HR: 0800h
AN: PP31C-0534    [Abstracts]
TI: Effect of CO₃^2- ions activity on the distribution of UO₂²⁺ ions between coral skeleton and seawater
AU: * Takaesu, Y
EM: k068307@eve.u-ryukyu.ac.jp
AF: Graduate School of Engineering and Science, University of the Ryukyus, 1 Senbaru Nishihara-cho, Okinawa, 903-0213, Japan
AU: Oomori, T
EM: oomori@sci.u-ryukyu.ac.jp
AF: Graduate School of Engineering and Science, University of the Ryukyus, 1 Senbaru Nishihara-cho, Okinawa, 903-0213, Japan
AU: Fujimura, H
EM: fujimura@sci.u-ryukyu.ac.jp
AF: Graduate School of Engineering and Science, University of the Ryukyus, 1 Senbaru Nishihara-cho, Okinawa, 903-0213, Japan
AB: In order to evaluate the paleo-ocean environmental change, trace element contents of coral skeleton have been used as proxy of temperature and salinity in seawater. For example, Sr/Ca and Mg/Ca ratios are useful parameters of past seawater temperature. During calcification, It is considered that uranium is incorporated into coral skeleton as cation UO₂²⁺ ions on Ca²⁺ sites. Dissolved uranium in seawater is ca.13nmol/l and present as uranyl carbonate complex ions such as UO₂(CO₃) ₃^4-, as well as free UO₂²⁺ ions, where complex formation affected by pH and/or CO₃^2- ions activity in seawater. Therefore uranium content of coral skeletons is expected to be a function of pH and/or CO₃^2- ions activity in seawater. We investigated distribution of UO₂²⁺ ions between coral skeleton and seawater. We incubated a coral colony ( Goniastrea aspera;18×26×15 cm³) at a temperature of 40 degrees centigrade in the water tank (27×38×24 cm³) under sunlight for 8 hours and collected seawater every two hours. We analyzed uranium, calcium, alkalinity and salinity in seawater by using ICP-MS, EGTA titration, HCl titration and salinometer respectively. pH is gradually increased from 8.17 to 8.54. Uranium concentration decreased from 13.22 nmol/l to 12.89 nmol/l until pH reached to 8.45. Calcium concentration kept decreasing from 10.54 nmol/l to 10.14 nmol/l. Logarithmic distribution coefficient λ is calculated by measured uranium and calcium; λ=log(U,i/U,f)/log(Ca,i/Ca,f). Activity of carbonate ions is calculated by using activity coefficient γ\ CO₃^2- = 0.021. Logarithmic distribution coefficient λ decreased from 2.6 to 0.6 when activity of CO₃^2- ions decreased from 5.0 nmol/l to 4.4 nmol/l. The result of this study suggested that distribution of UO₂²⁺ ions between coral skeleton and seawater was clearly influenced by activity of carbonate or pH in seawater.
DE: 4916 Corals (4220)
DE: 4924 Geochemical tracers
DE: 4999 General or miscellaneous
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting