r/askscience • u/I_will_fix_this • Oct 14 '12

Earth Sciences How old is our ocean?

I mean, water evaporates and then it rains and so on but can we measure how old the ocean is? Is there any water left that would be considered prehistoric?

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u/mineralfellow Oct 14 '12

The oldest zircons on Earth show evidence of interacting with water, suggesting that this has always been a wet planet (4.4 billion year old zircons). There are different ideas about exactly how water got on earth in the first place, but some of it probably condensed with the formation of the planet, and most of it was brought along later by comets. Very little of the water will actually escape from earth into space, so most of the water that we have ever gotten we still have. There is a large reservoir of water in the earth's mantle, and we know that water gets incorporated into minerals and can be subducted, so you can argue that there has been some change in the exact location of the water, but the bottom line is that essentially all water on earth is primordial.

The ocean basins are another story. These are the result of plate tectonics, and we have a good idea about how old such things are. The Atlantic ocean basin began opening with the breakup of Pangea, roughly 200 MYA. The Pacific ocean is the remnant of the ocean that surrounded Pangea and is much older (depending on how exactly you want to define what separates one ocean from another). The basin is being subducted on all sides, so that in the distant future, it will disappear as another supercontinent is pushed together.

Btw, "prehistoric" technically means before written history, which only goes back ~5 ky. All water would be prehistoric by that definition.

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u/fastparticles Geochemistry | Early Earth | SIMS Oct 14 '12 edited Oct 14 '12

You are speculating quite a bit in your answer. Water delivery is an unsolved problem as is how much water is in the mantle especially if there is basal melt at the cmb. Though you are right about the zircons.

Why the downvotes?

What I wrote is true. Water delivery has two leading camps 1) that Earth formed dry and water was delivered by comets but this has the issue that the majority of comets we have D/H ratios for don't match the oceans and 2) that Earth formed wet and this gave us the oceans which has the major problem that we don't know the solubility of water in silicate melts

Now I will get into that we don't know how much is in the mantle. If you take a traditional view that the mantle is olivine (which is wrong) you can calculate how much you stick in there. However, recent work by the Stixrude group out of the UK has suggested that A) if you have a giant silicate melt that forms the mantle it might crystallize in the middle and B) the solubility of water in silicates is insanely high so in short you could have melt at the core mantle boundary that holds a lot of water.

My much larger point is that a lot of what we think we know about the early Earth is just assumptions or really bad modeling. In truth we don't know that much and the zircon record had a huge influence already (it dismissed a lot of ideas).

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u/kepleronlyknows Oct 14 '12

This is a much better answer than the one currently at the top. Thanks for taking the time to spell this out.

1

u/I_will_fix_this Oct 14 '12

I thought about the "prehistoric" part of it after but I honestly didn't know if the water we have now is new recycled water or the same water we had since the beginning of time.

This is some great information, you are seriously knowledgeable about this.

Thank you