Лёд и Снег • 2012 • № 4 (120)
УДК 551.321+551.583.7
The IPICS «oldest ice» challenge: a new technology to qualify potential sites
© 2012 г. J. Chappellaz1, O. Alemany1, D. Romanini2, E. Kerstel2
1Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE), CNRS-Grenoble University, France; 2Laboratoire Interdisciplinaire de Physique (LIPhy), CNRS-Grenoble University, France chappellaz@lgge.obs.ujf-grenoble.fr
Статья принята к печати 25 октября 2012 г.
Antarctica, glacial-interglacial cycles, CO2, ice core drilling.
Антарктида, СО2, керновое бурение льда, ледниковые-межледниковые циклы.
Among the priorities of the International Partnerships in Ice Core Sciences (IPICS) core project of Past Global Changes (IGBP/PAGES), drilling ice as old as 1.5 million years is probably the most emblematic challenge. The search for a potential site in Antarctica hosting such old ice in good stratigraphic order is under way. Here we propose an innovative way to rapidly qualify potential sites. We plan to build a probe able to drill down to bedrock within one field season. The probe will embed a laser optical instrument measuring in real time key parameters such as the water isotopic composition of the ice and the concentration of one or more greenhouse gases.
Introduction
Understanding past climate variations provides solid physical understanding of the climate response to natural forcings, including the quantification of feedbacks (e.g., between climate and the carbon cycle) and the identification of non-linear responses and thresholds. Ice cores are exceptional archives of past climate and atmospheric composition, providing key information on climate forcing (orbital signal, volcanism, and solar activity), climate feedbacks (atmospheric greenhouse gas concentration, aerosols, dust...), and polar climate history through quantitative reconstructions of past temperature and precipitation accumulation.
Since the 1960s, intensive efforts have been dedicated to the recovery and analysis of ice cores from glaciers and polar ice sheets. In Greenland, the international NorthGRIP project reached 3085 m of depth, covering one full climatic cycle back to the end of the last interglacial period [20]. In Antarctica, the Vostok drilling down to 3300 m of depth -and handled under a unique joint collaboration between Russia, France and the USA - was instrumental to reveal the close relationship between climate and greenhouse gases at glacial-interglacial time scales [22]. The European Project for Ice Coring in Antarctica (EPICA) has completed the Dome C deep drilling, reaching 3260 m of depth and offering an exceptional 800,000 year long archive of climate and atmospheric composition evolution. The oldest part of the EPICA Dome C climatic record revealed surprisingly cool interglacial periods from 800,000 to 400,000 years ago [8, Fig. 1]. A strong imprint of obliquity, increasing from past to present, is found in Antarctic temperature [11], moisture origin [27] and greenhouse gas concentrations [17, 18]. The increased magnitude of interglacials may be attributed to a long-term modulation of the climate response (notably sensitive to the Antarctic water cycle) to obliquity amplitude.
The oldest part of the EPICA Dome C ice core has revealed exceptionally low values of CO2 from 650,000 to 800,000 years ago [18], questioning the stability of the strong Antarctic temperature - carbon cycle coupling on long time scales, and in contradiction with earlier hypotheses of a long term decreasing trend of CO2. (e.g., [21]).
The scientific need to extend ice cores records back in time
A key international challenge is now to place the past 800,000 years of climate variability of the EPICA Dome C record in the broader context of the past 2 million years (Myr). Marine records [16] evidence a dramatic reorganisation of the pattern of climate variability taking place around 1 Myr ago, with a shift from the «obliquity world» characterized by 40,000-year weak glacial-interglacial cycles to the «100,000-year world» with longer and stronger glacial-interglacial cycles (Fig. 1). The reasons for this major climate reorganization (the «Mid Pleistocene Transition», MPT) remain unknown and may be intrinsic to the climate - cryo-sphere - carbon cycle feedbacks. Two major hypotheses have been raised so far (see [12] for a recent review):
1. A non-linear ice sheet response to a long term cooling trend [3]. This long term cooling trend would be driven by a progressive long-term decrease in atmospheric CO2 concentration. The mechanism behind the non-linear ice sheet response itself could involve the Antarctic ice sheet behaviour [23]. Long term cooling gradually drove the East Antarctic ice sheet margin into the sea, changing the behaviour of the Antarctic ice sheet from a terrestrial ice sheet (ablation driven by melt) to a marine ice sheet (ablation driven by calving). Another mechanism could be related with the merging of continental North American ice sheets, below a certain northern hemisphere temperature threshold [4]. Finally, non-linear
Fig. 1. Antarctic records of the EPICA Dome C 6D (light blue, %o) [11], a proxy of Antarctic temperature, and a stack of Vostok and EPICA Dome C atmospheric Co2 (red, ppmv) [18-22] and Dome C CH4 concentrations (green, ppbv) [17] spanning the past 800,000 years.
The horizontal time scale is expressed in thousands of years. While the southern ocean should play a major role on glacial-interglacial CO2 variations, changes in CH4 may be mostly controlled by changes in wetlands and therefore northern hemisphere continental climate. For comparison, marine records of changes in benthic S180 reflecting variations in global ice volume are displayed (dark blue, %) [16] as well as the Earth's orbital parameters (yellow - precession; orange - obliquity, black - eccentricity) [2]. Note the strong similarities between variations in central Antarctic temperature and global ice volume records, and the long-term trend in benthic S180 combined with a change of periodicity around 1 Myr ago, shortly before the EPICA record stops.
Рис. 1. Временной ряд 6D, полученный по керну проекта EPICA на Куполе С (голубая линия, %) [11] и отражающий изменение температуры; сводная кривая концентрации атмосферного СО2, построенная по данным кернов Востока и EPICA (красная линия, 10-6 объёмная концентрация) [18-22]; кривая концентрации атмосферного CH4 по данным керна EPICA (зелёная линия, 10-9 объёмная концентрация) [17].
Возраст льда на горизонтальной оси дан в тыс. лет. Изменения CO2 при переходе от ледниковых к межледниковым периодам во многом контролируются процессами, протекающими в океанах Южного полушария, а изменения СН4 - процессами на заболоченных территориях и, следовательно, больше связаны с континентальным климатом Северного полушария. Для сравнения показаны: кривая б180 по бентосным фораминиферам в морских колонках, отражающая изменение объёма континентального льда (синяя линия, %) [16]; изменения орбитальных параметров Земли: (жёлтая линия - прецессия, оранжевая - наклон земной оси, чёрная - эксцентриситет) [2]. Отмечается хорошая согласованность между вариациями температуры в центральных районах Антарктиды и изменениями объёма континентального льда на Земле. Хорошо виден тренд в изменении б180 в морских колонках, а также смена характерных периодов вариаций примерно 1 млн лет назад, незадолго до того, как заканчивается палеоклиматический ряд, полученный по керну EPICA
processes in sea ice variations [26], changing the relationship between atmospheric temperature and the rate of accumulation and ablation of continental ice sheets, could also have played a role.
2. Changes in subglacial conditions that influence ice dynamics [6]. Geological observations reveal that the earliest northern hemisphere ice sheets had comparable extent but smaller volume before the MPT than after the MPT. Around the MPT, the crystalline Precambrian Shield bedrock became exposed by progressive glacial erosion, providing a higher friction substrate that enabled thicker ice sheet buildup and modified its response to orbital forcing.
The different formulated hypotheses to explain the MPT imply different amplitude and phase lags between orbital parameters, ice volume, climate at different latitudes and atmospheric CO2 concentrations. Obtaining continuous records of Antarctic temperature and accurate atmospheric composition (with higher resolution and accuracy than estimates from marine core boron isotopes for paleo-CO2 (e.g., [9]) back to the pre-MPT era is therefore essential to test the existing theories and to develop a data-based understanding of the last major Quaternary climate transition [12]. This urges us to obtain longer ice core records of Antarctic climate and global greenhouse gases.
The International Partnerships in Ice Core Sciences (IPICS), an international strategic programme gathering 25 nations and supported by IGBP/PAGES and SCAR, has identified for the next decade this major challenge: obtaining replicate Antarctic ice core climate and atmospheric composition records at least 1.5 Myr back in time. This requires:
1. To model the ice flow in order to identify the best locations where to find undisturbed records of climate back to 1.5 Myr, i.e. places with very low accumulation rate, simple ice flow regime, and sufficient ice thickness. This goal is currently being addressed by several international teams, using new data obtained during the International Polar Year 2007-2009.
2. To qualify the right location for deep drilling through radar data interpretation and in situ depth profiles of key climate variables.
Today, deep drilling operations are heavy and expensive: temporary camps to house ~30 drillers and scientists for several summer field seasons, transportation of tens of tons of drilling equipment including drilling fluids. The progress of deep drilling is at best 1 km/year, therefore requiring at least three drilling seaso
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