Boron isotope evidence for oceanic carbon dioxide leakage during the last deglaciation

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Abstract

Atmospheric CO2 fluctuations over glacial–interglacial cycles remain a major challenge to our understanding of the carbon cycle and the climate system. Leading hypotheses put forward to explain glacial–interglacial atmospheric CO2 variations invoke changes in deep-ocean carbon storage1,2, probably modulated by processes in the Southern Ocean, where much of the deep ocean is ventilated3. A central aspect of such models is that, during deglaciations, an isolated glacial deep-ocean carbon reservoir is reconnected with the atmosphere, driving the atmospheric CO2 rise observed in ice-core records4,5,6. However, direct documentation of changes in surface ocean carbon content and the associated transfer of carbon to the atmosphere during deglaciations has been hindered by the lack of proxy reconstructions that unambiguously reflect the oceanic carbonate system. Radiocarbon activity tracks changes in ocean ventilation6, but not in ocean carbon content, whereas proxies that record increased deglacial upwelling4,7 do not constrain the proportion of upwelled carbon that is degassed relative to that which is taken up by the biological pump. Here we apply the boron isotope pH proxy in planktic foraminifera to two sediment cores from the sub-Antarctic Atlantic and the eastern equatorial Pacific as a more direct tracer of oceanic CO2 outgassing. We show that surface waters at both locations, which partly derive from deep water upwelled in the Southern Ocean8,9, became a significant source of carbon to the atmosphere during the last deglaciation, when the concentration of atmospheric CO2 was increasing. This oceanic CO2 outgassing supports the view that the ventilation of a deep-ocean carbon reservoir in the Southern Ocean had a key role in the deglacial CO2 rise, although our results allow for the possibility that processes operating in other regions may also have been important for the glacial–interglacial ocean–atmosphere exchange of carbon.

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Figure 1: Location of cores PS2498-1 and ODP1238.
Figure 2: δ11B, pH and records from the SAA and the EEP during the last deglaciation.
Figure 3: and δ13C records from the SAA and the EEP during the last deglaciation compared to opal fluxes, and atmospheric δ13C.

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Acknowledgements

We thank the International Ocean Drilling Program for providing samples from ODP Leg 202, R. Gersonde and A. Mackensen for the PS2498-1 core material, J. F. McManus for sharing his unpublished benthic isotope data for ODP1238, and E. J. Rohling, M. P. Hain and C. Beaulieu for discussions. For the calibration of G. bulloides, we thank M. Kucera for providing core-top samples from the archives at the University of Tübingen, H. C. Bostock for samples from the National Institute for Water and Atmospheric Research, Wellington, and B. J. Marshall and R. Thunell for samples from the Cariaco Basin sediment trap time series. J. A. Milton, M. J. Cooper and A. Michalik provided assistance during ICP-MS analyses and sample preparation in the laboratory. C. Alt and M. T. Horigome helped with foraminifera picking. We thank the other members of ‘The B-Team’ at the National Oceanography Centre Southampton for their contributions. Financial support was provided by the European Community through a Marie Curie Intra-European Fellowship for Career Development to M.A.M.-B., the Universitat Autònoma de Barcelona through a Postdoctoral Research Grant to G.M., the Spanish Ministry of Science and Innovation (PROCARSO project CGL2009-10806) to G.M., P.Z. and P.G.M., a NERC PhD studentship awarded to M.J.H., a NOAA/UCAR Climate and Global Change Postdoctoral Fellowship to J.W.B.R., and NERC grant NE/D00876/X2 to G.L.F. G.M. was also supported by the Australian Laureate Fellowship project FL120100050 (E. J. Rohling).

Author information

M.A.M.-B., G.M., G.L.F. and P.Z. designed the study; G.M. and M.A.M.-B. produced the δ11B and trace element records for PS2498-1; M.A.M.-B. produced the δ11B and trace element records for ODP1238; G.M. produced the δ18O and δ13C data; M.J.H. developed the G. bulloides δ11B–pH calibration; J.W.B.R. sampled sediment core PS2498-1. M.A.M.-B. and G.M. wrote the first draft jointly, and all authors contributed to the interpretation and the preparation of the final manuscript.

Correspondence to M. A. Martínez-Botí or G. Marino.

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Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Comparison of PS2498-1 (blue) and ODP1238 (red) records.

a, δ11B. b, pH. c, .

Extended Data Figure 2 δ11B-derived compilation for the equatorial Pacific during the last deglaciation and Holocene.

Foraminifera-based record from the western equatorial Pacific21 (grey), Porites coral-based record from the central equatorial Pacific22,23 (as published in ref. 22, green), and foraminifera-based record from the EEP (this study, red). The records of refs 21, 22 have been smoothed by fitting a LOESS function with degrees of smoothing (span) of 0.2 and 0.4, respectively (Methods), to allow a better comparison with the ODP1238 record (see main text). ODP1238 is located in the EEP, and therefore represents a direct record of upwelling of CO2-rich waters, while the signal at central and western equatorial sites may have been modified during the westward transit of waters by, for example, equilibration with the atmosphere and/or nutrient utilization by the biological pump.

Extended Data Figure 3 Planktic δ18O and δ13C records from ODP1238.

a, Planktic δ18O records from OPD1238. Red, G. ruber sensu stricto (ss) 250–355 μm; green, G. sacculifer (mixed morphotypes) 355–425 μm; black, N. dutertrei 355–500 μm. b, Planktic δ13C records from OPD1238. To facilitate comparison between species, δ13C data has been normalized53. Red, G. ruber ss 250–355 μm; green, G. sacculifer (mixed morphotypes) 355–425 μm; black, N. dutertrei 355–500 μm.

Extended Data Figure 4 records from the SAA and the EEP during the last deglaciation, compared with indicators of dust input.

ac, SAA; df, EEP. a, δ11B-derived in core PS2498-1. b, Logarithm of the mass accumulation rates (MAR) of iron (Fe) in the sub-Antarctic site ODP109013. c, f, measured on a suite of Antarctic ice cores5. d, δ11B-derived in core ODP1238. e, Dust fluxes in the EEP29. Dust fluxes from ref. 29 were de-meaned and divided by their own standard deviation, and are displayed in standard deviation units.

Extended Data Figure 5 Age model for PS2498-1.

a, Chronology of SAA core PS2498-1. Carbon-14 calendar age/depth relationships in core PS2498-1. Grey shading indicates 95% confidence limits of calendar ages. b, PS2498-1 G. bulloides δ11B record plotted using the different chronologies described in Methods and compared with atmospheric CO2 (green; refs 5, 87, 88) and with Antarctic opal flux4 (orange) records. Red, constant ΔR = 300 yr; green, ΔR = 900 yr for intervals older than 16 kyr and ΔR = 300 yr for younger intervals6; magenta, variable ΔR correction38 (ranging between 500 and 900 yr between 13 and 16 kyr ago).

Extended Data Figure 6 Age model for ODP1238.

a, Radiocarbon ages for ODP1238 determined from N. dutertrei tests at LLNL-CAMS. b, Chronology of EEP core ODP1238. Orange circles, calendar ages; black line, linear fit; red line, third-order polynomial fit (Methods).

Extended Data Figure 7 Benthic and planktic δ18O stratigraphy for ODP1238.

a, Benthic δ18O stratigraphy for ODP1238 compared with other benthic δ18O stratigraphies from EEP cores. Black circles, unpublished benthic δ18O data for ODP1238 generated by J. F. McManus (LDEO, Columbia University); red line, site TR163-2240; blue line, sites RC13-140, RC23-22 and RC23-1544. b, Globigerinoides ruber δ18O stratigraphy for ODP1238 compared with other G. ruber stratigraphies from EEP cores. Black circles, ODP1238 (Methods); green squares, site TR163-197; red line, site TR163-2240; blue line, site ODP124041.

Extended Data Figure 8 δ11B–pH calibrations for G. bulloides and G. sacculifer.

a, Data tabulated; b, Data plotted. The green symbols and text show a new calibration for G. bulloides (with associated 2σ uncertainties). Horizontal error bars for core-top samples are 2σ of intra-annual variability in calculated monthly δ11Bborate, and for sediment trap samples reflect the range of δ11Bborate between December 2006 and February 2007. Vertical error bars represent the analytical reproducibility (2σ) as calculated using equation (1). The most recent PS2498-1 sample (2.2 kyr old) (black-filled circle) was not used in the calibration process, and is included to show its agreement with the calibration line. The red symbols and text show a calibration for G. sacculifer (with associated 2σ uncertainties). The calibration line incorporates both culture78 (empty symbols) and core-top (red-filled symbols) data17. Culture data analysed by N-TIMS (grey symbols and text)78 has been corrected by applying a laboratory offset of −3.32‰ (Methods) (the vertical grey arrow indicates an original N-TIMS calibration data point that falls outside the plot area). The ODP1238 late-Holocene average (black-filled square) was not used to produce the calibration equation, and is included to show its agreement with the calibration line. Horizontal error bars for core-top samples are 2σ of intra-annual variability in calculated monthly δ11Bborate, and for culture samples represent quoted uncertainties78 in pH. Vertical error bars represent quoted uncertainties in δ11B measurements17,78 (2σ). To calculate monthly pH variations at ODP1238, the method described in the G. bulloides calibration section has been used62 (with total alkalinity derived using the total alkalinity/salinity/temperature relationship for the ‘Equatorial upwelling Pacific Zone’ in ref. 72). The black line denotes a 1:1 relationship, that is, a pH sensitivity equal to that of borate ion. Heavily and lightly shaded regions around calibration lines represent 1σ and 2σ uncertainties, respectively.

Extended Data Figure 9 Effect of δ11B–pH calibration, total alkalinity and chronological uncertainties in and records.

a, PS2498-1 δ11B-based record calculated with the G. bulloides calibration equation (thick blue line), and its associated 2σ uncertainty (blue shaded envelope). b, PS2498-1 δ11B-based record assuming a constant total alkalinity of (i) modern values at PS2498-1 (blue), (ii) modern values minus 25 μmol kg−1 (green) and (iii) modern values plus 125 μmol kg−1 (red). c, PS2498-1 record calculated using (i) age derived from our age model (blue), (ii) age plus 0.5 kyr (green) and (iii) age minus 0.5 kyr (red). d, ODP1238 δ11B-based record calculated with the G. sacculifer calibration equation (thick red line), and its associated 2σ uncertainty (shaded red envelope). e, ODP1238 δ11B-based record assuming a constant total alkalinity of (i) modern values at ODP1238 (blue), (ii) modern values minus 25 μmol kg−1 (green) and (iii) modern values plus 125 μmol kg−1 (red). f, ODP1238 record calculated using (i) age derived from our age model (blue), (ii) age plus 0.5 kyr (green) and (iii) age minus 0.5 kyr (red). Note the different horizontal and vertical axes in each panel.

Extended Data Figure 10 Globigerinoides sacculifer Mg/Ca-based sea surface SST (red) and N. dutertrei Mg/Ca-based thermocline temperature (TT; green) at ODP1238.

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Martínez-Botí, M., Marino, G., Foster, G. et al. Boron isotope evidence for oceanic carbon dioxide leakage during the last deglaciation. Nature 518, 219–222 (2015) doi:10.1038/nature14155

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