Cosmogenic Carbon-14 In Glacial Firn and Ice
Cosmic rays, originating primarily outside of our solar system, are constantly bombarding the Earth. These energetic charged particles induce nuclear reactions in the Earth’s atmosphere and at the surface. One of the products of these reactions is carbon-14 (14C), the radioactive isotope of carbon commonly used for radiometric dating. Carbon-14 is produced in the atmosphere from nitrogen-14, and is responsible for the 14C content of atmospheric carbon dioxide (CO2), carbon monoxide (CO) and methane (CH4). 14C is also produced by cosmic rays directly in snow and glacial ice from oxygen-16. This new 14C rapidly reacts with O and H atoms in the ice to form CO, CO2 as well as smaller amounts of CH4.The amount of 14CO, 14CO2 and 14CH4 found in glacial ice cores is therefore determined by a combination of 14C included with atmospheric gases trapped in the air bubbles in the ice and 14C produced directly in the ice by cosmic rays. Ice core reconstructions of 14C content of past atmospheric CH4 can tell us about changes in natural methane emissions and their response to climate. 14C of CO2 in old air trapped in ice cores could be used for establishing the age of this ancient air. 14C of CO in glacial ice is a promising tracer of the past cosmic ray flux, as well as of the past atmospheric oxidizing capacity. However, to use any of these tracers in a meaningful way, we need to be able to distinguish between the 14C that was included in the ice with trapped air and 14C that was produced directly in the ice by cosmic rays.
Our group’s projects at Summit, Greenland and Taylor Glacier, Antarctica aim to understand the rates at which 14C is produced in ice at different depths, as well as how much of this 14C stays in the ice and how much escapes to the atmosphere from the thick snowpack that forms the upper layer of ice sheets. We are also hoping to initiate a new project at Dome C, Antarctica, which would use 14CO in ice cores at this very cold, low-snow-accumulation site to examine past variations in the flux of galactic cosmic rays for the last few thousand years.
This project has been supported by the Packard Foundation and by NSF Polar awards ARC-1203779 and PLR-1245659
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