Thread: Questions about our Oceans and their origin

I was studying the composition of igneous rock types and it struck me that chlorine was not a constituent. Igneous rocks do not contain enough chlorine to explain our salty oceans. Therefore, weathering of those rocks does not explain the the chlorine content of those oceans. Time went by and I came across this statement by Dr Mark McMenamin in his book, co-authored with his wife Dianna, Hypersea, on page 24: ---Quote--- ... for the last 700 million years there has been little change in the proportions of the major ions of marine water. Good evidence now exists that the chemical composition of the oceans has stayed pretty much constant for a long time.... The use of oxygen isotope variations to determine ancient seawater temperature, a proven technique, assumes constant marine salinity for hundreds of millions of years. ---End Quote--- This brought the question of the salt content of our oceans to my thinking once again. Now, normally McMenamin cites his books very thoroughly, but not this particular passage in this regard. Searching then revealed this paper on the Lunar and Planetary Institute web page by Y. Miura: IMPACT ORIGIN OF CHLORINE-BEARING MATERIALS OF SALTY SEA-WATER OF EARLY EARTH, COMPARED WITH THOSE ON MARS AND THE MOON (http://www.lpi.usra.edu/meetings/bom...8/pdf/3001.pdf) This is a short paper and on page two is this chart: ---Quote--- 1) Chlorine (Cl) concentration: Fusion crusts of Meteorites (in Air of Earth; craters on Mars, & meteorite fragments on the Moon) 2) Distribution to the surface: Huge impacts to collect Cl over surface (in Air of Earth; craters on Mars) 3) Rain-fall to form salty ocean water: Rain-fall by cooling from hot vapor, & Melting to salty ocean water (mainly on the Earth) Fig.4. Process to form salty sea-water from meteoric origin of chlorine element on Earth, Mars and the Moon. ---End Quote--- OK, but I still have questions. Are Na and K in equilibrium in the ocean today; in the past or deep past? I assume that chlorine as a gas or free ion in the atmosphere is too heavy to escape and the amount of chlorine on this planet is the total accumulation of all extraterrestrial impacting bodies since Earth formation, is that correct? Is it truly tenable that the mineral content of the ocean has remained reasonably constant for 700 million years as McMenamin suggests? What is the chemical path for chlorine from supernova to impact with our atmosphere. There is evidence that rock weathering rates increased with the colonization of the land by plants during the Devonian; was there a ready supply of chlorine to react with the additional Na and K or did the extraterrestrial sources of chlorine eventually catch up to the weathering rate, or was it always in step. I have oversimplified, I am sure, for the sake of brevity, but, please....

I would think that because material from our solar system built our planet the molecules that fall into our atmosphere would build up at a static rate. For every drop of mercury, for instance, would drop at the same percentage of accumulation as lithium. What might cause variations in ocean salinity is plate techtonics. In the 700 million years timeframe the ocean floor would move several hundred miles. Some heavier elements would subduct, but this wouldn't change things very much. It would just be an interesting variable to the neverending equation.

I don't think igneous rocks are the only types present on earth. Would sedimentary rocks explain the salinity ?

The problem with that is that sedimentary rocks are usually, by definition, made of the remains of other rocks, usually igneous. I could see ones of semi-organic origin (like limestone) being different, but they still don't change the basic premise of there being a finite amount available in the environment. I mean, any salt returning to the sea from a sedimentary rock was still in the sea to begin with.

Not to mention that it's unlikely that extraterrestrial sources provided a very large contribution, at least not without going so far back in history as to blur the notion of what "extraterrestrial material" is even supposed to mean. There's also the valid point mentioned above, that most of the building blocks of the planet would have been more or less homogenous.

I'd think though that the low chlorine content of igneous rocks isn't that big a problem, because the constant slow seepage into the sea could over time build up to quite a large concentration.

I'd imagine that is what happened, and it hasn't reached a point of hypersalinity because the limiting factor of the available metals for it to bind with to make the salt provides a point where the whole system would hit an equilibrium, which (so the reasoning goes) we'd have hit a long time ago.

Of course, this is just wild speculation in response to a fascinating question. Maybe the halidophile bacteria represent a time when the chemistry of the planet was such that the equilibrium point was much higher. Maybe not.

Perhaps this surprise indicate a fault in what we know about Earth's long ago history. More likely this is a paper which grasps at straws to try to find importance. Another long ago paper assumed the ocean salt NaCl percentage about two billion years ago was the same as the percent salt in human blood, which tends to be about the same as the salt percentage in the fluids of most living things. If so the percent of ocean salt has increased considerably in the past two billion years, if not the past 0.7 billion years.
If the average NaCl content of basalt has decreased from 3 parts per million to 2 parts per million in the past 2 billion years: That, perhaps, means the oceans got a billion tons of salt from each billion times a million = 10E15 tons of basalt. Additional chlorine likely came from asteroid hits and other sources we have not considered. I like multiple reasons. There is a rather large deposit of calcium chloride = gypsum = plaster of Paris, on the surface at White Sands National Monument, New Mexico. Theories on how it got there may be helpful in understanding this dilemma. Neil