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

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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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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 oceans14. 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 studies58,1316.
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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... The eastern South Pacific is particularly important for the carbon cycle as aged CO 2 -rich Pacific Deep Water (PDW) arrives from the north (Kawabe and Fujio 2010;Siani et al. 2013). During the last glacial period, CO 2 -rich PDW-influenced Circumpolar Deep Water (CDW) isolated from the atmosphere and partially flowed into the South Atlantic through the Drake Passage, likely affecting CaCO 3 dissolution not only in the Pacific sector but also in the Atlantic sector of the Southern Ocean (Siani et al. 2013;Ronge et al. 2016;Ullermann et al. 2016;Allen et al. 2020;Yu et al. 2020;Iwasaki et al. 2022). ...
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The chemical composition of benthic foraminifera from marine sediment cores provides information on how glacial subsurface water properties differed from modern, but separating the influence of changes in the origin and end-member properties of subsurface water from changes in flows and mixing is challenging. Spatial gaps in coverage of glacial data add to the uncertainty. Here we present new data from cores collected from the Demerara Rise in the western tropical North Atlantic, including cores from the modern tropical phosphate maximum at Antarctic Intermediate Water (AAIW) depths. The results suggest lower phosphate concentration and higher carbonate saturation state within the phosphate maximum than modern despite similar carbon isotope values, consistent with less accumulation of respired nutrients and carbon, and reduced air-sea gas exchange in source waters to the region. An inversion of new and published glacial data confirms these inferences and further suggests that lower preformed nutrients in AAIW, and partial replacement of this still relatively high-nutrient AAIW with nutrient-depleted, carbonate-rich waters sourced from the region of the modern-day northern subtropics, also contributed to the observed changes. The results suggest that glacial preformed and remineralized phosphate were lower throughout the upper Atlantic, but deep phosphate concentration was higher. The inversion, which relies on the fidelity of the paleoceanographic data, suggests that the partial replacement of North Atlantic sourced deep water by Southern Ocean Water was largely responsible for the apparent deep North Atlantic phosphate increase, rather than greater remineralization.
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Micropaleontological and geochemical analyses reveal distinct millennial-scale increases in carbonate preservation in the deep Southeast Atlantic (Cape Basin) during strong and prolonged Greenland interstadials that are superimposed on long-term (orbital-scale) changes in carbonate burial. These data suggest carbonate oversaturation of the deep Atlantic and a strengthened Atlantic Meridional Overturning Circulation (AMOC) during the most intense Greenland interstadials. However, proxy evidence from outside the Cape Basin indicates that AMOC changes also occurred during weaker and shorter Greenland interstadials. Here we revisit the link between AMOC dynamics and carbonate saturation in the deep Cape Basin over the last 400 kyr (sediment cores TN057-21, TN057-10, and Ocean Drilling Program Site 1089) by reconstructing centennial changes in carbonate preservation using millimeter-scale X-ray fluorescence (XRF) scanning data. We observe close agreement between variations in XRF Ca/Ti, sedimentary carbonate content, and foraminiferal shell fragmentation, reflecting a common control primarily through changing deep water carbonate saturation. We suggest that the high-frequency (suborbital) component of the XRF Ca/Ti records indicates the fast and recurrent redistribution of carbonate ions in the Atlantic basin via the AMOC during both long/strong and short/weak North Atlantic climate anomalies. In contrast, the low-frequency (orbital) XRF Ca/Ti component is interpreted to reflect slow adjustments through carbonate compensation and/or changes in the deep ocean respired carbon content. Our findings emphasize the recurrent influence of rapid AMOC variations on the marine carbonate system during past glacial periods, providing a mechanism for transferring the impacts of North Atlantic climate anomalies to the global carbon cycle via the Southern Ocean.
Article
A growing body of evidence suggests that respired carbon was stored in mid-depth waters (∼1–3 km) during the last glacial maximum (LGM) and released to the atmosphere from upwelling regions during deglaciation. Decreased ventilation, enhanced productivity, and enhanced carbonate dissolution are among the mechanisms that have been cited as possible drivers of glacial CO2 drawdown. However, the relative importance of each of these mechanisms is poorly understood. New approaches to quantitatively constrain bottom water carbonate chemistry and oxygenation provide methods for estimating historic changes in respired carbon storage. While increased CO2 drawdown during the LGM should have resulted in decreased oxygenation and a shift in dissolved inorganic carbon (DIC) speciation towards lower carbonate ion concentrations, this is complicated by the interplay of carbonate compensation, export productivity, and circulation. To disentangle these processes, we use a multiproxy approach that includes boron to calcium (B/Ca) ratios of the benthic foraminifera Cibicidoides wuellerstorfi to reconstruct deep-water carbonate ion concentrations ([CO3²⁻]) and the uranium to calcium (U/Ca) ratio of foraminiferal coatings in combination with benthic foraminiferal carbon isotopes to reconstruct changes in bottom water oxygen concentrations ([O2]) and organic carbon export. Our records indicate that LGM [CO3²⁻] and [O2] was reduced at mid water depths of the eastern equatorial Pacific (EEP), consistent with increased respired carbon storage. Furthermore, our results suggest enhanced mixing of lower Circumpolar Deep Water (LCDW) to EEP mid water depths and provide evidence for the importance of circulation for oceanic-atmospheric CO2 exchange.
Article
CO 2 escaped from the deep Why did the concentration of atmospheric carbon dioxide rise so much and so quickly during the last deglaciation? Evidence has begun to accumulate suggesting that old, carbon-rich water accumulated at depth in the Southern Ocean, which then released its charge when Southern Ocean stratification broke down as the climate there warmed. Basak et al. present measurements of neodymium isotopes that clearly show that the deepwater column of the glacial southern South Pacific was stratified, just as would be necessary for the accumulation of old, carbon-rich water. Their data also show that North Atlantic processes were not the dominant control on Southern Ocean water-mass structure during that interval, as has been thought. Science , this issue p. 900