ITMEMS Statement on Coral Bleaching

Pam Muller pmuller at
Wed Jan 6 10:07:17 EST 1999

Re: reefs in warmer times in the geologic record

1. Pleistocene interglacial reefs probably didn't have to contend with
a doubling of the rate of fixed nitrogen into terrestrial (and
ultimately aquatic) ecosystems, sedimentation associated with
deforestation and other anthropogenic changes in land use practices, the
plethora of new diseases transported and mixed by ships traveling
worldwide, and a 10-15% increase in biologically damaging/mutagenic UV
radiation.  And while they did experience somewhat higher CO2 levels,
certainly not the rate and magnitude of increase that may bring
Eocene-like atmospheric concentrations in the next century.

2. In Cretaceous and Eocene limestone, corals are generally quite common
and diverse, but it is my understanding from the literature and field
observations that corals were far less important as reef builders,
especially in shelf environments, during those warmer times.  They were
supposedly most common in shelf margin environments where temperatures
were at least more stable if not a bit cooler.

3. There are several hypotheses for the dominance of rudists as
bank/platform-top builders of reefs in the Cretaceous, all providing
possible insight into the 21st century prognosis for coral reefs.  
Higher CO2 levels shift the calcification advantage away from aragonite
towards calcite (see, e.g., Chapter 2 in Birkeland's Life and Death of
Coral Reefs). Steve Stanley at Johns Hopkins thinks that higher Ca/Mg
ratios also favors calcite production.  Many rudists produced both.  Also,
the more advanced organ-level physiology of the bivalve rudists may have
enabled them to be more tolerant of temperature, salinity and
sedimentation extremes. Some European specialists still dispute whether
rudists had algal symbionts and , if they are right, "bleaching" was not
an issue. E. Gili and P. Skelton have several recent papers on the
rudist/coral story.

4. As for the Late Paleocene-Eocene, coralline algae (variable Mg calcite)
and larger forams (low Mg calcite) tended to be the dominant
shelf carbonate producers.  Stan Frost contended that corals really didn't
shift fully to reef-building mode until the mid Oligocene.  That is
consistent with global cooling/falling atmospheric CO2 levels (and falling
ca/mg ratios in seawater associated with slowing rates of seafloor
spreading, if I understand the Stanley hypothesis correctly).

Thus, the paleo-record does provide insight, and the indication is not
necessarily hopeful for the future of coral reefs.  Though many coral
species will probably survive and thrive - they just won't necessarily
have the production potential to build reefs.  The losers will likely be
the reef-dependent taxa (corals and others).


Pamela Hallock Muller
Department of Marine Science
University of South Florida
140 Seventh Avenue South
St. Petersburg, FL 33701 USA
pmuller at
Phone: 727-553-1567
FAX: 727-553-1189 	NOTE NEW AREA CODE!!!

"Discovery consists of seeing what everybody has seen and thinking what
nobody has thought."  - Albert Szent-Gyorgyi -

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