[Coral-List] Is Palau's low pH - high diversity the norm of the past?

[Charles Birkeland]

Anne Cohen presented an interesting description of the diverse and healthy
coral communities in low pH waters on Palau’s beautiful heavily-eroded
karsts. From the perspective of geologic time, could this be the normal
situation?  Kleypas et al. 2001 reminded us that reef-production in the
Neogene and recent times have been extraordinary because scleractinian
corals experienced atmospheric CO2 in the low 3-digit concentrations only
in the Neogene and recent times. For the 190 million years previous to the
Neogene, scleractinians experienced dense atmospheric CO2 in the 4-digit
concentrations and ocean pH was generally well below 8. Earth perhaps
experienced low CO2 like now in the Carboniferous, but that was about 80
million years before the first scleractinians.


Scleractinians kept showing increases in diversity (occasionally set back
by events) while reef-production was meagre during low pH and high CO2 from
the late Jurassic to just before the Neogene. As Kleypas et al. (2001)
wrote, biological and geological performances of corals are not clearly
associated with each other. However, there was strong reef-production in
the late Triassic and mid-Jurassic, also during low pH and high CO2. So the
correlation of reef-building with pH and CO2 is back and forth.
Reef-building is more consistently associated with the Mg/Ca ratio in
seawater (i.e., lower as the intensity of new basalt production as seafloor
is more rapidly produced with faster plate tectonics, because
basalt-production takes up Mg – Stanley and Hardie 1998).  High Mg density
in seawater when continents are not moving fast is good for aragonitic
deposition, so the Triassic, Jurassic and Neogene sclearactinians produced
massive reefs. But during the Cretaceous, when the continents were moving
relatively fast, aragonitic skeletons were not facilitated so the diverse
scleractinians used calcite structures for substrata, such as the large
3-dimensional rudist bivalves (or the karsts in Palau).


I liked Bob Buddemeier and Bob Kinzie’s Reef Encounter 23 comment on the
reef-production in the Neogene and recent times as being unusually strong.
They said “Like obesity, a massive reef accumulation may be the result of
remaining stationary for too long under good conditions.”


Anne Cohen’s concern with bioerosion might also be spot on.  The reefs of
the Triassic and Jurassic had bioerosion, but Gary Vermeij (1987) found
that the diversity and abundance of bioeroders, and especially the depth of
bioerosion, accelerated in the Cretaceous and Cenozoic. Gary might say that
while pH probably has the same characteristics in the Cretaceous as now,
bioeroders might coevolve in an arms race with coral defenses and become
more effective as they diversify and bore deeper. Peter Glynn and Eric
Manzello have produced a very good review of increased future bioerosion in
a book coming out about September (“Coral Reefs in the Anthropocene”.
Pamela Hallock’s chapter covers much of the discussion above.) I think
Peter and Derek tell of a number of processes in addition to pH that
stimulate bioerosion and that bioerosion may be more detrimental in the
future than pH by itself or the decline in coral calcification.


The report by Barkley, Cohen, Golbuu et al. on the lack of effect of lower
pH on coral richness and the increasing effects of bioerosion struck me as
unusual for recent times, but possibly normal during most of the past and
the future.  Reefs may not matter to corals, but they are important for us
and so many organisms that dwell on and in them.


Chuck