Isotopic constraints on marine and terrestrial N2O emissions during the last deglaciation

Abstract

Nitrous oxide (N2O) is an important greenhouse gas and ozone-depleting substance that has anthropogenic as well as natural marine and terrestrial sources1. The tropospheric N2O concentrations have varied substantially in the past in concert with changing climate on glacial–interglacial and millennial timescales2,3,4,5,6,7,8. It is not well understood, however, how N2O emissions from marine and terrestrial sources change in response to varying environmental conditions. The distinct isotopic compositions of marine and terrestrial N2O sources can help disentangle the relative changes in marine and terrestrial N2O emissions during past climate variations4,9,10. Here we present N2O concentration and isotopic data for the last deglaciation, from 16,000 to 10,000 years before present, retrieved from air bubbles trapped in polar ice at Taylor Glacier, Antarctica. With the help of our data and a box model of the N2O cycle, we find a 30 per cent increase in total N2O emissions from the late glacial to the interglacial, with terrestrial and marine emissions contributing equally to the overall increase and generally evolving in parallel over the last deglaciation, even though there is no a priori connection between the drivers of the two sources. However, we find that terrestrial emissions dominated on centennial timescales, consistent with a state-of-the-art dynamic global vegetation and land surface process model that suggests that during the last deglaciation emission changes were strongly influenced by temperature and precipitation patterns over land surfaces. The results improve our understanding of the drivers of natural N2O emissions and are consistent with the idea that natural N2O emissions will probably increase in response to anthropogenic warming11.

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Figure 1: Changes in tropospheric N2O and climate proxies during the last glacial–interglacial cycle and the last deglaciation.
Figure 2: N2O emissions during the last deglaciation.

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Acknowledgements

Financial support was provided by the Swiss National Science Foundation (NSF) and the US NSF, including a Swiss NSF Fellowship for Prospective Researchers (139404) to A.S., US NSF Grant PLR08-38936 to E.J.B. and US NSF Grant PLR08-39031 to J.P.S. Further support came from the Marsden Fund Council from New Zealand Government funding, administered by the Royal Society of New Zealand. We thank C. Buizert, X. Faïn, J. Lee, L. Mitchell and P. Rose for fieldwork, R. Roth for providing the Bern3D Earth System Model run, J. Schwander for providing the NEEM firn air cylinder, S. Jaccard for comments and A. Ross for lab assistance. We thank B. Otto-Bliesner and Z. Liu for providing climate data from the TraCE-21ka model computation, which was carried out at the Oak Ridge Leadership Computational Facility, sponsored by the US Department of Energy, and the National Center for Atmospheric Research Supercomputing Facility, sponsored by the US NSF. The TraCE-21ka project was supported by the US NSF and the US Department of Energy.

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Contributions

J.P.S., V.V.P. and E.J.B. initiated and led the Taylor Glacier project. T.K.B., E.J.B., D.B., V.V.P., H.S., A.S. and J.P.S. performed fieldwork. A.S. set up the experimental apparatus for N2O isotopes and carried out the measurements, with great support from T.K.B., E.J.B. and J.S. Intercomparison measurements were performed by J.S. at the University of Bern and by T.K.B. at Oregon State University. D.B. and T.K.B. developed the timescale for Taylor Glacier. A.S. and F.J. performed the box modelling. R.S. and F.J. produced and interpreted the LPX-Bern model results. All authors discussed the results and contributed to the manuscript, which was written by A.S.

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Correspondence to Adrian Schilt.

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Extended data figures and tables

Extended Data Figure 1 Isotopic composition of marine and terrestrial N2O sources.

Field data of δ15N (relative to atmospheric N2) and δ18O (relative to VSMOW) of marine (blue triangles60,61) and terrestrial (green crosses62,63) N2O sources. Blue and green bars indicate the ranges as used in the box model (Extended Data Table 2). The mean tropospheric value of all Taylor Glacier data (orange diamond, with the orange box indicating the full range of the data) is enriched in heavy isotopes in both δ15N and δ18O relative to the approximate corresponding isotopic composition of the total source (black diamond) owing to the fractionation by the stratospheric sink.

Extended Data Figure 2 Comparison of Taylor Glacier and other data.

N2O concentration (diamonds), δ15N (triangles) and δ18O (crosses) data from Taylor Glacier (from the apparatus for N2O isotopes in grey, and from the apparatus for CO2 isotopes36 in orange) compared with a Taylor Glacier intercomparison measurement (red) and Talos Dome data (green) from the University of Bern, and with published data from Taylor Dome (blue4). Taylor Glacier and Talos Dome data from the University of Bern were corrected by +2.28 p.p.b. for the N2O concentration, −0.80‰ for δ15N and +0.36‰ for δ18O on the basis of intercalibration measurements made by Oregon State University and the University of Bern using firn air (Methods). The Taylor Glacier δ18O data from Oregon State University (grey crosses) were corrected by +1.38‰ on the basis of measurements with bubble-free ice and NOAA-1 standard gas (Extended Data Fig. 7c and Methods). Error bars indicate ±1σ.

Extended Data Figure 3 Effect of marine N2O cycle and inventory on tropospheric N2O concentration and δ15N under changing emissions.

Response of tropospheric N2O and δ15N to exponential increases in N2O emissions with timescales of 100, 200, 500 and 1,000 yr (from left to right; the maximum increase rate in the Taylor Glacier N2O data is indicated by the dotted grey line). Results from the two-box model (dashed lines show results without explicit representation of the ocean) are very similar to the results from the extended box model (with ocean, solid lines; Methods). The marine and terrestrial emissions are increased in parallel, that is, the marine fraction is always 37% of the total N2O emissions and the isotopic composition of emitted N2O remains constant. The decrease in δ15N is caused by imbalances between the sources and the stratospheric sink.

Extended Data Figure 4 Consistency of the calculated marine and terrestrial emissions with the Taylor Glacier δ18O data.

a, δ18O evolution for different initial marine fractions (red, 17%; purple, 37%) of the total emissions when calculated using the marine and terrestrial N2O emissions determined on the basis of the Taylor Glacier N2O concentration and δ15N data. In a Monte Carlo approach only scenarios with the same mean value as the Taylor Glacier δ18O data (green, with ±1σ error bars) were considered, which narrowed the possible δ18O isotopic composition of the sources. b, δ15N and δ18O of marine (triangles) and terrestrial (crosses) sources from modern field data (black, as in Extended Data Fig. 1), as well as the values needed to explain the Taylor Glacier data with different initial (at 16 kyr bp) marine fractions (red, 17%; purple, 37%). An initial marine fraction of 74% would require δ18O isotopic compositions outside the observed range (Extended Data Table 2), suggesting that such a high marine fraction is rather unlikely. Note that the Taylor Glacier data can be explained for an initial marine fraction of 74% when considering δ15N only, but only with rather extreme model parameters (Extended Data Fig. 5).

Extended Data Figure 5 Evolution of marine and terrestrial N2O emissions under different scenarios.

Sensitivity of marine (blue in large panels) and terrestrial (green in large panels) N2O emissions to initial marine fractions (red circles at 16 kyr bp) set to 17% (a), 37% (b) and 74% (c) in accordance with low, best and high estimates of the modern natural N2O budget1. The uncertainty bands related to the emissions (blue and green shaded areas) result from the Monte Carlo approach and indicate ±1σ of all solutions. For all scenarios, the maximum absolute changes in the marine fractions (black in large panels) over the last deglaciation are 7% or less. Dashed lines in the small panels show the distributions of the parameters as allowed for in the Monte Carlo approach (priors; Extended Data Table 2), and solid lines indicate the distributions of the parameters which allow for a reproduction of the Taylor Glacier N2O concentration and δ15N that respects the prescribed initial marine fractions (posteriors).

Extended Data Figure 6 Standard gases for δ15N, δ18O and N2O concentration.

The δ15N and δ18O values of 6.18‰ and 44.16‰ of the NOAA-1 standard gas collected at Niwot Ridge, Colorado, were assigned by linear extrapolation of the data from Cape Grim, Tasmania31, to the collection date of NOAA-1 (11 December 2008), on the basis of the assumption that N2O and its isotopes are well mixed in the troposphere owing to the rather long atmospheric lifetime. The N2O concentration of 322.32 ± 0.14 p.p.b. of the NOAA-1 standard gas was directly determined by the National Oceanic and Atmospheric Administration (NOAA-2006A scale; linear extrapolation of the Cape Grim data would lead to 320.9 p.p.b.). To test the calibrations for the N2O concentration and isotopic compositions, a second standard gas, NOAA-2, which was collected at Niwot Ridge on 5 October 1988, was measured against NOAA-1. The results (red crosses) were in good agreement with the Cape Grim data, in particular when taking into account the interannual scatter observed in the archived air.

Extended Data Figure 7 Stability in the course of the measurement series, characterization of the amount dependency of the measurement system, and tests with bubble-free ice.

a, NOAA-3 standard gas measurements performed at the end of each measurement day. No significant drifts were observed in the course of the measurement series, and the standard deviations for δ15N and δ18O were respectively 0.14‰ and 0.32‰ (n = 31), as indicated by the grey areas (±1σ) around the means (dashed lines). b, NOAA-1 standard gas measurements resulting in similar peak areas to the preceding ice-sample measurement routinely performed throughout the measurement series. These measurements covering the full range of peak areas from ice samples did not reveal any significant amount dependency. The mean and standard deviation (dashed lines and grey areas) for δ15N were 6.24‰ ± 0.22‰ and those for δ18O were 44.18‰ ± 0.59‰ (±1σ, n = 58), in agreement with the expected values (solid lines). c, Measurements of different amounts of NOAA-1 standard gas which was stored in the extraction pots while pieces of bubble-free ice were grated. Dashed lines and grey areas indicate the means and standard deviations (±1σ, n = 10), and solid lines indicate the expected values. On the basis of these measurements with bubble-free ice and the results shown in b, N2O, δ15N and δ18O were not corrected for amount dependency. However, during the extraction of standard gas over ice, a −1.38‰ offset was introduced in δ18O. Accordingly, all Taylor Glacier δ18O values were corrected by +1.38‰.

Extended Data Table 1 Equations forming the basis of the two-box model used to calculate marine and terrestrial N2O emissions
Extended Data Table 2 Parameters for the two-box model used to calculate marine and terrestrial N2O emissions

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Schilt, A., Brook, E., Bauska, T. et al. Isotopic constraints on marine and terrestrial N2O emissions during the last deglaciation. Nature 516, 234–237 (2014). https://doi.org/10.1038/nature13971

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