HR: 1340h
AN: PP33A-0913    [Abstracts]
TI: Controls on $^{231}$Pa/$^{230}$Th in the Indian Ocean: Circulation or Productivity?
AU: * Thomas, A L
EM: alext@earth.ox.ac.uk
AF: Department of Earth Science, University of Oxford, Parks Road, Oxford, OX1 3PR United Kingdom
AU: Henderson, G M
EM: gideonh@earth.ox.ac.uk
AF: Department of Earth Science, University of Oxford, Parks Road, Oxford, OX1 3PR United Kingdom
AU: McCave, N
EM: mccave@esc.cam.ac.uk
AF: Department of Earth Science, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AB: ($^{231}$Pa$_{xs}$/$^{230}$Th$_{xs}$)$_{0}$ ratios in marine sediments are a potential proxy of palaeoproductivity and/or past ocean circulation. Studies in the Atlantic have demonstrated the particular potential of this proxy to assess the rate of past circulation AŸA›A›ƒ_sAªA›ƒ,ª�" an important environmental variable which is otherwise difficult to reconstruct $^{[1, 2]}$. In this study, we present new water-column and sediment data from the western Indian Ocean to improve understanding of the controls on ($^{231}$Pa$_{xs}$/$^{230}$Th$_{xs}$)$_{0}$ and test the use of this proxy. The Indian Ocean differs from the Atlantic because deep waters forming in the Southern Ocean and transported into the basin have high rather than low Pa and Th concentrations $^{[3]}$. The ability to reconstruct past rates of flow into the Indian Ocean would be a powerful new use of Pa/Th, but no data presently exist to test this application. Water samples (10 litre), surface sediments, and a 5.5m Kasten core were collected on the Charles Darwin cruise CD154 from the area East of Madagascar. The down-core record, from10$\circ$S extends to 150 ka, and is coupled with a stable isotope stratigraphy and a sortable silt record. Water and sediment samples were analysed for isotopes of Pa, Th and U on a Nu Instruments MC-ICP-MS. The water-column dataset (26 samples) indicate that Pa concentrations are high in NADW transported into the Indian Ocean from the Atlantic, while opal scavenging has lowered Pa concentrations in deeper water masses. Sedimentary $^{230}$Th data indicate that there has been significant sediment focusing and that sediment mass fluxes are low. Preliminary sedimentary Pa/Th data show little relation to the sortable silt record. Ongoing Pa/Th and Ba data will complete this downcore record and allow comparison of a Pa/Th record with independent proxies of current speed and productivity in the same core. [1] E.F. Yu, et al, Nature 379, 689-694, 1996. [2] J.F. McManus, et al, Nature 428, 834-837, 2004. [3] M. M. Rutgers van der Loeff, and G. W. Berger. Deep-Sea Research Part I: Oceanographic Research Papers. 40(2): 339-357, 1993.
DE: 4825 Geochemistry
DE: 4860 Radioactivity and radioisotopes
DE: 4875 Trace elements
DE: 4267 Paleoceanography
DE: 4283 Water masses
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting