Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: Huldrich on 06/03/2015 20:22:16
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Hello, I am reading the book of Kent C. Condie, Earth as an Evolving Planetary System. I have a master in physics, not in geology, so some stuff is difficult to understand. In addition, my native language is not English. There is something in the book that I really need to understand, but I can't really get the real meaning out of it. Maybe someone can help me. Here is the text:
"Although the Sr isotope record of seawater for the Phanerozoic is relatively
well known (Veizer et al., 1999), except for the Neoproterozoic, the record for
Precambrian seawater is still incomplete. Although several orders of variation
in the Sr isotopic composition of ancient carbonates is recognized, only the first order
changes that occur over times of 500 Ma or more can be resolved in the
Precambrian record. Veizer and Compston (1976) were the first to suggest that
the growth rate of continental crust can be tracked with Sr isotopes using the
87Sr/86Sr ratio in marine carbonates. Prior to 2.5 Ga, 87Sr/86Sr ratios fall near
the mantle growth curve (0.702), suggesting that the oceans were largely buffered
by mantle input, and that little continental crust existed, at least above sea
level."
What I do not understand is the sentence: "the oceans were largely buffered by mantle input". What does this mean within this context? Especially "buffered by" does not give any sense to me. I googled for this expression and did not find a sense that could be applied to this. Does it mean something like "protected against"?
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the oceans were largely buffered by mantle input
There is a concept in chemistry of a "buffer solution", which resists changes in pH (ie H+ concentration) despite processes being applied which would normally change the pH.
If the article is using "buffer" in a similar sense, they could be saying that the Strontium isotope ratios in the ocean were the same as the ratio in the Earth's mantle. Isotope ratios in the sea could not change, because Sr was continually being washed out of the Earth's rocks, keeping the isotope ratios in the sea constant. This would apply despite the deposition of marine calcium carbonate rocks (strontium is immediately below calcium in the periodic table, and could be extracted from seawater by marine organisms, and deposited with the calcium carbonate rocks).
Speculation: 2.5 billion years ago, the Moon was much closer, and tides were much higher than today. This could have continually eroded the mantle, mixing it with seawater, and keeping the ocean concentration of soluble ions similar to the concentrations in the Earth's mantle. It would also have caused rapid erosion of any continental rocks.
I saw some estimates that in the past, tides could have been as high as 1km, although I couldn't clearly identify how long ago this is thought to have happened. (It sounds a bit like the movie Interstellar - a planet like a billiard ball, awash with kilometer-high tides...)
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Hi Evan, thanks for your answer. You may be right that "buffered" has a similar meaning than in chemistry: the ratios were kept stable by some not further precised process. As for the Moon, it was closer to the Earth in the past, which may have led to tides as high as 1km. This also heated the mantle because of enormous frictions in it. However, the mantle is below the crust and not directly in contact with seawater, except in subduction zones where crust, and with it seawater, is pushed into the mantle. So the mantle cannot be eroded by tides. It is thought that the strontium input into the sea came from washing out the continent. So when the ratio was high, continental surface was high too. This can be correlated to the formation of supercontinents.
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I think what the author was suggesting is that the earth was largely oceanic with little continental material. Mantle material would have been available at spreading centers and covering much of the ocean floor (i.e. the ocean floor is mafic igneous material that is derived from the mantle- with a thin sediment cover). Seawater would be in constant contact with mantle derived mafic rock, which would have buffered the Sr isotopic content.
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Seawater would be in constant contact with mantle derived mafic rock, which would have buffered the Sr isotopic content.
So you mean that the 87Sr/86Sr ratio was the same than that of the mantle because the mafic rocks derived from the mantle were the only source of strontium input into the ocean?
Apparently, I have a problem with the word "buffer". I understand it as a system of storage, for instance in IT, that can equalize high variations of input; or as a mechanical system that can absorb collisions, for instance shock dampers in cars. So what is the buffer in the geologic context, the ocean or the mantle?
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My guess is that buffering means the mantle was the sole or major source of Sr. If the mantle contains many, many times as much Sr as the sea, and was able to exchange Sr with the sea at a fast enough rate, then it will swamp out any other more subtle effects on the isotopic ratio.
These numbers are made up to illustrate: Imagine the sea contains roughly 100 tons of dissolved Sr and the mantle has 1000000 tons of Sr; if 50 tons of Sr can be exchanged between the sea and mantle every year, then the isotopic ratio in the sea should be held at the same ratio as the mantle. Even if there were some event that somehow magically made it such that the sea contained 100 tons of 100% Sr-87, this would have no major effect on the overall isotopic ratio (sea+mantle), and the odd distribution would wash out of the sea in a few years. (If the mantle is 10:7:83 Sr-88:Sr-77:Sr-76, the ratio in the sea would be roughly:
Sr-86 Sr-87 Sr-88
year 0 0 100 0
year 1 5 53 42
year 2 8 30 62
year 3 10 18 72
year 4 10 12 78
year 5 10 9 86
year ∞ 10 7 88
Given this fictional 10000:1 reservoir and 50% annual exchange, the isotopic concentrations are "buffered" from even catastrophic forcing events.