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

C. Buizert (1) , P. Martinerie (2) , V. Petrenko (3) , J. P. Severinghaus (4) , C. M. Trudinger (5) , Emmanuel Witrant (6) , J. L. Rosen (7) , A. J. Orsi (4) , M. Rubino (1, 5) , D. M. Etheridge (1, 5) , L. P. Steele (5) , C. Hogan (8) , J. C. Laube (8) , W. T. Sturges (8) , V. A. Levchenko (9) , A. M. Smith (9) , I. Levin (10) , T. J. Conway (11) , E. J. Dlugokencky (11) , P. M. Lang (11) , K. Kawamura (12) , T. M. Jenk (1) , J. W. C. White (13) , T. Sowers (14) , J. Schwander (15) , T. Blunier (1)

1 Centre for Ice and Climate [Copenhagen]
2 LGGE - Laboratoire de glaciologie et géophysique de l'environnement
3 Department of Earth and Environmental Sciences [Rochester]
4 SIO - UC San Diego - Scripps Institution of Oceanography
5 Centre for Australian Weather and Climate Research/ CSIRO Marine and Atmospheric Research
6 GIPSA-SLR - GIPSA - Systèmes linéaires et robustesse
7 department of geosciences
8 School of Environmental Sciences [Norwich]
9 Institute of materials engineering (Blackburn, Australia)
10 Institut für Umweltphysik [Heidelberg]
11 ESRL - NOAA Earth System Research Laboratory
12 NiPR - National Institute of Polar Research [Tokyo]
13 INSTAAR - Institute of Arctic Alpine Research [University of Colorado Boulder]
14 EESI - Earth and Environmental Systems Institute
15 Physics Institute [Bern]

Air was sampled from the porous firn layer at the NEEM site in Northern Greenland. We use an ensemble of ten reference tracers of known atmospheric history to characterise the transport properties of the site. 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 depth-diffusivity reconstruction. We define an objective root mean square criterion that is minimised in the model tuning procedure. 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. A comparison between two replicate boreholes drilled 64 m apart shows differences in measured mixing ratio profiles that exceed the experimental error. We find evidence that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. The ice age- gas age difference (1 age) at the firn-ice transition is calculated to be 182+3−9 yr. 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.

Domaines: Sciences de la Terre; Océan, Atmosphère

C. Buizert, P. Martinerie, V. Petrenko, J. P. Severinghaus, C. M. Trudinger, et al.. Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland. Atmospheric Chemistry and Physics, 2012, 12, pp.4259-4277. ⟨10.5194/acp-12-4259-2012⟩. ⟨hal-00843779⟩