Paper No. 6
Presentation Time: 9:30 AM


BIERMAN, Paul, Department of Geology and Rubenstein School of Environment and Natural Resources, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405 and SHAKUN, Jeremy D., Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA 02467,

Marine sediments preserve material shed from the continents over time. Continental sediment usually contains >100,000 atoms g-1 of in-situ produced 10Be, the result of subaerial exposure to cosmic rays, primarily neutrons which penetrate only a meter or two into regolith. Once Earth’s surface is covered by glacial ice, 10Be production ceases and glacial erosion removes the highly-dosed, near-surface material first before excavating material at depth containing progressively less 10Be.

To understand when the Greenland Ice Sheet eroded pre-glacial, Tertiary regolith, we isolated 30 samples of quartz from a core at ODP site 918, off the southeastern margin of Greenland and extracted in-situ produced 10Be. The age model for the core suggests that we analyzed sediment ranging in age from <0.1 to ~7 My, when ice-rafted debris first appeared at this site. To estimate 10Be concentration at deposition, we decay-corrected measured 10Be concentrations using core model ages.

Measured 10Be concentrations are low, 2100 to 40,000 atoms g-1. Decay-corrected concentrations are highest in the oldest sediment (~7 My, 170,000 atoms g-1) and decrease steadily to values between 10,000 and 50,000 atoms g-1 between 3.5 and 2.6 My. At 2.6 My, a single sample has a decay-corrected 10Be concentration of 160,000 atoms g-1. Between 2.6 My and 0.8 My, the decay-corrected concentration of 10Be generally declines first reaching contemporary values for ice-contact and fiord sediment (thousands of atoms g-1) with a step-like decrease at 0.8 My.

The isotopic data clearly show progressively deeper erosion of once-stable Tertiary regolith between 7.3 and 2.6 My by ice caps or small ice sheets. We interpret the high 10Be concentration of sediment at 2.6 My, which coincides with the onset of large-scale Northern Hemisphere glaciation, as reflecting Greenland-wide expansion of ice and the consequent continent-wide erosion and export of highly-dosed near-surface regolith. After 2.6 My, progressive erosion of regolith by the ice, results in decreasing 10Be concentration. By 0.8 My, 10Be concentrations in the core are similar to those issuing from the Greenland Ice Sheet today, suggesting that most near-surface regolith had been removed by then and that the ice sheet was eroding rock, the 10Be in which was produced in the Tertiary by deeply penetrating muons.