Dissertation Abstract
The influence of firn air transport processes and radiocarbon production on gas records from polar firn and ice
Buizert, Christo 2011
Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen (Denmark), 187 pp.
Air bubbles found in polar ice cores preserve a record of past atmospheric composition up to 800 ka back in time. The composition of the bubbles is not identical to the ancient atmosphere, as it is influenced by processes prior to trapping, within the ice sheet itself, and during sampling and storage. Understanding of these processes is essential for a correct interpretation of ice core gas records. In this work we focus on transport processes in the firn layer prior to bubble trapping, and in situ cosmogenic radiocarbon production in ice.
First, we present a review of firn air studies. We describe the firn air sampling process, the relevant physical characteristics of firn, the different mechanisms of air transport, and the effects of firn air transport on gas records.
Second, we present a characterization of the firn air transport properties of the NEEM site in Northern Greenland. Firn diffusivity needs to be reconstructed using reference tracers of known atmospheric history; we present a new calibration method that uses ten tracers simultaneously. We find that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. Six state-of-the-art firn air transport models are tuned to the NEEM site; all models fit the data within a 1 sigma Gaussian distribution. We present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. There are major differences in the way the models handle advective transport, and that diffusive fractionation of isotopes in the firn is poorly constrained by the models.
Third, we describe an empirical method to calculate the magnitude of diffusive fractionation (DF) of isotopes in the ice core record. Our method 1) requires little computational effort, 2) uses only commonly available ice core data, 3) does not require knowledge of the (unknown) true atmospheric history, and 4) is arguably more accurate than a full modeling study. We apply our method to CH4, CO2 and N2O mixing ratios found in ice cores, and find that C-13 of CH4 is the only trace gas isotopic signal for which the diffusive correction should always be applied during transitions. We apply the DF correction to published C-13 of CH4 records over the last glacial termination and the 8.2 kyr event.
Fourth, we describe in detail the development of the CIC firn air model. We derive expressions for the air velocity in the open porosity, the bubble trapping rate, and for the pressurization of closed bubbles due to firn compaction, and provide a complete mathematical description of trace gas mass transfer in the open porosity, and its numerical implementation using the Crank-Nicolson method.
Fifth, we combine cosmic ray scaling and production estimates with a 2-D ice flow line model to study cosmogenic C-14 production at Taylor Glacier, Antarctica. We find that 1) C-14 production by thermal neutron capture in air bubbles is negligible, 2) including ice flow patterns caused by basal topography can alter surface C-14 activities by up to 25% and 3) at high ablation margin sites, solar variability modulates the strength of the dominant spallogenic production by 10%. Uncertainties in published production rates are the largest source of error. The results presented here can inform ongoing and future C-14 and ice flow studies at ice margin sites, including important paleoclimatic applications such as the reconstruction of past C-14 of CH4.
First, we present a review of firn air studies. We describe the firn air sampling process, the relevant physical characteristics of firn, the different mechanisms of air transport, and the effects of firn air transport on gas records.
Second, we present a characterization of the firn air transport properties of the NEEM site in Northern Greenland. Firn diffusivity needs to be reconstructed using reference tracers of known atmospheric history; we present a new calibration method that uses ten tracers simultaneously. We find that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. Six state-of-the-art firn air transport models are tuned to the NEEM site; all models fit the data within a 1 sigma Gaussian distribution. We present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. There are major differences in the way the models handle advective transport, and that diffusive fractionation of isotopes in the firn is poorly constrained by the models.
Third, we describe an empirical method to calculate the magnitude of diffusive fractionation (DF) of isotopes in the ice core record. Our method 1) requires little computational effort, 2) uses only commonly available ice core data, 3) does not require knowledge of the (unknown) true atmospheric history, and 4) is arguably more accurate than a full modeling study. We apply our method to CH4, CO2 and N2O mixing ratios found in ice cores, and find that C-13 of CH4 is the only trace gas isotopic signal for which the diffusive correction should always be applied during transitions. We apply the DF correction to published C-13 of CH4 records over the last glacial termination and the 8.2 kyr event.
Fourth, we describe in detail the development of the CIC firn air model. We derive expressions for the air velocity in the open porosity, the bubble trapping rate, and for the pressurization of closed bubbles due to firn compaction, and provide a complete mathematical description of trace gas mass transfer in the open porosity, and its numerical implementation using the Crank-Nicolson method.
Fifth, we combine cosmic ray scaling and production estimates with a 2-D ice flow line model to study cosmogenic C-14 production at Taylor Glacier, Antarctica. We find that 1) C-14 production by thermal neutron capture in air bubbles is negligible, 2) including ice flow patterns caused by basal topography can alter surface C-14 activities by up to 25% and 3) at high ablation margin sites, solar variability modulates the strength of the dominant spallogenic production by 10%. Uncertainties in published production rates are the largest source of error. The results presented here can inform ongoing and future C-14 and ice flow studies at ice margin sites, including important paleoclimatic applications such as the reconstruction of past C-14 of CH4.