Is it possible to have a Terran-like world where the seas and oceans do not have high levels of salt?
Could, in fact, oceans and seas exist that were fresh-water?
Excuse my ignorance.
Assumptions:
If salt were rarer than on Earth, and if there were some mechanism to remove dissolved salt, then it would be possible. The Great Lakes are actually freshwater inland seas, so you could use that as a model. If salt were rare, there would be a food-chain to recover it since salt is needed for life. Salt-consuming microbes would consume the dissolved salt, and then be eaten by larger animals. Upon death, salt would be released back into the cycle as a part of decomposition.
If you answered Sodium Chloride, then simply have a world with very little Sodium and Chlorine. If you answered 'an ionic compound formed from an acid and a base', then you either have to get rid of all of the acids and bases or find a mechanism to sequester them, and that'd be difficult on a rocky (terran) world.
would be the most likely geological mechanism for sequestration, and the numbers couldn't really work out unless you have giant, below-sea-level inland basins with occasional catastrophic floods followed by long dry periods in which the sea water evaporates leaving a halite crust in the basin. Saltwater is about 5% salt by volume, whereas fresh water is less than .05%. I believe that the number of cycles to change the salinity from the original to final values given the volume of the ocean and the volume of total water is
$$c = \frac{\log_{10} \left(\frac{s_0}{s_f}\right)}{\log_{10}\left(\frac{V_t}{V_o}\right)} = \frac{2}{\log_{10}\left(\frac{V_t}{V_o}\right)}$$
If the inland basins were one tenth the volume of the oceans, it'd take almost 50 cycles to make the oceans fresh. For this method to work, there could be no way for basin water to escape to the ocean which would be a very strange topology. There is a reason that all of the salt lakes on Earth aren't that big. Additionally, the entire basins would be very salty unless you had rain pulling the salt to the bottom of the basin.
A life based mechanism might be more likely, but that would only apply to the most recent eighth of Earth's history. You'd need a group of halophiles moving salt from the ocean to sequestration somewhere. Perhaps if humans stop putting waste salt into the environment but keep pulling salt from the ocean, we'd eventually qualify.
We evolved from ocean creatures. Our blood's pH is regulated like the sea, our wounds heal faster in the ocean, and the same is true for most animals. While Earth creatures need salt to live, it's very likely that on your world, creatures would have different mechanisms to deal with fluid regulation and neural communication.
Some exoplanet are though to have oceans hundreds to thousands of kilometers deep without any land. It is conceivable that all the dissolved salt will sink to the bottom of these oceans due to the density of salt water vs fresh water. Also when there is this much water, there is a good chance the salinity is so low, it would be undetectable even if the salt does not sink.
It's possible, but would have to be through a contrived water cycle.
The IRL oceans are salty primarily due to the way silt and other non- water contaminants get moved during the natural water cycle (see NOAA page here).
The possible freshwater ocean setup I can imagine would require a system with almost perpetual rainfall into the ocean basins, with water flowing AWAY from the oceans, and possibly into an area where evaporation is highly facilitated. Such a world would have very turbulent airways, persistent rain, and essentially no sunlight. Plants would be primarily aquatic, and would generally draw energy from geothermal sources above most others (as photosynthesis would likely not exist in any noticeable quantity).
Here is my scheme to freshen the water of your world. Sequester the salt in geological formations. This actually happened / happens on Earth and so you can keep everything as it is now.
Ice age. Frozen water is fresh. As water freezes and accumulates as ice, the residual water gets saltier.
Evaporation of residual water and sequestration as sedimentary beds. Salt mines are located within thick beds of salt, presumably deposited by ancient events of this sort. It is a surprise to me that when the water comes back, the salt deposited does not just dissolve. Here is a working salt mine supplying Cleveland. The mine is directly underneath Lake Erie, which is freshwater.
http://fox8.com/2013/01/31/fox-8-explores-salt-mine-below-lake-erie/
Here is the Jacuzzi of Death: a site below the Gulf of Mexico where underground salt deposits are dissolving up into the water above. Presumably this is limited by a lack of water movement and mass action: once the water reaches saturation less salt dissolves from below. I do not understand why saturated salt solutions do not just pull in fresh water osmotically but that process must be slow.
http://www.latimes.com/science/sciencenow/la-sci-sn-jacuzzi-death-brine-20161102-story.html
These underwater lakes formed over millions of years as a much shallower Gulf of Mexico evaporated and left behind massive beds of salt. Over time, the salt layers became submerged and buried. Under the weight of these sediments, the salt layers shift and crack the shale above, allowing oil, gas and brine to escape.
The result is a super-salty brine so dense it doesn’t easily mix with the seawater around it
So: your world is frozen long enough for any dissolved salt to be deposited in sedimentary formations. The good thing about writing this is it is basically a riff on the Ice Age, which is familiar ground. Or your world could get super hot with all water present in the atmosphere as vapor - that would be trickier to write but also more novel - as once all water was vapor you might get a Venusian-type runaway greenhouse effect.
When the world melts (or cools?), fresh rain and runoff from melting ice will fill the bodies of water. Over time, these bodies will presumably get salty as the oceans have from residual salt on the land. But the Caspian Sea is 5 million years old, and still only a third as salty as the ocean.
ADDENDUM/ I have been musing over this concept. After you lock up much of the water as ice in the north you would probably still have a very salty equatorial sea, or a sea at a crustal low point. Relieved of the over lying water pressure, a volcanic event could happen in this sea, boiling off the water and covering what was left with basaltic flows. A Deccan flow type event could serve double duty and also trigger the end of the ice age.
Yes, it's quite possible, if salt were not so prevalent on the planet.
Presumably, life on that planet would have evolved into a form that wasn't so dependent on salt, just as different fish have evolved to either flourish in salty water, or in fresh water.
When considering an alternate world of your own making, remember that it doesn't have to be identical to earth in climate or chemical makeup. Within reasonable limits, life would adapt to that planet's conditions.
I'd imagine two or more continents connecting to trap a section of ocean between them, and the through the sodium cycle most of the salt would be sucked into the mantle. If the water level of the trapped ocean were higher than normal ocean level, there would be no in flowing currents to add salt, so the only water source would be salt less rainwater. The problem is the converging continents would meet long before plate tectonics removed a noticeable amount of salt, so you would have to come up with some factor to hold the continents in place.
