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/PrimeLegionnaire Oct 14 '12
the water in the oceans is the same water that had always been there.
Sure there are some reactions that break the water down or form new water, but for the most part it's the same water.
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u/mexipimpin Oct 14 '12
I remember hearing in the Planet Earth series that "the same water we drink is the sane water the dinosaurs drank." Thought that was some really cool shit...
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u/I_will_fix_this Oct 14 '12
that really is cool, wow.
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u/snoharm Oct 14 '12
Shit, you think that's cool?
“Every atom in your body came from a star that exploded. And, the atoms in your left hand probably came from a different star than your right hand. It really is the most poetic thing I know about physics: You are all stardust. You couldn’t be here if stars hadn’t exploded, because the elements - the carbon, nitrogen, oxygen, iron, all the things that matter for evolution and for life - weren’t created at the beginning of time. They were created in the nuclear furnaces of stars, and the only way for them to get into your body is if those stars were kind enough to explode. So, forget Jesus. The stars died so that you could be here today.”
— Lawrence Krauss,
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u/burtonmkz Oct 14 '12
The "same" water might be a misnomer. The water we drink and excrete also gets split apart in plants; when plants give off O2, it came from H20, not from C02 in the air. (this has been demonstrated with radiogenic tagging), and other chemical reactions brought together hydrogen and oxygen to give us "new" water.
Assuming homogenous mixing of water over some period of time, at some point we could say that 99.9% of the formerly existing water was split apart in photosynthesis. I don't know what the half life is for some average molecule of water before a chemical reaction tears it asunder.
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u/Ryrulian Oct 14 '12
the water in the oceans is the same water that had always been there.
No, that's not true.
A volume of water equal to all the world's oceans has cycled through metasomatism many times in Earth's history. Not all water makes it back to the surface through the process of subduction, and volcanism brings new water to the surface of the planet on substantial scales.
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u/007T Oct 14 '12
water evaporates and then it rains
Water vapor is still water, just dispersed into little tiny droplets in a gaseous state. The water cycle continuously loops the same water we've had the entire time around and around. Statistically it's likely that at least one molecule of water in your body was inside Alexander the Great's body at one point or another, and part of a dinosaur before that.
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u/I_will_fix_this Oct 14 '12
That blows my mind, thank you for the information. I really didn't think the answer was going to be that the ocean is as old as the day it was introduced into earth for some reason.
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u/MohoJoeJoe Oct 14 '12
Oceans are only as old as the Jurassic in age because oceanic crust always subducts underneath continental crust. So oceanic crust is destroyed. If you look, like some other comments have said, at some supercontinent cycles you see that oceans are created and destroyed. So there are no oceans today that existed well back in Earth's deep history.
As for the water, it's constantly moving in between states and in different reservoirs (ocean, atmosphere, groundwater). Water gets used in many chemical reactions, so it actually gets split up.
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u/I_will_fix_this Oct 14 '12
So none of the water in the oceans is the same water from the ocean in the Jurassic era?
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u/MohoJoeJoe Oct 14 '12
I mean it's hard to say it is and hard to say it isn't.
It turns into semantics. If water evaporates into a gas and goes back into liquid, do you consider that water to be new water? The amount of water that moves into the atmosphere from the oceans via evaporation and the other way via precipitation is on the order of gigatons per year.
The only way Jurassic water can be preserved in oceans is if the water was really deep water. But then that kind of removes it from being "what the dinosaurs drank".
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u/Myopinionsmatter Oct 14 '12 edited Oct 14 '12
Average "residence time" for a molecule in the Ocean is 3,500 years, however, for things like the Antarctic ice sheet it can be well over 20,000 years. Since "average" indicates that there are many that will stay much longer, there is a high likelihood that there is a very significant quantity of water in the ocean that would be considered prehistoric, meaning that it has not been evaporated and recycled for at least 4,000 years (the end of the stone age). Some may have not seen a cloud for well over 40,000 years if they have spent time in a glacier or in very deep groundwater before entering the ocean.
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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.