Geoscience Reference
In-Depth Information
seawater being pH-sensitive) suggest a glacial sur-
face ocean 0.1 to 0.2 pH units higher than during
interglacial periods (e.g. Sanyal
et al.
1995 ; Hönisch
and Hemming 2005 ; Foster 2008 ). Of course, changes
in the strength of the biological pump only re-
partition
C
T
, primarily vertically, meaning that
deep-ocean pH should be slightly lower during gla-
cial periods, all other things being equal (see Section
2.4.2).
subsequent carbonate burial removes carbon from
the atmosphere-ocean system. Thus, the silicate
weathering cycle is ultimately responsible for
sequestering carbon in the long term until a balance
between sources and sinks is restored. In the case of
large anthropogenic fossil fuel emissions (e.g. a total
of 5000 Pg C), it will take hundreds of thousands of
years for atmospheric CO
2
to return to climatically
relevant levels of, say, 400 ppmv in the future. Note
that the exact timing is difi cult to forecast, mostly
because of uncertainties in weathering parameteri-
zations (e.g. Uchikawa and Zeebe 2008 ).
2.3.7 Carbonate and silicate mineral
weathering
Weathering of carbonate minerals on continents
may be represented by
2.4 Long-term changes during earth's
history (quasi-steady states)
CaCO
+
CO
+
H O
®+
Ca
2
+
2HCO
-
(2.3)
In this section, we discuss long-term changes of
ocean chemistry during earth's history. As men-
tioned above, this chapter's focus is on past changes
in ocean chemistry, not solely on past ocean acidii -
cation events (to be discussed in Section 2.5). Over
several thousands of years, the carbonate mineral
saturation state of the oceans is controlled by the
balance of carbonate mineral weathering on conti-
nents (input to the ocean) and carbonate burial
(output) in ocean sediments (see Section 2.3.5 and,
e.g., Broecker and Peng 1989; Zeebe and Westbroek
2003 ; Ridgwell and Schmidt 2010 ). This balance
helps to establish fairly constant atmospheric CO
2
concentrations and ocean carbonate chemistry con-
ditions on timescales >10 000 years. However, the
entire system may not be in steady state with, for
instance, long-term processes such as silicate weath-
ering l uxes (hence the term 'quasi'-steady state).
3
2
2
3
whereas the reverse reaction (Eq. 2.2) represents the
precipitation and subsequent burial of carbonates
in marine sediments. As described in Section 2.3.5,
carbonate weathering (input to the ocean) and bur-
ial (output) are balanced via calcite compensation
on a relatively short timescale (~10 000 yr). Note
that for each mole of CO
2
taken up during CaCO
3
weathering, one mole of CO
2
is also released during
CaCO
3
precipitation. The net carbon balance for the
combined ocean-atmosphere system on timescales
over which carbonate weathering is balanced by
carbonate burial is therefore zero. For this reason,
carbonate weathering and burial are often ignored
in models of the long-term carbon cycle over mil-
lions of years (see, however, Ridgwell
et al.
2003 ).
On the other hand, silicate mineral weathering
and subsequent burial as calcium carbonate in
marine sediments may be represented by
2.4.1 The Holocene
CaSiO
+
CO
+
H O
®
CaCO
+
SiO
+
H O
(2.4)
It is instructive to consider ocean chemistry changes
during the Holocene (the ~12 000 yr period prior to
industrialization), not because of large variations
and/or acidii cation events but because of the
remarkable stability of the carbon cycle. It illustrates
the stark contrast to the current anthropogenic dis-
ruption, representing a large and rapid carbon per-
turbation relative to the natural balance of the
Holocene (see Fig. 2.2A). Ice core records reveal that
Holocene atmospheric CO
2
varied at most between
~260 and ~280 ppmv, with
p
CO
2
gradually rising
3
2
2
3
2
2
which shows that on a net basis one mole of carbon
in the form of CO
2
is removed from the atmosphere
and buried as CaCO
3
in sediments. This cycle is bal-
anced by input from volcanic degassing and net
organic oxidation on a timescale of 10
5
to10
6
years
(see above).
When the cycle is out of balance, for instance dur-
ing enhanced mineral weathering in response to
elevated atmospheric CO
2
, silicate weathering and
