Abstract
The Indian Ocean provides a source of salt for North Atlantic deep-water convection sites, via the Agulhas Leakage, and may thus drive changes in the ocean’s overturning circulation1,2,3. However, little is known about the salt content variability of Indian Ocean and Agulhas Leakage waters during past glacial cycles and how this may influence circulation. Here we show that the glacial Indian Ocean surface salt budget was notably different from the modern, responding dynamically to changes in sea level. Indian Ocean surface salinity increased during glacial intensification, peaking in glacial maxima. We find that this is due to rapid land exposure in the Indonesian archipelago induced by glacial sea-level lowering, and we suggest a mechanistic link via reduced input of relatively fresh Indonesian Throughflow waters into the Indian Ocean. Using climate model results, we show that the release of this glacial Indian Ocean salinity via the Agulhas Leakage during deglaciation can directly impact the Atlantic Meridional Overturning Circulation and global climate.
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Data availability
The geochemical datasets produced in this study are freely available at the data repository Pangea at https://doi.pangaea.de/10.1594/PANGAEA.955609. Source data are provided with this paper.
Code availability
Python code was used to calculate the stacks and analyse the ANICE-SELEN image outputs. Both are freely available on Zenodo, for stack calculation (https://zenodo.org/record/7478552 and https://doi.org/10.5281/zenodo.7478552), and ANICE-SELEN image processing (https://zenodo.org/record/7471500 and https://doi.org/10.5281/zenodo.7471500).
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Acknowledgements
We thank the members and crew of IODP expedition 361 for their work in collecting and providing the samples. This work was mainly funded through UK NERC grant NE/P000878/1 provided to S.N. by S.B. J.W.B.R. received funding for this work from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement 805246) which also funded M.D.D. S.N. received further funds from the Ministry of Science and Technology, Taiwan (MOST 111-2636-M-002-020) granted to H. Ren. X.Z. and Y.S. were funded through the Natural Science Foundation of China (41988101 and 42075047) and German Helmholtz Postdoc Program (PD-301). X.Z. also acknowledges support from the National Key Scientific and Technological Infrastructure project “Earth System Science Numerical Simulator Facility” (EarthLab). H.T.M. was funded through the Ministry of Science and Technology, Taiwan (110-2811-M-002-647 and 111-2811-M-002-116).
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S.N. collected, analysed and interpreted the data. S.N. wrote the paper, with support from all co-authors. J.W.B.R., M.B.A. and S.B. supported the collection, analysis and interpretation of the data, and provided the grant. X.Z. and Y.S. conducted the general circulation model simulations and supported the interpretation of the results. M.D.D. computed the data stacks and supported the interpretation of the results. B.d.B. and H.T.M. conducted model runs and postprocessing of sea-level and land/sea ratio reconstructions and supported the interpretation of the results. The samples were collected by S.B. and I.R.H. on IODP expedition 361. All authors provided comments on the final paper.
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Extended data figures and tables
Extended Data Fig. 1 U1476 sea surface hydrography reconstructions in comparison to U1476 benthic oxygen isotopes.
(a), (c), and (f) benthic foraminifera δ18O51 on inverted y-axes (light blue lines), (b) planktonic δ18Ocarbonate from G. ruber on inverted y-axis (green line, n = 371; this study), average analytical error indicated to the right of the record, (d) planktonic Mg/Ca with log-scale y-axis and (e) calculated SSTs (red line, n-383; this study) with Monte-Carlo propagated errors including analysis, age model, and equation calibration errors plotted as red shaded envelope, (g) calculated ice-volume corrected δ18Osw-ivc for the surface ocean as proxy for salinity, derived from the δ18OG.ruber and SST records (dark green; this study) with propagated errors including Monte-Carlo SST-, and δ18OG.ruber -analysis errors plotted as dark green shaded envelope. The centre of each deglaciation is indicated by dashed vertical lines and labelled T1 to T16. Note the increases in SST and δ18Osw-ivc during glacial periods (grey shaded bars).
Extended Data Fig. 2 Derivation and alternative derivations of the newly presented climate proxies.
(a) Atmospheric CO2 from Antarctic ice cores53 (pink), and by calculation using linear transfer from the LR04 benthic δ18O stack48 (dark red). (b) LR04 benthic δ18O stack48. (c) Resulting SST calculations using the method of ref. 9. with ice core CO253 (pink), and LR0448-based CO2 (dark red). (d) Difference between δ18O of global ice volume49 (blue) and δ18O of U1476 seawater (this study) prior to the ice volume correction (green). The large difference suggests that there is a substantial local salinity signal which cannot be explained by whole ocean changes. (e) Age model comparison using benthic δ18O for all sites in Fig. 2 (refs. 23,28,29,51). (f) U1476 salinity proxy (dark green; this study); dark green diamonds highlight times of salinification onset, and (g) GMSL49 (pink) highlighting sea level stands at times of salinification onset in pink dots. The analysis corresponds to Fig. 2e,f, and results in the same average GMSL stand (dashed pink line) at which salinification occurs. Deglaciations are plotted as dashed vertical lines, and intervals of rising δ18Osw-ivc prior to each glacial termination are indicated with vertical grey bars. Note y-axes in (b), (d), and (e) are inverted.
Extended Data Fig. 3 Indian Ocean salinity and temperature stack construction.
(a) Age model comparison using benthic δ18O for all cores included in the stacks on an inverted y-axis. The colours of the labels correspond to the coloured lines in all panels. References for all cores are listed in Fig. 1 and Methods. For comparison, the LR04 benthic δ18O stack48 is plotted in a dark red line on top. (b) All SST records which are included in the SST stack. (c) All salinity proxy records which are included in the salinity stack. (d) z-scores of all SST records included in the SST stack, the SST stack (thick black line) with 95th percentile error envelope (grey shaded band), and an SST stack excluding U1476 data with 95th percentile envelope (orange shaded band). (e) z-scores of all salinity proxy records included in the salinity stack, the salinity stack (thick black line) with 95th percentile error envelope (grey shaded band), and a salinity stack excluding U1476 data with 95th percentile envelope (orange shaded band). Note that there is no difference in the onset of the salinification and warming between the black and orange stacks suggesting that U1476 does not force the stack calculations. Intervals of rising δ18Osw-ivc prior to each glacial termination are indicated with vertical grey bars.
Extended Data Fig. 4 Salinity reconstruction using different global mean sea level corrections.
(a) δ18Osw-ivc records derived using 4 different GMSL reconstructions for the ice volume correction. U1476 δ18Obenthic51 is plotted in b) for lead-lag comparison. Grey bars highlight the early increase in δ18Osw-ivc prior to terminations. The ref. 49 ice volume correction allowed modelled ice volume δ18O to be used (dark green). For other GMSL reconstructions65,66, a change of 1% in δ18Obenthic per 120 m GMSL, or maximum reconstructed GMSL was assumed. The colour of each δ18Osw-ivc record corresponds with the referenced scenario in the other panels. (c) Absolute δ18Osw-ivc amplitude of the glacial δ18Osw-ivc maxima compared to the preceding minima for all 4 δ18Osw-ivc scenarios. The colours of the bars correspond to the referenced scenarios in (a). Note that differences between different scenarios are relatively small. (d) Corresponding ice volume δ18O curves for the 4 GMSL reconstructions. The colours of each line correspond with the referenced scenario in (a). Intervals of rising δ18Osw-ivc prior to each glacial termination are indicated with vertical grey bars in (a). Note y-axes in (b) and (d) are inverted.
Extended Data Fig. 5 Surface hydrography in the source regions of the ITF and Indian Ocean.
(a) U1476 δ18Osw-ivc (dark green), (b) U1476 Mg/Ca-derived sea surface temperatures (orange) (this study), and (c) U1476 δ18Obenthic51 (thin blue line) from the western Indian Ocean. (d) Alkenone-derived sea surface temperatures (light pink) and (e) δ18Obenthic (thin blue line) from the South China Sea46. (f) Mg/Ca-derived sea surface temperatures (brown) and (g) δ18Obenthic (thin blue line) from the western Pacific warm pool47. Onset of U1476 glacial salinification highlighted in grey bars. Note that both South China Sea, and western Pacific warm pool records do not show a similarly consistent lead-lag pattern between their respective SST and δ18Obenthic data. Note δ18Obenthic data is presented on inverted y-axes.
Extended Data Fig. 6 Spectral analysis results for U1476 climate proxies.
Spectral analysis of U1476 SST (a), δ18Osw-ivc (b), and δ18Obenthic51 (c); as well as SST z-score stack (e), δ18Osw-ivc z-score stack (f), and LR04 δ18Obenthic stack for comparison (g). Peaks above the red line are significant at the 95% interval. Black dashed vertical lines indicate precession (23 kyr), obliquity (41 kyr), and eccentricity (100 kyr) periodicities. All records were subsampled to 1 kyr prior to analysis.
Extended Data Fig. 7 Cross spectral analysis results for U1476 climate proxies showing lead-lag characteristics.
Cross spectral analyses showing coherence (red) and phase (black) between U1476 SST, δ18Osw-ivc, and δ18Obenthic51 (top, a–c); and SST stack, δ18Osw-ivc stack, and LR04 δ18Obenthic stack48 (bottom, d–f). Coherence data above the red, and purple, line indicates significant coherence between datasets at the 95%, and 80%, significance level. Phase data at times of significant coherence above the black line (Phase = 0) indicate significant lead of the first variable. Precession (23 kyr), obliquity (41 kyr), and eccentricity (100 kyr) periodicities are indicated by vertical black dashed lines. All records were subsampled to 1 kyr prior to analysis.
Extended Data Fig. 8 Spectral and cross-spectral analysis results for previously published hydrography data from U1446 in the Bay of Bengal.
(a) Spectral analysis of δ18Obenthic, and (b) δ18Osw-ivc. (c) Phase and coherence between δ18Obenthic and δ18Osw-ivc. Black horizontal line shows Phase = 0; positive phase values indicate δ18Osw-ivc leads δ18Obenthic Coherence data above the red line indicates significant coherence between datasets at the 95% significance level. Precession (23 kyr), obliquity (41 kyr), and eccentricity (100 kyr) periodicities are indicated by vertical black dashed lines. All data that went into this analysis was previously published in (ref. 14). All records were subsampled to 1 kyr prior to analysis.
Extended Data Fig. 9 Indian Ocean hydroclimate proxies compared to U1476 salinity.
(a) 15° S July summer insolation58. (b)–(d), (f), (h), (j), (l), (n), (p) U1476 δ18Osw-ivc (this study). (c) U1476 δ18Obenthic51 on inverted y-axis. (e) Malawi Lake SST, proxy for African Monsoon variability59. (g) Speleothem δ18OCaCO3 from Dongge Cave, China, proxy for Southeast Asian Summer Monsoon variability61. (i) Speleothem δ18OCaCO3 from Bittoo Cave, India, proxy for Indian Summer Monsoon variability60. (k) TY93-929/P δ18Osw-ivc, Arabian Sea surface ocean salinity proxy40. (m) U1446 δ18Osw-ivc, Bay of Bengal sea surface salinity proxy14. (o) GeoB10038-4 δ18Osw-ivc43, eastern Indian Ocean. Grey vertical bars highlight the period of salinification in U1476 which is also visible in the Arabian Sea, Bay of Bengal, and eastern Indian Ocean sea surface salinity proxies.
Extended Data Fig. 10 Changes in land surface as a function of changes in global mean sea level.
(a) Pixels occurring within the orange polygon were used to calculate the land/sea ratio in each time-slice map created by the ANICE-SELEN model. (b) Change in land to sea surface ratio in the Indonesian archipelago as modelled by the coupled ice sheet-topography model ANICE-SELEN for a subset of 9 glacial-interglacial cycles in respect to GMSL. Changes occurring from falling and rising sea level are plotted in blue and orange respectively – the hysteresis between falling and rising sea levels likely results from the delayed response of the solid Earth to changes in ocean water surface loading. (c) Slope of the land/sea surface ratio as a function of GMSL to highlight specific sea levels at which the rate of land exposure during sea level fall, or land flooding during sea level rise, is particularly fast with respect to the change in GMSL. Vertical blue shaded bars highlight the GMSL interval within which the change in ratios at GMSL fall is particularly fast. The map in (a) was plotted using the library Cartopy67 in Python. Outlines for countries are taken from ref. 68.
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Nuber, S., Rae, J.W.B., Zhang, X. et al. Indian Ocean salinity build-up primes deglacial ocean circulation recovery. Nature 617, 306–311 (2023). https://doi.org/10.1038/s41586-023-05866-3
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DOI: https://doi.org/10.1038/s41586-023-05866-3
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