>Natural rain is surprisingly acid in humid temperate regions, but
>becomes downright corrosive when sulfur dioxide and nitrous oxides
>are added to the air. These react with oxygen and water to form
>sulfuric and nitric acids. Marble and limestone statues and building
>stone are eroded rapidly under these conditions, and so must natural
>exposures of limestone. Under natural conditions, fossils in the
>limestone tend to be etched out in relief over a period of many
>years. In recent years, my impression is that the fossils aren't
>weathering out the way they used to, but are being weathered flush
>with the rock. What have you observed?

Natural carbonic acid (extremely dilute, in the form of rain) does a
wonderful job of preferentially dissolving nonbiogenic calcium
carbonate cement without attacking biogenic carbonate (fossils),
doesn't it? I have observed this occurring in a block of Wangaloa
(early Paleocene) shellbed I had lying outside my Geology Dept for a
few years. The shells, already partially exposed in this
beach-boulder, have slowly become better-defined. But it is a slow
process. There is a certain temperature range where it is most
effective, but it will still proceed at any above-freezing
temperature.

>But what about shell beds? Typically, in the U.S. Atlantic and Gulf
>Coastal Plains, the famous Tertiary shell beds are only a few feet
>thick and are composed of mixed shell and sand. (3 feet = about
>1 meter.) They are very porous and the pores are large, so
>groundwater can move readily through them. At most outcrops, these
>shell beds are weathered, with shells as soft as cheese. Only in the
>newest and deepest cuts are the shells in really good condition, and
>so far as I can tell, this has always been the case.

I see the same in Eocene-to-Miocene soft-matrix shellbeds here in New
Zealand, though groundwater tends to flow through the entirety of
such units (unless they are rendered impermeable by high clay/mud
content). If the exposures are not constantly cut back by erosion (eg
a riverbank), the rock becomes weathered and the fossils more and
more leached, until they are uncollectable. There is a big problem in
NZ with the spread of willows along streams; many classic riverbank
fossil sites have stabilized by these wretched trees, and now the
outcrops are overgrown by grass or scrub and what few fossils are
still accessible are falling apart; need earthmoving equipment to
salvage these localities...

>With acid rain, one would expect the process to be quicker than
>before, but would it be different in kind?

Possibly different. Acid rain may contain other chemical pollutants;
these "evaporites" may crystallize in the fossils, the pressure of
crystallization physically wedging the fossils apart ie causing them
to disintegrate. This happens naturally in parts of the world eg
Antarctica (though these are genuine evaporites, not windblown
pollutants), and such fossils (in this case Pliocene whales) are a
terminal mess.

Such chemical reactions are of course selfneutralizing, so the acid
will dissolve only a certain amount og carbonate before it is
neutralized; this means that groundwater will be neutralized rather
quickly.
--
Andrew Grebneff
Dunedin, New Zealand
64 (3) 473-8863
<[log in to unmask]>
Fossil preparator
Seashell, Macintosh & VW/Toyota van nut
I want your sinistral gastropods!
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