Skeletons and ocean chemistry: the long view - Ocean Acidification - page 79

devastating, impact on the abundance, diversity,
and evolution of calcifying organisms. More broadly,
the geological record indicates that calcifying organ-
isms have been subject to episodically changing Ω
throughout the past 542 Myr. Volcanism and associ-
ated thermal decomposition of organic matter have
generated ocean acidii cation over intervals that
were biologically long if geologically brief. Decreases
in surface-seawater Ω sustained for millions of years
have also been associated with the expansion of
anoxic subsurface waters and, hence, anaerobic het-
erotrophy. When ocean acidii cation and the expan-
sion of anoxic waters occur in tandem, the result
can be mass extinction that is differentially severe
for hypercalcii ers and other animals with limited
capacity to modulate the ionic composition of inter-
nal l uids. Our understanding of these events
remains imperfect, but the perspective they offer
can be used to better inform our expectations for the
future of reefs during our current anthropogenic
experiment.
Several generalities about the past seem relevant
to our environmental future:

tat loss, overi shing, and the impedance of
routes for migration.
(4) The timescale for recovery from ocean acidii ca-
tion is measured in geological time. Thus, even
assuming that inputs of CO 2 into the atmosphere
and oceans are ameliorated, diversity loss will
appear permanent on timescales relevant to the
human population.

(5) Physiological experiments and geological his-

tory are mutually illuminating. Studies of the
past can suggest relative biological vulnerabili-
ties, highlighting candidates for physiological
research. Physiology, in turn, provides an
important lens through which palaeontological
research can be focused.

4.5 Acknowledgements

We thank H. Pörtner, J. Erez, and J. Barry for useful
discussions of biomineralization and physiological
responses to ocean acidii cation, and J.-P. Gattuso
and R. Zeebe for helpful reviews of our chapter. We
also thank W. Kiessling for providing the diversity
and abundance data illustrated in Fig. 4.1. Research
was supported in part by the NASA Astrobiology
Institute (AHK) and the Agouron Institute (WWF).

(1) In assessing the vulnerability of the biota to
decreasing Ω, rate is key. When the rate of envi-
ronmental change is fast, the probability of
extinction is increased. Times of biological crisis
in the past were times, like today, when p CO 2
increased rapidly, not when p CO 2 was high (see
also Chapter 2 ).
(2) There is no clear reason to expect that the com-
ing century will see a 'sixth extinction' compa-
rable to those at the end of the Permian and
Cretaceous. Nonetheless, the loss of vulnerable
taxa from ocean ecosystems could affect eco-
logical function for many millennia. The past
tells us that there will be winners and losers in
a changing ocean. Corals, and therefore the
coral reef communities that harbour so much of
marine diversity, may well be among the
losers.
(3) In the past, extinction was more pronounced
when several biological challenges were
imposed at once. In similar fashion, the conse-
quences of ocean acidii cation will be amplii ed
by global warming, declining levels of dis-
solved oxygen (Brewer and Peltzer 2009), habi-

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