Can you throw seawater over your shoulder if you spill the salt and want to ward off evil water spirits? Well, on average, seawater is about 3.5% salinity, so if you're feeling superstitious, then go for it! It's hard to imagine the ocean without salt, but have you ever wondered why the sea is salty?
All of the sources that feed into the oceans worldwide are considered freshwater, so it can feel like a mystery without knowing more. While the water in streams, rivers, and glacial melt is freshwater, often it contains sediment and minerals that have eroded from the earth. The concentration of these minerals is too low to affect the freshwater, but once it feeds out into the ocean, the sediment it carries along, both dissolved and solid, helps contribute to the ocean's salinity.
This process has been taking place for hundreds of millions of years. A slow transfer of minerals and ions from the earth's crust, mainly because of precipitation, has increased the amount of salt in the ocean. If you sampled the water from a stream, it would contain trace amounts of sediment, ions, and minerals, so little in fact, that it would seem negligible when added to the unimaginably vast ocean. However, this is happening at a constant rate and has been happening for millions of years which is why the ocean has a salinity level of 3.5%, and it will continue to creep up ever so slightly over time. Thankfully, this process is too slow for us to measure in real-time, so it isn't anything to fret about.
You might be wondering why the oceans are getting saltier if freshwater is the only thing running into them. Wouldn't that dilute the oceans and make the seas less salty? Great question, and it's one that I had as well. It turns out that the natural precipitation cycle, mostly evaporation, is what keeps the salinity level of the ocean almost constant. Water enters and leaves our ocean at a balanced rate, being deposited by rivers, rain and glacial melt while evaporating or freezing at a more or less constant rate. This keeps the water level relatively similar but doesn't allow the ions and minerals in the salty saltwater to be removed. When water is evaporated, it's just the water leaving the ocean, not the salt.
While it's hard to detect, the fresh water that enters the ocean through estuaries and runoff carries trace amounts of ions and minerals in it. Not enough to make it saltwater, considering it's potable and supports freshwater animals, but enough that over the course of hundreds of millions of years, the salinity of the ocean has risen to what it is today. So for the next time you visit the beach and get a mouth full of saltwater, just know that what you're tasting is actually the earth and not the ocean. Well, it technically is the ocean, but what you're tasting was transferred from the earth's crust to the sea.