COMPILED REPORTS OF THE
U.S. ICE CORE RESEARCH WORKSHOP
1.1 SPECIALTY GROUP REPORT: STABLE ISOTOPES
Stable isotopes in ice, d180, dD and deuterium excess (d = dD-8 d180), provide a set of tools for monitoring the hydrologic cycle at the time that the snow was deposited. These parameters track the history of water from its evaporation over the ocean to its deposition as snow at the ice core site. Stable isotope information in ice cores can be separated into two categories: (1) stable isotope ratios or delta values, d180 and
dD, and (2) deuterium excess, which is the combination of both delta values. Delta values are controlled primarily by the temperature difference between the evaporation site and the snow deposition site, and thus can be used to infer paleotemperature changes, changes in the elevation of the ice sheet and changes in the accumulation rate of snow on the ice sheet. It is important when examining time series of d180 and dD values in ice cores to remember that conditions at the evaporation site,
i. e. air and water temperatures, isotopic composition of the surface ocean and multiplicity of moisture source areas, must always be considered. Deuterium excess values are controlled primarily by conditions at the moisture source region, specifically the air and sea temperatures, the moisture content of the atmosphere and the turbulence of the surface boundary layer. Deuterium excess values can be used to reconstruct changes in meteorological conditions over the ocean as well as to infer shifts in the moisture source regions from one area of the ocean to another. While deuterium excess has only recently been employed in ice core research, our understanding of its response to paleoenvironmental changes is improving rapidly. These stable isotope tools are useful on all of the time scales available in ice cores. 5180, 8D and deuterium excess commonly exhibit seasonal cycles, and can be used to track Holocene climate changes. 8180 has been one of the primary tools in identifying glacial/interglacial transitions as well as shorter term, rapid climatic reversals.
In low and mid-latitude glaciers, the relationship between d180 and dD and surface temperature is not as simple as on the high latitude ice sheets. At low latitudes a d-T relationship may be weak or non-existent. In these areas, isotopic fractionation accompanying weaker vapor loss from air masses during their crossing of broad continental areas where plant transpiration and surface evaporation can add to the atmospheric moisture must be considered. Nonetheless, the same basic reasons for pursuing stable isotopes in deep ice cores on the ice sheets apply to these glaciers/ice caps; namely, the length and temporal detail of the record. The scientific community should be aware that temperature is not the only signal contained in stable isotopes in ice cores, a fact which also applies to the deep ice cores in the ice sheets.
The overall objective is to obtain the maximum amount of paleoenvironmental information contained in the d180, dD and deuterium excess records in ice cores. To do this,, we must deconvolute the isotope signal and focus on whichever of the primary controlling variables is most important or most desired. These primary signals include:
- changes in climate on the ice sheet
- changes in climate at the moisture source area
- changes in elevation of the ice sheet
- changes in the location of the moisture source area(s) over the ocean
- ice flow effects, or changes in the site of snow deposition relative to the site of core collection.
Note that specific problems such as the discharge of ice and/or meltwater from the ice sheets into the moisture source areas are included in changes in climate at the moisture source area and/or on the ice sheet.
To accomplish this objective, combinations of parameters must be used. For example, the total gas content of the ice can be used to help constrain suspected elevation changes, and deuterium excess can be used in conjunction with d180 or dD to determine if changes at the moisture source area contribute significantly to the d180 changes seen in the ice core. Other paleoenvironmental records from outside of the ice sheets should also be employed. Examples include d180 and faunal assemblages in ocean sediment cores, geological evidence of glacial advances and retreats and other paleoenvironmental changes on the continents, and the wealth of paleoenvironmental changes recorded in lake cores.
The goal is to fully utilize the information in stable isotopes in ice cores, to sharpen and focus what can be learned. The ice core community realizes the pitfalls in simplistic interpretations of 8 values in ice cores. It is now time to go beyond such interpretations and extract more reliable and meaningful information from these isotopes.
III. Scientific Plan for State of the Art Measurement/lnterpretation
Recently, substantial changes have been made in how stable isotope values in ice cores are interpreted and in what is being measured. Improvements in theories of fractionating processes combined with a concerted effort on surveys of surface snow have led to more definite interpretations by the French of isotopes in the Vostok core, as well as wider use of the data, for example as an indicator of accumulation rates. The combination of isotopes in spatial surveys of surface snow and shallow cores, with isotopes in deep ice cores, has proven to be very successful. Another major change has been an increased focus on hydrogen isotope ratios,
dD, as a complement to the traditional d180 values and as a part of the measurement of deuterium excess values. At the present time there is no laboratory in the U.S. measuring dD and deuterium excess values in ice cores. This deficiency should be addressed in the near future.
Our recommendations for future research stress the importance of surface surveys and shallow to intermediate depth ice cores, as a necessary complement to deep ice cores. Isotope delta values from these samples can fulfill two important needs: the need for better calibration of delta values and deuterium excess by comparing these values with modem environmental records, and the need for more isotope records on the decade to century time scale. Changes in climate and environment taking place over decades to centuries are of most direct interest to assess man's impact on climate and the impact of climate change on man. The resulting d changes, however, are not well understood, due to a lack of data and due to the nature of the d changes: they are usually small, are superimposed on a spectrum of longer term climate changes, and unlike the major glacial-interglacial changes, the short term climate fluctuations may manifest themselves in different ways in different parts of the globe. Nonetheless, improving our understanding of these changes will greatly improve what we can learn from stable isotopes in deep ice cores.
Our longer range ice core research plan consists of the following elements:
A. A deep core to bedrock at/near Summit in Greenland to obtain a long paleoenvironmental record. This would be the first detailed terrestrial record covering several glacial-interglacial cycles and allow study of the three Milankovitch cycles of ~ 2 x 104, 4 x 104 and 105 years. To assure bedrock was reached one needs to penetrate bedrock and recover some bedrock core. It is highly desirable that the results from such a core can be compared with those from a core nearby, analyzed independently, to safeguard against analysis problems and ice flow related artifacts in the deep core record. Comparison of the two deep cores with each other and with existing intermediate and deep Greenland ice cores will indicate whether additional intermediate cores and/or cores penetrating the last glacial are needed. An array of shallow cores and snow pit studies is needed to determine spatial variability.
B. A deep core to bedrock in the Ross Sea drainage of West Antarctica to obtain a long paleoenvironmental record. This deep core will explore whether ice from the previous penulimate interglacial and preceding glacial is present in West Antarctica. This tests the hypothesis that the West Antarctic ice sheet disappeared during the previous interglacial, giving rise to ~ 6 m higher sea level. Interpretation of the results of this core will be facilitated by the high resolution record of the intermediate Siple Station core and the knowledge gained from ice dynamics studies at the Siple Coast. The interpretation of the isotopes in this core requires at least one additional core penetrating ice from the last glacial period and several well situated shallow cores and pits at varying distances to study present day spatial d variations. This spatial variability study as well as a survey of surface and bedrock topography, ice flow and accumulation should precede the deep drilling and lead to the selection of the most suitable drill site.
We do not address the problem of constructing a time scale for the core d records. We believe other techniques like continuous acidity and particle measurements, electroconductivity and/or H202 are more suitable than d analyses for this purpose. Detailed comparison of the Greenland and Antarctic long d records may separate global from local influences in both cores.
C. Though climate change is often global it may express itself differently in mid and low-latitudes than at the Poles. An array of cores at these latitudes is therefore required to show the changes in climate and environment at these more inhabited latitudes that correlate with recent changes in the polar d records. This defines spatial variability both zonally and
latitudinally. Comparison with local pollen, lake sediment and other paleoenvironmental records will help interpret the observed d changes. These cores provide transfer functions to translate the long records of polar d changes into global climate/environmental change. Ice caps and glaciers in Southern Alaska, the high Andes, the
Himalaya, China and New Guinea need to be studied. In general shallow to intermediate drilling will be sufficient to reach bedrock.
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