HR: 1340h
AN: PP33A-0910    [Abstracts]
TI: A 475 kyr record of extraterrestrial $^3$He and $^{230}$Th in North Atlantic sediments: caveats to derive MAR from these tracers.
AU: * Fourre, E
EM: elise.fourre@cea.fr
AF: Laboratoire des sciences du climat et de l'environnement, Orme des merisiers, Gif sur Yvette, 91191 France
AB: Extraterrestrial $^3$He is delivered to the earth surface by interplanetary dust particles (IDPs) characterized by a very high $^3$He/$^4$He ratio (2.4$\times$10$^{-4}$), several orders of magnitude higher than ratios measured in any terrigeneous matter. Several studies in the past ten years have opened up the potential of IDP-derived extraterrestrial $^3$He as a constant flux proxy: the global average flux is around (0.8-1)$\times$10$^{-12}$ cm$^3$ STP cm$^{-2}$ kyr$^{-1}$ over at least the last 200 kyr, and probably on a longer timescale (e.g. Marcantonio et al 2001, Paleoceanography 16, 260- 267; Higgins 2001, Columbia Univ. PhD; Winckler et al 2004, QSR 23, 1873-1878). Since $^3$He is a stable isotope, this method would not be limited by radioactive decay as it is the case for $^{230}$Th. However most of the studies on extraterrestrial $^3$He in deep-sea sediments have been conducted in regions of low sedimentation rate and specially of low terrigeneous supply. About sixty samples spanning the 475 kyr SU92-10 core from the North Atlantic Ocean (44$\deg$5N, $24\deg$5W, 3108m depth) have been selected to study helium and thorium isotopes in less favourable conditions: sedimentation rate around 1cm kyr$^{-1}$, high terrigeneous input, winnowing suspected. $^3$He/$^4$He ratios between 5.7$\times$10$^{-7}$ and 7.7$\times$10$^{-9}$ have been measured: in such case terrigeneous $^3$He cannot be neglected and deconvolution of the different components of the helium signal is necessary. Another consequence of the low concentration of IDPs is the poor reproducibility of the measurements preventing the interpretation of any fine structure of the $^3$He signal. The helium carrier phase in IDPs is not precisely known but Amari and Ozima (1985, Nature 317, 520-522) found that the helium resides mainly in the magnetic fraction of the sediments. We proceeded to systematic magnetic separations of 1g of decarbonated sediment to measure separately the magnetic fraction (less than 2% in mass but up to 80% of the $^3$He) and several aliquots of the non-magnetic fraction with a standard deviation improved by a factor 2 to 10 compared to replicates of the bulk decarbonated sediment. The first results show a cyclic pattern in the helium isotopic ratio, specially of the magnetic fraction, which, if confirmed, could be linked to variations in the detritic supply. Using MAR derived from $^{18}$O stratigraphy, a low cosmic $^3$He burial rate of (2.7 $\pm$ 1.0)$\times$10$^{-13}$ cm$^3$ STP cm$^{-2}$ kyr$^{-1}$ has been calculated. The $^{230}$Th$_{xs}^{\circ}$ measurements suggest an even larger winnowing effect at this core location. However work is still going on to interpret the thorium data: with a maximum of 2.2 dpm g$^{-1}$ for $^{230}$Th, the activities measured are very low making the $^{230}$Th$_{xs}^{\circ}$ determination all the more difficult. Among the hypotheses we are working on to explain thorium anomalies, one is the loss of $^{234}$U by diffusion out of the sediments and the role of the carbonates (between 40% and 97%) towards the U-Th isotopes.
DE: 9325 Atlantic Ocean
DE: 4860 Radioactivity and radioisotopes
DE: 4863 Sedimentation
DE: 4870 Stable isotopes
DE: 1040 Isotopic composition/chemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting