Past changes in seawater lead (Pb) isotopes record
the temporal evolution of
anthropogenic pollution, continental weathering inp
uts, and ocean current transport. To
advance our ability to reconstruct this signature,
we present methodological developments
that allow us to make precise and accurate Pb isoto
pe measurements on deep-sea coral
aragonite, and apply our approach to generate the f
irst Pb isotope record for the glacial to
deglacial mid-depth Southern Ocean.
Our refined methodology includes a two-step anion e
xchange chemistry procedure
and measurement using a
207
Pb-
204
Pb double spike on a ThermoFinnigan Triton TIMS
instrument. By employing a 10
12
Ω
resistor (in place of a 10
Ω
resistor) to measure the low-
abundance
204
Pb ion beam, we improve the internal precision on
206,207,208
Pb/
204
Pb for a 2 ng
load of NIST-SRM-981 Pb from typically ~420 ppm to
~260 ppm (2 s.e.), and the long term
external reproducibility from ~960 ppm to ~580 ppm
(2 s.d.). Furthermore, for a typical 500
mg coral sample with low Pb concentrations (~6-10 p
pb yielding ~3-5 ng Pb for analysis), we
obtain a comparable internal precision of ~150-250
ppm for
206,207,208
Pb/
204
Pb, indicating a
good sensitivity for tracing natural Pb sources to
the oceans. Successful extraction of a
seawater signal from deep-sea coral aragonite furth
er relies on careful physical and chemical
cleaning steps, which are necessary to remove anthr
opogenic Pb contaminants and obtain
results that are consistent with ferromanganese cru
sts.
Applying our approach to a collection of late glaci
al and deglacial corals (~12-40 ka
BP) from south of Tasmania at ~1.4-1.7 km water dep
th, we generated the first intermediate
water Pb isotope record from the Southern Ocean. Th
at record reveals millennial timescale
variability, controlled by binary mixing between tw
o Pb sources, but no distinct glacial-
interglacial Pb isotope shift. Mixing between natur
al endmembers is fully consistent with our
data and points to a persistence of the same Pb sou
rces through time, although we cannot rule
out a minor influence from recent anthropogenic Pb.
Whereas neodymium (Nd) isotopes in
the Southern Ocean respond to global ocean circulat
ion changes between glacial and
interglacial periods, Pb isotopes record more local
ised mixing within the Antarctic
Circumpolar Current, potentially further modulated
by climate through changing terrestrial
inputs from southern Africa or Australia. Such deco
upling between Pb and Nd isotopes in the
Southern Ocean highlights their potential to provid
e complementary insights into past
oceanographic variability.