# What is the origin of the ocean's salt?

We know we cannot live without salt, but an investigation into the origins of ocean's salt leads one to believe that this issue is not fully resolved yet.

What geological phenomenon caused the earth to contain so much of salt? I see questions on SE that ask about the origin of earth's water. In a similar vein did an extraterrestrial body "dump" salt on earth or is salt native to Earth?

It appears from these links that Jupiter's Moon Europa does contain a salty ocean - http://www.space.com/20078-jupiter-moon-europa-ocean-surface.html and http://www.planetary.brown.edu/pdfs/2440.pdf.

From a geochronology perspective what causes a planet to "develop" salt concentrations ?

• You may have more luck splitting this question up — maybe just asking the first one for now. It seems like a lot to take on. You might also mention what have you read to lead you to these questions, because there's lots out there. – kwinkunks Dec 5 '14 at 14:25
• @kwinhunks - apologies but I am not completely at fault. I live next to the sea and so I think about the ocean all the time :) – gansub Dec 5 '14 at 14:33
• You don't need to apologize, and I don't think you're 'at fault'. I wondered what kind of investigations you'd done, because questions like "I read X (specific quote, citation), can you help me understand Y?" usually elicit good answers. – kwinkunks Dec 5 '14 at 14:39
• gansub — Personally I think it's a more compact question, especially if you have read anything supporting (or not) the space salt theory. – kwinkunks Dec 5 '14 at 14:45

As one could imagine, there is a considerable amount of conjecture as to precisely Earth got its salt content. Ultimately, it came from the primordial planetary nebula and concentrations of the ions involved would have come from the various ingredients of the accretionary process. This answer focuses on how salt may have first formed, hence accumulated.

Ocean salinity is almost a chicken and egg situation - was their salt before the first waters or vice versa? According to current research, the latter appears to be true, especially in the case of the dominant $\ce{NaCl}$ species.

Knauth (2005) (2) contends that $\ce{Cl-}$ ions were outgassed through volcanic eruptions in the form of $\ce{HCl}$ and $\ce{Na+}$ were theorised to have been leached out of rocks (1). The exact amount of these ions and salt content are not entirely known (3), but the process may have started even as far back as the Hadean, and certainly in the Archean (3) and the early water bodies were believed to have been up to twice as saline as today (2).

In terms of halite deposits, this requires evaporation of a brine, which according to Knauth (2005) (2) and Sleep (2010) (3) required large enough continental masses to be able to enclose water bodies - the amount of this occurring in conjectural for Earth, as the halite produced would be unlikely to be preserved (3).

Knauth (2005) (2) expanded on the implications of this for other worlds with:

Inasmuch as chlorine is a common element throughout the galaxy and follows the water during atmospheric outgassing, it is likely that early oceans on other worlds are also probably so saline that evolution beyond the microbial stage is inhibited unless long-lived continental cratons develop.

Partly suggesting that the formation of salt is possible on any world - such as the salt found in Martian sediments, as per the article Magnesium sulphate salts and the history of water on Mars (Vaniman et al. 2004).

References

(1) Einsele, G. 1992 Sedimentary Basins: Evolution, Facies, and Sediment Budget (Book)

(2) Knauth, L.P. 2005 Temperature and salinity history of the Precambrian ocean: implications for the course of microbial evolution Palaeogeography, Palaeoclimatology, Palaeoecology

(3) Sleep, N. 2010, The Hadean-Archaean Environment Cold Spring Harbor Perspectives in Biology

• great answer. Couple of points remain. Am I right that emergence of the dominant species NaCl on earth is not particularly clear yet ? Are we the only planet with NaCl as the dominant species? – gansub Dec 7 '14 at 0:54
• From what I have read, neither are clear at this stage. – user889 Dec 7 '14 at 1:40
• @gansub technically, we are the only planet with liquid oceans, so yes, we the only planet with NaCl as the dominant species. – Gimelist Dec 7 '14 at 7:26

I'd like to add a bit to the existing answer and maybe break it down to several simple things:

1. Chloride likes water. The chloride ion, Cl- really likes being in water. It's one of the most incompatible elements in mantle melts, and it readily partitions into fluids exsolved from magmas. Basically, it's enriched in silicic rocks (that are forming most of the crust) and then it's just a small step to remove the chloride from the magmas to the fluids, which then end up in the ocean or in the atmosphere (and then the ocean because it rains). Once it's in the water, it's very hard to remove it from there.
2. Chlorides are very soluble. Sodium chloride, magnesium chloride, calcium chloride, copper chloride, and a many other chlorides are very soluble in water. Therefore, if you take any acidic water with some amount of chloride in it, and react it with rocks, it's going to leach stuff away. The fluid will become rich in all of that stuff. Particularly sodium, magnesium, calcium and potassium because these are the most common chloride-soluble elements in the crust. I would add that potassium is usually held more tightly to solid phases such as micas, so it's somewhat less mobile than sodium when bound to silicates.
3. Sodium chloride has no major solid sink. Calcium chloride is more soluble than sodium chloride. Based on this simplistic approach, you would expect to have more calcium to be more abundant in the ocean than sodium. Thing is, it forms calcium carbonate. Whether chemically precipitated or biogenic, a lot of calcium is removed as calcite and aragonite from the oceans. Magnesium goes that way too. Sodium does not precipitate. Therefore, it stays in the ocean.