Geoscience Reference
In-Depth Information
CHAPTER 4
Skeletons and ocean chemistry:
the long view
Andrew H. Knoll and Woodward W. Fischer
4.1 Introduction
the poles and at high altitude (Petit
et al
. 1999 ).
Therefore, deep-time estimates of
p
CO
2
rely on
models, broadly constrained by geochemical proxy
data. For example, the widely applied models of
Berner and colleagues (e.g. GEOCARB III; Berner
and Kothavala 2001; Berner 2006; Fig. 4.1C) esti-
mate l uxes of carbon from one reservoir to another,
based on geochemical proxies (mainly isotope
ratios and abundances of sedimentary carbonate
and organic carbon), and then calculate successive
steady states of the system through time. Additional
parameters are considered, including estimates of
carbon l uxes due to erosion, river run-off, plant
evolution, volcanic weathering, global CO
2
degas-
sing, and land area; these also inl uence the model
results.
These models suggest that atmospheric
p
CO
2
was
not wildly different from pre-industrial modern lev-
els back into the Miocene (23-5 Myr ago), but was
moderately higher earlier in the Cenozoic, and
higher yet—perhaps i ve to eight times the present
atmospheric level (PAL)—during the warmest parts
of the largely unglaciated Mesozoic Era (252-65 Myr
ago). Modelled
p
CO
2
during the Late Palaeozoic ice
age is, as might be predicted, low, but earlier
Palaeozoic estimates exceed 10 times PAL, with val-
ues in some iterations (Berner and Kothavala 2001)
spiking as high as 25 times PAL during the later
Cambrian Period (~500 Myr ago; Fig. 4.1). An inde-
pendent biogeochemical model (COPSE; Bergman
et al
. 2004) suggests a similar history, but with less
extreme late Palaeozoic and Mesozoic values.
Geochemical proxies for ancient
p
CO
2
(the C-isotopic
composition of alkenones, soil carbonates, and
organic matter; the distribution of stomata in the
epidermis of fossil leaves; and the stable isotope
In present-day seas, animals, algae, and protozoa
are threatened by ocean acidii cation, amplii ed in
many regions by seawater warming and hypoxia
( Doney
et al
. 2009). Many species may be affected
adversely by 21st-century environmental change,
but a decade of research suggests that the hypercal-
cifying animals responsible for reef accretion may
be especially vulnerable to an acidity-driven
decrease in the saturation state (Ω; see Box 1.1)
of surface seawater with respect to calcite and
aragonite.
The geological record reveals that natural changes
in the marine carbonate system have affected the
evolution and abundance of calcifying organisms
throughout the Phanerozoic Eon (542 million years
(Myr) ago to the present). This being the case, we
can use our understanding of the dynamic behav-
iour of the carbon cycle and the stratigraphic com-
ings and goings of reef-building organisms to
inform us about what, if any, lessons can be drawn
from the long-term past and applied to our near-
term future.
4.2
A record of atmospheric
CO
2
and
p
past global change
If there is one thing that geology makes clear it is
that the earth and its biota are in a continual state of
change. Because of its relationship to climate, the
partial pressure of CO
2
(
p
CO
2
) in the atmosphere
has been of particular interest to geologists and
geochemists, but direct measurement of ancient
CO
2
levels is impossible for intervals older than
those recorded in glacial ice preserved today near
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