salt contained in them could be sufficient to bring the salinity of the Precambrian seas to about five percent. Only in the course of subsequent geological history did the salinity of the seas gradually decrease to today’s value due to geochemical processes. (Geophysical Research Letters, 2022; doi: 10.1029 / 2021GL095748)

Those: Geophysical Research Letters

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