Geoscience Reference
In-Depth Information
Interestingly, our suggestion that the PETM
carbon input had a moderate impact on surface-
ocean saturation state is consistent with the
results of Gibbs
et al.
(2006), who studied the orig-
ination and extinction of nanoplankton during
the PETM. They concluded that the perturbation
of the surface-water saturation state across the
PETM was not detrimental to the survival of most
calcareous nanoplankton taxa. In contrast, the
Palaeocene-Eocene boundary marks a major
extinction event of benthic foraminifera, affecting
30 to 50% of species globally (e.g. Thomas 2007).
It is not clear, however, whether the benthic
extinction was caused by changes in oxygenation,
bottom-water temperatures, carbonate undersat-
uration as a result of the carbon input, and/or
other factors (for discussion see Ridgwell and
Schmidt 2010). In summary, the direct effects of
ocean acidii cation on surface calcii ers during
the PETM may have been limited because of a
relatively 'slow' carbon input rate (slow on
human timescales, rapid on geological time-
scales). Possible acidii cation effects on benthic
organisms are as yet difi cult to quantify because
of competing effects from other environmental
changes (see also Chapter 4).
As mentioned above, among the ocean acidii ca-
tion events hitherto identii ed in earth's history, the
PETM may be the closest analogue for the future.
Yet the evidence suggests that the rate of carbon
input from human activities may exceed the rate of
carbon input during the PETM. Thus, at present it
seems that the ocean acidii cation event humans
may cause over the next few centuries is unprece-
dented in the geological past for which sufi ciently
well-preserved palaeorecords are available.
successful scientii c experiment—in the sense of the
above dei nition—requires purposeful, and in most
cases clever and careful planning, design, and exe-
cution of the experiment. Moreover, the operator is
usually able to terminate the experiment at any time
if so desired (except in the case of an ill-designed
experiment). The fact that humans are emitting CO
2
is a consequence of a fossil fuel-based economy. It
seems that the only resemblance to an experiment is
that the outcome is as yet unknown.
In this regard, the geological record can provide
valuable information about the response of the
earth system to massive and rapid carbon input,
which should ultimately lead to improved future
predictions. In particular, studies of past changes
in ocean chemistry teach us a lesson about the
effects that ocean acidii cation may have on marine
life in the future. In addition, they provide the
necessary background for assessing the current
anthropogenic perturbation in the context of
earth's history. Our assessment shows that when
studying the past, a good understanding of the rel-
evant timescales involved is of utmost importance.
For instance, short-term carbon cycling on a times-
cale of 10 to 100 yr and long-term carbon cycling
on a timescale of millions of years involve two dis-
tinct cycles with vastly different reservoir sizes
and different sets of controls on atmospheric CO
2
and ocean chemistry. Thus, the pertinent times-
cales of palaeo-
p
CO
2
and palaeochemistry records
require thorough examination to qualify as appro-
priate future analogues.
Our survey of long-term changes of ocean car-
bonate chemistry during earth's history (quasi-
steady states) revealed that natural variations are
generally slow and small on timescales relevant to
the near future (see Fig. 2.6 for a summary). Because
the ocean saturation state is usually well regulated
and decoupled from pH over tens of thousands of
years, past events that involve geologically 'rapid'
changes of ocean carbonate chemistry are of partic-
ular interest. Among the ocean acidii cation events
discussed here, the PETM may be the closest ana-
logue for the future. However, the anthropogenic
rate of carbon input appears to be greater than dur-
ing any of the ocean acidii cation events identii ed
so far, including the PETM.
2.6 Conclusions
Fossil fuel burning and the resulting input of CO
2
to
the atmosphere has been referred to in the literature
as a global 'geophysical experiment'. By dei nition,
an experiment is 'An operation carried out under
controlled conditions in order to discover an
unknown effect or law, to test or establish a hypoth-
esis, or to illustrate a known law'. In fact, none of
this applies to fossil fuel burning. Conducting a
