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GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L12606, doi:10.1029/2006GL026020, 2006
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– Adélie Coast Bottom
Radiogenic isotope fingerprint of Wilkes Land–
Water in the circum-Antarctic Ocean
Tina van de Flierdt,1 Sidney R. Hemming,1,2 Steven L. Goldstein,1,2
and Wafa Abouchami3
Received 10 February 2006; revised 2 May 2006; accepted 12 May 2006; published 24 June 2006.
[ 1 ] Wilkes Land – Adélie Coast Bottom Water
encompasses the bottom waters formed around the coast
of East Antarctica in the area east of Prydz Bay and west of
the Ross Sea (90E– 150E). Here we show that these
bottom waters have not only characteristic physical
properties, but also carry a distinct radiogenic isotope
signal that traces their dispersal. The Nd, Hf and Pb isotopic
compositions of Circum-Antarctic ferromanganese nodules
record the isotopic composition of ambient seawater. Those
with the lowest Nd and Hf isotopic ratios and the highest Pb
isotopic ratios are from the deep Australian-Antarctic Basin,
and they fingerprint Wilkes Land – Adélie Coast Bottom
Water. The data illustrate the potential value of these isotope
systems as tracers of present and past deep and bottom
waters in the Circum-Antarctic realm and elsewhere.
Citation: van de Flierdt, T., S. R. Hemming, S. L. Goldstein,
and W. Abouchami (2006), Radiogenic isotope fingerprint of
Wilkes Land – Adélie Coast Bottom Water in the circum-Antarctic
Ocean, Geophys. Res. Lett., 33, L12606, doi:10.1029/
2006GL026020.
1. Introduction
[2] The Southern Ocean plays a crucial role in the
present-day global ocean current system. The connection
among the major ocean basins provided by the Antarctic
Circumpolar Current (ACC) not only permits efficient
global water-mass exchange, but also dominates transport
of heat, fresh water, and other properties that influence
climate. Furthermore, it is the Southern Ocean where the
densest waters in the global thermohaline circulation system
are formed. Its estimated production rate of 8 – 21 Sv is
equal to, or even greater than, the production of North
Atlantic Deep Water (NADW: 13– 18 Sv; 1Sv = 106 m3s 1
[e.g., Broecker et al., 1998; Orsi et al., 1999; Ganachaud
and Wunsch, 2000; Jacobs, 2004]). While most monitoring
studies have been focused on the Weddell Sea, it has
become increasingly clear that bottom water is formed at
many sites around Antarctica, including the Weddell Sea,
the Enderby Land – Amery Shelf - Prydz Bay area, the
Wilkes Land - Adélie Coast area, and the Ross Sea (see
Figure 1 and recent summaries by Orsi et al. [1999], Rintoul
et al. [2001], and Jacobs [2004]). Bottom waters formed
around Antarctica, together with (well-characterized)
NADW, are exported to the global ocean, and hence their
composition is of major interest for understanding global
deep water circulation patterns.
2. Radiogenic Isotope Fingerprint of Southern
Ocean Water Masses
[3] In the absence of radiogenic isotope tracer information of Southern Ocean seawater (with the exception of the
Drake Passage [Piepgras and Wasserburg, 1982]), ferromanganese nodules have been shown to be very useful and
reliable recorders of ambient seawater signals, for Nd, Hf,
and Pb isotopes [Frank, 2002]. These isotope systems may
have slightly different sources and cycling behavior in the
ocean, but share the characteristic that newly formed water
masses carry a distinct signature [Frank, 2002; Goldstein
and Hemming, 2003, and references therein]. This signature
predominantly originates from weathering and erosion of
continental material (Nd and Hf [e.g., van de Flierdt et al.,
2004]), but Pb may also involve significant hydrothermal
inputs [e.g., Vlastélic et al., 2001]. Another important
difference between the globally coupled Nd and Hf isotope
signal in seawater [Albarède et al., 1998] and dissolved Pb,
is the shorter residence time of Pb in seawater (50 – 200 yrs)
compared with Nd and Hf (500– 2000 yrs) [Frank, 2002;
Goldstein and Hemming, 2003, and references therein].
[4] Previous studies on surface scrapings of ferromanganese nodules from the Southern Ocean for their Nd and Pb
isotopic compositions revealed a strong geographic control
on the seawater signal, suggesting a major imprint of
NADW on the eastern Atlantic and Indian Ocean sectors
of the Southern Ocean [Abouchami and Goldstein, 1995;
Albarède et al., 1997]. Reinvestigation of some of the
nodules led Vlastélic et al. [2001] to suggest a Ross Sea
or Weddell Sea bottom water influence in the Indian Ocean
sector. Here we report new Hf isotope data on many of the
same circum-Antarctic nodules. In the context of the new
data we argue that a large fraction of the bottom seawater
signal in the Indian Ocean sector of the Southern Ocean is
derived from local bottom water sources around the Wilkes
Land – Adélie Coast area.
3. Methods and Samples
1
Lamont-Doherty Earth Observatory of Columbia University, New
York, USA.
2
Department of Earth and Environmental Sciences, Columbia University, Palisades, New York, USA.
3
Max-Planck-Institut für Chemie, Mainz, Germany.
Copyright 2006 by the American Geophysical Union.
0094-8276/06/2006GL026020$05.00
[5] To characterize the Hf isotopic composition of deep
and bottom waters in the Southern Ocean, we analyzed 26
surface scrapings from ferromanganese nodules (Table S1)
previously characterized for their Pb and Nd isotopic
compositions by Abouchami and Goldstein [1995] and
Albarède et al. [1997]. Most samples are located north of
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than the Atlantic (67W to 30E; eHf = 4.5) and Pacific
(149E to 67W; eHf = 4.4) sectors (Figures 2 and 3). The
five Indian sector nodules with the lowest eHf values are
derived from the Australian-Antarctic Basin, and these also
show elevated 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb
ratios [Abouchami and Goldstein, 1995] (Figure 3). Three
out of these 5 nodules exhibit some of the lowest eNd
values (eNd = 8.8 to 9.3 [Albarède et al., 1997]) and
the highest 208Pb/204Pb ratios (208Pb/204Pb = 39.32 to 39.40
[Abouchami and Goldstein, 1995; Vlastélic et al., 2001])
observed in the Southern Ocean (Figures 2 and 3). These
three nodules grew from bottom waters which have present
day temperatures below 0C (Figure 4).
5. Bottom Water Sources Around East Antarctica
Figure 1. Circum-Antarctic area south of 40S with
locations of investigated ferromanganese nodules (symbols
as in Figures 2 – 4; IO-TC1578-14A: yellow circle). White
lines with arrows indicate the main eastward flow in the
Antarctic Circumpolar Current (ACC) associated with the
Subtropical Front and the Subpolar Front, the Weddell
Gyre, and the Ross Gyre. Small white arrows in the Indian
Ocean sector reflect westward flowing currents south of the
ACC.
65S and south of 49S within the eastward flowing ACC
(Figure 1) and at depths below 3000 m. 50– 100 mg of
powdered nodule material was leached for 15 minutes in a
mixture of 6 M HCl and trace HF and the separation of Hf
was carried out with a fluorite precipitation step followed by
anion and Ln Spec ion exchange chromatography [e.g., van
de Flierdt et al., 2004]. All samples were analyzed on the
Axiom MC-ICP-MS at Lamont and details are in the
electronic supplement.1
[7] Physical and chemical properties of bottom waters
around East Antarctica suggest the existence of multiple
source areas beyond the Weddell Sea and the Ross Sea [e.g.,
Orsi et al., 1999; Rintoul et al., 2001; Jacobs, 2004]. Due to
strong topographic control on pathways, most newly formed
bottom waters are restricted to individual basins. For
example, Weddell Sea Bottom Water (WSBW) is restricted
to the Weddell-Enderby Basin west of 20E and is
only recorded by nodule IO-PC1277-14 (eHf = 4.5,
eNd = 7.9; Table S1 and Figure 4). Weddell Sea Deep
Water (WSDW), on the other hand, which is recorded by
nodule IO-TC1277-36 (eHf = 4.2, eNd = 8.8; Table S1 and
Figure 4) is exported to the north and to the east, but does
not enter the Australian-Antarctic Basin directly, since there
is a pronounced westward flow of waters south of the
Kerguelen Plateau (Figure 1) [Rintoul, 1998]. The westward
flowing part of Ross Sea Bottom Water (RSBW is recorded
by nodules TC42-6 and E38-8; eHf = 3.8 to 3.9, eNd =
7.2 to 7.9; Table S1 and Figure 4) enters the eastern part
4. Results
[6] A striking observation is that the overall variation of
authigenic Hf isotopes in the Southern Ocean ferromanganese nodules is small (Figures 2 and 3 and Table S1).
The mean Hf isotopic composition of the 26 samples is
eHf = 4.2 ± 1.4 (2s) (Table S1 and Figures 2 and 3). Nodule
IO-TC1578-14A from the NE corner of the Weddell
Sea (Table S1 and Figure 1) is not included in this
average value, because its eHf of 12.1 together with its
unradiogenic eNd of 11.3 [Albarède et al., 1997] defines a
position in Nd-Hf isotope space that indicates a detrital
rather than an authigenic origin (Figure 2). We therefore
eliminate this sample from further discussion. For the rest of
the data the average seawater Hf isotopic composition in the
Indian sector (30E to 149E; eHf = 3.5) is slightly lower
1
Auxiliary material is available at ftp://ftp.agu.org/apend/gl/
2006gl026020.
Figure 2. Neodymium versus Hf isotopic composition of
Circum-Antarctic ferromanganese nodules. The Southern
Ocean has been subdivided into an Atlantic sector (67W to
30E; blue triangles), an Indian sector (30E to 149E; red
squares), and a Pacific sector (149E to 67W; green
diamonds). Nodule IO-TC1578-14A plots outside the
diagram area (eHf = 12.1, eNd = 11.3). Nd data are
from Albarède et al. [1997].
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gradually fades eastward and shoals southward [e.g., Orsi et
al., 1999]. It is intriguing that the water depths of the
samples with the least radiogenic Nd and Hf isotope ratios,
as well as the most radiogenic Pb isotopes (Table S1 and
Figures 2 and 3) are between 3950 and 4620 m, and
therefore much deeper than the entrainment level of NADW
at 2500 to 3000 m. Furthermore, these samples are found far
to the east of the Atlantic sector, in the Australian-Antarctic
Basin in the Indian Ocean sector (Figure 1).
[10] Vlastélic et al. [2001] therefore concluded that the
radiogenic Pb isotope signal in the Indian Ocean sector must
be derived from Antarctica and suggested WSBW and
RSBW as sources. This is however an unlikely explanation
as these water masses do not enter the central and western
part of the Australian-Antarctic Basin, as outlined above.
Instead the signal is much more simply attributable to
Wilkes Land - Adélie Coast Bottom Water.
Figure 3. 208Pb/204Pb ratios versus Hf isotopic compositions of Circum-Antarctic ferromanganese nodules. Lead
data are from Abouchami and Goldstein [1995]. Symbols
are the same as in Figure 2.
of the Australian-Antarctic Basin but is overprinted west of
143E.
[8] Large parts of the Australian-Antarctic Basin are
filled with Adélie Land Bottom Water, a water mass with
lower salinity, higher oxygen content, and lower potential
temperature compared to RSBW. This water mass is thought
to form predominantly in the Adélie depression between
142.5E and 145.5E [Rintoul, 1998, and references
therein]. Bottom water at the location of four out of the
five nodules from the Australian-Antarctic Basin has a
temperature of less than 0.3C (Figure 4), and the three
nodules with the lowest Nd and Hf isotopic compositions
and the highest Pb isotope ratios (E-PhC 54-1, E-PhC 54-3,
and E-TC 45-42; Table S1) are derived from waters below
3950 m with temperatures less than 0C (LEVITUS and
WOA01 databases), consistent with Adélie Land Bottom
Water. However, we label this water mass here as Wilkes
Land – Adélie Coast Bottom Water, as the entire shelf
between these two locations on East Antarctica may be its
source region. The notion of a local water mass filling the
abyssal part of the Australian-Antarctic Basin is also
supported by high CFC contents [Orsi et al., 1999], as well
as by provenance studies suggesting rapid dispersal of
Antarctic-derived marine sediment to the AustralianAntarctic Basin by bottom waters below 3500 m [e.g.,
Dezileau et al., 2000].
7. New Interpretation of the Nd-Hf-Pb Isotope
Signal in the Australian-Antarctic Basin
[11] All Southern Ocean nodules fall on the global
‘‘seawater array’’ between the Atlantic and Pacific endmembers. Thus a simple mixture of Atlantic and Pacific
sources can explain a large portion of the overall Nd-Hf
(and Pb) isotopic variability in the Southern Ocean
(Abouchami and Goldstein [1995], Albarède et al. [1997],
and this study). However, such a mixture fails to explain the
geographical detail of the data in the Indian Ocean sector,
calling for a local source affecting the radiogenic isotope
signature of bottom waters. Here we suggest that the slight
offset of the data in the Australian-Antarctic Basin compared to the rest of the Southern Ocean data is due to local
inputs from glacially weathered old continental crust.
[12] Indeed, large parts of East Antarctica are characterized by old basement ages as evidenced from a survey of the
6. Previous Interpretations of the Nd-Pb Isotope
Signal in the Australian-Antarctic Basin
[9] Abouchami and Goldstein [1995] and Albarède et al.
[1997] inferred inflow of NADW to the Southern Ocean as
a source for lower Nd and higher Pb isotope signal in the
Indian sector, and this would be consistent with the slightly
lower Hf isotope ratios. It was noted by Vlastélic et al.
[2001] however that for Pb the NADW signal can be traced
to the eastern Atlantic sector but not further east in the
Indian Ocean. Indeed, when NADW intrudes the northern
rim of the ACC, the salinity maximum associated with it
Figure 4. Nd isotope data set for all surface scraping of
ferromanganese nodules from the Southern Ocean south of
49S and below 2500 m water depth (Albarède et al. [1997],
Frank et al. [2002], and this study) versus temperature of
the water mass the nodules grew in (WOA01 database). All
groupings are primarily based on the physical properties of
the water masses the samples were recovered from.
Symbols are the same as in Figure 2.
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Nd and Hf isotopic compositions of the detrital fraction of
marine core top sediments in direct proximity to Antarctica.
Erosional products from Dronning Maud Land, Prydz Bay,
and Wilkes Land display a very unradiogenic Nd and Hf
isotopic composition (eNd = 11 to 23, eHf = 5 to 30)
in contrast to the more radiogenic Nd and Hf isotope
composition (eNd = +1 to 5, eHf = +4 to 5) found around
West Antarctica. Core top sediments from the Ross and
Weddell Seas show intermediate values (eNd = 3 to 9,
eHf = 3 to 8; sediment results: unpublished data from
M. Roy and T. van de Flierdt, 2004).
8. Potential Mechanisms for the Acquisition of
the Radiogenic Isotope Fingerprint of Wilkes
– Adélie Coast Bottom Water
Land–
[13] Sanudo-Wilhelmy et al. [2002] investigated the
importance of coastal sources for dissolved trace metal
levels in the Weddell Sea and attributed elevated coastal
concentrations to benthic remobilization, including particle
resuspension and diffusive fluxes, and continental
weathering, including shelf ice erosion. Taking the sum of
these processes, we may explain the observed Nd-Hf-Pb
isotope signal of Wilkes Land - Adélie Coast Bottom Water.
[14] The low Hf isotope signature of Wilkes Land - Adélie
Coast bottom water, compared to WSBW, RSBW and
Circumpolar Deep Water (CDW), may be explained by a
weathering signal derived from the old continental crust of
East Antarctica, which gets imprinted on local bottom waters.
It has been suggested by van de Flierdt et al. [2002] that
glacial grinding of old zircons has the potential to release an
unradiogenic Hf isotope signal to the ocean. In addition to
destruction of zircons, enhanced alteration of other less
resistant unradiogenic phases such as biotites or K-feldspar
could also contribute to lowering the seawater Hf isotopic
composition [Bayon et al., 2006]. Incongruent weathering on
the Antarctic continent may also play an important role in
creating the radiogenic Pb isotope signal in the Southern
Ocean from 0 to 149E [Vlastélic et al., 2001]. Weathering of
fresh granitic rock initially releases radiogenic Pb consistent
with a correlation of Pb isotope ratios with water depth in the
Indian Ocean sector of the Southern Ocean [Vlastélic et al.,
2001].
[15] In contrast, the Nd isotopic composition of seawater
more directly reflects the bulk composition of continental
sources. Along the perimeter of East Antarctica the continental Nd isotope signal is likely to assert some influence on
the seawater signal through exchange of Nd along the
margin with newly forming bottom waters [e.g., Lacan
and Jeandel, 2005]. Figure 4 includes all available
authigenic Nd isotope data south of 49S and below
2500 m water depth (Albarède et al. [1997]; Frank et al.
[2002], and this study), and nicely illustrates that the main
controls on the Nd isotopic composition of the Southern
Ocean are geography and water depth. The lowest values
are found in bottom waters of the Indian and Atlantic
sectors of the Southern Ocean where the formation regions
are characterized by old continental rocks (Enderby Land,
Prydz Bay, Wilkes Land, Adélie Coast). In comparison to
Wilkes Land – Adélie Coast Bottom Water, WSBW and
RSBW appear to have a slightly more radiogenic Nd isotope
signature, overlapping with CDW. CDW becomes predom-
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inant above 3700 m, and has a much more variable
isotopic composition due to entrainment of NADW, return
flow of Indian and Pacific deep waters, and mixing with
regional bottom waters and overlying subsurface water
masses.
9. Conclusions
[16] Water mass characteristics in the deep AustralianAntarctic Basin indicate a strong bottom water source in the
region of Adélie Coast and Wilkes Land. By combining
new and previously published results on surface scrapings
of authigenic ferromanganese nodules from the Southern
Ocean, we show that bottom waters in the AustralianAntarctic basin have the lowest Hf and Nd isotopic
compositions and the highest Pb isotopic compositions yet
observed in the Southern Ocean. We suggest that this
signature is the distinct radiogenic isotope fingerprint of
Wilkes Land – Adélie Coast Bottom Water, a water mass
formed in vicinity to the old, glacially weathered continental
crust of East Antarctica.
[17] In contrast to physical water mass properties, a
radiogenic isotope signature can be traced back in time.
Thus, one important implication of our finding is that, given
the availability of suitable archives, radiogenic isotopes can
provide valuable insights into formation and spreading of
Antarctic Bottom Waters in the past ocean.
[18] Acknowledgments. This study was supported by NSF grants
OPP 00-88054 and ANT 05-48918, and the Comer Science and Education
Foundation. We thank G. Bayon and D. Vance for insightful reviews. We
also thank Marty Q. Fleisher, N. Gary Hemming, Anna Cipriani, Allison
M. Franzese, and Jennifer M. Cole for their help in keeping the labs and the
mass specs running smoothly. This is Lamont contribution 6933.
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