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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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... It is widely accepted that the magnitude of cooling and continental ice expansion at the LGM was amplified by the reduction in atmospheric CO 2 (e.g. Shakun et al., 2012;Marcott et al., 2014), and that more CO 2 was sequestered within the ocean at that time than is the case today (Yu et al., 2020;Curry and Oppo, 2005;Gebbie et al., 2015). ...
... There is also evidence for a shutdown in the export of AABW from the Weddell Sea region, which would further shift deep water ε Nd towards more radiogenic values (Huang et al., 2020). This reduction in NCW is consistent with evidence from benthic foraminiferal d 13 C (McCorkle et al., 1998;Hodell et al., 2003;Lund et al., 2015;Curry and Oppo, 2005;Gebbie et al., 2015;Sikes et al., 2017), d 18 O (Sikes 2017), and Cd/Ca (Marchitto et al., 1998;Umling et al., 2019), reconstructed seawater carbonate ion concentrations (Yu et al., , 2020 and south Atlantic and Southern Ocean sediment distributions during the LGM (Diekmann et al., 1999). ...
... The radiogenic ε Nd data of core TT1811-34GGC indicate that the lower circulation cell was made up of a greater proportion of recirculated PDW throughout the last glacial period, not just during the LGM (Wilson et al., 2020;Yu et al., 2020). The offset in ε Nd between our core T1811-34GGC and ODP Site 1088 suggests the inability of NCW from the South Atlantic to mix into the main body of LCDW across the entire last glacial period, even at times when AMOC may have been relatively strong (Bohm et al., 2015). ...
Article
The chain of events surrounding the initiation and intensification of the last glacial cycle remain relatively poorly understood. In particular, the role of Southern Ocean paleocirculation changes is poorly constrained, in part, owing to a paucity of sedimentary records from this region. In this study we present multiproxy data e including neodymium isotope and sortable silt measurements e for paleocirculation changes within the deep (3167 m water depth) Indian sector of the Southern Ocean from a new sediment core, TT1811-34GGC (41.718?S, 80.163?E). We find a tight coupling between circulation changes, Antarctic climate, and atmospheric CO2 concentrations throughout the last 118,000 years, even during the initial stages of glacial inception of Marine Isotope Stage (MIS) 5.4 to 5.1. We find that periods of cooling correspond to reductions in the entrainment of North Atlantic-sourced waters within the deep Southern Ocean, as evidenced by more radiogenic neodymium isotope values of deep water bathing our core site. Cooling also corresponds to generally slower bottom water flow speeds, as indicated by finer sortable silt size fractions. A reduction in entrainment of North-Atlantic sourced waters occurred during MIS 5.4e5.1, when Atlantic circulation was strong, suggesting a Southern hemisphere control on paleocirculation changes at that time. We hypothesise that expanded Southern Ocean sea-ice during MIS 5.4 increased the density of the deep Southern Ocean, reducing the ability of Atlantic-sourced waters to mix into Lower Circumpolar Deep Water. This led to an expanded contribution of Pacific Deep Water within the lower circulation cell and increased stratification within the deep Southern Ocean. These paleocirculation changes can help account for the reduction in atmospheric CO2 across the MIS 5.5 to 5.4 transition, and in doing so help explain the chain of events surrounding the decent into the last glacial period.
... However, more records from the Southern Indian Ocean, particularly along these two separate pathways and water column data are required to validate the above hypothesis. On the contrary, similar values with more radiogenic glacial ε Nd (ε Nd ¼ À6.8 ± 0.4) in South-East Atlantic , equatorial Indian Ocean (Piotrowski et al., 2009), and Arabian Sea (present study) suggest reduction in the export/shallowing of NADW (Rutberg et al., 2000;Yu et al., 2020). ...
... To quantify the fractional contributions of NADW and AABW during G-I periods in the Arabian Sea, we performed a Nd isotope mass balance calculation following the method of Rahaman et al. (2020) with the assumptions that Nd isotope behaves quasiconservatively and the mixing of the water masses at this core site is binary. Since the isotopic values of the end-members have changed during the G-I cycles (Yu et al., 2020), we have calculated water mass fractions separately for the interglacial and glacial stages. The late Holocene period (0e5 ka) is taken as a representative of the warm interglacials and the last glacial maximum (LGM) (18e22 ka) as a representative of the cold glacials. ...
... The endmember details of water masses are given in Table S5. The Holocene ε Nd and [Nd] end-member values of AABW and NADW, À8.0 ± 1.0, 25.1 pmol/kg, and À13.5 ± 0.5, 17.5 pmol/kg, respectively, were taken from published literature (Howe et al., 2016;Yu et al., 2020). However, the glacial NADW (GNADW) ε Nd end member value is not well constrained because of the large variability (À10.5 to À13.9) observed in the North Atlantic during the glacial periods (Bohm et al., 2015). ...
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Global overturning circulation plays a vital role in atmospheric CO2 and climate variability during glacial-interglacial (G-I) cycles; however, the exact mechanism remains elusive due to inadequate knowledge on past deep water circulation in the global ocean. Since no deep water is formed in the northern Indian Ocean, it ventilates from the south and acts only as a host for deep water circulation. Absence of any active deep water formation makes the northern Indian Ocean an ideal location to assess the extent of southern source waters and its role on past CO2 variability during the G-I climate cycles. This study provides the first record of deep water circulation in the Arabian Sea, the northwestern Indian Ocean, during the past 136 ka based on authigenic Nd isotope record (εNd). The Arabian Sea εNd record shows large variability ranging from −8.8 to −6.5 with more radiogenic values during the glacial stages (MIS 2 & 6) and less radiogenic values during the interglacial stages (MIS 1 & 5) indicating changes in water mass sources. The observation of more radiogenic εNd values similar to the glacial Antarctic Bottom Water (AABW) indicates enhanced flow of AABW (95–100%) and substantial reduction and/or almost complete retreat of North Atlantic Deep Water (NADW, 0–5%) during the glacials, whereas less radiogenic values indicate enhanced flow of NADW (∼20–40%) during the interglacials. The Arabian Sea εNd record followed exactly similar pattern to that of the equatorial Indian Ocean (EIO). However, amplitude of their variations differed significantly during the interglacials (MIS 1 & 5); the Arabian Sea εNd values were more radiogenic than the EIO. This suggests that during the interglacials, the Arabian Sea received more fraction of AABW through the western pathway, whereas the EIO received more fraction of NADW through the central pathway. This highlights differences in deep water exports from the Southern Ocean to the Arabian Sea and the EIO during the interglacials whereas export of similar water masses and its uniform distribution up to the northern Indian Ocean during the glacials. Our findings of significant G-I changes in AABW and NADW exports to the Indian Ocean and intra-basinal differences in their distribution have important implications for regional biogeochemical processes, paleo-redox conditions in the water column, carbon sink (organic and inorganic) and atmospheric CO2 variability during the G-I climate transitions.
... Buoyancy fluxes associated with sea ice production and melt play a primary role in Southern Ocean circulation (Abernathey et al., 2016;Pellichero et al., 2018), and changes in sea ice extent can potentially influence the basin-wide organization of water masses (Ferrari et al., 2014). While we do not see evidence for a dramatic shoaling of NADW at any time over the last glacial cycle, increased mid to deep gradients in εNd and δ 13 C could indicate the presence of more southern-sourced deep water, potentially with a larger contribution from the Pacific (as was observed at the LGM by Yu et al., 2020). Increased sea ice can also influence δ 13 C values through its influence on the regional extent of air-sea gas exchange (Broecker & Maier-Reimer, 1992;Lynch-Stieglitz & Fairbanks, 1994;Lynch-Stieglitz et al., 1995;Marchitto & Broecker, 2006). ...
... Using the B/Ca proxy for carbonate ion, Yu et al. (2020) also found evidence for a greater fraction of Pacific-sourced water in the deep Cape Basin. While they argue that this results from a shoaling of NADW, which our data do not support, we similarly do find evidence for a larger fraction of PDW relative to NADW at deep Sites RC11-83/TN057-21. ...
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A common conception of the deep ocean during ice age episodes is that the upper circulation cell in the Atlantic was shoaled at the Last Glacial Maximum compared to today, and that this configuration facilitated enhanced carbon storage in the deep ocean, contributing to glacial CO2 draw‐down. Here, we test this notion in the far South Atlantic, investigating changes in glacial circulation structure using paired neodymium and benthic carbon isotope measurements from International Ocean Discovery Program Site U1479, at 2,615 m water depth in the Cape Basin. We infer changes in circulation structure across the last glacial cycle by aligning our site with other existing carbon and neodymium isotope records from the Cape Basin, examining vertical isotope gradients, while determining the relative timing of inferred circulation changes at different depths. We find that Site U1479 had the most negative neodymium isotopic composition across the last glacial cycle among the analyzed sites, indicating that this depth was most strongly influenced by North Atlantic Deep Water (NADW) in both interglacial and glacial intervals. This observation precludes a hypothesized dramatic shoaling of NADW above ∼2,000 m. Our evidence, however, indicates greater stratification between mid‐depth and abyssal sites throughout the last glacial cycle, conditions that developed in Marine Isotope Stage 5. These conditions still may have contributed to glacial carbon storage in the deep ocean, despite little change in the mid‐depth ocean structure.
... . It has been proposed that deep-water, formed due to increased surface salinity driven by subdued freshwater flux during the last deglacial cold events, may have played a key role in global heat distribution and carbon cycles (Okazaki et al., 2010;Rae et al., 2014Rae et al., , 2020Yu et al., 2020). However, some studies argue that during the deglacial cold events, only enhanced intermediate-water ventilation occurred above a depth of ∼2,000 m in the North Pacific driven by the active polynya formation and brine rejection during sea ice formation in the Okhotsk Sea and/or western Bering Sea, while the deep ocean remained isolated (Gong et al., 2019;Jaccard & Galbraith, 2013;Max et al., 2014;Ohkushi et al., 2003). ...
... Previous studies suggested that North Pacific deep-water could form during the last deglacial abrupt cold events due to increased surface salinity driven by subdued local freshwater flux (Okazaki et al., 2010;Rae et al., 2014Rae et al., , 2020Yu et al., 2020). Precipitation in the North Pacific mid-latitudes is mainly shaped by the storm tracks, the position of which can be affected by the equator-to-pole temperature gradient (Brayshaw et al., 2008;Shaw et al., 2016). ...
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... The Pacific Ocean, which is the biggest marine carbon reservoir at the end of the global ocean conveyor belt, hosts the oldest and carbon-rich deep water in the global ocean owing to accumulated organic matter remineralization (Yu et al., 2020). The corrosive Pacific deep water (Sexton and Barker, 2012) results in shallower preservation depth of calcite (rarer and more soluble aragonite is excluded in our analysis), which hereafter is the only carbonate mineral considered in this article, in the Pacific sediments than that in the Atlantic and Indian ocean sediments (Berger et al., 1976;Biscaye et al., 1976;Kolla et al., 1976). ...
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... 2− ], with highest values during the LGM as seen in many studies for middepth records from the North Atlantic Ocean (Yu et al., 2008(Yu et al., , 2010(Yu et al., , 2020. The δ 13 C in the infaunal species O. umbonatus is on average ∼1‰ lower during the last glacial (∼40-19 ka) compared to the Holocene (Figure 4e). ...
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... A comparison of our ACC strength record to a high resolution carbonate saturation reconstruction from the Subantarctic South Atlantic 49 indicates a correspondence of stronger ACC with reduced carbonate saturation during the major Antarctic warm intervals (Fig. 4b-d). These changes in inflow of Pacific-type deep-water masses likely influenced the carbonate chemistry in the subantarctic South Atlantic competing with North Atlantic sourced-deep waters at millennial time-scales 49,55 . The contribution of Antarctic Bottom Water circulation to the modern ACC is modest, but potentially increased during glacial periods when expanded sea ice favored its production and increased its salinity 12,40 . ...
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... 2− ] and neodymium isotopes (Yu et al., 2020). Our δ 14 R record, taken together with published records, suggests that the deep Southern Ocean became more isolated from the upper ocean and the atmosphere during the LGM. ...
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