Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland

Abstract

Compacted snow (firn) preserves a continuous record of atmospheric composition up to a century back in time. Firn air transport modeling is essential for interpretation of firn gas records. Each site needs to be characterised individually through a tuning procedure, in which the effective diffusivity at each depth is adjusted to optimise the agreement between modeled and measured mixing ratios of a selected reference gas (usually CO<sub>2</sub>). We present the characterisation of the NEEM site, Northern Greenland (77.45° N 51.06° W), where an ensemble of ten reference tracers is used to constrain the diffusivity reconstruction. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the model tuning, and define a root mean square criterion that is minimised in the tuning. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1σ Gaussian distribution. The modern day Δage, i.e. the difference between gas age and ice age, is calculated to be 182 ± 8 yr. We find evidence that diffusivity does not vanish completely in the firn lock-in zone, as is commonly assumed. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records

Controls on the movement and composition of firn air at the West Antarctic Ice Sheet Divide

“…In addition, we collected two sets of surface samples (through the firn sampling equipment) and another set from each hole at a common depth (nominally 70 m). As in nearly all (Buizert et al, 2011) previous campaigns, compositions of the samples from the two holes agreed within uncertainties. …”

“…Uncertainties associated with the tuning of the diffusivitydepth profile can be reduced by more sophisticated multispecies techniques (Buizert et al, 2011), but real variability in the composition of firn air extracted from the lock-in zone is a more difficult problem.…”

“…2). A quantitative analysis of thermal signals in the WAIS-D firn air will be presented elsewhere (Orsi et al, 2011).…”

Continuous measurements of methane mixing ratios from ice cores

“…We use the Center for Ice and Climate firn air model (Buizert et al, 2011) to model gas transport in the porous firn. The model was adapted to allow calculation of mixing ratios in the closed pores, using the porosity parameterization of Goujon et al (2003).…”

Multiple greenhouse-gas feedbacks from the land biosphere under future climate change scenarios

“…Prescribed cCO 2 , global temperatures and Nr inputs stabilize in RCP2.6, whereas in RCP8.5, they increase throughout the twenty-first century. 34 . The dashed grey line is a spline of observational Southern Hemisphere data.…”