HR: 0830h
AN: PP11A-0214    [PDF]
TI: The Lithium Isotope Composition of Planktonic Foraminifera
AU: * Hathorne, E C
EM: e.c.hathorne@open.ac.uk
AF: Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AU: James, R H
EM: r.h.james@open.ac.uk
AF: Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AU: Harris, N B
EM: n.b.w.harris@open.ac.uk
AF: Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AB: The temporal record of the lithium isotopic composition of seawater has the potential to provide an important proxy of the relative rates of weathering and hydrothermal processes. One of the most powerful types of evidence for changes in ocean chemistry comes from analyses of foraminiferal calcium carbonate. Here, we explore the utility of foraminifera as recorders of the Li isotopic composition of seawater. The Li isotopic composition of foraminifera tests has been determined by multicollector inductively coupled plasma mass spectrometry (Nu Instruments) using a sample-standard bracketing technique. The external precision of this technique is $\pm$ 0.3 $\permil$ (2$\sigma$), based on fifteen analyses of seawater over a period of 9 months. Planktonic foraminifera ($\sim$10 mg) have been picked from surface sediments of the equatorial Pacific and the North Atlantic. Samples from the equatorial Pacific are Holocene/Pleistocene in age; those from the North Atlantic are from the Holocene. The foraminifera were subject to; cleaning in methanol and water, oxidation (hydrogen peroxide and sodium hydroxide), and leaching in weak acid (0.001M nitric acid). Additional reductive and refractory-phase cleaning steps (respectively, hydrous hydrazine/ammonia and DTPA solutions) had no effect on $\delta$$^{7}$Li or Li/Ca. The $\delta$$^{7}$Li value of the foraminifera ranges from 27 to 31$\permil$; slightly lighter than modern-day seawater (31.1 $\permil$). Different species have consistently different $\delta$$^{7}$Li values; {\it O. universa} have $\delta$$^{7}$Li values within error of seawater, while {\it Gr. truncatulinoides} record the lightest $\delta$$^{7}$Li (27.1$\pm$ 0.3$\permil$). This indicates that there are species specific vital effects on foraminiferal $\delta$$^{7}$Li. Samples of the same species from different latitudes in the North Atlantic have the same $\delta$$^{7}$Li, suggesting that there is no temperature effect on foraminiferal $\delta$$^{7}$Li. Furthermore, with the exception of {\it G. sacculifer}, there appears to be no variability in foraminiferal $\delta$$^{7}$Li with test size. Similarly to foraminiferal $\delta$$^{7}$Li, foraminiferal Li/Ca varies between different species. Foraminifera with lower Li/Ca generally have higher $\delta$$^{7}$Li, suggesting that species specific vital effects are due to differences in calcite precipitation rates; i.e. there is a kinetic control on foraminiferal Li/Ca and $\delta$$^{7}$Li. {\it O. universa} have lowest Li/Ca (and highest $\delta$$^{7}$Li), suggesting that the bulk of their calcite is laid down in a more open system.
DE: 4267 Paleoceanography
DE: 4805 Biogeochemical cycles (1615)
DE: 4825 Geochemistry
DE: 4855 Plankton
DE: 4894 Instruments and techniques
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting