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In autumn, leaves that fall on concrete make marks that remind me of fossils.

source: http://tanialove.com/musing/?p=373

Is this caused by a chemical reaction that occurs between the concrete and the leaf? Or would a leaf leave a stain on any surface as it decays? What circumstances can intensify the process?

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    $\begingroup$ Tannins in the leaf are leached out by water. Getting leaf wet then rolling a car tire on top would extract more fluid from the rehydrated leaf. en.wikipedia.org/wiki/Tannin $\endgroup$ – MaxW Nov 14 '15 at 18:01
  • $\begingroup$ If you check shortly after a rain you'll see wet, brown puddles. I suspect that the reason that they leave such a strong mark on concrete is that it can act as a base and tannins are mildly acidic, and their products are colored and relatively insoluble. Check out blackwater aquariums for more extreme examples of this coloration, though not involving concrete. $\endgroup$ – Jason Patterson Nov 14 '15 at 19:52
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The shape of the leaf is consistent with a type of maple tree leaf, and the brown coloring is characteristic of the water soluble tannins that would be extracted by water. For comparison, here are two photographs, one of tanic acid in water and one of a dried mixture of tannins. The photos were taken from this Wikipedia article. Although the tannic acid in the photograph below was extracted from different parts of a redwood tree, it is a ubiquitous tannin found in most hardwood tree leaves and the color is characteristic of many such tannins.

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