IN SITU PRODUCED 10BE IN MARINE SEDIMENT RECORDS 7 MILLION YEARS OF GREENLAND ICE SHEET EROSION IN RESPONSE TO CHANGING CLIMATE

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.