Hi,

My question is about salinity of seas. Why is the Red Sea (45 promilles) saltier than the World Ocean (35 promilles)? And why is the Dead Sea even saltier - 67 promilles? What are the natural causes of these differences?

I would put both instances down to evaporation, or some other process relating to it.

 

Doing a search on both seas does reveal information on reasons for the different salinities, probably best to start there.

How does the salt get into the sea? That's the question you want to answer.

It makes sense for the Dead Sea to increase salt concentration from evaporation and reduction of tributary input since it is fairly isolated, the Red Sea has about a 100k wide connection to the Gulf of Aden arm of the Indian Ocean, which makes me skeptical that evaporation could be the main cause there.

the Red Sea has about a 100k wide connection to the Gulf of Aden arm of the Indian Ocean, which makes me skeptical that evaporation could be the main cause there.

 

I actually was thinking the same thing when I first read the OP. Wikipedia (while I don't completely trust it) mentions that "due to the effects of the water circulation pattern, resulting from evaporation and wind stress. Salinity ranges between 3.6 and 3.8%." I'm going to try and see if I can find a more in depth explanation somewhere else, but that could be a good starting to point for explaining the salinity level.

 

As for why there is salt in sea water in the first place... there's a lot of salt around, and plenty of water for it to dissolve into.