Why is seawater salty, when water from the lake isn’t?


Why is seawater salty, when water from the lake isn’t?

In: Earth Science

Well funny enough sea water contains a whole lotta salt. Fresh water Lake water does not. Lake water (as in the Great Lakes) come off of melted icebergs that used to be on land. While other fresh water lakes come from melted snow off mountains. And other come from man made creations. Then there’s the Dead Sea which is sooooo salty nothing can live in it. I guess a sea but still an isolated body of water with high concentration of salt.
The sea just has loads of dissolved salt in it from salt deposits it wears away at

Rivers erode the land they run over, picking up minerals (such as salt) as they go. Most lakes aren’t the ultimate stopping point for this water, just a layover. The minerals in the water get carried on as the lakes drain into further rivers, and ultimately, to the ocean. There are a few exceptions: the Dead Sea, in Israel/Jordan, and the Great Salt Lake, in Utah, come to mind. In these oceans and the couple of lakes with no outlet, these minerals build up over millions of years, consentrating as the water evaporates but the solute stays

Water generally flows from higher elevations to lower elevations. In most places, the ocean is the lowest elevation. As water moves toward the ocean from snow melt to the water table to rivers, salts and minerals dissolve in it. Over a long period of time, these salts and minerals reach the ocean. When ocean water evaporates into clouds to continue the water cycle, the salts do not (edit) ~~dissolve~~ evaporate and remain in the ocean. Over billions of years of this process occurring, the planet’s ocean basin accumulated the salt it has today.

Starting with definitions, a lake is completely surrounded by land. It fills with water from rain, melting glaciers, and/or from the water table. A sea is a body of water that is largely surrounded by land, but is connected to the ocean. Think of the Mediterranean Sea or the South China Sea.

Now, it’s possible for a lake to be salty. The only one off the top of my head is the Dead Sea, which is actually a lake. What happens there is that are rivers flowing into the dead sea. Along the way, they dissolve salt and minerals from the surrounding area. The water evaporates from the sea, leaving behind the minerals and that’s how you get a salty lake.

For comparison, most lakes have an outlet. A river or stream where the water keeps moving, so those same minerals don’t build up. The dead sea doesn’t have an outlet.

As others have said, lakes usually have an output so that there is water (with a small amount of dissolved salts) constantly running through them. The exception would be [terminal lakes](https://en.wikipedia.org/wiki/Endorheic_basin) which do not run out to the sea and so do become salty to various different extents over time.

As for seawater, it’s not quite as simple as everyone else has made out, but not far off. It’s important to remember that the ocean is not the final resting place for salts, it’s just a way-station before they are passed on to the oceanic crust or the sediments which coat the crust. The length of time before this happens depends upon the individual ion in question — each one has its own cycle just like there is a water cycle and a carbon cycle.

This also explains why the balance of salts in seawater (which is the same pretty much everywhere in the oceans) is not simply a concentrated version of the river water which is supplied to the oceans. For example, although the dissolved ion content of rivers varies greatly (due to the particular bunch of rocks the rivers are running over/through before they get to the sea), chloride ions Cl^- are a fairly small part of the dissolved salts in all river inputs to the sea. Despite this, chloride is a major ion of seawater — along with sodium it makes up the vast majority of salt in the ocean. This is because chloride ions are removed *extremely slowly* from the oceans; far quicker than sodium ions Na^+ which are constantly being supplied *and removed* at a fairly high rate.

So seawater is salty not only because it has dissolved salts going into it (mainly from rivers), but also because some of those salts tend to hang around for a long time before getting removed. The level of any salt is a balance between the inputs and outputs of that particular salt. The oceans have been in a steady state balance for most salts for many millions of years, but this has not always been the case. It’s also not just been a case of building up all the salts over time, there’s a complex interplay between the factors which increase erosion and deliver salts to the sea (tectonics, climate, terrestrial volcanic activity) and the factors which remove salts (marine sedimentation, biological uptake, deep-sea hydrothermal activity, ocean circulation).

The history of ocean salinity has not been linear. The chemistry of the earliest oceans are extremely difficult to get a handle on, but we think that the oceans were significantly saltier than today until continents started to form, which helped to remove a lot of salts from the oceans (modern day salt flats and salt deposits within the crust — that all used to be part of the stuff circulating through the oceans).