Over the past eight hundred thousand years, glacial–interglacial cycles oscillated with a period of one hundred thousand years (‘100k world’1). Ice core and ocean sediment data have shown that atmospheric carbon dioxide, Antarctic temperature, deep ocean temperature, and global ice volume correlated strongly with each other in the 100k world2,3,4,5,6. Between about 2.8 and 1.2 million years ago, glacial cycles were smaller in magnitude and shorter in duration (‘40k world’7). Proxy data from deep-sea sediments suggest that the variability of atmospheric carbon dioxide in the 40k world was also lower than in the 100k world8,9,10, but we do not have direct observations of atmospheric greenhouse gases from this period. Here we report the recovery of stratigraphically discontinuous ice more than two million years old from the Allan Hills Blue Ice Area, East Antarctica. Concentrations of carbon dioxide and methane in ice core samples older than two million years have been altered by respiration, but some younger samples are pristine. The recovered ice cores extend direct observations of atmospheric carbon dioxide, methane, and Antarctic temperature (based on the deuterium/hydrogen isotope ratio δDice, a proxy for regional temperature) into the 40k world. All climate properties before eight hundred thousand years ago fall within the envelope of observations from continuous deep Antarctic ice cores that characterize the 100k world. However, the lowest measured carbon dioxide and methane concentrations and Antarctic temperature in the 40k world are well above glacial values from the past eight hundred thousand years. Our results confirm that the amplitudes of glacial–interglacial variations in atmospheric greenhouse gases and Antarctic climate were reduced in the 40k world, and that the transition from the 40k to the 100k world was accompanied by a decline in minimum carbon dioxide concentrations during glacial maxima.
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Allan Hills stable water isotope and gas data that support the findings of this study are available on the United States Antarctic Program Data Center (http://www.usap-dc.org/)with the following identifiers: DOI: 10.15784/601129 (ALHIC1502 stable water isotopes); DOI: 10.15784/601128 (ALHIC1503 stable water isotopes); DOI: 10.15784/601201 (heavy noble gases); DOI: 10.15784/601202 (CO2 concentration and δ13C-CO2); DOI: 10.15784/601203 (CH4 concentration); and DOI: 10.15784/601204 (elemental and isotopic composition of O2, N2 and Ar).
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We acknowledge US Ice Drilling Design and Operations (IDDO), driller M. Waszkiewicz, and Ken Borek Air for assistance with the field work. M. Kalk assisted with the CO2 measurements. We thank A. Menking and A. Buffen for helping with δ13C-CO2 measurements. This work received funding from National Science Foundation Grants ANT-1443306 (University of Maine), ANT-1443276 (Oregon State University), NSF-0538630 and ANT-0944343 (Scripps Institution of Oceanography), and ANT-1443263 (Princeton University). Y.Y. acknowledges the Princeton Environmental Institute at Princeton University through the Walbridge Fund, which supported the work upon which this material is partly based.
The authors declare no competing interests.
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Extended data figures and tables
Right, WorldView03 colour pan-sharpened imagery (copyright 2011, DigitalGlobe, Inc.) of the main ice field, Allan Hills Blue Ice Area (in true colour mode). Inset, Antarctica with hill shading (source: Australian Antarctic Data Centre, Map 13469; licensed under a Creative Commons Attribution 3.0 Unported License) on which our study area is marked by the black square. The black outcrop in the main image is the Allan Hills nunatak. The red arrow marks the local iceflow. Left, a magnified image of the drilling site from same source file. The imagery is enhanced (gamma-adjusted on ArcGIS 10) to highlight the colour contrast within the blue ice, which now has a greenish hue owing to the colour rendition. The brown lineations in the ice are exposed dust bands, providing a first-order tracer of surface ice stratigraphy. The locations of the cores reported in this work (see text) are marked with red circles. The location of the GPR profile in Extended Data Fig. 2 is shown as a yellow dashed line.
Extended Data Fig. 2 GPR profile proceeding from the SW to the NE, crossing (within less than 5 m) ALHIC1502 and ALHIC1503.
The exact transect location is shown in Extended Data Fig. 1. The location and depths of boreholes ALHIC1502 and ALHIC1503 (red bars) and ice drilled previously as BIT58 (grey bar) are indicated. The profile was collected with an 80-MHz MLF antenna in a step-and-collect survey style with a step size of 25 cm and a stacking rate of 64 scans. Standard post-processing steps were applied, including time-zero position correction, distance normalization, 50/110-MHz finite impulse response filter, background removal, and gain adjustment. Depth estimates from two-way travel time (TWTT) and migration use a radar travel velocity of 0.165 m ns−1. a, Non-migrated radargram showing dipping bed-parallel englacial stratigraphy and a strong dipping apparent bedrock reflection. Dashed blue line indicates the modelled location of the actual bedrock location as shown in b. b, Migrated radargram showing the ‘true’ location of the bedrock. Migration quality is poor at this location owing to the steeply dipping slope of the bedrock rise. Comparison between a and b suggests that the correct bedrock reflector location is translated downward and more steeply dipping than indicated in the non-migrated data. ALHIC1502 and ALHIC1503 borehole depths independently verify this interpretation.
Pair differences of δ18Oatm (Δδ18Oatm; red) and δ15N (Δδ15N; blue) are plotted against ΔδO2/N2 in ALHIC1502 and ALHIC1503 samples. The difference between two replicate samples is attributable to gas loss. The very weak dependence of Δδ18Oatm and Δδ15N on ΔδO2/N2 suggests that gas isotopes are relatively insensitive to gas loss fractionation in Allan Hills ice.
Using the two age models discussed in the text— the conservative approach (top and bottom left) and the proximity approach (top and bottom right)—CO2 and CH4 are plotted against δDice. Both of the age models show reduced range in the MPT and pre-MPT ice. We note, however, that the highest CO2 and CH4 values fall into the unassigned category in the strictest age model.
a, Ice core CO2 record between 0 and 800 ka versus LR04 benthic foram δ18O stack synchronized onto the AICC2012 timescale60,61. The result of a linear regression is shown and used to calculate the synthetic CO2 record between 1.2 and 2.0 Ma. b, A synthetic CO2 record between 1.2 and 2.0 Ma reconstructed from LR04 δ18O and the regression parameters calculated in a. The range of CO2 in this synthetic time series is 213–269 ppm. c–h, Fraction of the recovered CO2 variability (observed range/true range) of the synthetic CO2 time series (see Extended Data Fig. 5b) by 37 randomly selected samples, given different preferential preservation of interglacial ice. Here the term ‘interglacial’ is operationally defined as any sample with greater than 250 ppm CO2. Different multipliers indicate the multiple occurrence of the ‘interglacial’ ice to simulate varying degrees of ice preservation biases. This analysis shows that when the presence of interglacial ice is no more than eight times greater than that of glacial ice (c–f), we could expect 37 random samples to be likely to capture 66–98% (95% confidence interval) of the CO2 range in the ice.
Extended Data Fig. 6 Evaluating the fraction of δ18Oatm and δXe/Kr range captured by discrete samples.
a, b, Allan Hills δ18Oatm (a) and δXe/Kr (b) are potted against the δDice of the same depth, colour coded according to their age units. Vertical dashed lines represent the range of δDice observed in the 40k-world ice (−284 to −316‰). Notably, the range of δDice associated with nine δXe/Kr values from the 40k world is only about 40% of the entire δDice range in our 40k ice samples (−288 to −300‰); low δDice values (less than −300‰) are missing from the δXe/Kr samples. Thus, we do not expect the nine Xe/Kr samples to fully capture the range of Xe/Kr ratios in the 40k-world ice. On the other hand, the co-depth δDice values of δ18Oatm samples occupy 97% of the total range, implying that 29 δ18Oatm samples are likely to cover 70% of the variability preserved in the 40k-world ice, barring any diffusive smoothing of the δ18Oatm records.
a, δ15N; b, δO2/N2; c, δ18Oatm; d, δDice. The error bars associated with gas properties represent the pooled standard deviation (1σ) of all measurements: 0.010‰ for δ15N, 3.063‰ for δO2/N2, and 0.026‰ for δ18Oatm. Note that the range of δ18Oatm is less than 0.2‰. By comparison, glacial-to-interglacial δ18Oatm variability in the 100-kyr climate cycles is about 1.5‰21.
a, Partitioning function Z (ratio of the number gas molecules in the gas phase to that in the ice phase) of O2 (blue) and N2 (red) as a function of temperature. b, Characteristic time scale of the self-diffusion of water molecules in ice (black), of O2 permeation in ice (blue), and of N2 permeation in ice (red), plotted as a function of temperature. Note that for water molecules the length scale of diffusion (L) is 0.1 m, whereas for N2 and O2 L = 0.5 m, reflective of their respective sampling resolutions.
a, A contour map shows the correlation coefficient (R2) between δDice and d in core ALHIC1503 as a function of the number of adjacent stable water isotope data points included in the calculations, from 5 to 85 with a step of 10. The sampling resolution is approximately 10 cm. The high correlation coefficient (>0.7) between 132 and 142 m raises the suspicion that mixing has taken place in this interval. However, the possibility of mixing is not supported by CO2 and CH4. b–d, Cross-plots of δDice–CO2 (b), δDice–CH4 (c), and CO2–CH4 (d) in intervals between 132 and 140 m in ALHIC1503. The black solid lines are mixing lines based on two end members at 135.30 m and at 139.76 m, where the minimum and maximum CO2 values are measured, respectively. Most of the points do not fall onto the mixing line, implying that two-end-member mixing alone cannot explain the high correlation between δDice and d.
a, ALHIC1503; b, ALHIC1502. δDice (top), CO2 (middle), and δ13C of CO2 (bottom) in Allan Hills ice cores are plotted against depth. δDice and CO2 (both measured independently in smaller samples, plotted in red, and along with δ13C, plotted in blue) are also shown for comparison. Note the marked scale difference for CO2 and δ13C between ALHIC1503 and ALHIC1502.
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Yan, Y., Bender, M.L., Brook, E.J. et al. Two-million-year-old snapshots of atmospheric gases from Antarctic ice. Nature 574, 663–666 (2019) doi:10.1038/s41586-019-1692-3