HR: 0830h
AN: PP11A-0203    [PDF]
TI: Core-Top Calibration of Temperature Dependent Calcium Isotope Fractionation for ten Species of Planktonic Foraminifera
AU: * Sime, N G
EM: ngs20@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: De La Rocha, C L
EM: christina00@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: Galy, A
EM: albert00@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AB: Previous work has demonstrated the variable temperature dependence of calcium isotope fractionation ($\delta$$^{44}$Ca) in planktonic foraminifera up to 0.24$\permil$$\deg$C$^{-1}$. While the majority of this work has focused on cultured samples and plankton tows, there have been no detailed studies of core-top sediments. Here we present the results of $\delta$$^{44}$Ca ($^{44}$Ca/$^{42}$Ca) measurements made via MC-ICP-MS on ten species of planktonic foraminifera (spinose and non-spinose) from a suite of marine box-core surface samples, taken from sites between 61$\deg$S and 29$\deg$N with SST from 8 to 28$\deg$C. The $\delta$$^{44}$Ca-temperature relationship seen for all but three of the species was parallel to, but offset from, inorganic aragonite equilibrium by $+$0.15 to $+$0.20$\permil$, and was assumed to represent calcite equilibrium. In contrast to the results of previous studies, which found a much greater degree of temperature dependence on Ca isotope fractionation by {\it Globigerinella sacculifer, Neogloboquadrina pachyderma} (d) and {\it Globigerinoides ruber} than for other species, the core-top specimens of these three species showed the same $\delta$$^{44}$Ca-temperature relationship as the other foraminifera. In general, values for {\it Globorotalia inflata, Globorotalia truncatulinoides} and {\it Globigerina bulloides}, were offset from calcite equilibrium to more positive $\delta$$^{44}$Ca. The magnitude of this disequilibrium in some species may be as high as $+$0.70$\permil$, e.g. {\it G. bulloides}. Disequilibrium seems to be limited to deep-dwelling species and cold-water sites ($<$$\sim$12$\deg$C), and this may be tied to an increase in calcification rate at lower depths, initiated by the increasing [CO$_{3}$$^{2-}$], and causing further depletion of Ca in a closed Ca pool. Alternatively, it may be due to the addition of a later calcite crust. No difference in the magnitude of Ca isotope fractionation was observed between species bearing photosymbionts and those without them.
DE: 0330 Geochemical cycles
DE: 4267 Paleoceanography
DE: 4825 Geochemistry
DE: 4855 Plankton
DE: 4870 Stable isotopes
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting