Chemistry Reference
In-Depth Information
6
Geochemical Perspectives on
Coral Mineralization
Anne L. Cohen
Geology and Geophysics, MS#23
Woods Hole Oceanographic Institution
Woods Hole, Massachusetts 02543 U.S.A.
Ted A. McConnaughey
1304 Cedar Lane
Selah, Washington 98942 U.S.A.
INTRODUCTION
Corals open an exceptional window into many phenomena of geological,
geochemical, climatic, and paleontological interest. From the Paleozoic to the present,
corals provide some of the finest high-resolution archives of marine conditions. Corals
are likewise exceptional for chronometric purposes, and even the terrestrial
14
C timescale
has now been calibrated against coral
230
Th/
234
U. Corals also represent a testing ground
for basic ideas about mineralogy and geochemistry. The shapes, sizes, and organization
of skeletal crystals have been attributed to factors as diverse as mineral supersaturation
levels and organic mediation of crystal growth. The coupling between calcification and
photosynthesis in symbiotic corals is likewise attributed to everything from
photosynthetic alkalinization of the water, to efforts by the coral to prevent
photosynthetic alkalinization. Corals also leave a significant geochemical imprint on the
oceans. Their aragonite skeletons accept about 10 times more strontium than does calcite,
hence the proportion of marine aragonite precipitation affects the oceanic chemical
balance. Biological carbonates represent the biosphere's largest carbon reservoir, hence
calcareous organisms affect the ocean's pH, CO
2
content, and ultimately global
temperatures through the greenhouse gas connection. Finally, corals present some
geochemical puzzles for ecology and conservation. How do symbiotic corals obtain
nutrients in some of the most nutrient deficient parts of the planet? Are global
geochemical changes partially responsible for the widespread declines in coral reefs
during recent decades? We will address many of these issues, but will concentrate on
coral skeletal structure and calcification mechanism. These topics bear most directly on
the biomineralization process and generally affect the choice of skeletal materials and
analytical techniques used in geochemical investigations.
The coral reef is probably the planet's most spectacular biomineralization product.
These grand and complex ecosystems build on the accumulated skeletal debris of
countless generations of organisms, especially calcareous algae and symbiotic
foraminifera and corals. The algae produce much of the reef mass and help to cement it
together, while the corals build much of the erosion-resistant framework. Coral reefs
dominate much of the world's tropical coastline and cover abut 15% of the seabed
shallower than 30 m (Smith 1978).
Charles Darwin (1842) originally showed that reef corals grow almost exclusively in
shallow waters. He nevertheless hypothesized that reef sediments might sometimes
extend to great depths, having formed over millions of years near the sea surface, on the
flanks of subsiding volcanoes. Deep drilling in the Marshall Islands ultimately confirmed
Darwin's theory. Darwin didn't know why reef corals grew fastest in shallow water, but a
