Andrew Grebneff wrote,
> 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.

Interesting. What would this temperature range be?

I think that a good bit of the solution process takes place underground, at
the interface between acidic soil and alkaline limestone. When a road is cut
or a house built, sometimes erosion exposes dirty rock that already has
fossils etched out in relief. Also, fossils may be etched out along fissures
in the limestone.

In some places, myriads of fossils (silicified or not) wash out of the soil,
after who knows how many years of slow solution. After a few days or years
of heavy collecting, the fossils are generally never found in such numbers
again. At least, the Tennessee Valley sites reported to have large numbers
of the Mississippian blastoid (echinoderm) Pentremites generally are pretty
lame by the time someone tells me about them!

Acid rain has the potential to make fossil collection much more difficult.
Perhaps it interferes with the natural etching of fossils, perhaps it
enhances it -- My impression is that it interferes. But certainly, in porous
shell beds, the weathering zone will be deepened by long-term acid rain. The
fresh part of the shell bed, on the average, will be farther underground
than before. As collectors, how should we respond to this trend?

Cheers,
Andrew

Andrew K. Rindsberg
Geological Survey of Alabama