Geoscience Reference
In-Depth Information
CHAPTER 3
carbonate chemistry
James C. Orr
3.1 Introduction
Le Quéré
et al.
2009 ). If we would partition these
emissions equally per capita, each person on the
planet would be responsible for 4 kg per day of
anthropogenic CO
2
invading the ocean. To grasp
the size of the problem, this invisible invasion may
be compared with a recent, highly visible environ-
mental disaster. The ocean currently absorbs anthro-
pogenic carbon at a rate that is about a thousand
times greater than from when carbon escaped from
the BP Deepwater Horizon oil well that exploded
on 20 April 2010, releasing 57 000 barrels of petro-
leum per day into the Gulf of Mexico until it was
capped almost 3 months later. Of course, the form
of carbon released, the associated impacts, and the
duration of the carbon release differ greatly.
Anthropogenic ocean acidii cation is a chronic prob-
lem: it has been gradually increasing its intensity
for two centuries, and it is continuing.
This chapter is about the ongoing human-induced
shifts in fundamental ocean carbonate chemistry
that are occurring globally and are a growing con-
cern to scientists studying marine organisms. It
reviews the current state of ocean pH and related
carbonate system variables, how they have changed
during the industrial era, and how they are
expected to continue to change during this century
and beyond.
Surface-ocean pH has been relatively stable for
millions of years, until recently. Over the 800 000
years prior to industrialization, average surface-
water pH oscillated between 8.3 during cold peri-
ods (e.g. during the Last Glacial Maximum, 20 000
yr ago) and 8.2 during warm periods (e.g. just
prior to the Industrial Revolution), as reviewed
by Zeebe and Ridgwell in Chapter 2. But human
activities are upsetting this stability by adding
large quantities of a weak acid to the ocean at an
ever increasing rate. This anthropogenic problem
is referred to as ocean acidii cation because ocean
acidity is increasing (i.e. seawater pH is declin-
ing), even though surface-ocean waters are alka-
line and will remain so. The cause of the decline
in seawater pH is the atmospheric increase in the
same gas that is the main driver of climate change,
namely carbon dioxide (CO
2
).
Due to increasing atmospheric CO
2
concentra-
tions, the ocean takes up large amounts of anthro-
pogenic CO
2
, currently at a rate of about 10
6
metric
tons of CO
2
per hour (Brewer 2009), which is equiv-
alent to one-fourth of the current global CO
2
emis-
sions from combustion of fossil fuels, cement
production, and deforestation (Canadell
et al.
2007 ;
3.2
Basic chemistry under change
Ocean uptake of anthropogenic CO
2
helps limit the
level of CO
2
in the atmosphere but it also changes
the ocean's fundamental chemistry. That is, CO
2
is
not only a greenhouse gas; it is also an acid gas. It
reacts with seawater through a series of well-
understood reactions (e.g. Revelle and Suess 1957;
Broecker and Takahashi 1966; Stumm and Morgan
1970 ; Skirrow and Whiti eld 1975 ; Andersen and
Malahoff 1977). Like other atmospheric gases, CO
2
exchanges with its dissolved form in surface sea-
water, CO
2
(g)
«
CO
2
(aq). But CO
2
is exceptionally
soluble because its aqueous form reacts with water
to form carbonic acid, which dissociates producing
hydrogen ions. Most of the additional hydrogen
ions that are produced are neutralized when they
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