The concentration of atmospheric methane increased from around 360 ppbv at the last glacial maximum (similar to 20 ka before present) to about 700 ppbv in the pre-industrial era (similar to 200 years before present). The sources and/or sinks of methane must therefore have changed during this period; however, the relative sizes of the source- and sink-driven changes in methane concentration remain uncertain. We take the first "bottom-up" approach to identifying any chemical signals preserved in the ice record that could help us to determine these. Using an atmospheric chemistry-transport model, we explore the effects of source- and sink-driven changes in methane on a wide range of chemical species in the Antarctic boundary layer. Though we identify several potentially useful atmospheric signals, a simple and robust constraint on the sizes of the source- and sink-driven changes cannot be readily identified, owing to their preservation in the ice, limitations to the information they hold, and/or ambiguity surrounding their interpretation. This includes the mass-independent fractionation of oxygen isotopes in sulfates, and the concentration of formaldehyde, in which there has been considerable interest. Our exploration is confined to a domain in which NOx emissions and climate remain constant. However, given the uncertainties associated with the changes in these factors, we would anticipate that their inclusion would make it harder still to identify a robust signal. Finally, though formaldehyde cannot provide this, we propose how it might be used to synchronize the gas- and aqueous-phase Antarctic ice records and thus determine the relative phasing of glacial-interglacial changes in Southern Hemisphere CO2 and temperature.

Atmospheric hydroperoxides (ROOH) were measured at Summit, Greenland (72.97° N, 38.77° W) in summer 2003 (SUM03) and spring 2004 (SUM04) and South Pole in December 2003 (SP03). The two dominant hydroperoxides were H2O2 and CH3OOH (from here on MHP) with average (±1σ) mixing ratios of 1448 (±688) pptv, 204 (±162) and 278 (±67) for H2O2 and 578 (±377) pptv, 139 (±101) pptv and 138 (±89) pptv for MHP, respectively. In early spring, MHP dominated the ROOH budget and showed night time maxima and daytime minima, out of phase with the diurnal cycle of H2O2, suggesting that the organic peroxide is controlled by photochemistry, while H2O2 is largely influenced by temperature driven exchange between the atmosphere and snow. Highly constrained photochemical box model runs yielded median ratios between modeled and observed MHP of 52%, 148% and 3% for SUM03, SUM04 and SP03, respectively. At Summit firn air measurements and model calculations suggest a daytime sink of MHP in the upper snow pack, which decreases in strength through the spring season into the summer. Up to 50% of the estimated sink rates of 1–5×1011 molecules m−3 s−1 equivalent to 24–96 pptv h−1 can be explained by photolysis and reaction with the OH radical in firn air and in the quasi-liquid layer on snow grains. Rapid processing of MHP in surface snow is expected to contribute significantly to a photochemical snow pack source of formaldehyde (CH2O). Conversely, summer levels of MHP at South Pole are inconsistent with the prevailing high NO concentrations, and cannot be explained currently by known photochemical precursors or transport, thus suggesting a missing source. Simultaneous measurements of H2O2, MHP and CH2O allow to constrain the NO background today and potentially also in the past using ice cores, although it seems less likely that MHP is preserved in firn and ice.

The source of bromine that drives polar boundary layer ozone depletion events (ODEs) is still open to some debate. While ODEs are generally noted to form under conditions of a shallow stable boundary layer, observations of depleted air under high wind conditions are taken as being transport-related. Here we report observations from Antarctica in which an unusually large cloud of BrO formed over the Weddell Sea. The enhanced BrO was observed over Halley station in coastal Antarctica, providing an opportunity to probe the conditions within an active 'bromine explosion' event. On this occasion, enhanced BrO and depleted boundary layer ozone coincided with high wind speeds and saline blowing snow. We derive a simple model to consider the environmental conditions that favour ODEs and find two maxima, one at low wind/stable boundary layer and one at high wind speeds with blowing snow. Modelling calculations aiming to reproduce the wider regional or global impacts of ODEs, either via radiative effects or as a halogen source, will also need to account for high wind speed mechanisms.

Gas phase formaldehyde concentrations were measured during the 2004–2005 CHABLIS campaign at Halley research station, Antarctica. Data coverage span from May 2004 through to January 2005, thus capturing the majority of the year, with a wintertime minimum of near or below the instrumental detection limit rising to between 50 and 200 pptv during the austral summer. Factors controlling HCHO concentration include local chemical sources and sinks, and exchange with the snow surface. The measured seasonality is in line with previous observations from Neumayer station, with maximum in summer and minimum during the winter months, but with lower absolute concentrations throughout the year. The gas-phase production of HCHO was dominated by methane oxidation and a steady-state analysis showed that reactions of iodine and bromine species substantially reduced the predicted HCHO levels based upon in situ chemistry. This indicates a substantial additional HCHO source to be present that could be explained by a snowpack source.

The EPICA ice core, drilled at Dome C in East Antarctica, provides an undisturbed record of the past eight climatic cycles. Dust particles in the atmosphere can affect temperature by absorbing or reflecting solar radiation, and previous work suggested that production, transport and deposition of dust is influenced by climatic changes on glacial-interglacial timescales. A new analysis of the record of dust deposits in the EPICA core shows that the flux in atmospheric dust content increasingly correlates with Antarctic temperature during glacial periods as the climate gets colder, indicative of a progressive coupling of the climates of Antarctica and lower latitudes. An observed 25-fold increase in glacial dust flux over all eight glacial periods may reflect a strengthening of South American dust sources, and longer dust-particle lifetimes in the upper troposphere due to a reduced hydrological cycle during ice ages. It has been suggested that production, transport, and deposition of dust is influenced by climatic changes on glacial-interglacial timescales. This paper presents a high-resolution record of aeolian dust from the EPICA Dome C ice core in East Antarctica, which suggests that dust is increasingly correlated with Antarctic temperature as the climate becomes colder, indicative of a progressive coupling of the climates of Antarctica and lower latitudes. Dust can affect the radiative balance of the atmosphere by absorbing or reflecting incoming solar radiation1; it can also be a source of micronutrients, such as iron, to the ocean2. It has been suggested that production, transport and deposition of dust is influenced by climatic changes on glacial-interglacial timescales3,4,5,6. Here we present a high-resolution record of aeolian dust from the EPICA Dome C ice core in East Antarctica, which provides an undisturbed climate sequence over the past eight climatic cycles7,8. We find that there is a significant correlation between dust flux and temperature records during glacial periods that is absent during interglacial periods. Our data suggest that dust flux is increasingly correlated with Antarctic temperature as the climate becomes colder. We interpret this as progressive coupling of the climates of Antarctic and lower latitudes. Limited changes in glacial-interglacial atmospheric transport time4,9,10 suggest that the sources and lifetime of dust are the main factors controlling the high glacial dust input. We propose that the observed ∼25-fold increase in glacial dust flux over all eight glacial periods can be attributed to a strengthening of South American dust sources, together with a longer lifetime for atmospheric dust particles in the upper troposphere resulting from a reduced hydrological cycle during the ice ages.

The influence of snow microstructure on thermal and radiative transfer in snow has not been thoroughly investigated as the tools necessary to efficiently measure microstructural geometry at millimeter resolution have not yet been available. Here we investigate the impact of snow microstructure on the thermal and radiative properties of snow and specifically determine the depth resolution of snow stratigraphy measurements needed to adequately model snow temperatures. To address this subject, detailed information on physical properties of the snow cover was collected at Summit, Greenland, in summer 2003. We present a new set of snow microstructure data, measured with a high‐resolution penetrometer (SnowMicroPen (SMP)). The penetration resistance can be used to estimate the thickness of individual layers but also reflects the thickness and the number of bonds in the snowpack. SMP is a motor‐driven, constant speed micropenetrometer acquiring 256 hardness measurements per millimeter. The data were used to describe the physical properties of snow and to compute snow temperatures in the topmost meter of the snowpack using the snow cover model SNOWPACK. The spatial resolution of the snow microstructure and the subsequent parametrization of SMP‐estimated density strongly affected the modeled temperature and temperature gradients. The SMP‐estimated density was used as an input to SNOWPACK and affected the thermal conductivity and radiative transfer in the snowpack. Our results thus show that highly resolved stratigraphic input data on the order of at least 1 cm are necessary to adequately simulate snow temperatures and snow metamorphism, including the observed formation of subsurface hoar in the high Arctic snow cover.

An interpretative modeling analysis is conducted to simulate the diurnal variations in OH and HO2+RO2 observed at Summit, Greenland in 2003. The main goal is to assess the HOx budget and to quantify the impact of snow emissions on ambient HOx as well as on CH2O and H2O2. This analysis is based on composite diurnal profiles of HOx precursors recorded during a 3-day period (July 7–9), which were generally compatible with values reported in earlier studies. The model simulations can reproduce the observed diurnal variation in HO2+RO2 when they are constrained by observations of H2O2 and CH2O. By contrast, model predictions of OH were about factor of 2 higher than the observed values. Modeling analysis of H2O2 suggests that its distinct diurnal variation is likely controlled by snow emissions and loss by deposition and/or scavenging. Similarly, deposition and/or scavenging sinks are needed to reproduce the observed diel profile in CH2O. This study suggests that for the Summit 2003 period snow emissions contribute ∼25% of the total CH2O production, while photochemical oxidation of hydrocarbon appears to be the dominant source. A budget assessment of HOx radicals shows that primary production from O(1D)+H2O and photolysis of snow emitted precursors (i.e., H2O2 and CH2O) are the largest primary HOx sources at Summit, contributing 41% and 40%, respectively. The snow contribution to the HOx budget is mostly in the form of emissions of H2O2. The dominant HOx sink involves the HO2+HO2 reaction forming H2O2, followed by its deposition to snow. These results differ from those previously reported for the South Pole (SP), in that primary production of HOx was shown to be largely driven by both the photolysis of CH2O and H2O2 emissions (46%) with smaller contributions coming from the oxidation of CH4 and the O(1D)+H2O reaction (i.e., 27% each). In sharp contrast to the findings at Summit in 2003, due to the much higher levels of NOx, the SP HOx sinks are dominated by HOx–NOx reactions, leading to the formation and deposition of HNO3 and HO2NO2. Thus, a comparison between SP and Summit studies suggests that snow emissions appear to play a prominent role in controlling primary HOx production in both environments. However, as regards to maintaining highly elevated levels of OH, the two environments differ substantially. At Summit the elevated rate for primary production of HOx is most important; whereas, at SP it is the rapid recycling of the more prevalent HO2 radical, through reaction with NO, back to OH that is primarily responsible.

Nature 440, 491–496 (2006); doi:10.1038/nature04614. In calculating the fluxes used in this paper, we made a mistake in the snow accumulation rate used. We calculated fluxes assuming that snow accumulation rates were expressed in metres water equivalent, but they were actually expressed in metres ice equivalent.

The first measurements of peroxy (HO2+RO2) and hydroxyl (OH) radicals above the arctic snowpack were collected during the summer 2003 campaign at Summit, Greenland. The median measured number densities for peroxy and hydroxyl radicals were 2.2×108molcm−3 and 6.4×106molcm−3, respectively. The observed peroxy radical values are in excellent agreement (R2=0.83, M/O=1.06) with highly constrained model predictions. However, calculated hydroxyl number densities are consistently more than a factor of 2 lower than the observed values. These results indicate that our current understanding of radical sources and sinks is in accord with our observations in this environment but that there may be a mechanism that is perturbing the (HO2+RO2)/OH ratio. This observed ratio was also found to depend on meteorological conditions especially during periods of high winds accompanied by blowing snow. Backward transport model simulations indicate that these periods of high winds were characterized by rapid transport (1–2 days) of marine boundary layer air to Summit. These data suggest that the boundary layer photochemistry at Summit may be periodically impacted by halogens.

Annually resolved ice core records from different regions over the Greenland ice sheet (GrIS) are used to investigate the spatial and temporal variability of calcium (Ca2+, mainly from mineral dust) and sodium (Na+, mainly from sea salt) deposition. Cores of high common inter-annual variability are grouped with an EOF analysis, resulting in regionally representative Ca2+ and Na+ records for northeastern and central Greenland. Utilizing a regression and validation method with ERA-40 reanalysis data, these common records are associated with distinct regional atmospheric circulation patterns over the North American Arctic, Greenland, and Central to Northern Europe. These patterns are interpreted in terms of transport and deposition of the impurities. In the northeastern part of the GrIS sea salt records reflect the intrusion of marine air masses from southeasterly flow. A large fraction of the Ca2+ variability in this region is connected to a circulation pattern suggesting transport from the west and dry deposition. This pattern is consistent with the current understanding of a predominantly Asian source of the dust deposited over the GrIS. However, our results also indicate that a significant fraction of the inter-annual dust variability in NE and Central Greenland is determined by the frequency and intensity of wet deposition during the season of high atmospheric dust loading, rather than representing the variability of the Asian dust source and/or long-range transport to Greenland. The variances in the regional proxy records explained by the streamfunction patterns are high enough to permit reconstructions of the corresponding regional deposition regimes and the associated circulation patterns.

Sea ice and dust flux increased greatly in the Southern Ocean during the last glacial period. Palaeorecords provide contradictory evidence about marine productivity in this region, but beyond one glacial cycle, data were sparse. Here we present continuous chemical proxy data spanning the last eight glacial cycles (740,000 years) from the Dome C Antarctic ice core. These data constrain winter sea-ice extent in the Indian Ocean, Southern Ocean biogenic productivity and Patagonian climatic conditions. We found that maximum sea-ice extent is closely tied to Antarctic temperature on multi-millennial timescales, but less so on shorter timescales. Biological dimethylsulphide emissions south of the polar front seem to have changed little with climate, suggesting that sulphur compounds were not active in climate regulation. We observe large glacial-interglacial contrasts in iron deposition, which we infer reflects strongly changing Patagonian conditions. During glacial terminations, changes in Patagonia apparently preceded sea-ice reduction, indicating that multiple mechanisms may be responsible for different phases of CO2 increase during glacial terminations. We observe no changes in internal climatic feedbacks that could have caused the change in amplitude of Antarctic temperature variations observed 440,000 years ago.

(1) Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany, (2) British Antarctic Survey, Cambridge, UK, (3) Laboratoire de Glaciologie et Geophysique de l’Environnement, Grenoble, France, (4) Stockholm University, Sweden, (5) University of Bern, Switzerland, (6) University of Copenhagen: Denmark, (7) University of Florence, Italy, (8) University of Venice, Italy (hufischer@awi-bremerhaven.de)