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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Fig. 1: Comparison of εNd and 14C ages at 3.8 and 5 km water depths in the South Atlantic Ocean.
Fig. 2: Modern and LGM Atlantic meridional [CO32–] transects.
Fig. 3: [CO32–] versus δ13C and εNd.
Fig. 4: South Atlantic [CO32–] reconstructions at 3.8 and 5 km water depths compared with atmospheric CO2 during the last 60 ka.

Data availability

We have chosen not to deposit the data at this time but declare that all data presented in this study are provided in the Supplementary Tables.

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Acknowledgements

We thank P. Wang for the long-time service with foraminifera shell picking in the laboratory. This work is supported by NSFC41676026, ARC Discovery Projects (DP140101393, DP190100894) and Future Fellowship (FT140100993) to J.Y., Future Fellowship (FT180100606) and Discovery (DP180100048) to L.M., NSFC (41991322 and 41930864) to Z.D.J. and Australian Laureate Fellowship (FL120100050) to E.J.R. The contribution of J.F.M. was supported in part by the US-NSF. J.Y. was supported in part by the “111” Project (BP0719030) when visiting P. Cai at Xiamen University. G.M. acknowledges support from the University of Vigo’s programme to attract excellent research talent, and a generous start-up package. Core materials were provided by ODP/IODP/DSDP, LDEO (N. Anest), NOC (G. Rothwell, D. Thornalley and I. N. McCave), GEREGE (N. Thouveny) and WHOI (E. Roosen) core repositories.

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J.Y. designed the project and wrote the paper. Z.D.J. and F.Z. managed shell picking. A.M.P. and J.F.M. arranged samples. A.M.P. assisted seawater neodymium data compilation. X.M. plotted Fig. 2. All authors commented on the manuscript.

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Correspondence to J. Yu.

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Extended data

Extended Data Fig. 1 New benthic B/Ca (red circles; unit: μmol/mol) against benthic δ18O (grey circles; unit: ‰) and the LR04 record59 (bold grey lines).

For MD96–2085 and RC11–86, G. inflata and G. sacculifer δ18O (crosses) are shown, after adjusted by +1.8‰ and +3‰, respectively. All benthic B/Ca shown are from this study. References for age models and δ18O are given in Supplementary Table 1.

Extended Data Fig. 2 New and published deep-water [CO32-] using benthic B/Ca along with qualitative proxies for 3–4 km cores from the South Atlantic.

For RC13–228, RC13–229, and TNO57–6, [CO32-] are from this study, and %CaCO3, >63 μm, and fragmentation are from ref. 26. MD07–3076 data are from ref. 28. Age models and δ18O references are given in Supplementary Table 1. All cores show lower deep-water [CO32-] during the LGM than the Holocene.

Extended Data Fig. 3 Deep-water [CO32-] based on benthic B/Ca along with qualitative [CO32-] proxies in core TNO57–21 from the abyssal depth (~5 km) in the South Atlantic.

Also shown are %CaCO3 for another two abyssal cores RC11–83 (41.6°S, 9.8°E, 4718m) and ODP 1089 (40.9°S, 9.9°E, 4621m). Data are from refs. 15,42,60. All cores suggest slightly higher [CO32-] at ~5 km in the South Atlantic during the LGM than the Holocene.

Extended Data Fig. 4 Endmembers for modern and LGM water masses.

#: Italic numbers are assumed values, and using other values would have little effect on mixing lines shown in Fig. 3, due to insensitivity of mixing curvature to DIC values (see Extended Data Fig. 6). *: This study; see Supplementary Tables 1 and 2 for cores used to define associated endmembers.

Extended Data Fig. 5 Meridional Pacific Ocean [CO32-] distribution.

a, [CO32-] transect. b, hydrographic sites11 used to generate a. Today, the core of PDW is located at ~1–2 km in the polar North Pacific with a [CO32-] of ~50 μmol/kg. The low [CO32-] signature can be traced in the Southern Ocean (~50°S) due to the southward transport (southward black arrows) of PDW at ~1–2 km33. During the LGM, the core of GPDW is thought to deepen to ~3 km (dashed half circle)34. Our study suggests that the southward transport (dashed arrows) of GPDW was more extensive. By the time when GPDW was transported to the Pacific sector of the Southern Ocean, its signals would be transported via ACC (circle with an inner dot; transport out of the page) to the South Atlantic Ocean. White circles indicate cores at 3–4 km from the equatorial Pacific Ocean shown in Fig. 3. These cores show lower [CO32-] than the abyssal South Atlantic waters (TNO57–21), indicating that GPDW likely had lower [CO32-] than GAABW.

Extended Data Fig. 6 Alternative scenarios that may contribute to interpretation of the LGM data.

a, as Fig. 3d, but only triple [Nd] of GNAIW. New mixing trend is shown by the blue region. b, as Fig. 3d, but only invoke various degrees of biological respiration (dashed horizontal arrows) associated with GPDW-GAABW-GNAIW mixtures. c, as Fig. 3d, but mixing (blue region) with an aged and hence lower [CO32-] (70 μmol/kg) GAABW. Note that these are just some examples that can potentially contribute to explaining the LGM data, and should not be treated as exhaustive. At present, uncertainties (for example, large endmember εNd ranges and largely unconstrained [Nd]) preclude quantification of mixing ratios and respiration effects and their relative importance. Nevertheless, the more radiogenic εNd at 3.8 km (Fig. 1) would require mixing with GPDW.

Extended Data Fig. 7 Mixing curvature to water-mass endmember DIC and Nd contents.

Effect of endmember DIC changes on (a) δ13C-[CO32-] and (b) εNd-[CO32-]. Relative to the reference cases (grey lines), DICGNAIW and DICGAABW are decreased and increased by 200 μmol/kg, respectively, to intentionally enlarge the DIC contrast between water masses. Effect of endmember [Nd] changes on (c) δ13C-[CO32-] and (d) εNd-[CO32-]. Endmember [Nd] are varied from 1/3 to 3× of the reference value for (c) GNAIW and (d) GAABW. To simplify the view, only GNIAW and GAABW are shown, and GNIAW εNd is only considered at −13.5. This figure suggests that mixing curvature is insensitive to endmember DIC changes, but sensitive to [Nd] changes.

Extended Data Fig. 8 Zonal distribution of [CO32-] in the Southern Ocean.

a, Seawater [CO32-] for three sectors of the Southern Ocean. b, Hydrographic sites (~50–60°S)11 used to generate a. In today’s Southern Ocean, [CO32-] is not zonally homogeneous. Instead, the low-[CO32-] PDW signature is seen in relatively restricted regions at ~1–2 km in the Pacific sector of the Southern Ocean. Via ACC, this signal would be transported to other sectors including the South Atlantic, although its influence is not very clearly seen due to strong vertical mixing that tends to erode any signal anomalies. Our study suggests that the influence of GDPW was more extensive and deeper (~3–4 km) in the Southern Ocean during the LGM. GPDW influence is recorded by [CO32-] and other proxies (for example, εNd and 14C) from the deep South Atlantic.

Extended Data Fig. 9 Scenarios for different GAABW δ13C values.

a, “Mackensen” effects that would affect both deep-water δ13C and [CO32-]. b, Habitat change that only affects deep-water δ13C. See “GAABW δ13C endmember” in Methods for details.

Extended Data Fig. 10 LGM-Holocene [CO32-] difference for cores from >3 km in the Atlantic.

sd: standard deviation.

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Supplementary Tables 1–7.

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Yu, J., Menviel, L., Jin, Z.D. et al. Last glacial atmospheric CO2 decline due to widespread Pacific deep-water expansion. Nat. Geosci. 13, 628–633 (2020). https://doi.org/10.1038/s41561-020-0610-5

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