Atmospheric composition one million years ago from blue ice in the Allan Hills, Antarctica

a.) Snapshots of atmospheric greenhouse gases and Antarctic climate in Earth’s oldest ice

We present direct measurements of atmospheric composition and Antarctic climate from the mid-Pleistocene (1 Ma) from ice cores drilled in the Allan Hills blue ice area, Antarctica. The 1-Ma ice is dated from the deficit in 40Ar relative to the modern atmosphere and is present as a stratigraphically disturbed 12-m section at the base of a 126-m ice core. The 1-Ma ice appears to represent most of the amplitude of contemporaneous climate cycles and CO2 and CH4 concentrations in the ice range from 221 to 277 ppm and 411 to 569 parts per billion (ppb), respectively. These concentrations, together with measured δD of the ice, are at the warm end of the field for glacial – interglacial cycles of the last 800 ky and span only about one-half of the range. The highest CO2 values in the 1-Ma ice fall within the range of in- terglacial values of the last 400 ka but are up to 7 ppm higher than any interglacial values between 450 and 800 ka. The lowest CO2 values are 30 ppm higher than during any glacial period between 450 and 800 ka. This study shows that the coupling of Antarctic temperature and atmospheric CO2 extended into the mid-Pleistocene and demonstrat es the feasibility of discontinuously extending the current ice core record beyond 800 ka by shallow coring in Antarctic blue ice areas.

J. A. Higgins, Kurbatov, A. V. , Spaulding, N. E. , Brook, E. , Introne, D. S. , Chimiak, L. M. , Yan, Y. , Mayewski, P. A. , and Bender, M. L. , “Atmospheric composition 1 million years ago from blue ice in the Allan Hills, Antarctica”, Proceedings of the National Academy of Sciences, 2015.

Figure 1. Records from the Allan Hills BIA.
Figure 1. Records of (A) CH4, (B) CO2, and (C) δD from the Allan Hills BIA (Site 27; black line and black symbols between 115 and 250 ka) compared with records from Vostok/EPICA Dome C (green, red, and blue lines) (11, 18–20). The range of gas and ice properties in the 1-Ma ice from Site BIT-58 is shown to the right (Tables S1–S4). Boxes around the 1-Ma data indicate an age uncertainty of ±89 ky (SE) for n = 6 measurements of ice below 117 m assuming an external reproducibility (1σ) of ±213 ky (Materials and Methods has additional details). D shows the stacked benthic foraminiferal δ18O record (4), and E shows a record of deep ocean temperature based on foraminiferal Mg/Ca (17). ppb, parts per billion.

b.) Ice core records from shallow ice cores in the Allan Hills blue ice area, Antarctica

Terrestrial meteorite ages indicate that some ice at the Allan Hills blue ice area (AH BIA) may be as old as 2.2 Ma. As such, ice from the AH BIA could potentially be used to extend the ice core record of paleoclimate beyond 800 ka. We collected samples from 5 to 10 cm depth along a 5 km transect through the main icefield and drilled a 225 m ice core (S27) at the midpoint of the transect to develop the climate archive of the AH BIA. Stable water isotope measurements (δD) of the surface chips and of ice core S27 yield comparable signals, indicating that the climate record has not been significantly altered in the surface ice. Measurements of 40Aratm and δ18Oatm taken from ice core S27 and eight additional shallow ice cores constrain the age of the ice to approximately 90–250 ka. Our findings provide a framework around which future investigations of potentially older ice in the AH BIA could be based.

N. E. Spaulding, Higgins, J. A. , Kurbatov, A. V. , Bender, M. L. , Arcone, S. A. , Campbell, S. , Dunbar, N. W. , Chimiak, L. M. , Introne, D. S. , and Mayewski, P. A. , “Climate archives from 90-250 ka in horizontal and vertical ice cores from the Allan Hills Blue Ice Area, Antarctica”, Quaternary Research, vol. 80, pp. 562 - 574, 2013.