MICROSCOPIC OBSERVATIONS OF THE AIR HYDRATE-BUBBLE. TRANSFORMATION PROCESS IN GLACIER ICE

Des examens microscopiques des inclusions d'hydrates d'air ont été faits sur des échantillons provenant de forages profonds a Dye-3 et Camp-Century, Groënland et Byrd Station en Antarctique. Les plus faibles profondeurs pour lesquelles les hydrates d'air sont observés à Dye-3 Camp Century et Byrd Station correspondent respectivement à 1092 m, 1099 m et 727 m. Pour les forages à Dye-3 et Camp Century, les profondeurs observées pour l'apparition des hydrates d'air sont en accord avec les calculs de Miller (1). Pour le forage à Byrd Station cette apparition a lieu environ 100 m moins profond que prévu par les calculs de Miller. Cette différence apparente peut être attribuée au flux ascendant de glace qui provient d'environ 5 km en amont de Byrd Station. Les observations de joints de grains et de phase et les expériences de déformation révèlent que l'énergie de joint de phases est supérieure à celle de joint de grains et que le processus de transformation de l'hydrate d'air en bulle est clairement lié à un mécanisme de nucléation induite par déformation. Ces résultats suggèrent que le processus de transformation hydrate d'air/bulle est étroitement contrôlé par un processus de restauration se produisant à la fois in situ et postérieurement à l'extraction.Microscopie examinations for air hydrate inclusions were made on specimens of the Dye-3 and Camp Century, Greenland and Byrd Station, Antarctica deep ice cores. The shallowest depths at which air hydrates are observed in the Dye-3, Camp Century and Byrd Station cores are at 1092 m, 1099 m and 727 m depths respectively. For the Dye-3 and Camp Century cores, the observed depths for air hydrate appearance agree with Miller's calculation [1]. For the Byrd Station core, the observed depth for the appearance is about 100 m shallower than the calculation result by Miller. This apparent difference at Byrd Station may be attributed to the general upward ice flow trajectory which begins about 5 km upstream from the Byrd Station location. The phase/grain boundary observations and deformation experiments revealed that phase boundary energy is much higher than grain boundary energy and that the transformation process from air hydrate to bubble is clearly related to the strain-induced nucleation process. These findings suggest that the air hydrate/bubble transformation process is strongly controlled by both in situ and post ice core recovery nucleation activation process

Variations of the CO2 concentration of occluded air and of anions and dust in polar ice cores

Analysis of impurities entrapped in natural ice is the most promising method for reconstructing the history of atmospheric composition before the period of direct measurement and offers the possibility of extending the record to at least 100,000 years B.P. We report here the present state of work in this field, with special emphasis on atmospheric CO2 concentration. After discussing the mechanism by which atmospheric gases are entrapped in ice, we report CO2 concentrations in ice core samples, up to 100,000 years old, from deep drilling projects in Greenland and the Antarctic. Results from ice deposited during the last 2000 years allow us to estimate the preindustrial atmospheric CO2 level, an important boundary condition for modelling the anthropogenic CO2 increase. Using older samples from a deep ice core drilled at Dye 3, Greenland, we show that the CO2 concentration was 180 to 200 ppmv at the end of the Wisconsin and increased during the transition to the Holocene to values in the 260 to 300 ppmv range. Detailed CO2 measurements on sections of the Wisconsin part of the Dye 3 core which, based on δ18O, were deposited during times of significant climatic variation, show that the δ18O variations were accompanied by simultaneous correlated rapid CO2 variations. Other parameters, including micro-particle concentration and Cl−, NO− 3 and SO2− 4 concentrations also showed significant variations which correlate with the measured δ18O shifts

Seasonal variations in the concentration of 10Be, Cl-, NO3-, SO42-, H2O2, 210Pb, 3H, mineral dust, and σ18O in Greenland snow

A detailed snow pit study at Dye 3, South Greenland, has been carried out in order to investigate the seasonal variations of 10Be, Cl−, NO3−, SO42−, H2O2, 210Pb, 3H, Mn and σ18O. Special emphasis was placed on understanding the causes of 10Be variations because this isotope can be used to trace the history of solar activity. A sampling interval of 5 cm was chosen to assure a mean time resolution of about 20 samples per year for the period 1978–1983. Four different categories of seasonal variations were identified: strong summer peaks (σ18O, H2O2, 3H), weak bimodal peaks (NO3−, SO42−, conductivity, 10Be), a fall peak (210Pb) and a winter-spring peak (Cl−, Mn). Although we are still far from a detailed understanding of the mechanisms which control the measured compositions, there are indications that different processes dominate at different seasons. NO3−, SO42−, Mn and Cl− variations suggest that aerosol transport is most important during winter-spring. In summer-fall there seems to exist a second period of long range transport from low latitude continental areas resulting in enhanced concentrations of 210Pb, 10Be and SO42−

High-resolution palaeoclimatology of the last millennium: A review of current status and future prospects

This review of late-Holocene palaeoclimatology represents the results from a PAGES/CLIVAR Intersection Panel meeting that took place in June 2006. The review is in three parts: the principal high-resolution proxy disciplines (trees, corals, ice cores and documentary evidence), emphasizing current issues in their use for climate reconstruction; the various approaches that have been adopted to combine multiple climate proxy records to provide estimates of past annual-to-decadal timescale Northern Hemisphere surface temperatures and other climate variables, such as large-scale circulation indices; and the forcing histories used in climate model simulations of the past millennium. We discuss the need to develop a framework through which current and new approaches to interpreting these proxy data may be rigorously assessed using pseudo-proxies derived from climate model runs, where the 'answer' is known. The article concludes with a list of recommendations. First, more raw proxy data are required from the diverse disciplines and from more locations, as well as replication, for all proxy sources, of the basic raw measurements to improve absolute dating, and to better distinguish the proxy climate signal from noise. Second, more effort is required to improve the understanding of what individual proxies respond to, supported by more site measurements and process studies. These activities should also be mindful of the correlation structure of instrumental data, indicating which adjacent proxy records ought to be in agreement and which not. Third, large-scale climate reconstructions should be attempted using a wide variety of techniques, emphasizing those for which quantified errors can be estimated at specified timescales. Fourth, a greater use of climate model simulations is needed to guide the choice of reconstruction techniques (the pseudo-proxy concept) and possibly help determine where, given limited resources, future sampling should be concentrated