Characterization of complex basal layering in an ancient Allan Hills ice core

Abstract. Ice cores from the Allan Hills blue ice area in East Antarctica contain the oldest known direct samples of the ancient atmosphere, with ages extending through the Pleistocene (0.01–2.58 Ma), Pliocene (2.58–5.33 Ma), and into the late Miocene (>5.33 Ma). These cores are challenging to interpret because their timescales are compressed and discontinuous; understanding sub-meter-scale layering and stratigraphic continuity is therefore critical for robust climate interpretation. To provide insight into the composition and origins of sub-meter-scale layering in Allan Hills ice, we apply a coordinated sampling approach to two sections of the ALHIC1901 ice core, located 3 m and 5 m above the bed and dated to 1.3–1.8 Ma. This study is the first high-resolution characterization of ice of this age in the Allan Hills, and so provides critical insight into the composition and variability within this ice. First, we use three-dimensional electrical conductivity measurements (3D ECM) to identify planar and steeply dipping layers across the large-diameter (241 mm) cores. We then construct a common dip-adjusted depth scale and apply it to measurements of trace elements, ions, particle size and concentration, greenhouse gases, major gas (N₂, O₂) isotope and elemental ratios, water-isotope ratios, and ⁴⁰Ar_atm ages. Replicate measurements from different positions within the core cross section agree closely once aligned on the dip-adjusted depth scale, indicating coherent planar layering. Furthermore, ⁴⁰Ar_atm dating demonstrates that the layers are consistent with isochronal layering at dating precision. Age differences can exceed 100 kyr across decimeter-scale layering. While some climate signal is preserved across layers in water isotope ratios, we conclude that the primary source of variability across most measurements in this near-bed Allan Hills ice is incorporated basal material. Multiple layers enriched in basal material exhibit distinct properties, suggesting that basal material is incorporated into multiple discrete ice packets rather than a single homogeneous unit. This study demonstrates a novel approach — using high-resolution, multi-proxy measurements to characterize steeply dipping layering — with broad applicability to future work at the Allan Hills and other ice core sites.