Firn Research Articles

Analyses of firn gas samples from Dronning Maud Land, Antarctica: Study of nonmethane hydrocarbons and methyl chloride

Firn air was sampled on the Antarctic plateau in Dronning Maud Land (DML), during the Norwegian Antarctic Research Expedition (NARE) 2000/2001. In this paper, we describe the analyses for methyl chloride and nonmethane hydrocarbons (NMHCs) in these firn air samples. For the first time, the NMHCs ethane, propane, and acetylene have been measured in Antarctic firn air, and concentration profiles for these gases have been derived. A one‐dimensional numerical firn air diffusion model was used to interpret the measured profiles and to derive atmospheric concentrations as a function of time. The atmospheric trends we derived for the NMHC and methyl chloride at DML cover the period from 1975 to 2000. Methyl chloride shows a decreasing trend of 1.2 ± 0.6 ppt per year (annual mean concentration 548 ± 32 ppt). For ethane we found an increasing trend of 1.6 ± 0.6 ppt per year (annual mean concentration 241 ± 12 ppt). The concentrations of propane and acetylene appear to be constant over the period 1975–2000, with annual mean concentrations of 30 ± 4 ppt for propane and 24 ± 2 ppt for acetylene.

Atmospheric variability of methyl chloride during the last 300 years from an Antarctic ice core and firn air

Measurements of methyl chloride (CH3Cl) in Antarctic polar ice and firn air are used to describe the variability of atmospheric CH3Cl during the past 300 years. Firn air results from South Pole and Siple Dome suggest that the atmospheric abundance of CH3Cl increased by about 10% in the 50 years prior to 1990. Ice core measurements from Siple Dome provide evidence for a cyclic natural variability on the order of 10%, with a period of about 110 years in phase with the 20th century rise inferred from firn air. Thus, the CH3Cl increase measured in firn air may largely be a result of natural processes, which may continue to affect the atmospheric CH3Cl burden during the 21st century.

Two methods for firn-area and mass-balance monitoring of Svalbard glaciers with SAR satellite images

AbstractThis paper presents two methods for glacier monitoring on Svalbard using synthetic aperture radar (SAR) satellite images. Both methods were developed on glaciers in the Kongsfjorden area. The first method monitors the firn area extent and the firn line over time by thresholding and filtering the SAR image. Manual detection of the threshold is preferable, but using a constant threshold for all images also gives adequate results. A retreat of the firn-line position is visible, especially on Kongsvegen, corresponding to consecutive years of negative mass balance. The second method applies a k-means classification to three clusters on the glacier surface. The areal extent of the resulting class on the upper part of the glacier correlates remarkably well with the independently measured mass balance of Kongsvegen, having a correlation coefficient of around 0.89 for the various glaciers. This is because the snow from the accumulation area influences the k-means classification. Thus, on glaciers where mass-balance values are available, new mass-balance values can be predicted solely from SAR images. For glaciers where no mass balance is available, the area change cannot be calibrated to absolute mass-balance values, but relative changes can be predicted.

Characteristics and potential climatic significance of “miniature ice caps” (crest- and cornice-type low-altitude ice archives)

AbstractLong-term ice-core records of Alpine glaciers are usually taken from cold-firn areas at high altitudes, as on Mont Blanc and Monte Rosa. Perennial ice bodies at lower altitudes, however, also bear information about the past. Recent findings from the remains of such ice (the Oetztal iceman found in Austria; wooden bows at Lötschen Pass, Switzerland) clearly indicate the hitherto little-recognized fact that small, more-or-less static perennial ice bodies which are cold and frozen to the underlying bedrock may contain very old ice and, hence, important palaeoclimatic information about warm periods with minimum ice extent in the Alps. Since autumn 1998, investigations have been initiated on a crest-type location or “miniature ice cap” at Piz Murtèl, Engadine, Swiss Alps. First results from shallow drilling, temperature data-logging, geodetic surveying, visual observation, finite-element modelling of simplified basic two-dimensional configurations and comparison to earlier measurements at similar sites provide promising perspectives concerning a little-studied phenomenon with considerable scientific-environmental research potential. Specific characteristics of the investigated site, and probably of many other comparable mountain sites, are: cold ice (about –4°C at 10 m depth), no basal sliding, small mass turnover, striking lack of a firn zone, accumulation mainly by superimposed ice, and direct access to old layers (centuries, millennia?) at the ice/bedrock interface.

Complete and accurate mass spectrometric isotope analysis of tropospheric nitrous oxide

We describe a manual extraction and purification method for mass spectrometric isotope analyses of tropospheric N2O. A theoretical framework to correct for (hydro)fluorocarbon and CO2 interferences is developed and verified experimentally. The standard deviation for analysis of one sample on a single day is 0.05‰ for δ15N and δ18O and 0.2‰ for the relative enrichment of the terminal (1δ5N) and central (2δ5N) nitrogen atoms. The isotopic composition of N2O in tropospheric background air could thus be measured with unprecedented precision on samples from six locations. We obtained overall average values of δ15N = (6.72 ± 0.12)‰ versus air N2 and δ18O = (44.62 ± 0.21)‰ versus Vienna Standard Mean Ocean Water. Neither a clear spatial pattern from 28°N to 79°N, nor a temporal trend over the course of 2 years was found. Within the experimental uncertainties, this is in line with small trends of 0.02 to 0.04‰/a derived from analyses of Antarctic firn air and isotopic budget calculations. Using an independent 2δ15N calibration of our working standard versus air N2, we find large differences in the position‐dependent 15N/14N ratios: The mean for all samples is 1δ15N = (−15.8 ± 0.6)‰ and 2δ15N = (29.2 ± 0.6)‰ versus air N2. In light of a new definition for oxygen isotope anomalies, we reevaluate the existing measurements and derive a 17O excess of Δ17O = (0.9 ± 0.1)‰.

The isotopic fingerprint of the pre-industrial and the anthropogenic N<sub>2</sub>O source

Abstract. We have performed high-precision measurements of the 18O and position dependent 15N isotopic composition of N2O from Antarctic firn air samples. By comparing these data to simulations carried out with a firn air diffusion model, we have reconstructed the temporal evolution of the N2O isotope signatures since pre-industrial times. The heavy isotope content of atmospheric N2O is presently decreasing for all signatures at rates of about -0.038 %o yr -1 for 1d15N, -0.046 %o yr -1 for 2d 15N and -0.025 %o yr -1 for d18O. The total decrease since pre-industrial times is estimated to be about -1.8%o for 1d15N at both positions and -2.2%o for 2d15N. Isotope budget calculations using these trends and recent stratospheric measurements allow to isotopically characterize the present and the pre-industrial global average N2O source, as well as the additional N2O emissions that have caused the global N2O increase since pre-industrial times. The increased fluxes from the depleted surface sources alone are insufficient to explain the inferred temporal isotope changes. In addition, the global average N2O source signature is calculated to be significantly depleted today relative to the pre-industrial value, in agreement with recent indications from soil emission measurements.

Evidence for the loss of snow-deposited MSA to the interstitial gaseous phase in central Antarctic firn

We have examined several MSA (methanesulfonic acid) records from the upper 200 m of the Antarcticice sheet and in particular the new Dome F profile. At all the four sites studied, concentration profilesexhibit similar patterns as a function of depth. They suggest that snow metamorphism and solid phasemigration are responsible for a marked release of gaseous MSA to interstitial firn air as well as probablyto the free atmosphere, in particular at extremely low accumulation sites. Snow acidity can also modifyMSA concentration. It is proposed that, below the upper few metres where the communication withthe free atmosphere is possible, gaseous MSA may remain in the firn layers and be entrapped later inair bubbles at pore close-off, i.e. when firn is transformed into ice. Chemical measurements on the firncore do not take into account the MSA released to the gaseous phase, but this fraction is measurable inice samples. In spite of these alterations occurring in the firn layers, relative changes of the atmosphericMSA concentration in the past are probably still there deep within the Antarctic ice sheet. However, for glacial periods, different processes have to be considered in relation to modified aerosol properties.

Laboratory determination of thermal diffusion constants for 29N 2/ 28N 2 in air at temperatures from −60 to 0°C for reconstruction of magnitudes of abrupt climate changes using the ice core fossil–air paleothermometer

Rapid temperature change causes fractionation of isotopic gaseous species in air in firn (snow) by thermal diffusion, producing a signal that is preserved in trapped air bubbles as the snow forms ice. Using a model of heat penetration and gas diffusion in the firn, as well as the values of appropriate thermal diffusion constants, it is possible to reconstruct the magnitude of a particular paleoclimate change. Isotopic nitrogen in air serves as a convenient tracer for such paleoreconstruction, because the ratio 29N 2/ 28N 2 has stayed extremely constant in the atmosphere for ≥10 6 years. However, prior to this work no data were available for thermal diffusion of 29N 2/ 28N 2 in air, but only in pure N 2. We devised a laboratory experiment allowing fractionation of gases by thermal diffusion in a small, tightly controlled temperature difference. A mass spectrometer was employed in measuring the resulting fractionations yielding measurement precision greater than was attainable by earlier thermal diffusion investigators. Our laboratory experiments indicate that the value of the thermal diffusion sensitivity ( Ω) for 29N 2/ 28N 2 in air is +(14.7 ± 0.5) × 10 −3 per mil/°C when the average temperature is –30.0°C. The corresponding value for 29N 2/ 28N 2 in pure N 2 that we find is +(15.3 ± 0.4) × 10 −3 per mil/°C at –30.6°C, in agreement with the previously available literature data within their large range of uncertainty. We find that an empirical equation, Ω = (8.656/ T K − 1232/ T K2) ± 3% per mil/°C, describes the slight variation of the sensitivity values for 29N 2/ 28N 2 in air with temperature in the range of –60 to 0°C. A separate set of experiments also described in this paper rules out adsorption as a candidate for producing additional temperature change-driven fractionation of 29N 2/ 28N 2 in the firn air. The combined newly obtained data constitute a calibration of the fossil–air paleothermometer with respect to isotopic nitrogen and will serve to improve the estimates of the magnitudes of past abrupt climate changes recorded in ice cores.

Satellite retrieval of mass balance: comparing SAR images with albedo images and in situ mass-balance observations

AbstractWe present an analysis of many European Remote-sensing Satellite (ERS) synthetic aperture radar (SAR) images of Vatnajökull, Iceland, by comparing them with Advanced Very High Resolution Radiometer (AVHRR) images, mass-balance observations and modelled firn stratigraphy. Summer SAR and AVHRR images both detect the surface firn line (or the transient snow-line when it lies below the firn line) as a distinct boundary.Winter SAR images of Vatnajökull generally display the late-summer surface firn line, and often also a transition at a higher elevation that possibly reflects a subsurface continuation of the firn line. On some winter images the firn line is not adequately detected, possibly due to a shallow firn pack. We found no differences in back-scatter between melting snow and firn, and for most years no differences in reflectance either. For Vatnajökull, equilibrium-line altitude is therefore not a particularly useful estimator of the mean specific mass balance (Bm). Unlike albedo images, SAR images do not display interannual variations of the signal within the accumulation area that are clearly related toBm. Hence, SAR images of Vatnajökull contain less information aboutBmthan albedo images. However, for some drainage basins, mean transient snow-line/firn-line altitude during the melting season can be used to inferBm.

Firn–ice transition-zone features of four polythermal glaciers in Svalbard seen by ground-penetrating radar

AbstractDetailed ground-penetrating radar (GPR) surveys on the firn–ice transition zones of Kongsvegen, Nordenskjoldbreen, Hansbreen and Werenskioldbreen, Svalbard, are presented. We discuss features seen in GPR radargrams in all polythermal glaciers down-glacier of the firn line. The firn line of a polythermal glacier in GPR data is easily detected with both 50 and 200 MHz radar. By comparison with satellite images, characteristic patterns of reflection are shown to be caused by superimposed ice, but the reflections from the interface between the superimposed ice and the underlying glacier ice are of unknown origin. Layered reflections within the superimposed ice and firn area are likely from layers of ice lenses and relatively low-density firn in the firn area, and from bubble-rich and bubble-poor ice in the superimposed-ice zone. Dipping foliations in the glacier ice down-glacier of the firn line show that the layered reflections from the firn line are preserved, though in much weaker form, even when the firn has been transformed to solid ice.

Evidence for the loss of snow-deposited MSA to the interstitial gaseous phase in central Antarctic firn

We have examined several MSA (methanesulfonic acid) records from the upper 200 m of the Antarcticice sheet and in particular the new Dome F profile. At all the four sites studied, concentration profilesexhibit similar patterns as a function of depth. They suggest that snow metamorphism and solid phasemigration are responsible for a marked release of gaseous MSA to interstitial firn air as well as probablyto the free atmosphere, in particular at extremely low accumulation sites. Snow acidity can also modifyMSA concentration. It is proposed that, below the upper few metres where the communication withthe free atmosphere is possible, gaseous MSA may remain in the firn layers and be entrapped later inair bubbles at pore close-off, i.e. when firn is transformed into ice. Chemical measurements on the firncore do not take into account the MSA released to the gaseous phase, but this fraction is measurable inice samples. In spite of these alterations occurring in the firn layers, relative changes of the atmosphericMSA concentration in the past are probably still there deep within the Antarctic ice sheet. However, for glacial periods, different processes have to be considered in relation to modified aerosol properties.

Velocity of radio-wave propagation in ice at Vostok station, Antarctica

AbstractDuring the austral summer field season of the Russian Antarctic Expedition in 1999/2000, wide-angle reflections experiments were performed in the vicinity of the Russian station Vostok. A 60 MHz ice radar system with 12-bit digital recording was used. The measurements were taken along two perpendicular lines directed south–north and east–west with a distance of 200 m between marks. We used a one-layer model (without snow–firn zone influence) for the calculations. We calculate that the average velocity of radio-wave propagation in the ice sheet is168.4 ± 0.5 m μs−1.The same velocity was derived from hyperbolic diffractions from internal discontinuities. The results allow more accurate depth interpretation of radio-echo soundings.

Reconstructing atmospheric histories from measurements of air composition in firn

This paper investigates the use of a numerical model of firn diffusion and bubble trapping in the reconstruction of atmospheric records from firn measurements. We describe the concept of mean age and effective age of tracers in firn air and how the growth rate of a tracer in the atmosphere can alter the effective age. We discuss an iterative method to assign effective ages to firn measurements for tracers with fairly simple atmospheric histories, taking into account atmospheric growth rate variations. We then develop a Bayesian synthesis inversion calculation for inverting firn concentration measurements. This calculation gives estimates of the atmospheric concentration record with uncertainties. The dating and inversion techniques are demonstrated here with carbon tetrachloride measurements from a long firn record from Law Dome, Antarctica. The techniques are then applied to measurements of a range of halocarbons in a companion paper by Sturrock et al. [2002].

Atmospheric histories of halocarbons from analysis of Antarctic firn air: Major Montreal Protocol species

Air samples extracted from Antarctic firn at Law Dome have been analyzed for the suite of halocarbons that contribute most of the anthropogenic chlorine and about half the anthropogenic bromine presently released into the stratosphere. The species, all included in the Montreal Protocol, are the chlorofluorocarbons (CFCs ‐11, ‐12, ‐113, ‐114, ‐115), hydrochlorofluorocarbons (HCFCs ‐22, ‐141b, ‐142b), halons (H ‐1211, ‐1301), CH3CCl3 (methyl chloroform) and CCl4 (carbon tetrachloride). The measurements were used to reconstruct the atmospheric history of these species since the 1930s, providing a record considerably predating existing in situ records or other conventional air archives, encompassing virtually the entire history of anthropogenic emissions of CFCs, HCFCs, and halons and giving early 20th century levels for CH3CCl3 and CCl4. Significant features of this study are (1) the narrow age spread (spectral width 5 years) of the individual firn air samples, which reveals rapid atmospheric changes, (2) the use of inversion techniques to infer past atmospheric composition with associated uncertainties, and (3) the low analytical detection limit (<0.1 ppt), which, together with the narrow air age spread, detects early background levels and resolves the time that industrial emissions first appeared in the southern hemisphere atmosphere. Integrity of the data is demonstrated by successful intercomparison of data from independent firn sites on Law Dome with common time‐series. An upper limit is given for the potential contribution to atmospheric levels of CH3CCl3 from nonindustrial sources. The atmospheric records produced from firn air are compared to calculations based on the history of their global emissions.

Photochemically induced production of CH 3Br, CH 3I, C 2H 5I, ethene, and propene within surface snow at Summit, Greenland

Measurements at Summit, Greenland, performed from June–August 1999, showed significant enhancement in concentrations of several trace gases in the snowpack (firn) pore air relative to the atmosphere. We report here measurements of alkenes, halocarbons, and alkyl nitrates that are typically a factor of 2–10 higher in concentration within the firn air than in the ambient air 1–10 m above the snow. Profiles of concentration to a depth of 2 m into the firn show that maximum values of these trace gases occur between the surface and 60 cm depth. The alkenes show highest pore mixing ratios very close to the surface, with mixing ratios in the order ethene>propene>1-butene. Mixing ratios of the alkyl iodides and alkyl nitrates peak slightly deeper in the firn, with mixing ratios in order of methyl>ethyl>propyl. These variations are likely consistent with different near-surface photochemical production mechanisms. Diurnal mixing ratio variations within the firn correlate well with actinic flux for all these gases, with a temporal offset between the solar maximum and peak concentrations, lengthening with depth. Using a snow-filled chamber under constant flow conditions, we calculated production rates for the halocarbons and alkenes that ranged between 10 3–10 5 and 10 6 molecules cm −3 s −1, respectively. Taken together, these results suggest that photochemistry associated with the surface snowpack environment plays an important role in the oxidative capacity of the local atmospheric boundary layer, and influences post-depositional chemistry, which in turn may affect the interpretation of certain aspects of the ice core records collected previously at Summit.

Shouldn’t snowpacks be sources of monocarboxylic acids?

We report the first measurements of the mixing ratios of acetic (CH 3COOH) and formic (HCOOH) acids in the air filling the pore spaces of the snowpacks (firn air) at Summit, Greenland and South Pole. Both monocarboxylic acids were present at levels well above 1 ppbv throughout the upper 35 cm of the snowpack at Summit. Maximum mixing ratios in Summit firn air reached nearly 8 ppbv CH 3COOH and 6 ppbv HCOOH. At South Pole the mixing ratios of these acids in the top 35 cm of firn air were also generally >1 ppbv, though their maximums barely exceeded 2.5 ppbv of CH 3COOH and 2.0 ppbv of HCOOH. Mixing ratios of the monocarboxylic acids in firn air did not consistently respond to diel and experimental (fast) variations in light intensity, unlike the case for N oxides in the same experiments. Air-to-snow fluxes of CH 3COOH and HCOOH apparently support high mixing ratios (means of (CH 3COOH/HCOOH) 445/460 and 310/159 pptv at Summit and South Pole, respectively) in air just above the snow during the summer sampling seasons at these sites. We hypothesize that oxidation of carbonyls and alkenes (that are produced by photo- and OH-oxidation of ubiquitous organic compounds) within the snowpack is the source of the monocarboxylic acids.

Processes and properties of snow–air transfer in the high Arctic with application to interstitial ozone at Alert, Canada

Recent measurements of reactive chemical species in snow and firn at polar sites have served to underscore the importance of air–snow transfer processes in understanding changes in atmospheric chemistry. In this paper we present the first quantitative assessment of the impact of physical processes in the snow on air–snow chemical exchange of ozone. Measurements of snow properties, interstitial ozone concentrations, and an ozone kinetic depletion experiment results are presented along with two-dimensional model results of the diffusion and ventilation processes affecting gas exchange at Alert, Nunavut, Canada. The Arctic snowpack at Alert will allow rapid exchange of gases with the atmosphere. Even under natural ventilation conditions with moderate winds, the entire pack is exposed to air movement and therefore available for chemical exchange processes. Both measurements and model results indicate that ozone undergoes rapid depletion in the top centimeters of the snow—approximately within the top 5 cm under diffusion alone, and in the top 10 cm or less during ventilation. Due to the higher permeability of the snowpack on the sea ice site as compared to the terrestrial site, it is possible that chemical exchange processes could be even more rapid over the sea ice in the greater Arctic than at the terrestrial site. A quantitative discussion of complications that arise from current firn air sampling techniques is presented and possible improvements for future measurements are suggested.

A long‐term record of carbonyl sulfide (COS) in two hemispheres from firn air measurements

Carbonyl sulfide (COS) was measured in polar firn air from one Arctic and two Antarctic locations. The air samples represent atmospheric composition from the early to mid‐20th century up to the present day. This provides the longest record to date of atmospheric COS. Southern Hemispheric (SH) concentrations appear to have been almost constant at 482±13 ppt over this period, apart from a slight rise in the earliest part of the record. Northern Hemispheric (NH) concentrations also showed relatively little variation with a mean of 525±17 ppt. Over the last ten years, however, NH concentrations appear to have declined by about 8±5%. Such a decline might be due to decreased carbon disulfide (CS2) emissions by the viscose‐rayon industry. The absence of any large trend in COS concentrations over the last fifty or more years argues against COS being the origin of reported increases in stratospheric sulfate aerosol.

Changes in the global atmospheric methane budget over the last decades inferred from 13C and D isotopic analysis of Antarctic firn air

The atmospheric trend of methane isotopic ratios since the mid‐20th century has been reconstructed from Antarctic firn air. High volume air samples were extracted at several depth levels at two sites in East Antarctica. Methane concentration and its 13C/12C and D/H ratios were determined by gas chromatography, mass spectrometry, and infrared spectroscopy. A firn air transport model was applied to reconstruct past atmospheric trends in methane and its isotopic composition. By subsequent application of an atmospheric model, changes in methane sources and OH sink compatible with the past atmospheric trends are explored. In step with increasing methane mixing ratios, δ13C increased by ∼1.7‰ over the last 50 years. These changes mainly reflect a shift in relative source strength toward the heavier anthropogenic methane source, such as biomass burning and methane of nonbiological origin. The δD (CH4) showed a period of decline between the 1950s and 1975, followed by a gradual increase of 0.55‰/yr, also toward the heavier anthropogenic source. Dependent on possible changes in the OH sink, to which δD of methane is very sensitive, the inferred isotopic trends of δ13C and δD over the last 50 years constrain the relationship between natural and anthropogenic sources over the last century. The observed δD minimum around 1975 suggests that the slowing down in the methane source growth took place during this period.

Thermal fractionation of air in polar firn by seasonal temperature gradients

Air withdrawn from the top 5–15 m of the polar snowpack (firn) shows anomalous enrichment of heavy gases during summer, including inert gases. Following earlier work, we ascribe this to thermal diffusion, the tendency of a gas mixture to separate in a temperature gradient, with heavier molecules migrating toward colder regions. Summer warmth creates a temperature gradient in the top few meters of the firn due to the thermal inertia of the underlying firn and causes gas fractionation by thermal diffusion. Here we explore and quantify this process further in order to (1) correct for bias caused by thermal diffusion in firn air and ice core air isotope records, (2) help calibrate a new technique for measuring temperature change in ice core gas records based on thermal diffusion [Severinghaus et al., 1998], and (3) address whether air in polar snow convects during winter and, if so, whether it creates a rectification of seasonality that could bias the ice core record. We sampled air at 2‐m‐depth intervals from the top 15 m of the firn at two Antarctic sites, Siple Dome and South Pole, including a winter sampling at the pole. We analyzed 15N/14N, 40Ar/36Ar, 40Ar/38Ar, 18O/16O of O2, O2/N2, 84Kr/36Ar, and 132Xe/36Ar. The results show the expected pattern of fractionation and match a gas diffusion model based on first principles to within 30%. Although absolute values of thermal diffusion sensitivities cannot be determined from the data with precision, relative values of different gas pairs may. At Siple Dome, δ40Ar/4 is 66 ± 2% as sensitive to thermal diffusion as δ15N, in agreement with laboratory calibration; δ18O/2 is 83 ± 3%, and δ84Kr/48 is 33 ± 3% as sensitive as δ15N. The corresponding figures for summer South Pole are 64 ± 2%, 81 ± 3%, and 34 ± 3%. Accounting for atmospheric change, the figure for δO2/N2/4 is 90 ± 3% at Siple Dome. Winter South Pole shows a strong depletion of heavy gases as expected. However, the data do not fit the model well in the deeper part of the profile and yield a systematic drift with depth in relative thermal diffusion sensitivities (except for Kr, constant at 34 ± 4%), suggesting the action of some other process that is not currently understood. No evidence for wintertime convection or a rectifier effect is seen.