UNIVERSITE TOULOUSE III - PAUL SABATIER Ecole ... - LEGOS

)¡ ¡ £¢$ ¡¢£ ¡ "$ £ £¢¤¢¡
The Nd IC of a rock depends both on its age and samarium to neodymium ratio. ε Nd
ranges from –40 in old continental formations (e.g.: Greenland; Taylor et al., 1992) to +10 in
new volcanic formations (e.g.: Iceland; O'nions and Grönvold, 1973). The Nd found in the
ocean is of terrigeneous origin. It is brought to the ocean in dissolved or particulate phases, by
rivers, the atmosphere or remobilization from margin sediments. Within the ocean, most of
the Nd is found in the dissolved phase, the particulate phase represents only 5 to 10% of the
total content (Jeandel et al., 1995). Although input processes are still not completely
understood, it seems that dissolved/particulate exchanges at the margins play a preponderant
role (Jeandel et al., 1998; Lacan and Jeandel, 2001; Tachikawa et al., 2002). In the vicinity of
lithogenic sources, ε Nd is thus used as a tracer of particle transport and dissolved particulate
exchanges (Grousset et al., 1988; Jeandel et al., 1995; Tachikawa et al., 1999b). Away from
them, it is conservative and used as a tracer of water masses (Piepgras and Wasserburg, 1987;
Piepgras and Jacobsen, 1988; Jeandel, 1993; Jeandel et al., 1998; Lacan and Jeandel, 2001).
Nd residence time in the ocean is around 500 to 1000 years (Tachikawa et al., 1999a). That
makes ε Nd suitable to trace intra and inter ocean currents.
ε Nd is widely used in paleo-oceanography. Signal variations stored in sediments (mostly
metalliferous), are classically interpreted as reflecting past variations in circulation. Recently,
several studies have suggested that fluctuations in the Nd sources, via variations in erosion
rates, could also affect ε Nd records (Vance and Burton, 1999; Frank et al., 2001; Tachikawa et
al., 2002).
The past Nd IC of the North Atlantic Deep Water (NADW) has been particularly well
studied (Burton et al., 1997; Abouchami et al., 1999; Vance and Burton, 1999; Rutberg et al.,
2000). The production of this water mass initiates the conveyor belt circulation, and thus is a
key factor controlling our climate (Broecker, 1991). NADW is formed out of four sources:
Denmark Strait Overflow Water (DSOW), Island Scotland Overflow Water (ISOW),
Labrador Sea Water (LSW) and Lower Deep Water (LDW, derived from Antarctic Bottom
Water, AABW). Those are formed by convection in high latitudes: the DSOW and the ISOW
in the Nordic Seas (Greenland, Island and Norwegian Seas), the LSW in the Labrador Sea and
the AABW around the Antarctic continent (Dickson and Brown, 1994). The few data
available before this work show that the Nd ICs of these sources are very distinct: ε Nd ≈ -8 in
the Nordic Seas overflow waters, ε Nd ≈ -14.7 in the LSW and ε Nd ≈ -11.7 in the AABW
(Piepgras and Wasserburg, 1987). ε Nd variations in the past or present NADW could thus be
used to evaluate the relative contribution of each of its source waters and to draw conclusions
about fluctuations in the conveyor belt activity, if it can be ascertained that the end-member
values are constant.
To further develop this potential application of ε Nd , we need to better understand how
these waters acquire their signature. So far, only three Nd IC measurements were available for
the Nordic Seas, which is insufficient to constrain the signature of the DSOW and ISOW. One
of the goals of the Signature/GINS cruise (IFRTP) was to further address these questions. The
cruise has been carried out on board the R/V Marion Dufresne, in July and August 1999, in
the North Atlantic and more particularly in the Greenland and Iceland Seas. Hydrological
parameters (θ, S, O 2 and Acoustic Doppler Currents) and chemical tracers: CFC, SF6, 129 I,
REE and Nd IC have been measured. In this paper, we present Nd ICs and REE
concentrations of seawater samples from the East Greenland Current (EGC), along the east
Greenland slope, between the Fram and Denmark straits. We use these results to explain the
ε Nd signature of the DSOW and to better constrain the circulation in the Denmark Strait.
2