ANCIENT, SLOWLY-ERODING SOIL PRESERVED BENEATH THE SUMMIT OF THE GREENLAND ICE SHEET

Cores that extend to the bed of ice sheets contain sediment-rich basal ice zones. In order to constrain better the source and character of sediment in the lower meters of the 3053-m long GISP2 ice core (the “silty ice zone”), we measured the concentration of meteoric ¹⁰Be, organic carbon, and nitrogen in sediment separated from 17 core samples. The samples represent a total of 3 m of ice including an 11 cm section directly above the bed.

Meteoric ¹⁰Be is a cosmogenic isotope formed in the atmosphere and delivered to Earth’s surface by precipitation and dry fall. It is an excellent tracer of sediment exposed to pedogenic processes; high concentrations of meteoric ¹⁰Be are indicative of soil and sediment originating within several meters of Earth’s surface. Organic carbon and nitrogen are also markers of soil formation.

Silt in the lowermost 6.5 m of the GISP2 ice core has high concentrations of meteoric ¹⁰Be (0.6 to 3.8 x 10⁸ atoms/g), significant percentages of organic carbon (0.3 to 1.7%), and an average C/N ratio of ~10, all consistent with derivation from a well-developed, cold-region soil. The highest concentrations of meteoric ¹⁰Be and TOC were measured in the bottom-most ice sample. The ¹⁰Be concentration is consistent with a soil inventory >10¹⁰ atoms/cm² implying a soil exposure time >10⁵ years before the soil was covered by ice. As Pleistocene interglacial periods were all short (<10⁴ year) and few, if any, exposed the bed under GISP2, meteoric ¹⁰Be data imply the preservation of soil formed during pre-Quaternary interglacial exposure. Concentrations of meteoric ¹⁰Be and total organic carbon co-vary and decrease with distance from the bed, a trend consistent with other data suggesting that the underlying soil is mixed, by ice deformation, with rock flour derived from bedrock erosion elsewhere.

The GISP2 core location has likely been ice covered, with the possible exception of a short (4-7 ky) exposure at MIS 11 (Nishiizumi et al., 2006), since 2.5 Mya (Larsen et al., 1994), providing a minimum limiting age for the paleosol underlying Summit, Greenland. The continued existence of this ancient soil indicates extremely low rates of sub-ice erosion at GISP2. Survival of the soil for several million years under the Greenland Ice Sheet is consistent with the ice at Summit having been frozen to the bed for most, if not all, of the Quaternary.