Geoscience Reference
In-Depth Information
ocean acidii cation events have been identii ed dur-
ing the Pleistocene. Briel y, the available Pleistocene
data indicate periodic variations in ocean carbonate
chemistry during the past 2 Myr. These variations
are part of the natural glacial-interglacial climate
cycle and are restricted within remarkably stable
lower and upper limits (between ~180 μatm and
~300 μatm for
p
CO
2
and between ~8.1 and ~8.3 for
surface-ocean pH
T
).
A few estimates of surface-ocean pH
T
and atmos-
pheric CO
2
are available for the Pliocene epoch
(Bartoli
et al.
pers. comm.; Pagani
et al.
2010 ). Stable
boron isotopes indicate variations in surface-ocean
pH
T
between ~8.0 and ~8.3 and a gradual
p
CO
2
decline from 4.5 Myr to 2 Myr with extreme values
ranging between ~200 and ~400 μatm (Bartoli
et al.
pers. comm.). Over the same time interval, alkenone
data suggest a similar
p
CO
2
decline with extreme
p
CO
2
values ranging between ~200 and ~525 μatm
( Pagani
et al.
2010 ). Alkenone-based
p
CO
2
estimates
derive from records of the carbon isotope fractiona-
tion that occurred during marine photosynthetic
carbon i xation in the past. Several lines of evidence
suggest that the carbon isotope fractionation
depends on CO
2
levels (e.g. Pagani
et al.
2010 ). Note
that these reconstructions have large uncertainties.
Nevertheless, taking the results at face value, one
may estimate the maximum change in the surface-
ocean saturation state of calcite (Ω
c
) over the past 4
Myr. The cold periods may be characterized by pH
T
= 8.3 and
p
CO
2
= 200 μatm, which yields Ω
c
= 6.1
(
T
= 15°C,
S
= 35). The warm Pliocene periods (~4°C
warmer than pre-industrial) may be characterized
by pH
T
= 8.0 and
p
CO
2
= 525 μatm, which yields Ω
c
= 4.6 (
T
= 19°C,
S
= 35). By and large, the combined
evidence for the Pliocene and Pleistocene suggests
that over the past 4 Myr, ocean carbonate chemistry
has experienced relatively slow changes on time-
scales >10 000 yr, with atmospheric CO
2
varying
roughly between 200 ppmv and 500 ppmv.
1998 ; Zeebe 2001 ). Deep-sea sediment cores reveal
that the long-term steady-state position of the cal-
cite compensation depth (CCD) over the past 100 to
150 Myr did not vary dramatically; it rather gradu-
ally deepened slightly toward the present (for a
summary see Tyrrell and Zeebe 2004). This suggests
a more or less constant carbonate mineral satura-
tion state of the ocean over the Cenozoic, except for
the Eocene-Oligocene transition (~34 Myr) when
the CCD rapidly deepened permanently by several
hundred metres. A recent study indicates a more
dynamic CCD on shorter timescales, for instance
during the Eocene in the Equatorial Pacii c (Pälike
et al.
pers. comm.). Nevertheless, on long timescales,
the carbonate chemistry of the ocean over the
Cenozoic may be reconstructed based on saturation
state estimates and palaeo-
p
CO
2
reconstructions
(e.g. Tyrrell and Zeebe 2004 ; Ridgwell 2005 ;
Goodwin
et al.
2009 ; Stuecker and Zeebe 2010 ).
The details of the reconstructions can vary sub-
stantially, mostly depending on the different pal-
aeo-
p
CO
2
estimates. However, several trends appear
to be robust. Atmospheric CO
2
concentrations were
higher during the early Cenozoic and have declined
from a few thousand ppmv to 200 to 300 ppmv dur-
ing the late Pleistocene (Fig. 2.3). While the surface-
ocean saturation state was nearly constant over this
period of time, surface-ocean pH
T
was lower during
the early Cenozoic (perhaps ~7.6) and has gradu-
ally increased to its modern value of about 8.2 (see
Fig. 2.3 and Tyrrell and Zeebe 2004 ; Ridgwell and
Zeebe 2005). Note that these are long-term trends
which do not resolve possible large short-term vari-
ations during ocean acidii cation events such as the
PETM (see below). Note also that low surface-ocean
pH in the past during multimillion year periods of,
for instance, the Palaeocene or Cretaceous, are no
analogues for the centuries to come because of dif-
ferent seawater carbonate mineral saturation states
(Section 2.4.5).
One could ask whether or not the long-term trends
in ocean carbonate chemistry throughout the
Cenozoic had an effect on the evolution of marine cal-
cifying organisms. Note, however, that if species evo-
lution was sensitive to carbonate saturation state,
little effect is to be expected because saturation
state appears to have been nearly constant over
the Cenozoic. Regarding coccolithophores, a trend
2.4.4
The Cenozoic and beyond
A common approach to reconstructing ocean chem-
istry over the Cenozoic (the past ~65 Myr) is based
on estimates of past atmospheric CO
2
concentra-
tions and the carbonate mineral saturation state of
the ocean (e.g. Sundquist 1986; Broecker and Sanyal
