If y'all are really interested in the general inorganic carbon cycle
     in the ocean, see any chemical oceanography text (e.g. Riley and
     Skirrow, 1965; Chapter 7).  Dissolved carbonate and carbon dioxide are
     relatively rare in seawater.  Bicarbonate is the more common ion.  The
     CO2 that enters the ocean participates in equilibria summarized by the
     following:
 
             (H2O + CO2)  ->  (H2CO3) ->  (HCO3- + H+) -> (CO3-- + 2H+)
                          <-          <-               <-
 
 
     All of these relationships are reversible, of course, and there are
     equilibrium constants for each one.  The tricky part is that CO2 is
     involved in both inorganic and organic cycles, so that the fate of an
     individual carbon atom over time is a biogeochemical thing.  Consider
     all of the runoff of organics into the ocean, the complex food webs,
     precipitation into shells and skeleta, dissolution of carbonate in
     acidic animal guts, bacterial activity, photosynthesis, and on and on.
 
     All of these things result in a slightly alkaline seawater (pH>8).
     The equilibria above result in a well buffered oceanic seawater
     system.  Add a little acid or alkali and it's absorbed and
     neutralized.  Its only when the system gets stressed thet the pH
     varies appreciably, and that's when you folks that have a seawater
     aquarium see problems.
 
     Gotta go.
 
     Bob Avent
 
 
______________________________ Reply Separator _________________________________
Subject: Shell Dissolution
Author:  JOhn a cramer <[log in to unmask]> at ~smtp
Date:    3/25/98 9:58 PM
 
 
Paul Monfils raises some interesting questions about calcium availability=
=2E =
 
I don't know if anyone has traced out the life cycle of a calcium atom in=
 
the oceans.  Certainly I don't know the details.  I suspect the
availability of carbonate ions is at least as important.  Indeed, Vermeij=
's
hint that the equilibrium calcium carbonate concentration decreases as
temperature increases is probably an empirical fact implying that carbon
dioxide availability is a controlling factor.  That is, the solubility of=
 
gases in water decreases with temperature increases, unlike the behavior =
of
salts.  That the salt calcium carbonate has a gas-like solubility suggest=
 
to me that the solubility of carbon dioxide dominates the concentration o=
f
calcium carbonate.  Where that leaves Paul's calcium ions I don't know. =
 
That raises another point.  The pH of fresh water is not 7.0 unless extre=
me
care is taken to prevent contact with carbon dioxide.  Water just sucks
that stuff up.  Our chemistry dept. still churns out water at about 6.0
and, if it is allowed to sit very long in the open air, that drops even
lower into the 5.5 range.  That is why so many soils have a pH in the are=
a
of 5.5.  This has nothing to do with acid rain which has even lower pH
values.  =
 
Incidently, this relates to the question of global warming.  The ability =
of
the oceans to absorb more carbon dioxide is unknown (and possibly
underestimated).  I think that means that no one has an answer to Paul's
question about where the calcium is and what it is doing.
Acidity is everywhere in freshwater.  That makes life tough for freshwate=
r
shells.  The ocean is basic and far more congenial to shells although, as=
 
people have noted, pH and calcium carbonate concentrations fluctuate
sustantially, depending on temperature and other conditions.   =