Impact of physical properties and accumulation rate on pore  close-off in layered firn

Abstract. Investigations into the physical characteristics of deep firn near the lock-in zone through pore close-off are needed to improve understanding of ice core records of past atmospheric composition. Specifically, the permeability and microstructure profiles of the firn through the diffusive column influence the entrapment of air into bubbles and thus the ice age–gas age difference. The purpose of this study is to examine the nature of pore closure processes at two polar sites with very different local temperatures and accumulation rates. Density, permeability, and microstructure measurements were made on firn cores from the West Antarctic Ice Sheet (WAIS) Divide, a site that has moderate accumulation rates with a seasonal climate archive, and Megadunes in East Antarctica, a site that is a natural laboratory for accumulation rate effects in the cold low-accumulation desert. We found that the open pore structure plays a more important role than density in predicting gas transport properties, throughout the porous firn matrix. For firn below 50 m depth at both WAIS Divide and Megadunes, finer-grained layers experience close-off shallower in the firn column than do coarser-grained layers, regardless of which grain size layer is the denser layer at depth. Pore close-off occurs at a critical open porosity that is accumulation rate dependent. Defining pore close-off at a critical open porosity for a given accumulation rate as opposed to a critical total porosity accounts for the pore space available for gas transport. Below the critical open porosity, the firn becomes impermeable despite having small amounts of interconnected pore space. The low-accumulation sites, with generally coarse grains, close off at lower open porosities (~<10%) than the open porosity (~>10%) of high-accumulation sites that have generally finer grains. The microstructure and permeability even near the bottom of the firn column are relic indicators of the nature of accumulation when that firn was at the surface. The physical structure and layering are the primary controlling factors on pore close-off. In contrast to current assumptions for polar firn, the depth and length of the lock-in zone is primarily dependent upon accumulation rate and microstructural variability due to differences in grain size and pore structure, rather than the density variability of the layers.

Download Full-text

A first chronology for the NEEM ice core

Climate of the Past Discussions ◽

10.5194/cpd-9-2967-2013 ◽

2013 ◽

Vol 9 (3) ◽

pp. 2967-3013 ◽

Cited By ~ 9

Author(s):

S. O. Rasmussen ◽

P. Abbott ◽

T. Blunier ◽

A. Bourne ◽

E. Brook ◽

...

Keyword(s):

Accumulation Rate ◽

Ice Core ◽

Conductivity Measurement ◽

Ice Cores ◽

Monte Carlo Analysis ◽

Heat Diffusion ◽

Ice Age ◽

Age Difference ◽

Electrical Conductivity Measurement ◽

Volcanic Origin

Abstract. A stratigraphy-based chronology for the North Greenland Eemian Ice Drilling (NEEM) ice core has been derived by transferring the annual layer counted Greenland Ice Core Chronology 2005 (GICC05) from the NGRIP core to the NEEM core using 787 match points of mainly volcanic origin identified in the Electrical Conductivity Measurement (ECM) and Dielectrical Profiling (DEP) records. Tephra horizons found in both the NEEM and NGRIP ice cores are used to test the matching based on ECM and DEP and provide additional horizons used for the time scale transfer. A thinning function reflecting the accumulated strain along the core has been determined using a Dansgaard–Johnsen flow model and an isotope-dependent accumulation rate parameterization. Flow parameters are determined from Monte Carlo analysis constrained by the observed depth-age horizons. In order to construct a chronology for the gas phase, the ice age–gas age difference (Δage) has been reconstructed using a coupled firn densification–heat diffusion model. Temperature and accumulation inputs to the Δage model, initially derived from the water isotope proxies, have been adjusted to optimize the fit to timing constraints from δ15N of nitrogen and high-resolution methane data during the abrupt onsets of interstadials. The ice and gas chronologies and the corresponding thinning function represent the first chronology for the NEEM core, and based on both the flow and firn modelling results, the accumulation history for the NEEM site has been reconstructed, providing the necessary basis for further analysis of the records from NEEM.

Download Full-text

A first chronology for the North Greenland Eemian Ice Drilling (NEEM) ice core

Climate of the Past ◽

10.5194/cp-9-2713-2013 ◽

2013 ◽

Vol 9 (6) ◽

pp. 2713-2730 ◽

Cited By ~ 85

Author(s):

S. O. Rasmussen ◽

P. M. Abbott ◽

T. Blunier ◽

A. J. Bourne ◽

E. Brook ◽

...

Keyword(s):

Accumulation Rate ◽

Ice Core ◽

Conductivity Measurement ◽

Ice Cores ◽

Heat Diffusion ◽

Ice Age ◽

Age Difference ◽

Volcanic Origin ◽

The North ◽

North Greenland

Abstract. A stratigraphy-based chronology for the North Greenland Eemian Ice Drilling (NEEM) ice core has been derived by transferring the annual layer counted Greenland Ice Core Chronology 2005 (GICC05) and its model extension (GICC05modelext) from the NGRIP core to the NEEM core using 787 match points of mainly volcanic origin identified in the electrical conductivity measurement (ECM) and dielectrical profiling (DEP) records. Tephra horizons found in both the NEEM and NGRIP ice cores are used to test the matching based on ECM and DEP and provide five additional horizons used for the timescale transfer. A thinning function reflecting the accumulated strain along the core has been determined using a Dansgaard–Johnsen flow model and an isotope-dependent accumulation rate parameterization. Flow parameters are determined from Monte Carlo analysis constrained by the observed depth-age horizons. In order to construct a chronology for the gas phase, the ice age–gas age difference (Δage) has been reconstructed using a coupled firn densification-heat diffusion model. Temperature and accumulation inputs to the Δage model, initially derived from the water isotope proxies, have been adjusted to optimize the fit to timing constraints from δ15N of nitrogen and high-resolution methane data during the abrupt onset of Greenland interstadials. The ice and gas chronologies and the corresponding thinning function represent the first chronology for the NEEM core, named GICC05modelext-NEEM-1. Based on both the flow and firn modelling results, the accumulation history for the NEEM site has been reconstructed. Together, the timescale and accumulation reconstruction provide the necessary basis for further analysis of the records from NEEM.

Download Full-text

Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities

Climate of the Past ◽

10.5194/cp-13-833-2017 ◽

2017 ◽

Vol 13 (7) ◽

pp. 833-853 ◽

Cited By ~ 13

Author(s):

Camille Bréant ◽

Patricia Martinerie ◽

Anaïs Orsi ◽

Laurent Arnaud ◽

Amaëlle Landais

Keyword(s):

Accumulation Rate ◽

Ice Cores ◽

East Antarctica ◽

Ice Age ◽

Age Difference ◽

Last Deglaciation ◽

Densification Rate ◽

Impurity Effects ◽

High Accumulation ◽

The Last Deglaciation

Abstract. The transformation of snow into ice is a complex phenomenon that is difficult to model. Depending on surface temperature and accumulation rate, it may take several decades to millennia for air to be entrapped in ice. The air is thus always younger than the surrounding ice. The resulting gas–ice age difference is essential to documenting the phasing between CO2 and temperature changes, especially during deglaciations. The air trapping depth can be inferred in the past using a firn densification model, or using δ15N of air measured in ice cores. All firn densification models applied to deglaciations show a large disagreement with δ15N measurements at several sites in East Antarctica, predicting larger firn thickness during the Last Glacial Maximum, whereas δ15N suggests a reduced firn thickness compared to the Holocene. Here we present modifications of the LGGE firn densification model, which significantly reduce the model–data mismatch for the gas trapping depth evolution over the last deglaciation at the coldest sites in East Antarctica (Vostok, Dome C), while preserving the good agreement between measured and modelled modern firn density profiles. In particular, we introduce a dependency of the creep factor on temperature and impurities in the firn densification rate calculation. The temperature influence intends to reflect the dominance of different mechanisms for firn compaction at different temperatures. We show that both the new temperature parameterization and the influence of impurities contribute to the increased agreement between modelled and measured δ15N evolution during the last deglaciation at sites with low temperature and low accumulation rate, such as Dome C or Vostok. We find that a very low sensitivity of the densification rate to temperature has to be used in the coldest conditions. The inclusion of impurity effects improves the agreement between modelled and measured δ15N at cold East Antarctic sites during the last deglaciation, but deteriorates the agreement between modelled and measured δ15N evolution at Greenland and Antarctic sites with high accumulation unless threshold effects are taken into account. We thus do not provide a definite solution to the firnification at very cold Antarctic sites but propose potential pathways for future studies.

Download Full-text

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

Atmospheric Chemistry and Physics ◽

10.5194/acp-12-4259-2012 ◽

2012 ◽

Vol 12 (9) ◽

pp. 4259-4277 ◽

Cited By ~ 110

Author(s):

C. Buizert ◽

P. Martinerie ◽

V. V. Petrenko ◽

J. P. Severinghaus ◽

C. M. Trudinger ◽

...

Keyword(s):

Ice Core ◽

Air Transport ◽

Ice Age ◽

Age Difference ◽

Advective Transport ◽

Transport Models ◽

Free Air ◽

Model Tuning ◽

Intercomparison Study ◽

Lock In

Abstract. 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 (Δage) 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.

Download Full-text

Optimisation of glaciological parameters for ice core chronology by implementing counted layers between identified depth levels

Climate of the Past Discussions ◽

10.5194/cpd-10-3585-2014 ◽

2014 ◽

Vol 10 (4) ◽

pp. 3585-3616 ◽

Cited By ~ 4

Author(s):

L. Bazin ◽

B. Lemieux-Dudon ◽

A. Landais ◽

M. Guillevic ◽

P. Kindler ◽

...

Keyword(s):

Bayesian Approach ◽

Accumulation Rate ◽

Ice Core ◽

Ice Cores ◽

Age Difference ◽

Sensitivity Tests ◽

New Type ◽

Lock In ◽

Ice Core Dating

Abstract. A~recent coherent chronology has been built for 4 Antarctic ice cores and the NorthGRIP (NGRIP) Greenland ice core (Antarctic Ice Core Chronology 2012, AICC2012) using a bayesian approach for ice core dating (Datice). When building the AICC2012 chronology, and in order to prevent any confusion with official ice cores chronology, it has been imposed that the AICC2012 chronology for NGRIP should respect exactly the GICC05 chronology based on layer counting. However, such a strong tuning did not satisfy the hypothesis of independence of background parameters and observations for the NGRIP core as required by Datice. We present here the implementation in Datice of a new type of markers that is better suited to constraints deduced from layer counting: the markers of age-difference. Using this type of markers for NGRIP in a 5 cores dating exercise with Datice, we have performed several sensitivity tests and show that the new ice core chronologies obtained with these new markers do not differ by more than 400 years from AICC2012 for Antarctic ice cores and by more than 130 years from GICC05 for NGRIP over the last 60 000 years. With this new parameterization, the accumulation rate and lock-in depth associated with NGRIP are more coherent with independent estimates than those obtained in AICC2012. While these new chronologies should not be used yet as new ice core chronologies, the improved methodology presented here should be considered in the next coherent ice core dating exercise.

Download Full-text

The WAIS-Divide deep ice core WD2014 chronology – Part 2: Methane synchronization (68–31 ka BP) and the gas age-ice age difference

Climate of the Past Discussions ◽

10.5194/cpd-10-3537-2014 ◽

2014 ◽

Vol 10 (4) ◽

pp. 3537-3584 ◽

Cited By ~ 3

Author(s):

C. Buizert ◽

K. M. Cuffey ◽

J. P. Severinghaus ◽

D. Baggenstos ◽

T. J. Fudge ◽

...

Keyword(s):

Ice Core ◽

Impurity Content ◽

Ice Cores ◽

Ice Age ◽

Age Difference ◽

Atmospheric Methane ◽

High Accumulation ◽

West Antarctic Ice Sheet ◽

Annual Layer ◽

West Antarctic

Abstract. The West Antarctic Ice Sheet (WAIS)-Divide ice core (WAIS-D) is a newly drilled, high-accumulation deep ice core that provides Antarctic climate records of the past ∼68 ka at unprecedented temporal resolution. The upper 2850 m (back to 31.2 ka BP) have been dated using annual-layer counting. Here we present a chronology for the deep part of the core (67.8–31.2 ka BP), which is based on stratigraphic matching to annual-layer-counted Greenland ice cores using globally well-mixed atmospheric methane. We calculate the WAIS-D gas age-ice age difference (Δage) using a combination of firn densification modeling, ice flow modeling, and a dataset of δ15N-N2, a proxy for past firn column thickness. The largest Δage at WAIS-D occurs during the last glacial maximum, and is 525 ± 100 years. Internally consistent solutions can only be found when assuming little-to-no influence of impurity content on densification rates, contrary to a recently proposed hypothesis. We synchronize the WAIS-D chronology to a linearly scaled version of the layer-counted Greenland Ice Core Chronology (GICC05), which brings the age of Dansgaard-Oeschger (DO) events into agreement with the U/Th absolutely dated Hulu speleothem record. The small Δage at WAIS-D provides valuable opportunities to investigate the timing of atmospheric greenhouse gas variations relative to Antarctic climate, as well as the interhemispheric phasing of the bipolar "seesaw".

Download Full-text

The SP19 chronology for the South Pole Ice Core - Part 2: gas chronology, Δage, and smoothing of atmospheric records

10.5194/cp-2020-71 ◽

2020 ◽

Cited By ~ 1

Author(s):

Jenna A. Epifanio ◽

Edward J. Brook ◽

Christo Buizert ◽

Jon S. Edwards ◽

Todd A. Sowers ◽

...

Keyword(s):

Ice Core ◽

Ice Age ◽

Age Difference ◽

South Pole ◽

The South ◽

West Antarctic Ice Sheet ◽

West Antarctic ◽

Abrupt Changes ◽

Analogous Data ◽

Lock In

Abstract. A new ice core drilled at the South Pole provides a 54 000-year paleoenvironmental record including the composition of the past atmosphere. This paper describes the SP19 chronology for the South Pole atmospheric gas record and complements a previous paper (Winski et al., 2019) describing the SP19 ice chronology. The gas chronology is based on a discrete methane (CH4) record with 20- to 190-year resolution. To construct the gas time scale abrupt changes in atmospheric CH4 during the glacial period and centennial CH4 variability during the Holocene were used to synchronize the South Pole gas record with analogous data from the West Antarctic Ice Sheet Divide ice core. Stratigraphic matching based on visual optimization was verified using an automated matching algorithm. The South Pole ice core recovers all expected changes in CH4 based on previous records. Smoothing of the atmospheric record due to gas transport in the firn is evident but relatively minor, despite the deep lock-in depth in the modern South Pole firn column. The new gas chronology, in combination with the existing ice age scale from Winski et al. (2019), allows a model-independent reconstruction of the gas age-ice age difference through the whole record, which will be useful for testing firn densification models.

Download Full-text

The WAIS Divide deep ice core WD2014 chronology – Part 1: Methane synchronization (68–31 ka BP) and the gas age–ice age difference

Climate of the Past ◽

10.5194/cp-11-153-2015 ◽

2015 ◽

Vol 11 (2) ◽

pp. 153-173 ◽

Cited By ~ 101

Author(s):

C. Buizert ◽

K. M. Cuffey ◽

J. P. Severinghaus ◽

D. Baggenstos ◽

T. J. Fudge ◽

...

Keyword(s):

Ice Core ◽

Impurity Content ◽

Ice Cores ◽

Ice Age ◽

Age Difference ◽

Atmospheric Methane ◽

High Accumulation ◽

Data Set ◽

Annual Layer ◽

West Antarctic

Abstract. The West Antarctic Ice Sheet Divide (WAIS Divide, WD) ice core is a newly drilled, high-accumulation deep ice core that provides Antarctic climate records of the past ∼68 ka at unprecedented temporal resolution. The upper 2850 m (back to 31.2 ka BP) have been dated using annual-layer counting. Here we present a chronology for the deep part of the core (67.8–31.2 ka BP), which is based on stratigraphic matching to annual-layer-counted Greenland ice cores using globally well-mixed atmospheric methane. We calculate the WD gas age–ice age difference (Δage) using a combination of firn densification modeling, ice-flow modeling, and a data set of δ15N-N2, a proxy for past firn column thickness. The largest Δage at WD occurs during the Last Glacial Maximum, and is 525 ± 120 years. Internally consistent solutions can be found only when assuming little to no influence of impurity content on densification rates, contrary to a recently proposed hypothesis. We synchronize the WD chronology to a linearly scaled version of the layer-counted Greenland Ice Core Chronology (GICC05), which brings the age of Dansgaard–Oeschger (DO) events into agreement with the U/Th absolutely dated Hulu Cave speleothem record. The small Δage at WD provides valuable opportunities to investigate the timing of atmospheric greenhouse gas variations relative to Antarctic climate, as well as the interhemispheric phasing of the "bipolar seesaw".

Download Full-text

Modelling the firn thickness evolution during the last deglaciation: constrains on sensitivity to temperature and impurities

10.5194/cp-2016-91 ◽

2016 ◽

Author(s):

Camille Bréant ◽

Patricia Martinerie ◽

Anaïs Orsi ◽

Laurent Arnaud ◽

Amaëlle Landais

Keyword(s):

Accumulation Rate ◽

Ice Cores ◽

Ice Age ◽

Age Difference ◽

Last Deglaciation ◽

High Accumulation ◽

Temperature Changes ◽

Gas Trapping ◽

Different Temperatures ◽

The Last Deglaciation

Abstract. The transformation of snow into ice is a complex phenomenon difficult to model. Depending on surface temperature and accumulation rate, it may take several decades to millennia for air to be entrapped in ice. The air is thus always younger that the surrounding ice. The resulting gas-ice age difference is essential to document the phasing between CO2 and temperature changes especially during deglaciations. The air trapping depth can be inferred in the past using a firn densification model, or using δ15N of air measured in ice cores. All firn densification models applied to deglaciations show a large disagreement with δ15N measurements in several sites of East Antarctica, predicting larger firn thickness during the Last Glacial Maximum, whereas δ15N suggests a reduced firn thickness compared to the Holocene. We present here modifications of the LGGE firn densification model, which significantly reduce the model-data mismatch for the gas trapping depth evolution over the last deglaciation, while preserving the good agreement between measured and modelled modern firn density profiles. In particular, we introduce a dependency of the activation energy to temperature and impurities in the firn densification rate calculation. The temperature influence reflects the existence of different mechanisms for firn compaction at different temperatures. We show that both the new temperature parameterization and the influence of impurities contribute to the increased agreement between modelled and measured δ15N evolution during the last deglaciation at sites with low temperature and low accumulation rate, such as Dome C or Vostok. However, the inclusion of impurities effects deteriorates the agreement between modelled and measured δ15N evolution in Greenland and Antarctic sites with high accumulation.

Download Full-text