Geoscience Reference
In-Depth Information
4.3.1
Skeletons and surface-water Ω
surprisingly, Kiessling's data show that widespread
reef development, common from early Neogene
time through to today, is not generally characteristic
of Phanerozoic oceans (Fig. 4.1A).
Reefs can be dei ned as discrete rigid carbonate
structures formed by
in situ
or bound components
that develop topographic relief upon the seal oor
( Wood 1999 ). Structures that i t this dei nition have
existed since the evolution of benthic microbial
communities more than 3 billion years ago
( Grotzinger and Knoll 1999 ; Allwood
et al
. 2009 ).
With the late Neoproterozoic emergence of complex
multicellularity, both animals and algae began to
participate in reef accretion, and through the
Phanerozoic Eon (the past 542 Myr) a number of
major taxa have contributed to reef formation.
Kiessling's (2009) compilation highlights several
i rst-order stratigraphic patterns in the composition
of reef biotas (Fig. 4.1B). First, microbial accretion
did not cease with the evolution of hypercalcifying
metazoans, but rather declined slowly and i tfully
through time, ceasing to be quantitatively impor-
tant only in the Cretaceous. Second, bryozoans,
bivalves, calcareous algae, and other groups have
contributed a moderate volume of reef carbonate
through time, with calcareous algae peaking in the
late Carboniferous and early Permian periods and
again in the Neogene, and rudist bivalves playing a
major role in Cretaceous reefs (to the extent that
rudist deposits i t Wood's dei nition of a reef; Gili
et al
. 1995). Throughout the Phanerozoic Eon, how-
ever, peaks in reef abundance correspond to times
of widespread and diverse hypercalcifying animals,
mainly massively calcifying sponges and cnidari-
ans, and calcareous algae. It is principally the epi-
sodic waxing and waning of these organisms that
gives rise to the widely applied concept of succes-
sive reef biotas. Insofar as hypercalcii ers should be
sensitive to factors that control Ω, we can ask
whether these factors were in play when successive
reef biotas expanded and collapsed.
Archaeocyathids, an extinct group of calcareous
sponges, were major contributors to reef accretion
in Early Cambrian oceans, but a major extinction
event near the Cambrian Stage 3-4 (Botomian-
Toyonian) boundary essentially wiped out the
group, beginning a nearly 50 Myr interval during
which metazoans played only a minor role in reef
A large body of experimental research supports the
hypothesis that the cost and effectiveness of forma-
tion of carbonate skeletons vary inversely with Ω
( Gattuso
et al
. 1999 ; Langdon and Atkinson 2005 ,
and references therein). Skeletal responses to ocean
acidii cation, however, vary across taxa, as might be
predicted from the basic features of skeletal physiol-
ogy. Because hypercalcii ers have only limited phys-
iological capacity to pump ions across membranes
to modify the composition of l uids from which
skeletal minerals are precipitated, they are particu-
larly vulnerable to decreasing Ω in ambient waters.
Ries
et al
. (2009) grew a variety of skeletal inverte-
brates and algae at a range of Ω. Not surprisingly, a
majority of the experimental species showed a
decline in skeleton formation with decreasing Ω.
Three arthropods, however, actually increased skel-
etal mass with decreasing Ω, and red and green
algae, as well as a limpet and a sea urchin, showed
an initial increase in calcii cation followed by decline
as Ω decreased beyond a threshold value. Gooding
et al
. (2009) also observed an increase in skeletal
mass in the sea urchin
Pisaster ochraceus
grown at
elevated
p
CO
2
and temperature. These responses are
consistent with the pattern of extinction and sur-
vival across the Permian-Triassic boundary, and
expectations for vulnerability to decreasing Ω. We
note, however, that changing calcii cation rate is
only one of many potential physiological responses
to ocean acidii cation—in some cases, skeletal com-
pensation is accompanied by decreased perform-
ance in other important aspects of growth or
metabolism ( Pörtner 2008 ; see Chapter 10 ).
4.3.2 Hypercalcii cation through time
Palaeontologists have long understood that neither
the abundance nor the taxonomic composition of
reefs has remained constant through time, prompt-
ing the generalization that six to eight successive
reef biotas have l ourished in Phanerozoic oceans,
separated by stratigraphic gaps during which meta-
zoans contributed little to reef accretion (e.g. Wood
1999 ; Copper 2002a ; Kiessling 2002 ; Ezaki 2009 ). In
a recent compilation of reef abundance and diver-
sity, Kiessling ( 2009 ) has rei ned this view. Perhaps
