HR: 0830h
AN: PP21C-1191    [PDF]
TI: A Record of Oceanic Lithium Isotope Composition for the Last 7Ma
AU: * Marriott, C S
EM: caedmonm@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford, Oxford, OX1 3PR United Kingdom
AU: Henderson, G M
EM: gideonh@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford, Oxford, OX1 3PR United Kingdom
AB: Continental weathering plays an important role in global climate change but has proved difficult to reconstruct for the past. New geological tools with which to assess the past rate and style of weathering are therefore urgently required. One such tool is Li isotope fractionation. Recent studies [1,2] have shown preferential release of $^{7}$Li into the aqueous phase and retention/adsorption of $^{6}$Li during weathering processes such as partial dissolution and secondary mineral formation. Lithium behaves conservatively in the oceans, with a residence time of $\sim$1Ma, so that a history of ocean Li isotope composition provides information about the average rate and style of global continental weathering on long timescales. The incorporation of lithium as a trace element in marine carbonates enables the construction of a record of oceanic Li-isotopic variation and is the focus of this work. Carbonate Li-isotope compositions are lighter than seawater by $\sim$8 per mil, but this fractionation is not temperature dependent. This has been demonstrated by measurement of Li isotopes in inorganically precipitated calcites (5-$30\deg$C) [3], in coralline aragonite (25-$30\deg$C) [3] and in benthic foraminifera {\it Uvigerina} (7-$23\deg$C). This lack of T-dependent fractionation suggests that the variation in the isotope composition of planktonic foraminifera will solely reflect changes in oceanic Li isotope composition, which in turn is strongly influence by changes in continental weathering. ODP site 758, located on the Ninetyeast Ridge in the Indian Ocean ($5\deg$N, $90\deg$E; 2925m), was sampled at 2m intervals, over a depth corresponding to the last 7Ma, to produce 55 samples with a temporal resolution of approximately 130Ka. Site 758 is previously well studied with an existing chronology and high resolution Sr, O and Nd isotope data. Individual foram species in the core top were first investigated to assess inter-species fractionation effects. Down core lithium isotope variation was examined by measurement of hand picked and fully-reductively cleaned {\it G. menardii}. {\it G. menardii} was chosen because of its abundance throughout the core enabling the required 30-40mg of carbonate to be readily picked. Lithium isotope composition will be measured using a well established MC-ICP-MS technique [2,3] with a precision of better than $\pm$1 per mil to provide a record of the Li isotope composition of seawater for the last 7Ma and to identify changes in continental weathering during this period. 1. Huh Y. et al. (2001) EPSL 194, 189-199. 2. Pistiner J. S. \& Henderson G. M. (In press) EPSL. 3. Marriott C. S. et al. (2002) GCA S1 66, A485.
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1065 Trace elements (3670)
DE: 1886 Weathering (1625)
DE: 4267 Paleoceanography
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting