A strange lake discovered at the bottom of the Red Sea

Sea water is salty, Everyone knows it. If the salt content is on average 35 g/L, the salinity is, however, variable. It can in fact be modified by local constraints such as inflows of continental fresh water and evaporation. The salt content of the Black Sea is thus rather low because of the contribution of several rivers, while the Mediterranean is oversalinized because of the strong evaporation it undergoes.

There are also small variations in surface salinity, which mainly depend on the climatic zone (tropical, intertropical, etc.), seasons and currents. In general, these variations balance out with depth. At the average level latitudes, strong evaporation means that the water is saltier at the surface than at depth. Then, descending, a rapid decrease in salinity is observed, thus defining a strong gradient vertical salinity called the halocline. Salinity, along with temperature, strongly influences the density of seawater, which generates strong stratification which mainly develops between 0 and 500 meters deep.

A hypersaline lake discovered on the bottom of the Red Sea

But there are extreme cases, relatively rare, where sea water is so salty that it literally “sinks” to the bottom under the effect of its high density to form strange underwater lakes. Recently, such a pool of brine was discovered at the bottom of the Gulf of Aqaba, north of the Red Sea.

While diving 1,770 meters deep, one of the ROV submarines of the OceanX expedition indeed fell on a vast expanse ofbrackish water 10,000m². The analyzes carried out within this real lake submarine revealed the presence of large communities of germs. And this is far from obvious, because this type of environment is particularly inhospitable. First, the salinity there is extremely high. It has indeed been measured at 160 PSU, or 160 grams of salt per kilo of water! Secondly, the quantity of oxygen there is almost nil, defining an environment anoxic. The study of living organisms in this type of environment is therefore particularly important for understanding the adaptability of life to very hostile conditions, an essential question for the search for traces of life on other planets.

An interesting geological archive containing the traces of several earthquakes and tsunamis

The other question that interested the scientists in charge of this oceanographic mission is the origin of this brine lake. Two hypotheses are put forward. The first is the dissolution of evaporitic rocks present in the bottom sediments of the Red Sea. Evaporites, such as salt or gypsumare rocks resulting from the evaporation of sea water. These levels would have formed at the time of the salinity crisis of the end of the Miocene, when the Red Sea was opening and then formed a shallow and still closed basin. The second hypothesis involves evaporation atair free of sea water at the current sebkhas, which are shallow, flood-prone depressions located on the coast. the liquid hypersaline produced at the level of the sebkhas would thus percolate through the coastal plateau to come and accumulate at the foot of the slope, at the bottom of the ocean.

This brine lake also contains an interesting geological archive, with the presence of sedimentary deposits which testify, according to the authors of the study published in NatureCommunicationsthe occurrence of several tsunamiscaused by powerful tremors of earth that have shaken the region over the last centuries.

This study shows all the scientific interest represented by this type of brackish lake, of which only a few dozen examples are currently known.