The Allan Hills site, nestled at the periphery of the East Antarctic ice sheet, offers a rare geological archive where ice originating from the Antarctic interior is trapped within mountainous terrain. Unlike traditional ice core extraction sites characterized by undisturbed horizontal stratigraphy, the deformation and complex flow patterns at Allan Hills cause stratigraphic layers to become uneven and disrupted. As a result, the ice at Allan Hills provides discrete snapshots of Earth’s climatic conditions rather than continuous chronological records, presenting a unique yet challenging opportunity to study ancient climate dynamics.

Employing sophisticated isotopic analysis focused on the ratios of noble gases contained within air bubbles trapped in this ancient ice, researchers have inferred significant ocean temperature shifts spanning millions of years. These gases serve as proxies that reflect temperature-dependent solubility changes in ocean waters, offering a window into deep ocean thermal history. Findings reveal a pronounced ocean cooling trend of approximately 2 to 2.5 degrees Celsius over the past three million years. Notably, this cooling occurred predominantly in deep ocean waters rather than at the surface, diverging from previously held positions based solely on surface temperature proxies.

This divergence in ocean temperature cooling patterns between surface and deep waters invites new hypotheses regarding heat transfer mechanisms within the ocean. The evidence suggests that early in this timeframe, during the onset of extensive northern hemisphere glaciation, the deep ocean cooled rapidly over about a million years before more gradual surface cooling ensued. This nuanced temporal offset points to complex ocean circulation and thermohaline processes influencing Earth’s long-term climate regulation, underscoring the intricate interplay between atmospheric, oceanic, and cryospheric systems.

Complementing these oceanographic insights, a concurrent study analyzed the same Allan Hills ice archives for direct measurements of two critical greenhouse gases: carbon dioxide (CO2) and methane (CH4). Results reveal an unexpected stability in long-term atmospheric concentrations, with CO2 levels generally remaining below 300 parts per million (ppm) over this extensive interval. Specifically, CO2 concentrations measured around 2.7 million years ago stood at approximately 250 ppm, with a subtle decline of nearly 20 ppm by one million years ago, while methane concentrations hovered around 500 parts per billion (ppb) without significant fluctuation.

These results challenge previous reconstructions based on sediment chemistry analyses that have posited higher historical CO2 levels, emphasizing the superior reliability of direct ice core measurements in reconstructing ancient atmospheric compositions. The direct greenhouse gas records from Allan Hills provide a refined framework for understanding the pre-industrial baseline of Earth’s atmospheric greenhouse gas concentrations, highlighting relative stability before the profound anthropogenic increases observed during the industrial era.

In vivid contrast, modern measurements underscore the magnitude of human impact, with atmospheric CO2 now averaging 425 ppm and methane ballooning to 1,935 ppb as of 2025 according to NOAA data. This stark difference accentuates the rapid pace of contemporary climate change and the unprecedented levels of greenhouse gases compared to millions of years of natural variability, emphasizing the urgency for robust climate mitigation strategies.

These insights collectively suggest that the gradual cooling trend observed over the past three million years cannot be attributed solely to changes in greenhouse gas concentrations. Instead, the research implicates additional factors such as alterations in Earth’s albedo—reflectivity changes driven by expanding ice sheets and shifts in vegetation cover—as well as evolving ocean circulation patterns. This holistic perspective reinforces the complexity of Earth’s climate system, where multiple interdependent components modulate long-term climate trajectories.

Research leaders Julia Marks-Peterson and Sarah Shackleton express optimism that this expanding chronological window into past greenhouse gas concentrations and ocean temperatures will refine climate models. By integrating these new empirical constraints, models can better simulate past climate states and improve predictions of future climate dynamics, especially in the context of anthropogenic perturbations.

The discovery of ice as ancient as six million years at the bottom of recently drilled cores underlines the vast potential for further extending the paleoclimate record. Ongoing and future campaigns facilitated by COLDEX aim to retrieve these older ice samples, employing innovative field techniques and core preservation methods designed to minimize contamination and stratigraphic disturbance, ensuring data integrity.

Future research directions extend beyond temperature and greenhouse gases to include investigations into other trace gases trapped within the ice, which may reveal insights into biosphere activity, volcanic emissions, and chemical weathering processes across geologic timescales. Moreover, efforts to elucidate the physicochemical conditions fostering the exceptional preservation of ancient ice are underway, vital for identifying optimal drilling locales across Antarctica’s complex glacial landscape.

COLDEX’s multidisciplinary approach harnesses advanced isotopic geochemistry, coupled with cutting-edge drilling technology and climate modeling, positioning it at the forefront of paleoclimate science. These achievements are supported by collaborative funding from the NSF’s Office of Polar Programs, the Science and Technology Center Program, and Oregon State University, alongside logistical support from the U.S. Antarctic Program, the Ice Drilling Program Office, and the Ice Core Facility in Denver.

In sum, the revelations stemming from the Allan Hills ice core studies represent a pivotal leap in our comprehension of Earth’s climate evolution. They underscore the value of integrating multiple temperature proxies with direct greenhouse gas measurements to unravel the complexities of natural climate variability and offer a refined baseline for assessing ongoing and future anthropogenic influences on the global climate system.

Subject of Research: Not applicable

Article Title: Broadly stable atmospheric CO2 and CH4 levels over the past 3 million years

News Publication Date: 19-Mar-2026

Web References:
https://doi.org/10.1038/s41586-025-10032-y

Image Credits: Julia Marks-Peterson

Keywords: Paleoclimate, Antarctic ice cores, noble gases, ocean temperature, greenhouse gases, carbon dioxide, methane, Allan Hills, Earth’s climate history, ice core analysis, deep ocean cooling, climate evolution

The project, spearheaded by Sarah Shackleton of Woods Hole Oceanographic Institution and John Higgins of Princeton University under the auspices of the National Science Foundation-funded Center for Oldest Ice Exploration (COLDEX), signifies a monumental achievement in climate science. Unlike prior ice core studies relying on deep drilling of more than 2,000 meters into the Antarctic interior, COLDEX scientists tapped into unique local topographical and climatological features at Allan Hills. The combination of rugged mountainous terrain, katabatic winds, and extraordinarily cold surface temperatures has preserved ancient ice layers near the surface, enabling the extraction of ice cores without the logistical burdens of ultra-deep drilling.

These ice cores capture minute quantities of air trapped within tiny bubbles formed when snow compresses into ice. The direct dating of ice using argon isotope ratios—signifying an innovative methodological breakthrough—allows precise age determination of the ice itself, circumventing uncertainties inherent in indirect dating techniques. This direct dating revealed ice samples aged approximately six million years, a period characterized by climatic conditions vastly different from the present, making this trove invaluable for reconstructing ancient atmospheric composition and temperature fluctuations.

Analyses of oxygen isotopes within the ice show a persistent cooling trend of approximately 12 degrees Celsius (22 degrees Fahrenheit) over the six million years preceding the present. This is the first direct quantitative assessment of long-term Antarctic cooling through this deep temporal lens, offering new context to the geological and paleoenvironmental records of global climate shifts. These data also provide essential insight into the progression of glaciation and greenhouse gas fluctuations through the Miocene and Pliocene, epochs critical for understanding the mechanisms driving Earth’s climate system.

COLDEX’s approach is distinguished by its ability to retrieve relatively shallow cores that represent non-continuous but extraordinarily ancient climate snapshots. By compiling these discrete temporal data points, the researchers have constructed an invaluable climatic “library” that enriches our understanding of polar climate dynamics over millions of years. This complements younger, more continuous ice core records obtained from conventional deep Antarctic drilling, enhancing the resolution of long-term climate reconstructions and offering clarity on how polar ice and atmospheric composition evolved during major climate transitions.

The logistical challenges of working in Allan Hills are formidable. The remote location demands extended field campaigns with teams enduring harsh Antarctic conditions. The combination of strong, persistent winds and bitter cold not only preserves the ice but also complicates collection efforts. Yet, the research team’s perseverance is opening new frontiers in ice core recovery and analysis, pushing the science of climate history into unprecedented territory.

The trapped atmospheric gases in these ancient ice cores hold keys to reconstructing past greenhouse gas concentrations, especially carbon dioxide and methane, thereby illuminating natural variability before significant anthropogenic influence. These reconstructions are crucial for benchmarking climate models and improving projections of future climate scenarios amid ongoing global warming. Moreover, the data offer insights into ocean heat content variation and its interaction with atmospheric changes, integral components of the Earth system.

Looking ahead, COLDEX is poised to expand its investigations in Allan Hills, with plans for additional drilling campaigns that could further extend the age range of recovered ice and deepen time-series coverage. These future efforts aim to refine our understanding of polar climate evolution and to resolve outstanding questions about ice sheet stability and response to climatic drivers over multimillion-year timescales.

This pioneering research exemplifies the interdisciplinary collaboration between geoscientists, glaciologists, chemists, and climatologists. It relies on cutting-edge methodologies in isotope geochemistry and ice core analysis, enabled by robust support from national science foundations and polar research programs. The success at Allan Hills underscores the importance of exploring diverse glacial environments to unlock Earth’s climatic past.

In summary, the discovery of six-million-year-old ice at Allan Hills fundamentally redefines the temporal limits of the Antarctic ice core record. It enables a direct and detailed investigation of Earth’s climatic conditions long before the Quaternary ice ages, offering an invaluable archive to decipher the natural variability and drivers of climate change. This work not only transforms our understanding of polar climate history but also serves as a critical benchmark for modeling future global climate trajectories.

Subject of Research:
Not applicable

Article Title:
Miocene and Pliocene ice and air from the Allan Hills blue ice area, East Antarctica

News Publication Date:
28-Oct-2025

Web References:
DOI: 10.1073/pnas.2502681122

References:
Shackleton, S., Higgins, J., et al. (2025). Miocene and Pliocene ice and air from the Allan Hills blue ice area, East Antarctica. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.2502681122

Image Credits:
COLDEX

Keywords:
Antarctic ice core, paleo climate, Miocene, Pliocene, ice dating, argon isotope, greenhouse gases, paleoclimatology, Allan Hills, COLDEX, ice drilling, climate history