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background state within about 100 000 years (Rohl
et al
. 2000 ; Nunes and Norris 2006 ). Shoaling of the
calcite compensation depth by as much as 2 km pro-
vides empirical evidence of ocean acidii cation
( Zachos
et al
. 2005 ).
Parallels to the present prompt the question of
how earth's biota fared across the PETM event. On
land, vascular plants record pronounced but tran-
sient species migrations, with only limited extinc-
tion (Wing
et al
. 2005 ). Palaeocene mammals suffered
extinctions, but new taxa appeared, including many
modern mammalian orders, most at initially small
size—marking the PETM as a time of pronounced
mammalian
turnover
rather than diversity decline
( Gingerich 2006 ).
Many marine taxa also display a pattern of pro-
nounced turnover but limited extinction (Scheibner
and Speijer 2008 ), with corals ( Kiessling 2001 ) and
various microplankton groups (Scheibner and
Speijer 2008) showing transient range expansion
toward the poles. Major extinction depleted the
diversity of deep-sea benthic foraminiferans
(Thomas 2007), but corals—a group considered
especially vulnerable to present-day ocean acidii -
cation (Kleypas
et al
. 1999 ; Hoegh-Guldberg
et al.
2007)—show little change in diversity. Diversity,
however, does not tell the whole story. In a compre-
hensive review of carbonate platforms along the
Palaeogene margins of the Tethyan Ocean, Scheibner
and Speijer (2008) demonstrated that shelf margin
reefs built by colonial corals and calcareous algae
declined markedly at the PETM. Solitary (but not
colonial) scleractinians occur in basal Eocene car-
bonates, but contribute relatively little to carbonate
accumulation. Across the same boundary, larger
benthic foraminifera expand dramatically.
Thus, combined warming and ocean acidii cation
55 Myr ago made only a limited long-term mark on
the marine biota. While acidii cation expanded the
volume of undersaturated deep-sea waters, skele-
ton formers persisted on the shelves. This persist-
ence, however, does not imply strict ecological
continuity. Coral reef ecosystems declined widely
and did not recover for hundreds of thousands of
years—a geological instant but almost impossibly
long by the standards of human civilization.
Migration may have played an important role in
taxonomic persistence on land and in the ocean, but
this required corridors for migration, no longer
unimpeded on land or, perhaps, in the sea. In sum-
mary, then, the PETM record may be reassuring on
an evolutionary timescale, but it raises concerns on
the ecological scales relevant to humans. Persistence
of coral species, perhaps in isolated populations
with little or no calcii cation (e.g. Fine and Tchernov
2007) may not ensure the continual accretion of
reefs, with their attendant ecosystem services.
4.2.2
End-Permian mass extinction
An earlier event interpreted in terms of ocean acidi-
i cation occurred 252 Myr ago, at the end of the
Permian. Estimates of
p
CO
2
change and global
warming coincide broadly with those for the PETM,
but the biological consequences were starkly differ-
ent. On land, a poorly resolved record of vertebrate
evolution suggests migration and increased taxo-
nomic turnover across the Permian-Triassic bound-
ary (Smith and Botha 2005), and land plants show
both poleward migration and regionally distinct
patterns of extinction, most pronounced at high
southern latitudes ( Rees 2002 ; Abu Hamad
et al.
2008). Marine ecosystems, however, were devastat-
ed—species loss is estimated at 90% or more, while
metazoan reefs and other ecosystems that had long
dominated the seal oor disappeared ( Erwin 2006 ).
A reasonable scenario for end-Permian mass
extinction invokes rapid, massive inl ux of CO
2
into
the atmosphere and oceans, in association with one
of the largest eruptions of l ood basalts known from
the geological record. At least 1.2 × 10
6
km
3
of basal-
tic volcanic rocks were deposited over what is now
western Siberia, largely accumulating in a million
years or less (Reichow
et al
. 2007 ). Comparison with
modern volcanoes, such as those in Hawaii, sug-
gests that this event might have released 10
17
to 10
19
mol CO
2
(equivalent to 10 to 1000 times the amount
of CO
2
estimated for the latest Permian atmosphere;
Wignall 2001), although integrated into an active
carbon cycle over a million years, this would
increase atmospheric levels by only twofold or less
( Knoll
et al.
2007). Comparisons with Hawaiian vol-
canism, however, probably underestimate CO
2
release from Siberian trap volcanism, very likely by
a wide margin. The Siberian magmas ascended
through thick carbonate and evaporate deposits,
