HR: 11:05h
AN: B21G-04 INVITED     [PDF]
TI: Reef Coral Biomineralization: A Geochemical Perspective
AU: * Cohen, A L
EM: acohen@whoi.edu
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543 United States
AB: The trace element and stable isotope composition of long-lived reef corals preserve high-resolution records of marine climate from the Paleozoic to the present. However, while skeletal chemistry responds to changing ocean conditions, the relationships between composition and external conditions seldom appear to obey thermodynamics. Deviations from chemical equilibrium, referred to as "vital effects", reflect the imprint of coral physiology on the kinetics of crystal growth. Although typically viewed as a hinderance, vital effects can provide a window into biomineralization mechanisms and processes. For example, the Sr/Ca ratio of bulk coral skeleton is lower than it would be if the skeleton were precipitated from seawater under equilibrium conditions. This deviation has been attributed to kinetically controlled processes associated with light-enhanced calcification. Because the offset from inorganic precipitates is not consistent, but varies within and amongst coral species, it has proven to be problematic for Sr/Ca-based paleothermometry. I examined the origins of this vital effect using SIMS ion microprobe to measure changes in the Sr/Ca ratio of crystals within a single sclerodermite, from the nucleation site through the tip of the fasciculus, a distance of 50 - 100 æm. The Sr/Ca ratio of seed crystals found in centers of calcification is close to that of inorganic aragonite precipitates formed at the same conditions. This indicates that the nighttime precipitation of these submicron, equant-shaped crystals, which constitutes the initial phase of the daily cycle of crystal growth, may occur within a space filled with seawater at precipitation rates slow enough for the system to approach equilibrium. Tight bundles of needleshaped crystals (the fasciculi) nucleate on the seed crystals and grow outward like spherulites to fill the calcifying space. The Sr/Ca ratio of these crystals decreases linearly as they grow. This observation supports a monocrystalline rather than polycyclic model of crystal growth. Furthermore, the rate of change of Sr/Ca along the fasciculus is greater than is predicted by a closed-system mass balance model. It is highest in fast-growing tropical corals during the summer, and lowest in slow-growing subtropical corals in winter. These observations support previous assertions of a link between Sr/Ca disequilibrium and light-enhanced calcification. However, the uniformity of crystal morphology along the length of the fasciculus suggests that a kinetic process may not be responsible. An alternative explanation involves hourly, daily and seasonal changes in the Sr/Ca composition of the calcifying fluid, brought about largely through changes in the light-driven and carrier-mediated transport of Ca2+ into the calcifying space.
DE: 1615 Biogeochemical processes (4805)
DE: 3670 Minor and trace element composition
DE: 4267 Paleoceanography
DE: 4825 Geochemistry
DE: 4875 Trace elements
SC: Biogeosciences [B]
MN: 2003 Fall Meeting