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Igloo U
Analytical Chemistry of Ice Cores The information you get from a 100,000-year-old glacial record depends on how you sample it
T
he Arctic Circle is about as far as you can get from the South Pole. But both of these cold, remote, and relatively barren regions provide irreplaceable opportunities for the analytical chemist. Both areas are home to the bestpreserved records of ancient climate conditions on Earth: glacial ice that is thousands of meters deep and more than 100,000 years old. Each year, a new layer of snow and ice traps dust, pollen, atmospheric radioisotopes, and gases in the ice, and each year the weight of the ice and snow in the upper layers compresses the lower layers. The result is an annual record that goes much further back in time than tree rings or carbon dating systems do. The composition of gases trapped in the layers of ice, isotope ratios for oxygen and carbon, trace metals, organic compounds, pollens and dust, and acidity of the ice can all indicate changes in temperature, precipitation, and atmospheric composition over time along with specific global events. Mapping them through time requires careful sample handling under difficult conditions and accurate dating of the layers.
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F ocus
The glacial record was impossible to sample until about 40 years ago. The first ice cores were drilled in the mid-1950s but weren't made available to the analytical community at large. In 1958 the "Little America V" core, the oldest one archived in the United States, was drilled from the Ross ice shelf in Antarctica. Much of it is still available for analysis at the new National Science Foundation (NSF) Na-
Tales from the Frozen North
In the time since his team began drilling the GISP2 core, Paul Mayewski of the University of New Hampshire and colleagues across the country have already analyzed enough of the core to make a number of discoveries about climate and oceanic changes during the past 110,000 years. Statistical correlation of analyte concentrations in short cores and surface-level "snow pits" dug near the GISP2 core with known recent climate and ocean conditions allows the researchers to model fluctuations in sea surface temperatures, air mass movement, solar activity, and other parameters in the long core record. From a central section of the core Mayewski's lab has measured Ca2*, MgK K*. NH; Na+, CI", NOi, and SOi by ion chromatography at nanogram/ gram concentrations. One of every 10 samples was analyzed in thefieldduring drilling, Mayewski says. Because the chromatograph doesn't work well in the extreme cold, the measurements were made in a van kept at 25 °C rather than in the core processing trench. "By now my lab has measured all eight major ions at 20,000 levels spanning the length of the core," says Mayewski. Other researchers have measured methanesulfonic acid (MSA), examined insoluble particles by scanning electron microscopy and laser light scattering, and used accelerator MS to determine oxygen and carbon isotopes, deuterium and deuterium excess, and cosmogenic isotopes. These analytes, along with C02 and CH4, provide signatures for specific
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tional Ice Core Laboratory (NICL) in Denver. Newer cores such as the Greenland Ice Sheet Project II (GISP2) core, which was completed in 1993, and samples from the Russian-French Vostok core from Antarctica are also stored there. NICL curator Geoffrey Hargreaves says, "NICL is the U.S. archive for all the NSF cores. We store everything from the Little America V core through the Taylor dome
sources and weather conditions. Calcium, for instance, is blown onto the ice in continental dust; high concentrations indicate aridity and high wind speed. NH4 and K\ taken together, come from large biomass-burning events such as forestfiresand can indicate periods of drought. Similarly, Na+, Cl~, and MSA come from marine sources. C02 and CH4, from biological sources,fluctuatewith temperature. Different concentration profiles for combinations of these analytes make it possible to trace the movement of individual air masses. Cosmogenic isotopes such as 10Be, 36 CL and 26A1 and spikes of transient stratospheric isotopes such as 7Be have also been found in the ice core layers. The researchers believe the extremely rare 7Be reaches the Earth during stratospheric injections into the troposphere and therefore marks solar flare events. The record of changes in 180 is being used as a "paleothermometer" after calibration by comparison with borehole temperatures, and deuterium and deuterium excess appear to fluctuate with ocean surface temperatures, wind strength, and other factors involved in the North Atlantic Oscillation, a regular seesaw in winter temperatures between western Greenland and northern Europe. Mayewski says that these indicators may explain some notable historical events. A model of the oscillation cycle derived from ice core measurements indicates that the Spanish Armada may have sunk because it set out to attack Great Britain during a year in the cycle when the seas were particularly rough around Europe. Had the
Analytical Chemistry, September 1, 1995
core drilled last year in Antarctica and a Siple dome test core for the West Antarctica Ice Sheet (WAIS) project." He estimates the facility currently archives ~ 45 "named" cores and shorter test cores. "At this point, we have ~ 11,300 m of ice." To get to the glaciers, the NSF Office of Polar Programs, along with the Department of Energy, the Department of Defense, the National Océanographie and At-
Spanish sailed a year or two earlier or later, they might have changed the course of history. Not surprisingly, the GISP2 core also contains some hard lessons about recent times. "The levels of nitrate and sulfates for the last hundred years are dramatically higher than for thousands of years before," Mayewski says. "We do see a decrease with the advent of the Clean Air Act." In the basemap project he's planning to start in Antarctica, analysis of the short 200year cores may identify the most sensitive sites for monitoring proxy indicators of ozone depletion. Comparison of analyte records from GISP2 and from the Antarctic Taylor dome core shows substantial correspondence for global atmospheric indicators such as C0 2 over extended periods of time. "Knowing whether, the two polar regions are synchronous or not means you may have to look for a global forcing agent, e.g., solar variability, to account for some of these events," Mayewski notes. In addition to the long temperature and atmosphere cycles, WC levels indicate regular climatic variations at 2200-, 550-, and 208-year intervals, and there are markers for 12- and 22-year solar cycles as well. "The common assumption is that the natural climate is constant," Mayewski says. "It's clear from the record at this point that that's not so. I think we're close tofiguringout what the natural climate would have done for the past 200 years without the increase in human activity. We can extrapolate into modern times from the older record and subtract out what did happen."
mospheric Administration, the National Center for Atmospheric Research, and other federal agencies, sponsors a number of expeditions to the Arctic each year and maintains the better publicized McMurdo research station in Antarctica. Established consortia for polar research include Paleoclimates of Arctic Lakes and Estuaries (PALE), the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL), the National Ice and Snow Data Center, the Institute of Arctic and Alpine Research, Byrd Polar Research Center at Ohio State University, and groups at Duke University and the Universities of Alaska, Colorado, Miami, New Hampshire, Rhode Island, and Washington, among others. Usually, researchers raise the money for a large project by calling on their colleagues to send in grant applications to NSF and the other federal agencies for work to be done on a prospective core. They use a variety of established methods, including IC, GC, HPLC, MS, light spectroscopy, atomic spectroscopies, and electrochemical methods, to analyze melted samples taken from specific regions of a core segment. Patrick Webber, director of Arctic System Science (ARCSS) research in the NSF Office of Polar Programs, says that although the NSF grants are awarded to individual research groups, NSF has established two general contracts for the projects, one with NICL for centralized ice core archiving, the other with the Polar Ice Coring Office (PICO), which is based at the University of Nebraska, for drilling and logistics.
is usually at the top of an ice cap, where ice may be 3000 m thick and was most likely deposited straight down in orderly and coherent layers. On the slope of a glacier, ice flows, avalanches, and runoff from higher up may produce an ice core with disorganized layers that don't allow date assignments. The drilling area may be narrowed down using ground-based radar systems. Resolution and length of the time record are two other important factors. The WAIS researchers are looking for two different types of drill sites: one with high layer and time resolution, comparable to that of the GISP2 core, the other with more layers for a given depth and therefore less spatial resolution than the GISP2
Volcanic ash and radioactive spikes produced by nuclear bomb tests are specific benchmarks for dating.
core. "For a high-resolution core, you want an area where it snows a lot every year. For a long time record, you want an area where it doesn't," Taylor says. Once a site is chosen, the expedition team drills several short test cores of up to a few hundred meters in length, and then the drill for the long ice core is set up. Drilling is a slow process; during the few Drilling down the years summer months when the ice is accessiObtaining a deep glacial ice core starts ble, a full season of drilling may produce with aerial radar surveys to locate a suitable drill site. Ken Taylor of the Desert Re- only ~ 1000 m of ice core, so a single long search Institute in Reno, NV, is one of the core can take several years to complete. researchers involved with the prospective Ice cores were originally drilled using WAIS project, which is now at the siting equipment leased from oil companies. The stage. "We use ice-penetrating radar in an current method uses an electromechaniaircraft at 150 knots to scan the refleccally driven wire line drill that PICO develtions office with different chemical charac- oped specifically for the GISP2 core. The teristics. It allows us to detect time horinew drill produces cores that are 5.2 in. in zons such as volcanic eruptions," by the diameter. iV-butylacetate is used as a drilldust layers settled in the ice. "Basically, we ing fluid to flush ice chips out of the cavity can map glacial features the way seismol- and prevent the hole from collapsing ogists map geological features for earth- inward. The solvent is a vast improvement quake prediction." He says the ideal site over the older alternatives such as kero-
sene and arctic diesel that was densified with fréons, Taylor notes. "Kerosene is a variable mixture with a lot of compounds. With some of the environmental analyses on the core samples, in particular, you might get a peak, go to check it against the kerosene list, and find that the compound you had was also present in the kerosene. Butylacetate is a single, readily identifiable compound, and it isn't of interest as an analyte." Timing is everything
Date assignments, at least preliminary ones, are performed at the coring site as the sections of ice core are drilled. Chronological transition zones that indicate when the climate has changed (e.g., the transition from the Younger Dryas to the Holocene era) are identified through density studies. Generally, dates are assigned visually by counting annual layers of physical markers such as dust and pollen, much the way tree rings are counted. Benchmarks for specific known events are also sought to provide a frame of reference; examples include ash for volcanic eruptions or radioactive spikes from isotopes released into the atmosphere during nuclear tests. However, the layers are not evenly spaced all the way down the core. At the top under the snow, says Hargreaves, the first 300-350 m is "firn," an open porous structure like expanded styrofoam. In this region of the GISP2 core, which has very high resolution, annual layers start out 20 cm thick, or 5 years/m. Further down, firn turns into ice. The middle of the GISP core records ~ 30 years/m. Further still, 40,000-year-old layers are ~ 1 cm thick. Near the bottom of the core, he says, "The researchers began tofindstructural problems—one layer would flow into another. At that point, they had to stop counting and model the rest of the ice core to the bottom to estimate the number of years it represented." The GISP2 core represents more than 200,000 years, 110,000 of which can be dated coherently. Processing and sampling
Preliminary processing methods at the coring site are critical for preserving the integrity of the cores. For example, the ice must be kept below -15 °C in order to keep gases trapped in the layers from mi-
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Focus grating. To keep everything as cold as pos sible, the research team digs a trench and puts up a plywood roof covered with snow to create a tunnel where processing can take place. As it's drilled, the core is brought up in 6-m sections, where possible, and cut into 2-m segments. Then thin slabs are sliced lengthwise from the sides of the core with a bandsaw for sampling, the core sec tions are sealed and labeled in polyethyl ene sleeves, slid into vapor-barriered and aluminized cardboard tubes, and stored in insulated shipping containers to send back to NICL by ship, "cold deck" (no heat) airplane flights, and freezer trucks at the end of the season. NICL, which was built in Denver two years ago to receive cores from the previ ous national ice core lab at the State Uni versity of New York-Buffalo, has its own core processing line and examination room that are kept at -20 to -24 °C, in addition to the main storage freezer, which is kept at -36 °C. The new facilities, which are run jointly by NSF, the U.S. Geo logical Survey, and the University of Col orado-Boulder, make it possible to do more of the processing later and to send fewer people out to the coring site, which should reduce the overall cost of an expe dition.
Major ions, MSA, stable isotopes (Ο, Η), and insoluble particles 8 cm 2
Stable isotopes and structural properties 8.6 cm 2 I Archive 10 cm2
Cosmogenic radionuclides 3.2 cm2
The research team usually takes sam ples from the core for its own use on site, but most of the core goes to NICL along with some packages of ice chips. Re searchers have to be careful not only to leave some ice from the same sections intact for other groups who chipped in for the project to study, but to take the ap propriate samples for their work. The vast majority of chemical analyses are per formed on melted samples from the core; once taken, the spatial resolution for those samples is irreversibly fixed. A group measuring oxygen isotope ra tios may need only a few microliters of melt water from each time interval but re quire extremely clean portions of the ice core to sample from. On the other hand, the liters of melt water they discard to get to the clean inside portion of the core are carefully packaged and shipped to other researchers who need bulk (e.g., 2-L) samples to measure 10Be at ultratrace concentrations but whose studies are un affected by the impurities in the outside ice. In practice, very little goes to waste; investigators get permission to take only as much as their proposals indicate, and sample sharing is often coordinated by the lead investigator for a project or by a pre formed committee within the ice coring community, says Hargreaves. Excess
C0 2 , CH4, u c or archive 23.3 cm 2
I
Laser light scattering 11.2 cm2
Isotopes of gases or archive 47.1 cm2
Archive 23.3 cm 2
Stable isotopes 2.3 cm2
ECM and visual stratigraphy on prepared surface
Figure 1 . GISP2 ice core cross-section diagram for primary analyses. Heavy lines indicate vertical saw cuts made on each piece of the core; light lines indicate cuts made at depths selected for sampling. (Adapted from "GISP2 Notebook," No. 3, Fall 1993, p. 4) 548 A
Analytical Chemistry, September 1, 1995
sample melt water is even shipped back to NICL for archiving. Taylor says coordinated sampling strat egies between research groups are impor tant to ensure consistent time mapping of different analytes within the same core. "Paul Mayewski [University of New Hampshire], the chief scientist for GISP2, has been working very hard to get every one to sample as much as possible from the same exact piece of ice," he says (Figure 1). This means that researchers from several groups travel to NICL and take samples from the same cross-section of ice at the same time. "Otherwise," he explains, "you have to slice up the ice core segment lengthwise to divide it up. It's harder to compare results that way, be cause small depth errors across the layer can lead to differences in time assign ments." Several methods are used in different laboratories to clean the ice. These range from simple scraping to melting either the inside or outside of the core. For trace metal determinations, says Taylor, some groups use elaborate cutting protocols that require cutting with progressively cleaner handsaws. The handsaws are tested for cleanliness by cutting deionized water ice and testing that ice for trace-metal con tamination before cutting the core. Drill ing fluid is usually less of a problem, says Hargreaves; at the coring site, the cores are allowed to sit so that most of the drill ing fluid evaporates, and the remaining traces can often be sublimed at the labs or NICL. Analytical methods Most chemical analyses are performed on melted samples more or less as they are for ordinary aqueous samples, but that doesn't mean analysis is completely straightforward. For one thing, the sample size and whether the method used is a continuous or batch process may affect the time resolution for a particular analyte. Absorption spectroscopies, for instance, can be performed continuously on a lengthwise sample as it is melted a little at a time from one end. Batch methods re quire separate samples to be taken from a number of layers to maintain the time resolution; this gets more difficult as the layers thin out in the older ice. Dust parti cles in the layers can be characterized
by laser light scattering at 90° or using a Coulter counter, which sizes particles easily but has poor inherent spatial resolution because it operates in batch mode. In all cases, says Taylor, methods are refined on the shorter test cores before they're used on the long cores. Melting the samples before analysis is convenient but presents fundamental problems in some cases. Taylor, who determines annual acidity levels as a measure of variations in atmospheric nitrogen and as a signal for volcanic eruptions and forest fires, says, "When you melt the ice, the dust can dissolve into ionic species, so you have to worry about the relationship between the acidity of the ice and the acidity of the melt water." To get around the problem of the phase change, he has devised an instrument, housed at NICL, to measure electroconductivity and dielectric properties in solid sections from the transition portion of the core. These are some of the few chemical analyses made on the solid cores. The sides of the ice core are microtomed to smooth them and the core sits horizontally on a long electrode while a carriage on rails moves two other electrodes side by side along the core. To measure acidity, dc electroconductivity measurements are taken between the two moving electrodes, with a potential difference of 2 kV. Spatial resolution along the core is < 1 cm. "In the solid phase," he notes, "most ions are locked in the ice lattice, so you only measure protons." To determine the total ion content of the core, dielectric properties are measured at frequencies ranging from 10 kHz to 1 MHz using the long electrode under the slab with an electrode on the moving carriage; the ice acts as the capacitor. The system is set up to keep the ice temperature stable to within 1-2 °C during measurement. One problem of working directly with the solid ice core segments, though, is the lack of a suitable calibration standard. "Glacial ice is a series of crystals (Figure 2) with a very different distribution from that of modern ice," says Taylor. "If you pour water into your freezer, you get a totally different substance with different electrical properties. No one has come up with a good calibration ice analogue." To calibrate his instrument, he takes natural
modern ice and measures the electroconductivity, then melts the ice and performs strong acid titration to measure proton concentration. Although most of the analyses are done in-lab, says NSF's Webber, "The big breakthrough in recent years has been the ability to do more of the chemistry in the field. Until recently, the only chemical measurements we could make were for universal indicators like CH4, C0 2 , and ozone with instruments launched in planes or balloons." Taylor's group, for instance, performed IC and electrochemical measurements in Greenland at the GISP2 site but, he says, the temperature and power stability in the processing trench were limited. However, says Webber, the introduction of portable IC and GC instruments, many of which now have their own power supplies, may make it possible to extend what can be done at the coring site and to bring results home along with the cores. "In 35 years of Arctic exploration, I've seen the speed and the detail of analysis increase enormously. It's changed the depth of our understanding for the natural processes taking place." T h e global record
One objective of archiving the NSF-sponsored ice cores at NICL is to allow researchers to go back and compare one ice core with another to map regional and global patterns for a given period. Hargreaves says some researchers are still requesting to look at ice from the 1968 Byrd core. Mayewski and other GISP2 researchers have developed statistical methods for correlating analyte concentrations with climate and ocean events. The GISP2 core appears to correlate well with another deep core nearby and with the Taylor dome core in Antarctica. A somewhat different Arctic program, PALE, is working to develop specific protocols for intersample comparisons on drilled lake sediment cores. These include modeling the relationships between the observable sediment layers and time, synthesizing data from cores taken at different sites, and calibrating paleoclimate processes in the sediment records using modern meteorological information. Mathieu Duvall, data coordinator for PALE, remarks that long ice cores are too expensive and difficult to drill in high
Figure 2. Single ice crystals in a horizontal thin section taken at 130.12 m from the Taylor dome ice core. Crystal size increases steadily with depth.
enough numbers to span the Arctic for local mapping, but numerous lake sediment cores have been drilled in areas where the sediment has formed identifiable annual layers of pollen and other substances. The lake cores go back only about 20,000 years, but there are ~ 150 lake core sites around the Arctic at this point. Duvall says PALE researchers are working to calibrate markers such as modern surface pollen deposition so that local variations (e.g., nonnative pollen blown into an Arctic region from the south) can be excluded. He and the PALE researchers are also in the process of developing a set of standards for reporting data and entering records in the PALE data archive, which is held with the National Geophysical Data Center in Boulder, CO, so that archived data from all the laboratories can be compared directly with greater certainty. Webber says the Western Antarctic Ice Sheet, which Taylor's group has been surveying for ice core sites, is NSF's next large ice core project, but he also anticipates an internationally sponsored core site in northern Greenland in the next few years. Mayewski adds that his team intends to create a basemap of shorter ice cores from WAIS similar to the PALE project. As more cores become available, some of PALE's unifying strategies may be useful for comparing ice cores from different regions and with different levels of time resolution. Deborah Noble
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