Yuzhen Yan

The Allan Hills blue ice area, East Antarctica, offers a unique opportunity to extend the ice core record beyond 800 thousand years (kyr), with ice as old as 6 million years recently recovered. The ice in this area demonstrates several peculiarities—such as strong layer thinning and folding—that warrant an in-depth investigation of its stratigraphy and the fidelity of the climate record it contains. Here, we present a high-resolution Continuous Flow Analysis (CFA) of two shallow ice cores from the Allan Hills (ALHIC 2201 and ALHIC 2302), spanning the upper 69 m and 46 m, respectively. Our CFA analysis includes methane, water stable isotopes, and particle concentrations, allowing us to characterize their variability and assess how well geochemical measurements are recorded and preserved in Allan Hills ice. Dating of the ice (using the ⁴⁰Ar chronometer) revealed ages ranging from ~150–1200 kyr with substantial age discontinuities and folding, highlighting the complex stratigraphy of ice in this region. To assess glacial-interglacial variability, we conduct descriptive statistical analyses of each measurement relative to the EPICA Dome C (EDC) deep ice core record, which served as a reference benchmark over the last 800 kyr. Relative to EDC, we find data representing warm climate states are overrepresented at Allan Hills, and the Allan Hills records typically exhibit a narrower distribution compared to the EDC record. These observations confirm previous suggestions of interglacial bias and glacial cycle averaging in some sections of Allan Hills records. Differences between ALHIC 2201 and ALHIC 2302 further suggest that coring location within the Allan Hills region affects signal preservation in the record. Our high-resolution investigation of this ice is a critical step toward improved interpretation of the discrete, multi-million-year records from the Allan Hills.

The Allan Hills blue ice area, East Antarctica, offers a unique opportunity to extend the ice core record beyond 800 thousand years (kyr), with ice as old as 6 million years recently recovered. The ice in this area demonstrates several peculiarities—such as strong layer thinning and folding—that warrant an in-depth investigation of its stratigraphy and the fidelity of the climate record it contains. Here, we present a high-resolution Continuous Flow Analysis (CFA) of two shallow ice cores from the Allan Hills (ALHIC 2201 and ALHIC 2302), spanning the upper 69 m and 46 m, respectively. Our CFA analysis includes methane, water stable isotopes, and particle concentrations, allowing us to characterize their variability and assess how well geochemical measurements are recorded and preserved in Allan Hills ice. Dating of the ice (using the ⁴⁰Ar chronometer) revealed ages ranging from ~150–1200 kyr with substantial age discontinuities and folding, highlighting the complex stratigraphy of ice in this region. To assess glacial-interglacial variability, we conduct descriptive statistical analyses of each measurement relative to the EPICA Dome C (EDC) deep ice core record, which served as a reference benchmark over the last 800 kyr. Relative to EDC, we find data representing warm climate states are overrepresented at Allan Hills, and the Allan Hills records typically exhibit a narrower distribution compared to the EDC record. These observations confirm previous suggestions of interglacial bias and glacial cycle averaging in some sections of Allan Hills records. Differences between ALHIC 2201 and ALHIC 2302 further suggest that coring location within the Allan Hills region affects signal preservation in the record. Our high-resolution investigation of this ice is a critical step toward improved interpretation of the discrete, multi-million-year records from the Allan Hills.

The Allan Hills blue ice area, East Antarctica, offers a unique opportunity to extend the ice core record beyond 800 thousand years (kyr), with ice as old as 6 million years recently recovered. The ice in this area demonstrates several peculiarities—such as strong layer thinning and folding—that warrant an in-depth investigation of its stratigraphy and the fidelity of the climate record it contains. Here, we present a high-resolution Continuous Flow Analysis (CFA) of two shallow ice cores from the Allan Hills (ALHIC 2201 and ALHIC 2302), spanning the upper 69 m and 46 m, respectively. Our CFA analysis includes methane, water stable isotopes, and particle concentrations, allowing us to characterize their variability and assess how well geochemical measurements are recorded and preserved in Allan Hills ice. Dating of the ice (using the ⁴⁰Ar chronometer) revealed ages ranging from ~150–1200 kyr with substantial age discontinuities and folding, highlighting the complex stratigraphy of ice in this region. To assess glacial-interglacial variability, we conduct descriptive statistical analyses of each measurement relative to the EPICA Dome C (EDC) deep ice core record, which served as a reference benchmark over the last 800 kyr. Relative to EDC, we find data representing warm climate states are overrepresented at Allan Hills, and the Allan Hills records typically exhibit a narrower distribution compared to the EDC record. These observations confirm previous suggestions of interglacial bias and glacial cycle averaging in some sections of Allan Hills records. Differences between ALHIC 2201 and ALHIC 2302 further suggest that coring location within the Allan Hills region affects signal preservation in the record. Our high-resolution investigation of this ice is a critical step toward improved interpretation of the discrete, multi-million-year records from the Allan Hills.

The Allan Hills blue ice area, East Antarctica, offers a unique opportunity to extend the ice core record beyond 800 thousand years (kyr), with ice as old as 6 million years recently recovered. The ice in this area demonstrates several peculiarities—such as strong layer thinning and folding—that warrant an in-depth investigation of its stratigraphy and the fidelity of the climate record it contains. Here, we present a high-resolution Continuous Flow Analysis (CFA) of two shallow ice cores from the Allan Hills (ALHIC 2201 and ALHIC 2302), spanning the upper 69 m and 46 m, respectively. Our CFA analysis includes methane, water stable isotopes, and particle concentrations, allowing us to characterize their variability and assess how well geochemical measurements are recorded and preserved in Allan Hills ice. Dating of the ice (using the ⁴⁰Ar chronometer) revealed ages ranging from ~150–1200 kyr with substantial age discontinuities and folding, highlighting the complex stratigraphy of ice in this region. To assess glacial-interglacial variability, we conduct descriptive statistical analyses of each measurement relative to the EPICA Dome C (EDC) deep ice core record, which served as a reference benchmark over the last 800 kyr. Relative to EDC, we find data representing warm climate states are overrepresented at Allan Hills, and the Allan Hills records typically exhibit a narrower distribution compared to the EDC record. These observations confirm previous suggestions of interglacial bias and glacial cycle averaging in some sections of Allan Hills records. Differences between ALHIC 2201 and ALHIC 2302 further suggest that coring location within the Allan Hills region affects signal preservation in the record. Our high-resolution investigation of this ice is a critical step toward improved interpretation of the discrete, multi-million-year records from the Allan Hills.

The Allan Hills blue ice area, East Antarctica, offers a unique opportunity to extend the ice core record beyond 800 thousand years (kyr), with ice as old as 6 million years recently recovered. The ice in this area demonstrates several peculiarities—such as strong layer thinning and folding—that warrant an in-depth investigation of its stratigraphy and the fidelity of the climate record it contains. Here, we present a high-resolution Continuous Flow Analysis (CFA) of two shallow ice cores from the Allan Hills (ALHIC 2201 and ALHIC 2302), spanning the upper 69 m and 46 m, respectively. Our CFA analysis includes methane, water stable isotopes, and particle concentrations, allowing us to characterize their variability and assess how well geochemical measurements are recorded and preserved in Allan Hills ice. Dating of the ice (using the ⁴⁰Ar chronometer) revealed ages ranging from ~150–1200 kyr with substantial age discontinuities and folding, highlighting the complex stratigraphy of ice in this region. To assess glacial-interglacial variability, we conduct descriptive statistical analyses of each measurement relative to the EPICA Dome C (EDC) deep ice core record, which served as a reference benchmark over the last 800 kyr. Relative to EDC, we find data representing warm climate states are overrepresented at Allan Hills, and the Allan Hills records typically exhibit a narrower distribution compared to the EDC record. These observations confirm previous suggestions of interglacial bias and glacial cycle averaging in some sections of Allan Hills records. Differences between ALHIC 2201 and ALHIC 2302 further suggest that coring location within the Allan Hills region affects signal preservation in the record. Our high-resolution investigation of this ice is a critical step toward improved interpretation of the discrete, multi-million-year records from the Allan Hills.

Traditional ice cores provide invaluable insights into changes in past atmospheric greenhouse gas levels, like carbon dioxide (CO ₂ ), and their relationship to climate over the last 800,000 years. Recently, ice as old as 6 million years (Ma) has been retrieved from the Allan Hills Blue Ice Area (BIA), Antarctica. However, CO ₂ levels in samples older than 3 Ma are anomalously elevated by post‐depositional processes. In this work, we explore the viability of using triple isotopic composition of atmospheric oxygen (Δ ¹⁷ O‐O ₂ ) as an independent approach for reconstructing atmospheric CO ₂ from such altered samples. Atmospheric Δ ¹⁷ O‐O ₂ and CO ₂ are dynamically linked via photochemically driven isotope exchange reactions in the stratosphere. While Δ ¹⁷ O‐O ₂ is also modulated by global biospheric productivity (GPP), its observed covariation with CO ₂ on glacial‐interglacial timescales indicate that Δ ¹⁷ O‐O ₂ , to first order, is affected by atmospheric CO ₂ levels. We employ this covariation to reconstruct atmospheric CO ₂ concentrations from both pristine and altered ice samples from the Allan Hills BIA back to 2.7 Ma. We find the Δ ¹⁷ O‐O ₂ accurately reproduces pristine CO ₂ values to within 18 ± 15 ppm (1σ) and confirms recent direct observations that long‐term mean values remained below 300 ppm for the entire Pleistocene epoch. Additionally, our results exhibit a secular ∼20 ppm decline in mean CO ₂ from the early to mid‐Pleistocene transition (2.7–1.2 Ma), followed by stabile values across the MPT. Finally, we obtain first‐order estimates of early‐mid Pleistocene (2.4–0.8 Ma) GPP using paired Δ ¹⁷ O‐O ₂ and CO ₂ measurements in pristine samples.

The Allan Hills blue ice area, East Antarctica, offers a unique opportunity to extend the ice core record beyond 800 thousand years (kyr), with ice as old as 6 million years recently recovered. The ice in this area demonstrates several peculiarities—such as strong layer thinning and folding—that warrant an in-depth investigation of its stratigraphy and the fidelity of the climate record it contains. Here, we present a high-resolution Continuous Flow Analysis (CFA) of two shallow ice cores from the Allan Hills (ALHIC 2201 and ALHIC 2302), spanning the upper 69 m and 46 m, respectively. Our CFA analysis includes methane, water stable isotopes, and particle concentrations, allowing us to characterize their variability and assess how well geochemical measurements are recorded and preserved in Allan Hills ice. Dating of the ice (using the ⁴⁰Ar chronometer) revealed ages ranging from ~150–1200 kyr with substantial age discontinuities and folding, highlighting the complex stratigraphy of ice in this region. To assess glacial-interglacial variability, we conduct descriptive statistical analyses of each measurement relative to the EPICA Dome C (EDC) deep ice core record, which served as a reference benchmark over the last 800 kyr. Relative to EDC, we find data representing warm climate states are overrepresented at Allan Hills, and the Allan Hills records typically exhibit a narrower distribution compared to the EDC record. These observations confirm previous suggestions of interglacial bias and glacial cycle averaging in some sections of Allan Hills records. Differences between ALHIC 2201 and ALHIC 2302 further suggest that coring location within the Allan Hills region affects signal preservation in the record. Our high-resolution investigation of this ice is a critical step toward improved interpretation of the discrete, multi-million-year records from the Allan Hills.

Ice core records from Antarctica document continuous variations in atmospheric greenhouse gases over the past 800,000 years, delineating the glacial-interglacial cycles that characterize the late Pleistocene epoch^1-3. Studies of blue ice areas⁴ have extended these records back to 2 million years (Myr)^5,6. The evolution of atmospheric greenhouse gases before this time thus remains uncertain. Here we use discontinuous ice core snapshots spanning 3.1-0.5 Myr ago (Ma) to show no marked change in mean methane (CH₄) and a small decline of about 20 ppm in carbon dioxide (CO₂) between 2.9 Ma and 1.2 Ma, followed by stable concentrations (±10 ppm) across the mid-Pleistocene Transition. Our findings are based on the shallow ice cores drilled in the Allan Hills Blue Ice Area (BIA), Antarctica⁷. The records are complicated by postdepositional processes and probably represent averages over glacial cycles weighted by climate-dependent differences in accumulation rates (which we assume to be constant). Samples aged 2.8-3.1 Myr, affected by respiration and corrected using stable carbon isotopes in CO₂ (δ¹³C), yield mean atmospheric CO₂ levels indistinguishable from the early Pleistocene (250 ± 10 ppm). Although palaeoclimate archives from Antarctic blue ice areas are complex, our records show that measurements of greenhouse gases in ice cores can be extended to the late Pliocene epoch, providing snapshots of Earth's climate system over a time of global cooling^7,8 and falling sea level⁹.

Currently, chronologically discontinuous ice cores from the Allan Hills Blue Ice Area (BIA), Antarctica, are our only direct insight into the atmospheric composition of periods beyond the continuous ice core record (800 ka BP). An accurate and precise greenhouse gas history beyond 800 ka would aid understanding of the mechanisms involved in the climatic transitions across the late Pliocene and early Pleistocene. Here we present carbon dioxide (CO2) and methane (CH4) results from a new core from the Allan Hills BIA (ALHIC1901). The bottom 25 m of ALHIC1901 contain 52 sampled depths with co-registered 40Aratm dates (Shackleton et al. in prep), measurements of δD of ice, δ18Oatm, and concentrations of CO2 and CH4 in trapped air. Of these samples, 25 are older than the continuous ice core record, with ages from 821 ± 80 ka to 2700 ± 270 ka. The bottom meter contains ice from the Pliocene with ages from 2700 ± 270 ka to 4000 ± 400 ka. The carbon isotope ratio of CO2 (δ13C-CO2) was measured on 18 samples to examine the possibility of input of non-atmospheric CO2 from oxidation of organic matter. Our results indicate that CO2 and CH4 levels were similar in the early Pleistocene to those found for the last 800 ka. A small decline of approximately 20 ppm is seen in CO2 across the Pleistocene, and no secular trend is observed in CH4. Pliocene-aged samples appear to contain a mixture of atmospheric CO2 and CO2 derived from respiration of organic matter at the glacier bed. Using an isotope mixing model we estimate that atmospheric CO2 was lower than 350 ppm at ~3.1 Ma,