Palaeoenvironmental History of the Last Glacial Cycle
1. Resumé
The principal focus for this project will involve ice-core drilling to reconstruct well-dated, high resolution time series of climate in Antarctica during the past 2000 years, contributing directly to the objectives of IGBP/PAGES (Eddy, 1992). There will also be a continuing effort to complete collaborative studies on the Greenland GRIP core and to develop the BAS programme towards a future European deep drilling programme in Antarctica aimed at extending these investigations through at least the last glacial cycle.
The 2000-year time-frame spans at least two periods of extreme climate fluctuation in the post-glacial warm period (Holocene), including the European "Little Ice Age" and the so-called "Mediaeval Warm Interval". The aim is to achieve an improved characterisation and dating of the natural climatic fluctuations that have occurred under similar boundary conditions to the present, and to obtain evidence on other factors that may have been connected with these changes. Geographically, the project will centre mainly at sites within the Atlantic sector of Antarctica. The BAS effort will contribute to our knowledge of the geographic scale and synchrony of such events, and may give evidence on causative factors. The project will include collaboration in deeper drilling, so that these records can be placed in a longer perspective, and processes examined under the much more radical climate re-organisations that have occurred on glacial-interglacial time-scales.
The investigations will be enhanced by parallel efforts within project ICD1 to improve the calibration of ice-core parameters and to set the data from individual drilling sites into a larger scale context, and by efforts to understand the air/snow transfer function in project ICD7 and the longer-range processes occurring during transport in ICD6. Contributions from the ice-modelling group will assist in the evaluation of sites for drilling, and should strengthen the strategy for dating ice cores at greater depth by placing constraints upon the age-depth relationships using physical principles such as continuity and momentum balance.
These activities are seen as a precursor to deeper drilling, planned as a wider European collaborative venture to achieve records of climate and ice sheet history extending into and beyond the last ice age, where data will be obtained that will help to elucidate the processes involved during the much more radical climate reorganisations of the last climate cycle. Preparative steps towards this larger venture (EPICA) are planned within the next five years, although associated shallow ice-core studies, which form part of the reconnaissance phase of EPICA will be undertaken as part of project ICD1 which is focused on the climate of the past 100 years.
2. Scientific Background
Ice cores can provide uniquely detailed evidence on past climate and in addition information about potentially linked environmental changes, on time-scales ranging from decades up to several hundred thousand years (Oeschger and Langway, 1989). Shallow to medium-depth ice core records have proved a valuable means to extend temporally the instrumental records of climate, contributing to a firmer characterisation and understanding of natural climate variations, so that the significance of on-going trends in relation to greenhouse gas forced warming can be assessed. Several periods of sustained and apparently geographically large-scale climate shifts have occurred during the past 2000 years. Ice cores can contribute to our knowledge of the global signature and synchroneity of climate changes during these periods, which should help to identify some of the causes of these fluctuations, and to elucidate the forcing mechanisms involved, in turn leading to more reliable computer models of climate change. Ice-core records therefore feature prominantly in the strategy of the PAGES project of IGBP.
The Antarctic Peninsula region is of special interest because it lies close to the Weddell Sea, one of the largest areas of sea-ice production in Antarctica. It therefore offers a unique observational platform to study the role played by sea ice and ocean-atmosphere exchange processes in climate change of the Southern Hemisphere (Peel and Mulvaney, 1992). In addition the region has several features that offer significant advantages for ice core studies, including a high annual snowfall rate which can allow accurate dating of individual annual layers through many hundreds of years (Mosley-Thompson, 1992; Peel, 1992), and a relatively dense network of weather stations, with records extending back to at least the International Geophysical Year (1957), that can support the calibration of the ice-core records. Recent confirmation of rapid climate events in at least the Greenland region, believed to be associated with sudden shifts in the strength of the thermohaline circulation in the Atlantic Ocean, have given impetus to the search for parallel changes in the southern hemisphere eg as in plans for EPICA (Jouzel et al., 1994). Areas of Antarctica strongly influenced by conditions in the South Atlantic Ocean should be well placed to detect signals of changes in the deep circulation of the Atlantic Ocean. Examination of the behaviour of the sea ice zone during significant climate events of the Holocene, and of the way that evidence for these changes has been recorded in ice cores over the past 2000 years, will form a foundation for interpreting the much larger shifts that have characterised a large part of the last glacial period.
Shallow to medium-depth ice cores from the Antarctic Peninsula region have so far demonstrated the distinctive regime of this sector of Antarctica, with larger trends and greater interannual variability than observed elsewhere, as seen also in the recent instrumental records (Sansom, 1989; King, 1994). The climate of the region appears to be largely decoupled from the remainder of Antarctica, in part reflecting strong sensitivity to variations in the circumpolar westerlies (Raper et al., 1984). The isotopic and chemical content of cores from low altitude sites, especially in areas adjacent to the Weddell Sea, have been revealed as sensitive indicators on variations in the ice cover in this zone (Peel and Mulvaney, 1992), and have the potential to yield important information on ocean/atmosphere exchange processes and their coupling to climate change. On the other hand evidence from the higher altitude sites appears less sensitive to such local influences and may provide the best representation of climate change in the region. A balanced approach to investigation of both low and high altitude sites should therefore help to elucidate complementary aspects of the description of and processes involved in climate change.
3. Research and Methodology
We shall address progressively longer time-scales so that the results of deeper drilling will benefit from a more thorough understanding of the more recent record and its relationship to climate trends recorded at a similar resolution by other media and in other parts of the world.
The last 2000 years [200-300 m drilling on Berkner Island]
The Weddell Sea zone has been identified as an area of particular interest in relation to detecting changes in the circulation of the Atlantic Ocean in the past and in ocean-atmosphere interactions in the marginal ice zone, which may be indicative of larger scale changes (Peel & Mulvaney, 1992). Berkner Island is an excellent platform to obtain a stable, high-resolution ice-core record that will be closely focused on the behaviour of the Weddell Sea (Wagenbach and others, in press). Drilling at both the north and the south domes is being planned as a joint project with Alfred-Wegener-Institute, to start in 1994/95, and should yield records up to 2000 years long. These records will complement those spanning a similar period from the Antarctic Peninsula, and help to establish a linkage to the Queen Maud Land sector of Antarctica, identified as a target zone for future European deep drilling.
The first objective is to undertake intermediate depth (150-300+ m) drilling at two summits, which provide sites with significantly different snow accumulation rates, and at different distances from local moisture and aerosol sources in the Weddell Sea. The drilling activity will be supported by shallow pit and core studies [ICD1], by a geophysical survey, and by the installation of an automatic weather station, with the possible later addition of an aerosol collection station [ICD7].
Comparison of the record with contemporaneous ice core records currently being analysed from the Antarctic Peninsula and from equatorial latitudes, will help to establish the significance of the polar climatic records in terms of more global climatic fluctuations. Low mean annual temperatures (~-25C) will ensure that there is generally negligible summer melting, which can complicate the interpretation of ice core data. A broad spectrum of analyses will be undertaken on the cores to characterise features of both the physical and chemical climate. Whilst some measurements will be made "in situ", the cores will be returned to laboratories in UK and Germany for detailed analysis. A particular effort will be made to achieve maximum accuracy in the dating.
The Holocene and late-glacial period [Drilling to bedrock on Berkner Island or Dyer Plateau]
A logical extension of the 300 m drilling at Berkner Island will be to achieve bedrock in a future (~1000 m) drilling. A preliminary flow-model indicates that a record of the most recent Greenland rapid climate event, the warming at the end of the Younger Dryas (~10.7 ka BP), should be preserved at South Dome, Berkner Island. A drilling to bedrock should therefore reveal, at perhaps the most sensitive location possible in Antarctica, whether it is possible to detect changes in the strength of the Atlantic thermohaline circulation, a postulated cause of the rapid climate events recorded most strongly in Greenland throughout the last glacial. The site will also allow an accurately dated record of the complete Holocene sequence, including the Climatic Optimum, when temperatures may have been 1-2C warmer than now. The site is also of glaciological interest as deeper parts of the core should provide clues to the behaviour of the Filchner-Ronne ice shelf during the LGM and early Holocene.
A record spanning at least 2000 years (and preferably the entire Holocene) is also needed from the spine of the Antarctic Peninsula. Dyer Plateau has been shown by Thompson and others (in press) to preserve an excellent stratigraphic record, capable of precise dating for at least many centuries. A high priority is to establish a detailed record of climate through the Holocene in this critical region adjacent to the marginal ice zone, together with a coarsely-resolved record of the deglaciation sequence. The record would form an important link in a chain of proxy records extending pole to pole, where a particular effort will be made to identify leads and lags between climatic shifts in different localities, possible pointers to the source areas of climatic changes. Drilling to bedrock (500-1000 m depth) is being considered as a further option for a possible collaborative venture.
No firm plans have yet been drawn up for either bedrock drilling, although options for drilling at one or both sides are being considered for the period up to the start of planned European deep drilling in Dronning Maud Land. If there are significant delays in obtaining funding for EIPCA then these somewhat smaller projects, involving 2-3 nations, could help to maintain momentum in the European ice drilling community. It is possible that major outlay on new drilling equipment could be avoided by borrowing the Australian "Istuk" type drill for 1000 m drilling.
The last climatic cycle and beyond [EPICA - European Project for Ice Coring in Antarctica]
BAS will be a full partner in EPICA, a long-term (~7 years) deep ice-core drilling project to derive high resolution records of climate change in Antarctica through up to several glacial to interglacial cycles. There will be particular emphasis on the periods of rapid and highly non-linear climatic variations during both the last glaciation and apparently during the Eemian interglacial in Greenland. In order to achieve its goals, deep drilling is planned at two sites, Dome Concordia and Dronning Maud Land, in East Antarctica to achieve the required temporal resolution on different time scales and to ensure a continent-wide perspective. The strategy for BAS ice-core activities will be closely coupled to the progress of funding of the various stages of EIPCA. However, more theoretical studies will consider how the strength and relative importance of various transport processes may have varied during the major climate shifts of the last glacial cycle, impacting atmospheric chemistry and in turn the ice-core record. These studies may include some modelling of the Antarctic stratosphere, with the possibility of also modelling some aspects of Glacial Maximum tropospheric chemistry later in the five-year period.
The first Phase of EPICA proposes a deep drilling at Dome Concordia, with a geophysical/geochemical reconnaissance of the Dronning Maud Land region (Atlantic Sector) occurring in parallel. Dronning Maud Land is a relatively unexplored part of Antarctica and currently lacks the basic geophysical data needed to model ice flow across this sector of the ice sheet. The aim of the reconnaissance will be to obtain the essential geophysical and geochemical information needed to locate a deep drilling site that will provide an optimum record through the last glacial cycle in the sector of Antarctica bordering the Atlantic Ocean.
The reconnaissance plans to follow a corridor several ice-thicknesses wide, along the ice divide through DML leading up to Dome F. BAS plans to contribute to the exploratory shallow coring activities that will be used to establish the present-day climate gradients across DML, and especially to identify the characteristics and range of influence of air masses tracking from the Atlantic Ocean. These studies will characterise the spatial and recent temporal pattern of deposition of a range of constituents that will be examined in the future deep core [Project ICD1]. The reconnaissance may also allow scope to carry out a remote aerosol and snowfall sampling programme in support of transfer-function studies [see Project ICD7] which will be needed to back up the interpretation of the future deep core.
BAS also proposes to contribute to the large-scale geophysical investigations that will be needed to characterise adequately this sector of the Antarctic ice sheet, in particular to the airborne radio-echo sounding programme needed to profile ice thickness along the ice divide. Ground parties could also deploy appropriate surface markers to help calibrate satellite data, eg. SAR. These data would be used to select the most appropriate drilling sites and provide input to numerical models of the area. It is now understood that ice flow near to divides and domes is not at all well described by the simple approximations used successfully in other areas of ice sheets, and that more refined models will be needed to constrain age-depth relationships in a mathematically more consistent manner. Such models may also help to explain some of the disruptions in the ice core record near bedrock, induced by flow over rugose bedrock.
The Dome C drilling will offer the first prospects for a multiple glacial-cycles' drilling at a dome site in Antarctica, where the Eemian interglacial should be recorded at comparable or better resolution than in the GRIP/GISPII cores. It will therefore be important for BAS to be involved - but in a way that will not overcommit our human resources. In the first instance this could involve deployment of the DEP with or without a BAS person on-site, together with some comparative detailed analysis of a range of ices covering different types of "chemical event" - to further improve the fundamental "calibration" of the DEP technique. Also, there may be scope for some SEM investigations on the more acidic ices that will arise from Dome C compared with the Summit core.
Drilling at the second site in the Atlantic sector is not likely to start before the end of the decade, and its detailed objectives are not yet defined. BAS should be a major player in this activity, which is likely to dominate our ice-core activities for several years once it is underway.
4. Wider implications
The proposed work aims to contribute to a more reliable description of past climatic changes, and to obtain evidence on possible causes, in a sensitive part of the global climate system. This will help us to identify the effects of anthropogenic climate forcing on present-day climate change and will in addition provide data needed for more rigorous testing of computer climate models, so that they can be used more confidently to predict future climate. More reliable long-range climate is needed to plan for future global economic development, so that suitable strategies to reduce possibly undesirable impacts of climate change can be devised, with least damage to quality of life.
5. References
Oeschger, H. and C.C. Langway, Jr. Eds. 1989. The environmental record in glaciers and ice sheets. Chichester: John Wiley, 287-318.
Eddy, J.A. Ed. 1992. The PAGES project: Proposed implementation plans for research activities. Global Change Report No. 19. IGBP Stockholm.
Jouzel, J., C. Hammer, H. Miller, G. Orombelli, D.A. Peel and B. Stauffer. 1994. European Project for Ice Coring in Antarctica. Science Plan. Prepared by EPICA Science Working Group.
King, J.C. 1994. Recent climate variability in the vicinity of the Antarctic Peninsula. Int. J. Climat. 14, 357-369.
Mosley-Thompson, E. 1992. Palaeoenvironmental conditions in Antarctica since A.D. 1500: ice core evidence. In Climate since A.D. 1500. Eds. Bradley, R.S. and P.D. Jones. London, Routledge, 572-591.
Peel, D.A. 1992. Ice core evidence from the Antarctic Peninsula region. In Climate since A.D. 1500. Eds. Bradley, R.S. and P.D. Jones. London, Routledge, 549-571.
Peel, D.A. and R. Mulvaney. 1992. Time-trends in the pattern of ocean-atmosphere exchange in an ice core from the Weddell Sea sector of Antarctica. Tellus, 44B, 430-442.
Raper, S.C.B., T.M.L. Wigley, P.R. Mayes, P.D. Jones and M.J. Salinger. 1984. Variations in surface air temperatures: part 3. The Antarctic. Mon. Weath. Rev. 112, 1341-1353.
Sansom, J. 1989. Antarctic surface temperature time series. J. Climate, 2, 1164-1172.
Thompson, L.G., D.A. Peel, E. Mosley-Thompson, R. Mulvaney, J. Dai, P.N. Lin, M.E. Davis and C.F. Raymond. (In press) Climate since 1500 AD on the Dyer Plateau: evidence for recent climate change. Ann. Glaciol., 20.
Wagenbach, D., W. Graf, A. Minikin, U. Trefzer, J. Kipfstuhl, H. Oerter and N. Blindow. (In press) Reconnaissance of chemical and isotopic firn properties on top of Berkner Island, Antarctica. Ann. Glaciol. 20.