Ice cores from the Allan Hills, Antarctica show relatively stable atmospheric CO2 and CH4 levels over the last 3 million years

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Here, we present new snapshots of carbon dioxide (CO 2 ) and methane (CH 4 ) between 3.1 and 0.4 million years ago (Ma) from shallow ice cores drilled in the Allan Hills Blue Ice Area (BIA). In the oldest ice (>1 Ma), mixing and thinning have attenuated the glacial-interglacial variability, and we reconstruct long-term averages. The data indicate that CO 2 and CH 4 levels in the early Pleistocene are within the range of variability observed for the last 0.8 Ma. Across the Pleistocene, no significant change in mean CH 4 is observed but we find a small, 25 ppm decline in CO 2 from 2.9 to 1.2 Ma followed by stable mean CO 2 across the mid-Pleistocene Transition. In late Pliocene samples (ranging from 2.8-3.1 Ma), trapped air is impacted by the addition of CO 2 from respired organic matter. Corrections using the stable carbon isotopes of CO 2 indicate that atmospheric CO 2 in these late Pliocene samples is within the range measured in the early Pleistocene (<300 ppm). Observed changes in greenhouse gases are small relative to both local and global cooling observed in the same ice cores (Shackleton et al., 2024a, b) and independent records from marine sediments, suggesting that other components of Earth’s climate system contributed to global cooling over the last 3 million years. Earth and environmental sciences/Climate sciences Earth and environmental sciences/Climate sciences/Palaeoclimate Figures Figure 1 Figure 2 Figure 3 MAIN TEXT Earth’s climate over the last 3 million years is characterized by gradual cooling and an increase in the amplitude and duration of glacial cycles. The greenhouse gases (GHGs) carbon dioxide (CO 2 ) and methane (CH 4 ) play an important role in regulating Earth’s climate but their histories are not well constrained for significant climatic intervals over the last 3 Ma due, in part, to the lack of direct atmospheric archives. Two intervals have received particular attention, the transition from the Pliocene to the Pleistocene (PPT; ~2.6 Ma), a period of continental ice sheet expansion in the northern hemisphere, and the mid-Pleistocene Transition (MPT; 1.2-0.8 Ma), a transition from small-amplitude, 40 ka glacial cycles to large amplitude, 100 ka cycles. Changes in atmospheric CO 2 have been invoked to explain both of these climate transitions; the PPT has been hypothesized to be associated with a drop in CO 2 to less than 280 ppm 1,2 , and the MPT associated with a decline in CO 2 3,4 . More broadly, trends in atmospheric CO 2 over the last 3 million years provide critical insights into the processes that control the global carbon cycle on geologic timescales 5,6 . Finally, CH 4 has been hypothesized to also play a role in past warm climates 7 , but no proxies for atmospheric CH 4 exist beyond the ice core record. Reconstructions of atmospheric CO 2 beyond the ice core record primarily rely on indirect methods that take advantage of the relationship between atmospheric CO 2 , aqueous carbonate chemistry (CO 2 , HCO 3 - , CO 3 2- , pH) and the isotopic composition of biogenic fossils. For example, the ratio of boron isotopes ( 11 B/ 10 B) recorded in the shells of planktonic foraminiferal calcite has been used to reconstruct local pH and atmospheric CO 2 8–14 (CO 2 -boron). Advances to the CO 2 -boron technique have gradually reduced uncertainty, allowing reconstructions of orbital-scale variability 14–16 . The ratio of 13 C to 12 C in alkenones in marine sediments has also been used to reconstruct atmospheric CO 2 14,17–21 (CO 2 -alk) due to the fact that the magnitude of carbon isotope fractionation depends on the ratio of intracellular and extracellular aqueous CO 2 . Unfortunately, CO 2 -alk and CO 2 -boron produce very different histories over the last 3 million years; CO 2 -boron declines from ~400 to 250 ppm while CO 2 -alk shows relatively constant levels around 250 ppm, though with larger uncertainties. Both approaches have potential for systematic biases related to evolutionary change that are inherent to the use of biogenic fossils as paleoenvironmental archives. Other relevant proxies include carbon isotopes in paleosols 22 (CO 2 -paleosol) (though these reconstructions are associated with very large uncertainties), and a new proxy for atmospheric CO 2 based on the δ 13 C of leaf wax from a sediment core from the Bay of Bengal (CO 2 -leafwax) 23 , which currently extends to 1.5 Ma. Here, using new Allan Hills ice cores that extend further back in time, and an improved sampling and dating strategy enabled by larger core diameter, we extend ice core-based observations of CO 2 and CH 4 to the entire Pleistocene, redate and re-interpret previously published Allan Hills data, and provide indirect constraints on CO 2 for the late Pliocene. This new work both improves our interpretations of Allan Hills BIA ice cores and offers new insights into past greenhouse gas variations. Interpreting Allan Hills paleoclimate records In the Allan Hills BIA, located in Victoria Land, on the western flanks of the Transantarctic Mountains, ice flow, subglacial topography, and ablation have exposed old ice at and near the surface of the East Antarctic ice sheet 24 . Shallow (100-200 m) ice cores drilled in the Allan Hills BIA and dated using the 40 Ar atm geochronometer 25 have revealed the existence of stratigraphically complex ice as old as 6 Ma 26,27 (Shackleton et al. 2024a). As discussed in Shackleton et al., (2024a), developing paleoclimate archives from ice that is thinned and folded is in its early stages, resulting in significant uncertainties in the amount of time represented in an individual sample, the extent of mixing between samples of different ages, and/or the possibility of preservation bias. Given these complexities, we take a minimalist approach of interpreting the data as representing discrete snapshots of East Antarctic climate and the atmosphere that extend from the Late Pliocene to the Late Pleistocene. In previous work on Allan Hills cores, 40 Ar atm ages could not be measured on the same depths as other climate properties due to sample size constraints. Consequently, results for climate properties of those cores (ALHIC1502 and 1503) were binned in broad age ranges 26,27 . We improve previous GHG reconstructions from Allan Hills in three ways. First, using a large-diameter (24.1 cm) core we co-register climate property measurements (CO 2 , CH 4 , and the isotopic composition of atmospheric oxygen (δ 18 O atm ), see Methods) with age ( 40 Ar atm ) determinations (Fig. 1). Second, by increasing the density of ages (79 samples in the bottom 25 meters), individual folds and age discontinuities are resolved (Shackleton et al. 2024a). Of the 79 samples analyzed, 54 are older than 800 ka, with ages ranging from 800 ± 80 ka to 4000 ± 400 ka. Third, we further interrogate samples with apparently elevated CO 2 using measurements of the δ 13 C of trapped CO 2 at the same depth (n = 31) and find that all samples from the bottom 8 m contain variable levels of non-atmospheric CO 2 (see Methods). Measured δ 13 C of CO 2 values are strongly correlated with 1/CO 2 , indicating that they represent a mixture of atmospheric CO 2 and a single source of organic carbon (Supplementary Fig. 6). Correcting for the contribution from respiration results in 21 additional estimates of atmospheric CO 2 that range in age from 700 ± 70 ka to 3100 ± 300 ka (excluding the deepest 4000 ka sample, see Methods). Comparison of these values with values from pristine ice of the same age reveals 13 indistinguishable and 8 samples with positive offsets of ~20-50 ppm (Supplementary Fig. 7). This offset is observed in both young (~800 ka) and old samples, suggesting that either these values are under corrected or that they reflect true periods of elevated CO 2 . Comparing data from samples with 40 Ar atm ages younger than 800,000 years to continuous ice core records from EPICA Dome C (EDC 28–30 ) reveals that there are at least two types of paleoclimate snapshots, broadly distinguished by proximity to bedrock: 1) short-exposure snapshots in shallow ice that preserve some of the true glacial-interglacial variability, and 2) long-exposure snapshots in near-basal ice that preserve climate signals that more closely represent the mean over a glacial cycle. The transition between these two types of snapshots coincides with both the appearance of ice older than 1 Ma and the first evidence for contamination of trapped CO 2 by respired organic carbon (~141 m depth in ALHIC1901). When comparing the variance in gas properties in the short-exposure snapshots to corresponding EDC records approximately 42% ± 0.7% of the CO 2 range observed in the EDC record from 400-800 ka is captured in these samples, while the range of CH 4 and δ 18 O atm encompasses 38% ± 0.7% and 23% ± 0.6% of the EDC range, respectively (Supplementary Fig. 2). Glacial-level values of all three gas properties are missing in our record, suggesting a preservation bias toward interglacial samples in this age range. This is likely due to very low deposition during glacial periods, consistent with accumulation rate reconstructions from nearby stratigraphically intact cores (ALHIC1903 and S27) 31 (See Methods) (Carter et al., in review). Ice that is younger than 800 ka but located stratigraphically below ice older than 1 Ma records much less variability in all climate properties and the values agree well with depth-weighted averages of these gases from the same period in EDC (Supplementary Fig. 3). We interpret these samples as representing long-exposure snapshots of the mean climate state. As these samples constitute all our oldest ice samples, we take a conservative approach and focus on long-term trends in the record rather than change in variance (Fig. 1). The high-resolution chronology of core ALHIC1901permits a reevaluation of the ages previously assigned to samples from core ALHIC1503, which is ~8 m away 27 . A comparison of measured properties plotted on depth reveals a depth offset between the two cores ranging from 9 to 12 m, implying layers are dipping at ~48° (see Methods, Supplementary Fig. 1). Applying the depth-age relationship from ALHIC1901 to ALHIC1503 results in the reassignment of 51 sample ages from ALHIC1503 and resolves an apparent discrepancy in the range of atmospheric CO 2 and CH 4 in the early Pleistocene and MPT between the two cores (Fig. 1). Additionally, results from ALHIC1901 indicate the presence of non-atmospheric CO 2 at a comparable depth to high CO 2 concentrations (260-280 ppm) measured from ALHIC1503, although sufficient samples to test this interference with stable isotope measurements are no longer available from ALHIC1503. Redated ALHIC1503 measurements are included in our discussion and, moving forward, their new age assignments should be used when plotting these data. Greenhouse gases across the MPT The greenhouse gas records from Allan Hills ice cores show no significant change in mean CO 2 or CH 4 across the MPT (Fig. 1), consistent with independent datasets 9,23,32 and modeling studies 33 . In addition, our CO 2 concentrations across the MPT agree with the CO 2 -leafwax record after applying a Gaussian-weighted smoothing over a 400,000 year window (Supplementary Fig. 8, see Methods). Our interpretation of deep samples representing long-exposure snapshots precludes a discussion of changes in minimum and maximum CO 2 levels across the MPT, a critical component of many hypotheses for this transition. However, we note that a decline in the average value of CO 2 in glacial periods would result in a decline in overall mean CO 2 unless balanced by higher CO 2 during interglacial periods. Further investigations of Allan Hills ice should focus on improving the chronology across the MPT and developing new techniques for constructing high-resolution records of trapped gases to characterize the change in variance across this important climate transition. Greenhouse gases across the Pleistocene Our record of mean atmospheric CO 2 from pristine samples (n = 28) indicates a gradual ~25 ppm decline in CO 2 from 250 to 225 ppm from 2.9 to 1.2 Ma. Our atmospheric CH 4 record indicates no significant change in a mean value of ~500 ppb across the same period (Fig. 1). Our measurements from the late Pliocene/early Pleistocene are ~100 ppm lower than the mean concentrations from CO 2 -boron reconstructions and generally within the range of CO 2 -alk, though the two CO 2 -alk records from this period differ (Fig. 3, see Methods). Our CO 2 record agrees quite well with model reconstructions for this period (Supplementary Fig. 9). Our Pliocene-aged samples impacted by respiration (n = 22) include 4 samples that range in age from 2.6-3.1 ± 0.3 Ma. We estimate the atmospheric CO 2 from these samples using a range of organic δ 13 C endmembers from -28‰ to -30‰, with -29‰ as the highest likelihood (see Methods). The mean CO 2 -corrected concentrations for three of the four Pliocene-aged samples are indistinguishable from the pristine samples from the same period, while one sample exhibits a slightly higher range of corrected concentrations from 290-320 ppm (Fig. 3). Due to the associated age uncertainty, only one sample is very likely from the Pliocene ( 40 Ar atm age of 3.1 ± 0.3 Ma), with an estimated CO 2 of 245 ± 40 ppm. Taken together, our samples from the late Pliocene indicate that CO 2 may have been lower than 280 ppm before the PPT. It is possible that a drop in CO 2 occurred slightly earlier, that the CO 2 threshold required to build northern hemisphere ice sheets is lower than previously thought, or that the onset of northern hemisphere glaciation had drivers other than CO 2 2,34 . The relatively modest change in average concentration of atmospheric CO 2 and no change in CH 4 stand in contrast to evidence of cooling over the Pleistocene. Mean ocean temperature (MOT) inferred from trapped noble gases in Allan Hills samples show a clear ~1.5° C cooling in the same set of samples (Shackleton et al. 2024b). Many independent temperature reconstructions from marine sediments indicate global cooling through this same period 35,36 . Our results from Allan Hills ice cores suggest other factors, in addition to the slight decline in CO 2 , such as changes in ocean circulation 37,38 , albedo from ice cover or vegetation 39,40 , and/or global geography and seaways 41,42 , may be key contributors to the global cooling of the late Pliocene to the mid-Pleistocene. Strong non-linearity in the ice sheet and vegetation responses to climate may contribute to the observation that at similar CO 2 concentrations the late Pliocene had smaller ice volume, and was much warmer than the preindustrial period. Implications for the global carbon cycle On timescales of hundreds of thousands to millions of years, the global carbon cycle strikes a balance between the CO 2 emitted from Earth’s interior and long-term carbon burial 45 . Earth’s surface temperature is believed to play a critical role in this balance as a controlling factor on silicate weathering rates 46 , though finding direct evidence for this negative feedback in the geologic record remains challenging. Our reconstruction of a ~25 ppm decline in atmospheric CO 2 from 2.9 to 1.2 Ma is consistent with an imbalance between sources and sinks of <1% 47 . This slight imbalance (if it exists) may represent a decline in CO 2 sources, an increase in sinks, or some combination of the two. The results presented here provide a quantitative constraint that can be evaluated against existing hypotheses which include a decline in volcanic outgassing, an increase in continental silicate weathering efficiency, or a weak silicate weathering feedback and declining seawater Ca + concentrations 48–51 . Future work refining the timing and magnitude of CO 2 changes over the Pleistocene from Allan Hills ice cores will permit further evaluation of these hypotheses. Interestingly, much of the observed decline in CO 2 could be explained by an internal redistribution of CO 2 associated with cooler ocean temperatures and increased CO 2 solubility; a 1.5° C decline in MOT would be expected to lower atmospheric CO 2 by ~12 ppm 52 . This mechanism only explains the decline in atmospheric CO 2 if the flux if sources and sinks are independent of climate. Our record provides the first direct measurement of the global CH 4 cycle in the early Pleistocene. Average CH 4 concentrations do not change across the Pleistocene and absolute values are similar to, but in the lower range, of those found in much of the 800 ka ice core record. This result is in contrast to a recent PlioMIP modeling study that employed a dynamic global vegetation model and found that warmer climates are associated with higher CH 4 emissions from all sources 7 . Though higher temperatures and humidity would reduce CH 4 lifetime and offset these increased sources 53 , the apparent stability of average atmospheric CH 4 through significant global cooling warrants further study. Conclusion Despite a warmer climate in the Late Pliocene and Early Pleistocene, our new record of atmospheric greenhouse gases from shallow ice cores in Antarctica indicate that average atmospheric CO 2 and CH 4 levels over this time period are all below pre-industrial values. Future studies will further refine this record, seeking orbital-scale variability that would allow us to establish peak interglacial values before the MPT. 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REFERENCES IN PREP Carter, A., Aarons, S., Schnaubelt, J., Tabor, C., Higgins, J., Shackleton, S., Epifanio, J., Morgan, J., Koornneef, J., Davies, G.R., Gabrielli, P., Choi, A., Severinghaus, J.P., Brook, E.J., Introne, D.S., Marks Peterson J., Sutter, J. Evidence for diminished Ross Ice Shelf and West Antarctic Ice Sheet during the Last Interglacial from ice cores at the Allan Hills, Antarctica. In Review. Shackleton, S., Hishamunda, V., Davidge, L., Bender, M., Higgins, J. 6 million-year-old ice and air from the Allan Hills blue ice area, East Antarctica. Submitted Shackleton, S. Hishamunda, V., Bender, M., Yan, Y., Carter, A., Morgan, J., Severinghaus, J., Aarons, S., Marks Peterson, J., Epifanio, J., Buizert, C., Brook, E., Kurbatov, A., Higgins, J. Global ocean heat content over the last 3 million years. Submitted Supplementary Figure 9 Supplementary Figure 9 is not available with this version. Additional Declarations There is NO Competing Interest. Supplementary Files MarksPetersonetal2024TableS1.xlsx Table S1 JMP3MillionYrCO2Supp.docx Supplementary Information Cite Share Download PDF Status: Published Journal Publication published 18 Mar, 2026 Read the published version in Nature → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5610566","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Physical Sciences - Article","associatedPublications":[],"authors":[{"id":392464689,"identity":"26d202c0-6976-4085-8303-d45796143e1b","order_by":0,"name":"Julia Marks Peterson","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABDUlEQVRIie3QsUrEMBjA8S8EkiVwa8qdPkMlcMfRoa/SEGhfoYNgXeoS6epjCL6AJXC3BFw7CFoKzldwqAhi9fAGIeVGwfyHQD6+H4QA+Hx/Ngans18jPLGOv4kIigPHxxAAeXt/LFnRqu7yhUF32+u2fc0f45g+1M+QR7JwkLU2+Mwyg5d2K8TCvkjNFA7BZk4SNooEBTNk2aRkHpQmYaAIR6Vxk6eOvo2EiZuUvo8kZrOODuhjgjSYoJHwkKcE9aVBmisCqHCTtVbj/7Is5HaD52CN1E0neLLJhIusaN32lzq6qK5K1A+5iWkl293uPDpxPuzrQHp/wexnnDjWDwSG/QUNE5s+n8/3f/sEfjpXyZsNlIYAAAAASUVORK5CYII=","orcid":"","institution":"Oregon State University","correspondingAuthor":true,"prefix":"","firstName":"Julia","middleName":"Marks","lastName":"Peterson","suffix":""},{"id":392464690,"identity":"a97e81e7-c119-49e5-9600-88b2f73e61ab","order_by":1,"name":"Sarah Shackleton","email":"","orcid":"https://orcid.org/0000-0001-5927-1954","institution":"Woods Hole Oceanographic 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Oceanography","correspondingAuthor":false,"prefix":"","firstName":"Ross","middleName":"","lastName":"Beaudette","suffix":""},{"id":392464697,"identity":"37fa2b4f-1db4-4185-88a1-6c341b6d09fe","order_by":8,"name":"Valens Hishamunda","email":"","orcid":"https://orcid.org/0000-0003-1043-6847","institution":"Princeton University","correspondingAuthor":false,"prefix":"","firstName":"Valens","middleName":"","lastName":"Hishamunda","suffix":""},{"id":392464698,"identity":"df8a5517-7595-49e6-9869-7b1a2f575f83","order_by":9,"name":"Austin Carter","email":"","orcid":"","institution":"University of California at San Diego","correspondingAuthor":false,"prefix":"","firstName":"Austin","middleName":"","lastName":"Carter","suffix":""},{"id":392464699,"identity":"4df503ab-d6f8-4930-b3e2-784c7a1f0a7f","order_by":10,"name":"Andrei Kurbatov","email":"","orcid":"https://orcid.org/0000-0002-9819-9251","institution":"University of Maine System","correspondingAuthor":false,"prefix":"","firstName":"Andrei","middleName":"","lastName":"Kurbatov","suffix":""},{"id":392464700,"identity":"5a677be8-50d3-4685-b38f-9d88be8a751d","order_by":11,"name":"Jenna Epifanio","email":"","orcid":"","institution":"Oregon State University","correspondingAuthor":false,"prefix":"","firstName":"Jenna","middleName":"","lastName":"Epifanio","suffix":""},{"id":392464701,"identity":"a324cd5d-5d9a-4a24-9c1e-7f881bfc8b9b","order_by":12,"name":"Jacob Morgan","email":"","orcid":"","institution":"University of California at San Diego","correspondingAuthor":false,"prefix":"","firstName":"Jacob","middleName":"","lastName":"Morgan","suffix":""},{"id":392464702,"identity":"649c39f9-a2a1-40c5-ab74-7c2be509cfd7","order_by":13,"name":"Mike Bender","email":"","orcid":"","institution":"Princeton University","correspondingAuthor":false,"prefix":"","firstName":"Mike","middleName":"","lastName":"Bender","suffix":""},{"id":392464703,"identity":"43bb75cf-a2e5-488e-a8fd-65e270142eba","order_by":14,"name":"Ed Brook","email":"","orcid":"","institution":"Oregon State University","correspondingAuthor":false,"prefix":"","firstName":"Ed","middleName":"","lastName":"Brook","suffix":""}],"badges":[],"createdAt":"2024-12-09 16:19:04","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5610566/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5610566/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41586-025-10032-y","type":"published","date":"2026-03-18T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":72203122,"identity":"9d89f4d0-122f-4c71-900b-f5a9c21cbc2c","added_by":"auto","created_at":"2024-12-23 16:16:53","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":249081,"visible":true,"origin":"","legend":"\u003cp\u003eFrom top to bottom: a-c) Measurements from this study are blue circles, redated measurements from ALHI1503 in red circles\u003csup\u003e27\u003c/sup\u003e, and the continuous ice core record for each gas parameter in grey (EDC CH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e28\u003c/sup\u003e, composite CO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e29,54\u003c/sup\u003e, and EDC δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003eat\u003c/sub\u003e\u003csup\u003e30\u003c/sup\u003e. d) Benthic stack of foraminiferal δ\u003csup\u003e18\u003c/sup\u003eO\u003csup\u003e55\u003c/sup\u003e and d\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003eice\u003c/sub\u003e values from ALHIC ice cores. Measurements are divided into “short” (x) and “long” (o) snapshots, with linear regressions fit to the long snapshots only. Open blue circles represent samples without a co-registered \u003csup\u003e40\u003c/sup\u003eAr\u003csub\u003eatm\u003c/sub\u003e age. Redated measurements from ALHI1503 are included. A linear regression model fit of the ALHIC1901 data with time for three intervals are plotted for each gas: pre-MPT (2.9 – 1.2 Ma), MPT (1.2 – 0.8 Ma). Only the pre-MPT CO\u003csub\u003e2\u003c/sub\u003e trend is statistically significant (R\u003csup\u003e2\u003c/sup\u003e = 0.48, p-value \u0026lt;\u0026lt; 0.05).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5610566/v1/de982d0313074325a8858f8c.png"},{"id":72203126,"identity":"80486c5b-e695-4a75-8a04-2505490ab173","added_by":"auto","created_at":"2024-12-23 16:16:54","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":46975,"visible":true,"origin":"","legend":"\u003cp\u003eCross-plot of measured CO\u003csub\u003e2\u003c/sub\u003e concentrations and δ\u003csup\u003e13\u003c/sup\u003eC values for a subset of samples from ALHIC1901. Samples with pristine CO\u003csub\u003e2\u003c/sub\u003e in (o) whereas samples with altered δ\u003csup\u003e13\u003c/sup\u003eC values in squares (\u0026nbsp; ). Samples colored by \u003csup\u003e40\u003c/sup\u003eAr\u003csub\u003eATM\u003c/sub\u003e age. The linear regression (red line, dashed lines = 95% confidence intervals) indicates a strong correlation (R\u003csup\u003e2\u003c/sup\u003e = 0.97, p-value \u0026lt;\u0026lt; 0.05). The y-intercept of the linear fit is -29‰, suggesting this to be the most likely signature of the organic source.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5610566/v1/10cb7e1fce2ddfaebc45cb89.png"},{"id":72202836,"identity":"5321acc4-5cf1-4af6-9742-963d0ff866ae","added_by":"auto","created_at":"2024-12-23 16:16:34","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":215462,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of pristine CO\u003csub\u003e2\u003c/sub\u003e (circles) and respiration-corrected CO\u003csub\u003e2\u003c/sub\u003e (open squares) from ALHIC1901 to CO\u003csub\u003e2\u003c/sub\u003e proxy reconstructions. Error bars in respiration-corrected CO\u003csub\u003e2\u003c/sub\u003e comes from uncertainties in the δ\u003csup\u003e13\u003c/sup\u003eC value of the respired organic carbon (-28‰ to -30‰). a) CO\u003csub\u003e2\u003c/sub\u003e-alkenone data (blue diamond) over the last 3.5 million years from\u003csup\u003e14,17,19,20\u003c/sup\u003e. b) CO\u003csub\u003e2\u003c/sub\u003e-paleosol data (green diamond) over the last 3.5 million years from\u003csup\u003e22\u003c/sup\u003e. c) CO\u003csub\u003e2\u003c/sub\u003e-boron data (red diamond) over the last 3.5 million years from\u003csup\u003e8,9,11–15\u003c/sup\u003e. The continuous ice core record is plotted in grey\u003csup\u003e29,54\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5610566/v1/639ec8458c22781a71549f81.png"},{"id":104952669,"identity":"c986ec68-b885-4e51-ad69-062e6496fddd","added_by":"auto","created_at":"2026-03-19 07:13:58","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1010220,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5610566/v1/7a3de238-9402-4430-941b-79a98ec3f69d.pdf"},{"id":72203030,"identity":"e947d7e7-52c7-4e98-b421-86b8a31db492","added_by":"auto","created_at":"2024-12-23 16:16:45","extension":"xlsx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":38038,"visible":true,"origin":"","legend":"Table S1","description":"","filename":"MarksPetersonetal2024TableS1.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-5610566/v1/0e2a5515048724d700eb57fb.xlsx"},{"id":72203048,"identity":"10216320-56a7-45c0-8b41-6382dc48477a","added_by":"auto","created_at":"2024-12-23 16:16:48","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":5056573,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"JMP3MillionYrCO2Supp.docx","url":"https://assets-eu.researchsquare.com/files/rs-5610566/v1/27cda9081aee8517f32cf682.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Ice cores from the Allan Hills, Antarctica show relatively stable atmospheric CO2 and CH4 levels over the last 3 million years","fulltext":[{"header":"MAIN TEXT","content":"\u003cp\u003eEarth’s climate over the last 3 million years is characterized by gradual cooling and an increase in the amplitude and duration of glacial cycles. The greenhouse gases (GHGs) carbon dioxide (CO\u003csub\u003e2\u003c/sub\u003e) and methane (CH\u003csub\u003e4\u003c/sub\u003e) play an important role in regulating Earth’s climate but their histories are not well constrained for significant climatic intervals over the last 3 Ma due, in part, to the lack of direct atmospheric archives. Two intervals have received particular attention, the transition from the Pliocene to the Pleistocene (PPT; ~2.6 Ma), a period of continental ice sheet expansion in the northern hemisphere, and the mid-Pleistocene Transition (MPT; 1.2-0.8 Ma), a transition from small-amplitude, 40 ka glacial cycles to large amplitude, 100 ka cycles. Changes in atmospheric CO\u003csub\u003e2\u003c/sub\u003e have been invoked to explain both of these climate transitions; the PPT has been hypothesized to be associated with a drop in CO\u003csub\u003e2\u003c/sub\u003e to less than 280 ppm\u003csup\u003e1,2\u003c/sup\u003e, and the MPT associated with a decline in CO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e3,4\u003c/sup\u003e. More broadly, trends in atmospheric CO\u003csub\u003e2\u003c/sub\u003e over the last 3 million years provide critical insights into the processes that control the global carbon cycle on geologic timescales\u003csup\u003e5,6\u003c/sup\u003e. Finally, CH\u003csub\u003e4\u003c/sub\u003e has been hypothesized to also play a role in past warm climates\u003csup\u003e7\u003c/sup\u003e, but no proxies for atmospheric CH\u003csub\u003e4\u003c/sub\u003e exist beyond the ice core record.\u003c/p\u003e\n\u003cp\u003eReconstructions of atmospheric CO\u003csub\u003e2\u003c/sub\u003e beyond the ice core record primarily rely on indirect methods that take advantage of the relationship between atmospheric CO\u003csub\u003e2\u003c/sub\u003e, aqueous carbonate chemistry (CO\u003csub\u003e2\u003c/sub\u003e, HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e, pH) and the isotopic composition of biogenic fossils. For example, the ratio of boron isotopes (\u003csup\u003e11\u003c/sup\u003eB/\u003csup\u003e10\u003c/sup\u003eB) recorded in the shells of planktonic foraminiferal calcite has been used to reconstruct local pH and atmospheric CO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e8–14\u003c/sup\u003e (CO\u003csub\u003e2\u003c/sub\u003e-boron). Advances to the CO\u003csub\u003e2\u003c/sub\u003e-boron technique have gradually reduced uncertainty, allowing reconstructions of orbital-scale variability\u003csup\u003e14–16\u003c/sup\u003e. The ratio of \u003csup\u003e13\u003c/sup\u003eC to \u003csup\u003e12\u003c/sup\u003eC in alkenones in marine sediments has also been used to reconstruct atmospheric CO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e14,17–21\u003c/sup\u003e (CO\u003csub\u003e2\u003c/sub\u003e-alk) due to the fact that the magnitude of carbon isotope fractionation depends on the ratio of intracellular and extracellular aqueous CO\u003csub\u003e2\u003c/sub\u003e. Unfortunately, CO\u003csub\u003e2\u003c/sub\u003e-alk and CO\u003csub\u003e2\u003c/sub\u003e-boron produce very different histories over the last 3 million years; CO\u003csub\u003e2\u003c/sub\u003e-boron declines from ~400 to 250 ppm while CO\u003csub\u003e2\u003c/sub\u003e-alk shows relatively constant levels around 250 ppm, though with larger uncertainties. Both approaches have potential for systematic biases related to evolutionary change that are inherent to the use of biogenic fossils as paleoenvironmental archives. Other relevant proxies include carbon isotopes in paleosols\u003csup\u003e22\u003c/sup\u003e (CO\u003csub\u003e2\u003c/sub\u003e-paleosol) (though these reconstructions are associated with very large uncertainties), and a new proxy for atmospheric CO\u003csub\u003e2\u003c/sub\u003e based on the δ\u003csup\u003e13\u003c/sup\u003eC of leaf wax from a sediment core from the Bay of Bengal (CO\u003csub\u003e2\u003c/sub\u003e-leafwax)\u003csup\u003e23\u003c/sup\u003e, which currently extends to 1.5 Ma.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHere, using new Allan Hills ice cores that extend further back in time, and an improved sampling and dating strategy enabled by larger core diameter, we extend ice core-based observations of CO\u003csub\u003e2\u003c/sub\u003e and CH\u003csub\u003e4\u003c/sub\u003e to the entire Pleistocene, redate and re-interpret previously published Allan Hills data, and provide indirect constraints on CO\u003csub\u003e2\u003c/sub\u003e for the late Pliocene. This new work both improves our interpretations of Allan Hills BIA ice cores and offers new insights into past greenhouse gas variations.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInterpreting Allan Hills paleoclimate records\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn the Allan Hills BIA, located in Victoria Land, on the western flanks of the Transantarctic Mountains, ice flow, subglacial topography, and ablation have exposed old ice at and near the surface of the East Antarctic ice sheet\u003csup\u003e24\u003c/sup\u003e. Shallow (100-200 m) ice cores drilled in the Allan Hills BIA and dated using the \u003csup\u003e40\u003c/sup\u003eAr\u003csub\u003eatm\u0026nbsp;\u003c/sub\u003egeochronometer\u003csup\u003e25\u003c/sup\u003e have revealed the existence of stratigraphically complex ice as old as 6 Ma\u003csup\u003e26,27\u003c/sup\u003e (Shackleton et al. 2024a). As discussed in Shackleton et al., (2024a), developing paleoclimate archives from ice that is thinned and folded is in its early stages, resulting in significant uncertainties in the amount of time represented in an individual sample, the extent of mixing between samples of different ages, and/or the possibility of preservation bias. Given these complexities, we take a minimalist approach of interpreting the data as representing discrete snapshots of East Antarctic climate and the atmosphere that extend from the Late Pliocene to the Late Pleistocene.\u003c/p\u003e\n\u003cp\u003eIn previous work on Allan Hills cores, \u003csup\u003e40\u003c/sup\u003eAr\u003csub\u003eatm\u003c/sub\u003e ages could not be measured on the same depths as other climate properties due to sample size constraints. Consequently, results for climate properties of those cores (ALHIC1502 and 1503) were binned in broad age ranges\u003csup\u003e26,27\u003c/sup\u003e.\u0026nbsp;We improve previous GHG reconstructions from Allan Hills in three ways. First, using a large-diameter (24.1 cm) core we co-register climate property measurements (CO\u003csub\u003e2\u003c/sub\u003e, CH\u003csub\u003e4\u003c/sub\u003e, and the isotopic composition of atmospheric oxygen (δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003eatm\u003c/sub\u003e), see Methods) with age (\u003csup\u003e40\u003c/sup\u003eAr\u003csub\u003eatm\u003c/sub\u003e) determinations (Fig. 1). Second, by increasing the density of ages (79 samples in the bottom 25 meters), individual folds and age discontinuities are resolved (Shackleton et al. 2024a). Of the 79 samples analyzed, 54 are older than 800 ka, with ages ranging from 800 ± 80 ka to 4000 ± 400 ka. Third, we further interrogate samples with apparently elevated CO\u003csub\u003e2\u003c/sub\u003e using measurements of the δ\u003csup\u003e13\u003c/sup\u003eC of trapped CO\u003csub\u003e2\u003c/sub\u003e at the same depth (n = 31) and find that all samples from the bottom 8 m contain variable levels of non-atmospheric CO\u003csub\u003e2\u003c/sub\u003e (see Methods). Measured δ\u003csup\u003e13\u003c/sup\u003eC of CO\u003csub\u003e2\u003c/sub\u003e values are strongly correlated with 1/CO\u003csub\u003e2\u003c/sub\u003e, indicating that they represent a mixture of atmospheric CO\u003csub\u003e2\u003c/sub\u003e and a single source of organic carbon (Supplementary Fig. 6). Correcting for the contribution from respiration results in 21 additional estimates of atmospheric CO\u003csub\u003e2\u003c/sub\u003e that range in age from 700 ± 70 ka to 3100 ± 300 ka (excluding the deepest 4000 ka sample, see Methods). Comparison of these values with values from pristine ice of the same age reveals 13 indistinguishable and 8 samples with positive offsets of ~20-50 ppm (Supplementary Fig. 7). This offset is observed in both young (~800 ka) and old samples, suggesting that either these values are under corrected or that they reflect true periods of elevated CO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eComparing data from samples with \u003csup\u003e40\u003c/sup\u003eAr\u003csub\u003eatm\u003c/sub\u003e ages younger than 800,000 years to continuous ice core records from EPICA Dome C (EDC\u003csup\u003e28–30\u003c/sup\u003e) reveals that there are at least two types of paleoclimate snapshots, broadly distinguished by proximity to bedrock: 1) short-exposure snapshots in shallow ice that preserve some of the true glacial-interglacial variability, and 2) long-exposure snapshots in near-basal ice that preserve climate signals that more closely represent the mean over a glacial cycle. The transition between these two types of snapshots coincides with both the appearance of ice older than 1 Ma and the first evidence for contamination of trapped CO\u003csub\u003e2\u003c/sub\u003e by respired organic carbon (~141 m depth in ALHIC1901). When comparing the variance in gas properties in the short-exposure snapshots to corresponding EDC records approximately 42% ± 0.7% of the CO\u003csub\u003e2\u003c/sub\u003e range observed in the EDC record from 400-800 ka is captured in these samples, while the range of CH\u003csub\u003e4\u003c/sub\u003e and δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003eatm\u003c/sub\u003e encompasses 38% ± 0.7% and 23% ± 0.6% of the EDC range, respectively (Supplementary Fig. 2). Glacial-level values of all three gas properties are missing in our record, suggesting a preservation bias toward interglacial samples in this age range. This is likely due to very low deposition during glacial periods, consistent with accumulation rate reconstructions from nearby stratigraphically intact cores (ALHIC1903 and S27)\u003csup\u003e31\u003c/sup\u003e (See Methods) (Carter et al., in review). Ice that is younger than 800 ka but located stratigraphically below ice older than 1 Ma records much less variability in all climate properties and the values agree well with depth-weighted averages of these gases from the same period in EDC (Supplementary Fig. 3). We interpret these samples as representing long-exposure snapshots of the mean climate state. As these samples constitute all our oldest ice samples, we take a conservative approach and focus on long-term trends in the record rather than change in variance (Fig. 1).\u003c/p\u003e\n\u003cp\u003eThe high-resolution chronology of core ALHIC1901permits a reevaluation of the ages previously assigned to samples from core ALHIC1503, which is ~8 m away\u003csup\u003e27\u003c/sup\u003e. A comparison of measured properties plotted on depth reveals a depth offset between the two cores ranging from 9 to 12 m, implying layers are dipping at ~48° (see Methods, Supplementary Fig. 1). Applying the depth-age relationship from ALHIC1901 to ALHIC1503 results in the reassignment of 51 sample ages from ALHIC1503 and resolves an apparent discrepancy in the range of atmospheric CO\u003csub\u003e2\u003c/sub\u003e and CH\u003csub\u003e4\u003c/sub\u003e in the early Pleistocene and MPT between the two cores (Fig. 1). Additionally, results from ALHIC1901 indicate the presence of non-atmospheric CO\u003csub\u003e2\u003c/sub\u003e at a comparable depth to high CO\u003csub\u003e2\u003c/sub\u003e concentrations (260-280 ppm) measured from ALHIC1503, although sufficient samples to test this interference with stable isotope measurements are no longer available from ALHIC1503. Redated ALHIC1503 measurements are included in our discussion and, moving forward, their new age assignments should be used when plotting these data.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGreenhouse gases across the MPT\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe greenhouse gas records from Allan Hills ice cores show no significant change in mean CO\u003csub\u003e2\u003c/sub\u003e or CH\u003csub\u003e4\u003c/sub\u003e across the MPT (Fig. 1), consistent with independent datasets\u003csup\u003e9,23,32\u003c/sup\u003e and modeling studies\u003csup\u003e33\u003c/sup\u003e. In addition, our CO\u003csub\u003e2\u003c/sub\u003e concentrations across the MPT agree with the CO\u003csub\u003e2\u003c/sub\u003e-leafwax record after applying a Gaussian-weighted smoothing over a 400,000 year window (Supplementary Fig. 8, see Methods). Our interpretation of deep samples representing long-exposure snapshots precludes a discussion of changes in minimum and maximum CO\u003csub\u003e2\u003c/sub\u003e levels across the MPT, a critical component of many hypotheses for this transition. However, we note that a decline in the average value of CO\u003csub\u003e2\u003c/sub\u003e in glacial periods would result in a decline in overall mean CO\u003csub\u003e2\u003c/sub\u003e unless balanced by higher CO\u003csub\u003e2\u003c/sub\u003e during interglacial periods. Further investigations of Allan Hills ice should focus on improving the chronology across the MPT and developing new techniques for constructing high-resolution records of trapped gases to characterize the change in variance across this important climate transition.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGreenhouse gases across the Pleistocene\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOur record of mean atmospheric CO\u003csub\u003e2\u003c/sub\u003e from pristine samples (n = 28) indicates a gradual ~25 ppm decline in CO\u003csub\u003e2\u003c/sub\u003e from 250 to 225 ppm from 2.9 to 1.2 Ma. Our atmospheric CH\u003csub\u003e4\u003c/sub\u003e record indicates no significant change in a mean value of ~500 ppb across the same period (Fig. 1). Our measurements from the late Pliocene/early Pleistocene are ~100 ppm lower than the mean concentrations from CO\u003csub\u003e2\u003c/sub\u003e-boron reconstructions and generally within the range of CO\u003csub\u003e2\u003c/sub\u003e-alk, though the two CO\u003csub\u003e2\u003c/sub\u003e-alk records from this period differ (Fig. 3, see Methods). Our CO\u003csub\u003e2\u003c/sub\u003e record agrees quite well with model reconstructions for this period (Supplementary Fig. 9). Our Pliocene-aged samples impacted by respiration (n = 22) include 4 samples that range in age from 2.6-3.1 ± 0.3 Ma. We estimate the atmospheric CO\u003csub\u003e2\u003c/sub\u003e from these samples using a range of organic δ\u003csup\u003e13\u003c/sup\u003eC endmembers from -28‰ to -30‰, with -29‰ as the highest likelihood (see Methods). The mean CO\u003csub\u003e2\u003c/sub\u003e-corrected concentrations for three of the four Pliocene-aged samples are indistinguishable from the pristine samples from the same period, while one sample exhibits a slightly higher range of corrected concentrations from 290-320 ppm (Fig. 3). Due to the associated age uncertainty, only one sample is very likely from the Pliocene (\u003csup\u003e40\u003c/sup\u003eAr\u003csub\u003eatm\u003c/sub\u003e age of 3.1 ± 0.3 Ma), with an estimated CO\u003csub\u003e2\u003c/sub\u003e of 245 ± 40 ppm. Taken together, our samples from the late Pliocene indicate that CO\u003csub\u003e2\u003c/sub\u003e may have been lower than 280 ppm before the PPT. It is possible that a drop in CO\u003csub\u003e2\u003c/sub\u003e occurred slightly earlier, that the CO\u003csub\u003e2\u003c/sub\u003e threshold required to build northern hemisphere ice sheets is lower than previously thought, or that the onset of northern hemisphere glaciation had drivers other than CO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e2,34\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe relatively modest change in average concentration of atmospheric CO\u003csub\u003e2\u003c/sub\u003e and no change in CH\u003csub\u003e4\u003c/sub\u003e stand in contrast to evidence of cooling over the Pleistocene. Mean ocean temperature (MOT) inferred from trapped noble gases in Allan Hills samples show a clear ~1.5° C cooling in the same set of samples (Shackleton et al. 2024b). Many independent temperature reconstructions from marine sediments indicate global cooling through this same period\u003csup\u003e35,36\u003c/sup\u003e. Our results from Allan Hills ice cores suggest other factors, in addition to the slight decline in CO\u003csub\u003e2\u003c/sub\u003e, such as changes in ocean circulation\u003csup\u003e37,38\u003c/sup\u003e, albedo from ice cover or vegetation\u003csup\u003e39,40\u003c/sup\u003e, and/or global geography and seaways\u003csup\u003e41,42\u003c/sup\u003e, may be key contributors to the global cooling of the late Pliocene to the mid-Pleistocene. Strong non-linearity in the ice sheet and vegetation responses to climate may contribute to the observation that at similar CO\u003csub\u003e2\u003c/sub\u003e concentrations the late Pliocene had smaller ice volume, and was much warmer than the preindustrial period.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eImplications for the global carbon cycle\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOn timescales of hundreds of thousands to millions of years, the global carbon cycle strikes a balance between the CO\u003csub\u003e2\u003c/sub\u003e emitted from Earth’s interior and long-term carbon burial\u003csup\u003e45\u003c/sup\u003e. Earth’s surface temperature is believed to play a critical role in this balance as a controlling factor on silicate weathering rates\u003csup\u003e46\u003c/sup\u003e, though finding direct evidence for this negative feedback in the geologic record remains challenging. \u0026nbsp;Our reconstruction of a ~25 ppm decline in atmospheric CO\u003csub\u003e2\u003c/sub\u003e from 2.9 to 1.2 Ma is consistent with an imbalance between sources and sinks of \u0026lt;1%\u003csup\u003e47\u003c/sup\u003e. This slight imbalance (if it exists) may represent a decline in CO\u003csub\u003e2\u003c/sub\u003e sources, an increase in sinks, or some combination of the two. The results presented here provide a quantitative constraint that can be evaluated against existing hypotheses which include a decline in volcanic outgassing, an increase in continental silicate weathering efficiency, or a weak silicate weathering feedback and declining seawater Ca\u003csup\u003e+\u003c/sup\u003e concentrations\u003csup\u003e48–51\u003c/sup\u003e. Future work refining the timing and magnitude of CO\u003csub\u003e2\u003c/sub\u003e changes over the Pleistocene from Allan Hills ice cores will permit further evaluation of these hypotheses. Interestingly, much of the observed decline in CO\u003csub\u003e2\u003c/sub\u003e could be explained by an internal redistribution of CO\u003csub\u003e2\u003c/sub\u003e associated with cooler ocean temperatures and increased CO\u003csub\u003e2\u003c/sub\u003e solubility; a 1.5° C decline in MOT would be expected to lower atmospheric CO\u003csub\u003e2\u003c/sub\u003e by ~12 ppm\u003csup\u003e52\u003c/sup\u003e. This mechanism only explains the decline in atmospheric CO\u003csub\u003e2\u003c/sub\u003e if the flux if sources and sinks are independent of climate.\u003c/p\u003e\n\u003cp\u003eOur record provides the first direct measurement of the global CH\u003csub\u003e4\u003c/sub\u003e cycle in the early Pleistocene. Average CH\u003csub\u003e4\u003c/sub\u003e concentrations do not change across the Pleistocene and absolute values are similar to, but in the lower range, of those found in much of the 800 ka ice core record. This result is in contrast to a recent PlioMIP modeling study that employed a dynamic global vegetation model and found that warmer climates are associated with higher CH\u003csub\u003e4\u003c/sub\u003e emissions from all sources\u003csup\u003e7\u003c/sup\u003e. Though higher temperatures and humidity would reduce CH\u003csub\u003e4\u003c/sub\u003e lifetime and offset these increased sources\u003csup\u003e53\u003c/sup\u003e, the apparent stability of average atmospheric CH\u003csub\u003e4\u003c/sub\u003e through significant global cooling warrants further study. \u0026nbsp;\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eDespite a warmer climate in the Late Pliocene and Early Pleistocene, our new record of atmospheric greenhouse gases from shallow ice cores in Antarctica indicate that average atmospheric CO\u003csub\u003e2\u003c/sub\u003e and CH\u003csub\u003e4\u003c/sub\u003e levels over this time period are all below pre-industrial values. Future studies will further refine this record, seeking orbital-scale variability that would allow us to establish peak interglacial values before the MPT. However, a mean atmospheric CO\u003csub\u003e2\u003c/sub\u003e value of ~250 ppm in the Late Pliocene is difficult to reconcile with interglacial maxima that exceed 400 ppm, as suggested by some proxy records, unless the glacial-interglacial range was large, which is not consistent with most interpretations of the benthic oxygen isotopic record and its implications for ice volume variability in the early Pleistocene. Our records thus suggest that other processes – ocean gateways, albedo, ocean circulation – play an important role in explaining higher late Pliocene/early Pleistocene temperatures.\u003c/p\u003e"},{"header":"REFERENCES","content":"\u003col\u003e\n \u003cli\u003eDeConto, R. M. et al. Thresholds for Cenozoic bipolar glaciation. Nature 455, 652\u0026ndash;656 (2008).\u003c/li\u003e\n \u003cli\u003eVizca\u0026iacute;no, M., Rupper, S. \u0026amp; Chiang, J. C. H. 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Nat Commun 10, 4342 (2019).\u003cbr\u003e\u003cbr\u003e\u003cstrong\u003eREFERENCES IN PREP\u003c/strong\u003e\u003c/li\u003e\n\u003c/ol\u003e\n\u003col start=\"1\" type=\"1\"\u003e\n \u003cli\u003eCarter, A., Aarons, S., Schnaubelt, J., Tabor, C., Higgins, J., Shackleton, S., Epifanio, J., Morgan, J., Koornneef, J., Davies, G.R., Gabrielli, P., Choi, A., Severinghaus, J.P., Brook, E.J., Introne, D.S., Marks Peterson J., Sutter, J. Evidence for diminished Ross Ice Shelf and West Antarctic Ice Sheet during the Last Interglacial from ice cores at the Allan Hills, Antarctica. In Review.\u003c/li\u003e\n\u003c/ol\u003e\n\u003col start=\"2\" type=\"1\"\u003e\n \u003cli\u003eShackleton, S., Hishamunda, V., Davidge, L., Bender, M., Higgins, J. 6 million-year-old ice and air from the Allan Hills blue ice area, East Antarctica. Submitted\u003c/li\u003e\n\u003c/ol\u003e\n\u003col start=\"3\" type=\"1\"\u003e\n \u003cli\u003eShackleton, S. Hishamunda, V., Bender, M., Yan, Y., Carter, A., Morgan, J., Severinghaus, J., Aarons, S., Marks Peterson, J., Epifanio, J., Buizert, C., Brook, E., Kurbatov, A., Higgins, J. Global ocean heat content over the last 3 million years. Submitted\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Supplementary Figure 9","content":"\u003cp\u003eSupplementary Figure 9 is not available with this version.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5610566/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5610566/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe role greenhouse gases play in the evolution of Earth's climate over the last 3 million years is uncertain beyond the continuous ice core record (800,000 years). Here, we present new snapshots of carbon dioxide (CO\u003csub\u003e2\u003c/sub\u003e) and methane (CH\u003csub\u003e4\u003c/sub\u003e) between 3.1 and 0.4 million years ago (Ma) from shallow ice cores drilled in the Allan Hills Blue Ice Area (BIA). In the oldest ice (\u0026gt;1 Ma), mixing and thinning have attenuated the glacial-interglacial variability, and we reconstruct long-term averages. The data indicate that CO\u003csub\u003e2\u003c/sub\u003e\u0026nbsp;and CH\u003csub\u003e4\u003c/sub\u003e\u0026nbsp;levels in the early Pleistocene are within the range of variability observed for the last 0.8 Ma. Across the Pleistocene, no significant change in mean CH\u003csub\u003e4\u003c/sub\u003e is observed but we find a small, 25 ppm decline in CO\u003csub\u003e2\u003c/sub\u003e from 2.9 to 1.2 Ma followed by stable mean CO\u003csub\u003e2\u003c/sub\u003e across the mid-Pleistocene Transition. In late Pliocene samples (ranging from 2.8-3.1 Ma), trapped air is impacted by the addition of CO\u003csub\u003e2\u003c/sub\u003e\u0026nbsp;from respired organic matter. Corrections using the stable carbon isotopes of CO\u003csub\u003e2\u003c/sub\u003e indicate that atmospheric CO\u003csub\u003e2\u003c/sub\u003e\u0026nbsp;in these late Pliocene samples is within the range measured in the early Pleistocene (\u0026lt;300 ppm). Observed changes in greenhouse gases are small relative to both local and global cooling observed in the same ice cores (Shackleton et al., 2024a, b) and independent records from marine sediments, suggesting that other components of Earth’s climate system contributed to global cooling over the last 3 million years.\u003c/p\u003e","manuscriptTitle":"Ice cores from the Allan Hills, Antarctica show relatively stable atmospheric CO2 and CH4 levels over the last 3 million years","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-12-23 16:06:36","doi":"10.21203/rs.3.rs-5610566/v1","editorialEvents":[],"status":"published","journal":{"display":false,"email":"[email protected]","identity":"nature","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"nature","sideBox":"Learn more about [Nature](http://www.nature.com/nature/)","snPcode":"","submissionUrl":"","title":"Nature","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"05752187-5a6f-4112-9760-8852fc9b6748","owner":[],"postedDate":"December 23rd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":41837495,"name":"Earth and environmental sciences/Climate sciences"},{"id":41837496,"name":"Earth and environmental sciences/Climate sciences/Palaeoclimate"}],"tags":[],"updatedAt":"2026-03-19T07:13:53+00:00","versionOfRecord":{"articleIdentity":"rs-5610566","link":"https://doi.org/10.1038/s41586-025-10032-y","journal":{"identity":"nature","isVorOnly":false,"title":"Nature"},"publishedOn":"2026-03-18 04:00:00","publishedOnDateReadable":"March 18th, 2026"},"versionCreatedAt":"2024-12-23 16:06:36","video":"","vorDoi":"10.1038/s41586-025-10032-y","vorDoiUrl":"https://doi.org/10.1038/s41586-025-10032-y","workflowStages":[]},"version":"v1","identity":"rs-5610566","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5610566","identity":"rs-5610566","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}