According to the NOAA, " Ocean salt primarily comes from rocks on land".
Rocks on land are the major source of salts dissolved in seawater. Rainwater that falls on land is slightly acidic, so it erodes rocks. This releases ions that are carried away to streams and rivers that eventually feed into the ocean.
However, the rivers and streams themselves are not particularly salty. This suggests that the ocean is salty because the tiny amounts of salt brought in by rivers and streams accumulated in the ocean over time, while in the rivers and streams themselves the water is constantly being replenished and the salt levels don’t accumulate. (This raises the question as to freshwater lakes and ponds.)
But if that’s the case, then one might think this would be an ongoing process, and that the salt levels in the ocean would be constantly increasing, albeit at an extremely slow pace. Is this correct? (Similar might apply to all sorts of mineral and chemical levels, for similar reasons.)
There is a point past which it cannot get any saltier, at which point salt will be deposited at the bottom. The world ocean is not at that point currently. Salt is sometimes removed from the world ocean when portions of it become isolated and dry up, dropping all of their salt which is sometimes then covered by other, undissolvable, sediments. The net effect is that some of the water eventually goes back to the ocean and the salt stays on land or under ground.
However, I don’t know if the world ocean would be at the saturation point were it not for these instances of salt deposition in places such as the Mediterranean and Gulf of Mexico.
But absent this, I assume that the ocean salt level is constantly increasing. I’m not aware of major salt deposition events currently happening.
But over long periods of time they deposit large amounts of salt into the oceans. And humans cannot be helping the situation with all of the salt that must be in our waste water, but compared to the ocean it’s most likely still a small add over a short period of time. We are talking millions of years here, it adds up eventually.
But all of the glacier melt is essentially fresh water, which dilutes the sea water. Is the average ocean salinity rising or falling? There are certainly going to be local effects, you can’t expect the Mediterranean or the Gulf of Mexico to be diluted right away by Antarctic ice.
That’s a good point which I hadn’t considered. A quick google implies that average river salinity is around
0.00012 versus .035 for the ocean which would mean that as long as around 1/300th of the water coming in is not evaporated but instead adds to ocean levels that it would dilute the ocean instead of saltify it. Since most of the current ocean level rise is due to expansion of the water, and there are several feet of runoff per year, that’s a difficult calculation since the amount of sea level rise due to melting water is still around a millimeter a year.
I recall some discussion of this topic once, in terms of “how lucky are humans/life to exist on earth?” The world is constantly over geological time creating new salt sinks, where oceans become trapped and their salt subsumed into the crust of the earth. This happened with the ocean that ran up the center of North America millions of years ago, which is why areas of the mid-west from Louisiana to Saskatchewan have buried salt deposits a few hundred feet down. If this didn’t happen on cue, over millions of years the oceans would become too salty for life, plankton would die off, and the oxygen cycle would be greatly impaired causing us all to go extinct. The author in the discussion suggested that the Great Barrier Reef, the Gulf of Mexico, and other geological formations were Mother Nature preparing the next great salt sinks. So short answer to your question - over eons, the salt level of the oceans fluctuates; but theoretically, some will eventually be buried by natural processes.
But in general, yes - there are areas of Saskatchewan (and other areas of mid-west N. America) where excessive irrigation has cause salt to leach up to the surface, to run off with the spring rains. The farmers of Egypt, for example, found that after the Aswan Dam was built and stopped regular flooding, that irrigating the soil was creating a salt buildup as evaporation left behind the small amounts of dissolved salt in the fresh water. It’s very small amounts but it accumulated. The solution was to overwater and let the runoff carry the salt away to the sea. (Which is why most places don’t have this problem - other places get enough rain from time to time that the salt washes away.)
the author’s point was that if all the salt available (Salt Lake, Dead Sea, midwest salt deposits, etc.) was dissolved in the oceans, life would cease.
Hasn’t almost all fresh water previously been in the ocean and then evaporated out and redeposited on land? The salinity of the ocean should reflect eons of salt being leeched from the land by fresh water that returns to the ocean and then cycles back to the land as fresh water from precipitation.
Geology has taken care of excess salt in the past. That doesn’t mean it will in the future. There is no “one way or the other” geology will take care of “excess salt”. It’s only “excess” to us. Geology doesn’t care one way or the other.
“Mother Nature is preparing the next salt sink” phrasing in your previous post could be a harmless flourish, or it could be pure nonsense. We can look at the past and calculate some rough probabilities for future salt deposits, but the next few hundred million years could be unlucky on this front instead of the luck in the past and the oceans could get inhospitable to life.
It does. As well as salt deposited back into the crust in various ways. (Trapped seas, ocean floor subduction, etc.) But I think Ludovic was referring to the direction of the arrow (saltier, less salty) right at this moment.
Is there a threshold of salt concentration where salt water is inhospitable to life as we know it even on a theoretical basis, or just the current life that exists in the oceans? eg. should some organisms theoretically be able to live in water that is fully saturated in salt? I am wondering if life would be able to adapt to slowly increasing salt concentrations over time, or if there is some sort of cutoff where current known biological mechanisms for dealing with salt would fail? It seems like microbes at least should be able to survive, at least in areas where fresh water are being introduced similar to what occurs in the Dead Sea.
Freshwater lakes and ponds all have outflow rivers or streams. A lake without any outflows ends up even saltier than the ocean, like the Dead Sea or the Great Salt Lake.
Because the ocean is essentially water + salt [+ fish + microplastics + boats] it becomes markedly more saline during the past glacial maxima that took more or less every 100,000 years during the Pleistocene, as the water content was drawn into glaciers and surface ice.
There is a Wikipedia article on Palaeosalinity, but it is pretty poorly structured and doesn’t address the OP question.
But there is a level of “salty” which effectively excludes life as we know it. Dead sea… is there anything in the Great Salt Lake? The article I was reading about this mentioned that despite the total amount of salt on earth being far greater than is necessary to make the oceans dead, this has never happened - for various reasons, salt has been sequestered geologically roughly as fast (in geological time) as it enters the oceans.
Brine shrimp (a.k.a. sea monkeys) live in the Great Salt Lake. There are fish that live in rivers that flow into the lake and some get washed into it during flooding events. They’ll survive there until the salinity of the lake goes back up.