Last glacial atmospheric CO2 decline due to widespread Pacific deep-water expansion

Abstract

Ocean circulation critically affects the global climate and atmospheric carbon dioxide through redistribution of heat and carbon in the Earth system. Despite intensive research, the nature of past ocean circulation changes remains elusive. Here we present deep-water carbonate ion concentration reconstructions for widely distributed locations in the Atlantic Ocean, where low carbonate ion concentrations indicate carbon-rich waters. These data show a low-carbonate-ion water mass that extended northward up to about 20° S in the South Atlantic at 3–4 km depth during the Last Glacial Maximum. In combination with radiocarbon ages, neodymium isotopes and carbon isotopes, we conclude that this low-carbonate-ion signal reflects a widespread expansion of carbon-rich Pacific deep waters into the South Atlantic, revealing a glacial deep Atlantic circulation scheme different than commonly considered. Comparison of high-resolution carbonate ion records from different water depths in the South Atlantic indicates that this Pacific deep-water expansion developed from approximately 38,000 to 28,000 years ago. We infer that its associated carbon sequestration may have contributed critically to the contemporaneous decline in atmospheric carbon dioxide, thereby helping to initiate the glacial maximum.

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Articles
https://doi.org/10.1038/s41561-020-0610-5
1Research School of Earth Sciences, The Australian National University, Canberra, Australian Capital Territory, Australia. 2State Key Laboratory of Loess
and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, China. 3Climate Change Research Centre, University of
New South Wales, Sydney, New South Wales, Australia. 4CAS Center for Excellence in Quaternary Science and Global Change, Xi’an, China. 5Open Studio
for Oceanic–Continental Climate and Environment Changes, Qingdao National Laboratory for Marine Science and Technology, Qingdao, China.
6Lamont–Doherty Earth Observatory, Columbia University, New York, NY, USA. 7State Key Laboratory of Marine Geology, Tongji University, Shanghai,
China. 8Department of Earth Sciences, University of Cambridge, Cambridge, UK. 9Ocean and Earth Science, National Oceanography Centre, University
of Southampton, Southampton, UK. 10Centro de Investigación Mariña, GEOMA, Palaeoclimatology Lab, Universidade de Vigo, Vigo, Spain.
✉e-mail: jimin.yu@anu.edu.au
Ocean circulation and the carbon cycle are intricately linked,
thus ocean circulation reconstructions can provide impor-
tant insights into the mechanisms of past atmospheric CO2
changes. Circulation in the deep (more than ~2.5 km) Atlantic dur-
ing the Last Glacial Maximum (LGM; 18–22 thousand years ago
(ka)) is traditionally viewed as following a mixing model between
deep waters formed in the basin’s polar regions, without much con-
tribution from waters from other oceans1–4. Using this long-held
ocean circulation model, however, it is difficult to explain the
observed older radiocarbon (14C) ages and more radiogenic neo-
dymium isotopic (εNd) signatures at ~3.8 km than at ~5 km in the
LGM South Atlantic5,6 (Fig. 1). Burke etal.7 showed that sluggish
recirculation of southern-sourced waters combined with reduced
mixing with 14C-rich northern-sourced waters can contribute to old
14C ages at ~3.8 km, in the absence of interocean water-mass inter-
actions. Yet, additional mechanisms are probably needed to fully
explain the depth structure and large magnitude of 14C age changes,
along with the more radiogenic εNd signal observed at 3.8 km
(Fig. 1). Pacific Deep Water (PDW) can notably affect deglacial εNd
signatures in the Drake Passage (Southern Ocean)8, but their role in
the deep South Atlantic during the LGM remains unexplored. PDW
stores a large amount of respired carbon9,10, thus temporal changes
in its volumetric extent would have important implications for past
atmospheric CO2 levels.
Deep-water carbonate ion concentrations ([CO32–]) can provide
critical information about past deep ocean circulation and dissolved
inorganic carbon (DIC) changes. In the modern Atlantic, contrast-
ing [CO32–] signatures between water masses reflect ocean circula-
tion patterns11 (Fig. 2). Also, past DIC changes may be quantified
from [CO32–] reconstructions12. Here we present deep-water [CO32–]
reconstructions for extensive locations in the Atlantic to deci-
pher the role of ocean circulation in the glacial atmospheric CO2
decrease. We focus on deep South Atlantic hydrography, which
remains incompletely understood despite intensive studies5–8,13–16.
First meridional [CO32–] transect for the LGM Atlantic
We have reconstructed deep-water [CO32–] using benthic B/Ca for
the Holocene (0–5 ka) and LGM samples from 41 cores (Fig. 2 and
Extended Data Figs. 1–3). Five cores at 3.0–4.2 km and an abyssal
core at ~5 km from the South Atlantic were chosen to investigate the
reasons for the 14C and εNd anomalies at 3.8 km water depth (Figs. 1
and 2a). Thirty additional cores from widely spread locations (1.1–
4.7 km, 36° S to 62° N) in the Atlantic and five cores at 3–4 km from
the equatorial Pacific provide a broader context of water-mass sig-
natures. Benthic B/Ca is converted into deep-water [CO32–] using
species-specific global core-top calibrations17. The uncertainty asso-
ciated with [CO32–] reconstructions is ~5 μmol kg–1 (ref. 17). Detailed
information about the samples and analytical methods along with
new (n = 173 samples) and compiled (n = 260 samples) data is given
in Methods and Supplementary Tables 1–7.
Figure 2c shows the first meridional [CO32–] transect for the
deep Atlantic during the LGM (Methods). Given the locations of
the studied cores, this transect mainly reflects [CO32–] distributions
for eastern Atlantic basins. Future work is needed to investigate the
extent of zonal homogeneity in the LGM Atlantic. Above ~2.5 km,
the [CO32–] of glacial North Atlantic waters reached up to ~140 μmol
kg–1, which is ~20 μmol kg–1 higher than in modern North Atlantic
Deep Water (NADW)11. These waters likely represent the previously
Last glacial atmospheric CO2 decline due to
widespread Pacific deep-water expansion
J. Yu 1,2 ✉ , L. Menviel 3, Z. D. Jin 2,4,5, R. F. Anderson 6, Z. Jian7, A. M. Piotrowski8, X. Ma2,
E. J. Rohling 1,9, F. Zhang 2,4, G. Marino1,10 and J. F. McManus6
Ocean circulation critically affects the global climate and atmospheric carbon dioxide through redistribution of heat and carbon
in the Earth system. Despite intensive research, the nature of past ocean circulation changes remains elusive. Here we pres-
ent deep-water carbonate ion concentration reconstructions for widely distributed locations in the Atlantic Ocean, where low
carbonate ion concentrations indicate carbon-rich waters. These data show a low-carbonate-ion water mass that extended
northward up to about 20° S in the South Atlantic at 3–4 km depth during the Last Glacial Maximum. In combination with radio-
carbon ages, neodymium isotopes and carbon isotopes, we conclude that this low-carbonate-ion signal reflects a widespread
expansion of carbon-rich Pacific deep waters into the South Atlantic, revealing a glacial deep Atlantic circulation scheme differ-
ent than commonly considered. Comparison of high-resolution carbonate ion records from different water depths in the South
Atlantic indicates that this Pacific deep-water expansion developed from approximately 38,000 to 28,000 years ago. We infer
that its associated carbon sequestration may have contributed critically to the contemporaneous decline in atmospheric carbon
dioxide, thereby helping to initiate the glacial maximum.
NATURE GEOSCIENCE | VOL 13 | SEPTEMBER 2020 | 628–633 | www.nature.com/naturegeoscience
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