From: Steven M Smith (smsmith@usgs.gov)
Date: Mon Nov 17 2003 - 10:14:35 EST
I wrote:
<<As a geochemist, I would postdict that fossil shells from mollusks, which
were alive during these periods, would contain higher metal contents (such
as iron) or different compositions than mollusk shells from other periods.
In other words, we could use the chemistry of fossil shells as a proxy for
the changing chemistry of the ocean.>>
Mike DePangher replied:
<I think there might be two problems with using Fe in carbonates as an
indicator of Fe in seawater. Although Fe++ is easily incorporated into many
carbonate minerals (there's a good stain for it in thin section), I don't
know of any that take Fe+++. Is the oxidation state of Fe critical in Art's
model? There might also be problems involving conversion of primary
skeletal aragonite into calcite. Cheers. >
Hi Mike,
Thanks for your reply. Yes, because of the oxidation state, I wouldn't
expect Fe+++ to substantially substitute for calcium in the aragonite
shells. In fact, I wouldn't expect Fe++ to be a major component of the
shells despite the ease of substitution. I simply expect that under
different ocean chemical conditions, mollusk shells would pick up minute
amounts of trace elements that could be a response to those environmental
conditions. I simply think that a comparison of trace elements over time
in mollusk shells might have a story to tell IF there were a major addition
of metals (like iron) to the ocean at definable periods. (You will notice
that in the original message, I didn't commit myself to Fe as the only or
best indicator in those shells. However, I see that my statements were not
clear that I was talking about trace elements in the shells and not the
major shell chemistry.) I freely admit that I'm speculating ... but that's
how new ideas get proposed and experiments get started.
If I remember correctly from the discussion several years ago (a memory is
a dangerous thing to depend on), Art wasn't proposing a model but simply
speculating on some recently published ideas. From what I've heard (i.e.
hearsay), the Fe ocean-seeding idea is still a highly controversial subject
with some overstated claims on all sides. I am not familiar enough with
the literature to tell you if the oxidation state of iron is critical for
the planktonic blooms.
Steve
_____________
Steven M. Smith, Geologist, U.S. Geological Survey
Box 25046, M.S. 973, DFC, Denver, CO 80225
Office: (303)236-1192, Fax: (303)236-3200
Email: smsmith@usgs.gov
-USGS Nat'l Geochem. Database NURE HSSR Web Site-
http://pubs.usgs.gov/of/1997/ofr-97-0492/
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