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This work was financially supported by the Deutsche
Forschungsgemeinschaft under its Research Grants program
(H.B.) and Research Training Group GRK2112 - Molecular
Biradicals: Structure, Properties and Reactivity (B.E.).
M.-A.L. thanks the National Sciences and Engineering Research
Council of Canada for a postdoctoral fellowship. G.B.-C. thanks
the Alexander von Humboldt Foundation for a postdoctoral
fellowship. We thank R. Bertermann for NMR experiments,
K. Radacki for crystallographic insight, and C. Mahler for mass
spectrometry. X-ray data are available free of charge from the
Cambridge Crystallographic Data Centre under reference
CCDC-1578524 (5), -1578525 (1), -1578526 (8), -1578527 (6),
and -1578528 (7). Further spectroscopic, crystallographic, and
computational data are included in the supplementary materials.
Materials and Methods
Figs. S1 to S54
Tables S1 and S2
9 October 2017; accepted 12 December 2017
Breakup of last glacial deep
stratification in the South Pacific
Chandranath Basak,1*†‡ Henning Fröllje,1,2‡ Frank Lamy,3 Rainer Gersonde,3
Verena Benz,3 Robert F. Anderson,4 Mario Molina-Kescher,5 Katharina Pahnke1
Stratification of the deep Southern Ocean during the Last Glacial Maximum is thought
to have facilitated carbon storage and subsequent release during the deglaciation as
stratification broke down, contributing to atmospheric CO2 rise. Here, we present
neodymium isotope evidence from deep to abyssal waters in the South Pacific that
confirms stratification of the deepwater column during the Last Glacial Maximum. The
results indicate a glacial northward expansion of Ross Sea Bottom Water and a Southern
Hemisphere climate trigger for the deglacial breakup of deep stratification. It highlights
the important role of abyssal waters in sustaining a deep glacial carbon reservoir and
Southern Hemisphere climate change as a prerequisite for the destabilization of the water
column and hence the deglacial release of sequestered CO2 through upwelling.
The Southern Ocean (SO) has long been rec- ognized as a key player in regulating at- mospheric CO2 variations and hence global climate based on the tight coupling be- tween Southern Hemisphere (SH) temperatures and atmospheric CO2 concentrations (1).
Nevertheless, the mechanisms involved are not
fully understood. The most promising explana-
tion includes changes of the biological pump and
its interaction with ocean circulation in the SO,
where CO2 sequestration and release occur be-
cause of the production of new and upwelling of
old deep waters (2–4). The SO therefore acts as
a component of ocean-atmosphere interactions
that is sensitive to changes in climate and the
stability of the water column. Evidence exists
for a stratified deep SO during the Last Glacial
Maximum (LGM) (5, 6), which led to diminished
gas exchange as compared with that of today and
the presence of radiocarbon-depleted deep waters
in the South Pacific (7 ) overlying better ventilated
bottom waters (7–9). With the onset of SH warm-
ing during the last deglaciation, which was approx-
imately coincident with Heinrich Stadial 1 (HS1),
the SO water column became destratified and
well mixed, releasing sequestered CO2 to the
atmosphere and contributing to the deglacial
atmospheric CO2 rise (4–7). The proposed de-
stratification mechanisms include southward-
shifting westerlies and enhanced upwelling (5),
as well as sea ice retreat, associated buoyancy
flux changes, and increased mixing of northern-
and southern-sourced waters (10). Here, we show
evidence for a sharp geochemical boundary be-
tween deep and bottom waters in the Pacific
sector of the SO during the LGM. This deep
stratification is defined by distinct neodymium
(Nd) isotope signatures of the deep and abyssal
waters during the LGM, in contrast to a homog-
eneous Nd isotopic composition of the deep to
abyssal South Pacific today (11). We further sug-
gest a role for SH climate in triggering the break-
up of deep stratification, thus setting the stage
for upwelling and release of sequestered carbon.
We used Nd isotopes (143Nd/144Nd, expressed
as eNd) from fossil fish teeth and debris and
planktic foraminifera from deep (3000 to 4000 m)
to abyssal (>4000 m) water depths in the South
Pacific in order to reconstruct the history of the deep-
water column structure over the past 30,000 years
(Fig. 1 and supplementary materials). The eNd
signature of modern Circumpolar Deep Water
(CDW) (eNd = –8 to –9) (11, 12) is largely deter-
mined by mixing between North Atlantic Deep
Water (NADW) with eNd = –13.5 near its source
(13) and North Pacific Deep Water (NPDW) with
eNd = –3.5 (14), with additional contributions
from Antarctic Bottom Water (AABW) with
eNd = –9 and –7 in the South Atlantic (12) and
the South Pacific (11), respectively. It is well
documented that end-member eNd signatures
of NPD W and NADW remained constant at least
for the past 2 million years (15 ). Ross Sea Bottom
Water (RSBW) acquires its eNd from Antarctic
shelf sediments. Because the Ross Sea shelf re-
ceives its sediments through glacier transport, a
substantial change in the shelf sediment lithol-
ogy would require a change in provenance and/
or flow direction of the glaciers. Thus, we assume
that the RSBW end member remained constant
and that the observed seawater changes are re-
lated to water mass mixing. Previous studies sug-
gest that glacial-interglacial and shorter-term eNd
changes of South Atlantic deep water were con-
trolled by changes in NADW export (16–18). We
present evidence from South Pacific deep (E11-2,
PS75/073-2, PS75/056-1, and PS75/059-2; 3109
to 3613 m water depth) and abyssal (PS75/054-1;
4085 m water depth) (Fig. 1 and table S1) sedi-
ment cores that indicates instead a strong ad-
ditional SH climate control on the development
900 23 FEBRUARY 2018 • VOL 359 ISSUE 6378 sciencemag.org SCIENCE
1Max Planck Research Group for Marine Isotope Geochemistry,
Institute for Chemistry and Biology of the Marine Environment
(ICBM), University of Oldenburg, Carl-von-Ossietzky-Strasse
9-11, 26129 Oldenburg, Germany. 2Department of Geosciences,
University of Bremen, Klagenfurter Strasse 2-4, 28359 Bremen,
Germany. 3Alfred Wegener Institute, Helmholtz Centre for Polar
and Marine Research, Am Handelshafen 12, 27570 Bremerhaven,
Germany. 4Lamont-Doherty Earth Observatory of Columbia
University, 61 Route 9W, Palisades, NY 10964, USA. 5GEOMAR
Helmholtz Centre for Ocean Research Kiel, Wischhofstraße 1-3,
24148 Kiel, Germany.
*Corresponding author. Email: email@example.com
†Present address: California State University, 9001 Stockdale
Highway, Bakersfield, CA 93311, USA. ‡These authors contributed
equally to this work.