HR: 0800h
AN: B21B-0872    [Abstracts]
TI: Modelling Coral Biomineralization: Mixed Kinetic/Equilibrium Trace Element Coprecipitation Models Reveal new Complexity
AU: * Sinclair, D J
EM: djsweb1971@yahoo.com.au
AF: GEOTOP UQAM-McGill, Universite du Quebec a Montreal C.P. 8888 Succ. Centre-Ville, Montreal, QC H3C 3P8 Canada
AU: Risk, M
EM: riskmj@mcmaster.ca
AF: School of Geography and Geology, McMaster University 1280 Main St West, Hamilton, ON L8S 4L8 Canada
AB: A mixed kinetic/equilibrium steady state model of trace-element co-precipitation in coral skeleton is presented, and tested against high spatial resolution observations of coral trace-element composition made previously by LA-ICP-MS. The model is implemented in PHREEQC, and simulates physicochemical precipitation from a small pocket of seawater which is isolated by the coral, and modified by enzyme exchange of 2 H$^{+}$ for Ca$^{2+}$. The model assumes that all aqueous trace-element species in the calcifying fluid are in full equilibrium and that selected trace element species compete kinetically for precipitation with the major aqueous species (Ca$^{2+}$ for cation substituents and CO$_{3}$$^{2-}$ for anion substituents). No equilibrium is assumed for the CaCO$_{3}$ skeleton. Carbon is supplied to the system by diffusion of CO$_{2}$ into the high-pH calcifying fluid, and trace elements are continuously replenished through the addition of fresh seawater. The rate at which the coral operates the enzyme pump, and the rate at which it replenishes the seawater component are independent variables, and the steady state trace-element composition of the skeleton/calcifying fluid are evaluated over a 2D grid of variables spanning realistic rates of pumping and seawater influx. It is assumed that variations in the trace element composition of the coral skeleton are the result of shifts in the steady-state caused by changes to these variables. The results indicate an unexpected complexity in the response of the trace elements. First order predictions suggest that increasing the rate of calcification by increasing the enzyme pumping should result in a mutual dilution of most trace element species by pumped Ca$^{2+}$ and diffused CO$_{2}$. However, for high rates of pumping and low seawater replenishment, the model predicts a change in the trace-element response of the system as high CO$_{3}$$^{2-}$ concentrations drive calcification and deplete Ca$^{2+}$. This added complexity makes rationalizing observations with models more difficult.
DE: 4808 Chemical tracers
DE: 4830 Higher marine organisms
DE: 4842 Modeling
DE: 4875 Trace elements
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting