The earth’s oceans were life carriers in more ways than one: Only their increased salinity could have brought carbon dioxide levels and the climate of the early earth into the habitable range, as a simulation suggests. Accordingly, the saltier water absorbed less CO2 and froze later, which in turn promoted a warmer climate and compensated for the much weaker radiation from the young sun at the time, as researchers report.
Actually, the earth should have been rather cold and not very habitable in its early days. The young sun emitted around 20 to 25 percent less light and heat around four billion years ago. So the young earth should actually have been too cold for liquid water and life. Instead, however, their climate was mild and seas covered the entire planet.
Does the ocean solve the faint young sun paradox?
How was this possible? This contradiction, also known as the paradox of the young weak sun, has not been clearly clarified. Although some hypotheses assume that an increased concentration of greenhouse gases how methane or carbon dioxide in the primordial atmosphere could have compensated for the lack of radiation, this has not yet been clearly proven.
Another explanation for the paradox could now have been found by Stephanie Olson from Purdue University in Indiana and her colleagues. They have investigated whether and how the salinity of the oceans influences the Earth’s climate. It is already known that an increased content of dissolved salts inhibits the absorption of gases into the water – a saltier ocean absorbs less CO2 or methane and thus increases their content in the air. “In addition, a higher salinity lowers the freezing point of the water and thus prevents the formation of sea ice,” the researchers explain.
Primeval earth in three variants
But so far it is unclear how salty the primordial seas were. “But we have every reason to believe that the salinity of the oceans has changed over the course of Earth’s history,” writes the team. On the one hand, evaporation, hydrothermal vents, as well as weathering and other geochemical processes can change the salinity of seawater. On the other hand, dissolved sodium and chloride ions only remain in the ocean water for an average of around 80 to 98 million years and therefore have to 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 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 percent less solar radiation than today and had an atmosphere dominated by CO2 and methane.
More warmth and less ice
The result: Even a slightly higher salinity in the primordial sea would have had a positive effect on the climate development of the early Earth. “Increased ocean salinity led to warming, especially at high latitudes, and reduced sea ice cover,” the team reports. 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 primeval sea with less salt. The sea ice area was about 71 percent smaller.
With the same CO2 content and the same solar radiation, an early Earth with today’s ocean salinity of 3.5 percent would have been almost completely glaciated and would have retained only a strip of open water at the equator. “But if you increase the salinity to five percent, the model results in a warm climate with surface temperatures of a good 20 degrees and only seasonal ice at the poles,” say Olson and her team.
In addition, a saltier ocean lowers the CO2 threshold at which the planet falls into the “snowball” status of a global ice age. “The threshold at which the earth suddenly tilts between different climate states depends on the salinity,” say the scientists.
“Salt of the Earth” as the key ingredient
According to the research team, the primeval ocean could have played a more important role in the early Earth climate than previously thought. “Our results raise the exciting possibility that a saline primordial ocean may have at least partially compensated for the young Sun’s fainter luminosity,” write Olson and her colleagues. “Then salt would have been an essential ingredient for the livability of the early earth.”
It is still unclear whether the primeval ocean was actually saltier than it is today. According to the researchers, however, extensive occurrences of primeval saline sediments make this quite likely. According to some studies, the salt bound in them could be enough to bring the salinity of the Precambrian seas to around five percent. Only in the course of later geological history did the salinity of the seas gradually drop to today’s value as a result of geochemical processes. (Geophysical Research Letters, 2022; doi: 10.1029/2021GL095748)
Source: Geophysical Research Letters