David Campbell and Allen Aigen merely hint at the marvelous complexity of
CO2 chemistry in seawater. The executive summary is: Every chemical in the
system is affected when the concentration of any other chemical is changed,
and the concentration of CO2 in seawater is indeed rising as it absorbs part
of the excess CO2 produced by human activities.

As to its significance, perhaps it is best to remind ourselves that
Globigerina ooze covers about half the earth's surface in the deep sea, and
that this ooze is made of calcium carbonate. So if seawater begins to
dissolve calcium carbonate, there is an awful lot down there for it to
dissolve. We should probably not worry too much about the fate of mollusks
just yet.

The amount (concentration) of calcium carbonate that seawater can hold
depends on temperature and pressure. Mollusk shells regularly dissolve back
into seawater in the deepest part of the ocean, and also in cold shallow
water as on the coasts of Scotland and Labrador. The mollusks that live
under these conditions have adaptations (such as periostraca) to maintain
their shells. If ocean water changes, we may notice it first as a southward
shift in the distribution of these mollusks.

A further complexity is that calcium carbonate comes in two main mineral
forms, calcite and aragonite, and aragonite is not as stable as calcite in
the ocean. Most gastropods and bivalves are made of aragonite, as well as
all scaphopods and chitons. Bivalves made almost entirely of calcite include
the oysters (ostreids and gryphaeids) and scallops (pectinids), and some
others combine both minerals in their shells. So the depth at which
aragonite tends to dissolve is shallower than the corresponding depth for
calcite.

The capacity of seawater to absorb carbon dioxide without much changing is
high. Although it must have a limit, I suspect that we will reach another
limit first: All this CO2 enters the atmosphere before it dissolves in
seawater, and that concentration is going up even faster. The atmosphere
will probably be unbreathable before the oceans become unbuffered by the
increase in CO2. At that point, presumably the amount of CO2 entering the
atmosphere will decrease.

Andrew K. Rindsberg
Geological Survey of Alabama

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