How the salt in the oceans would have made the Earth habitable very early

The composition of theatmosphere has a direct impact on the climate, this is now self-evident. The multitude of paleoclimatic studies has also highlighted the major influence of atmospheric composition on the evolution of the Earth’s climate, and this since the origin of the Earth. However, the composition of the oceans played an equally important, even crucial, role in maintaining favorable climatic conditions for the appearance of life at the beginning of our planet’s history. The chemical exchanges between the ocean and the atmosphere are indeed very intense and make it possible to regulate certain processes, in particular at the level of greenhouse gas levels. In this context, the quantity of salt present in the oceans would be a factor that should not be overlooked.

The salinity of the oceans influences the climate

Ocean salinity and climate are two related parameters. If it is currently a question of the influence of climate on salinity due to global warming and the evaporation it generates, the reverse is also true: the salinity of the oceans has the capacity to influence the climate. This is demonstrated by a new study, published in the journal Geophysical Research Letters.

The oceans have the capacity to store gases in dissolved form. This is particularly the case for CO₂ we talk so much about right now. However, the solubility of gases in water is intimately dependent on the concentration of ions dissolved in the oceans. Knowing that the salinity of the oceans is defined by the concentration of ions sodium (N / A⁺) and chloride (Cl^–), it appears that the solubility of gases decreases with increasing salinity: a salty ocean will therefore be less able to capture greenhouse gaseswhich will remain in the atmosphere.

The multiple effects of salt

The effect of salt does not stop there, however. Salinity also directly influences water density (salt water is denser thanpure water), which impacts the ocean circulation circuits, thereby altering the temperature exchanges between the water and theair, especially at high latitudes. A well-known effect, salt also lowers the freezing point of seawater, preventing the formation of ice on the surface of the oceans. This phenomenon limits the formation of sea ice : the higher the salinity, the lower the temperature will have to be reached for the ice to begin to form.

All of these effects are closely linked and their combination means that a variation in salinity can have pronounced effects on ocean circulation, on the transport of heat towards high latitudes and on the formation of the pack ice. However, even small variations in the ice cover can lead to significant changes in the climate through the modification of the reflective surface represented by the ice.

A large number of causes can lead to variations in the quantity of salt in the oceans: variation in the level of water, variation in external inputs (rivers, etc.) or at the level of hydrothermal sources, increase or decrease in the rate of alteration of continental rocks… All these variables make it easy to imagine that the salinity of the oceans could have changed significantly during the history of the Earth.

A saltier Archean ocean would have resulted in a warm climate despite a still faint Sun

Based on a climate model taking into account oceanic and atmospheric dynamics, the team of American scientists shows that the saltier the oceans, the warmer the climate. The results of the study suggest that an increase in the amount of salt from 20 to 50 g/kg in ocean water would lead to a 71% reduction in sea ice. These estimates are valid in the current configuration, but also for the past.

During the Archean, which corresponds to the first geological period of the Earth, scientists show that the presence of a saltier ocean than the current one could have allowed the maintenance of a warm climate, in association with high rates of greenhouse gas (CO₂ and CH₄) in the air. This conclusion is important since at this time of the Earth’s youth, the Sun was then much less bright (about 20% less than today). The presence of a very salty ocean could thus have prevented the Earth from turning into a ball of ice, ensuring in particular favorable conditions for the appearance and development of life.