Studies of Material in Polar Ice

E. L. FIREMAN

Smithsonian InstitutionAstrophysical Observatory

The high-elevation regions of the polar ice sheetsare the most remote places on Earth. In these regions,sediments accumulate at the lowest rate on Earth(McCorkell et al., 1967), and the annual accumula-tion layers in the ice sheet provide excellent timemarkers for determining the deposition rate. Sedi-ments that accumulate at the lowest rate shouldcontain the highest percentage of extraterrestrialmaterial. However, even the most slowly accumulat-ing sediments may be predominantly terrestrial. Fine-grained dust is carried by winds over large distancesfrom its locality of origin. On the other hand, verylittle is known about the extraterrestrial material ar-riving at the Earth. The only substances definitelyidentified as extraterrestrial are meteorites. What isknown from a variety of studies (Whipple, 1961) isthat interplanetary material ranging in size fromless than 1 /L to more than 1 km in size enters theEarth's atmosphere and that much of it wholly orpartially disintegrates into dust before reaching theEarth's surface. If we could collect dust with asignificant extraterrestrial component in sufficientamounts to apply many analytical techniques, in-cluding isotope analysis, we might be able to demon-strate which fraction is extraterrestrial and learnabout its chemistry, mineralogy, amount in space,history, and origin. Since some of these studies re-quire grams of material, we undertook the collectionand study of material from large volumes of polarice.

Collections. We collected the soluble materialfrom 5 million liters and the insoluble material frommore than 10 million liters of melted ice. This was nottoo difficult because we were able to use two subsur-face wells at Camp Century, Greenland, from which10 1/day of water could be processed. Camp Cen-tury is located at 77°10'N. 61°08'W. at an elevationof approximately 2,000 m. One well (Fig. 1), usedfor the camp's water supply, was made by jets ofsteam. The melted water was almost immediatelypumped into three 5,000-gal aluminum storagetanks. Our first collection was made by removingabout 400 g of material from the bottom of thetanks after more than 10 million liters of water hadbeen stored in the tanks. Next, we installed a filtersystem between the well and the tanks and passed 5million liters of water through cellulose and nylonfilter papers of 8-, 3-, and 0.45- j pore size. Thematerial was later removed from the filter paperby dissolving the paper in an appropriate solvent

and centrifuging; 2.6>< 10 g of material was ob-tained per liter of water. Most of this material wasorganic: oil from the pump, rubber from the hose,and undissolved filter paper. The weight was re-duced to 6 X10-5 g/l by low-temperature ashing.There was no difference between the collections madewith different pore sizes or between those made oncellulose and nylon papers.

The second well (Fig. 2) was made by a heat ex-changer used to eliminate waste heat from the camp.After the camp stopped using this well, we installed apump and an electrical heater at its bottom andpumped the water through either a filter unit identicalto that used in studies at the first well or two ion-ex-change columns in series. Approximately 5 millionliters of water were passed through the filters, andabout the same amount was passed through the ion-exchange columns. One column contained 40 litersof cation exchange resin; the other contained 30 litersof an anion-cation mixture. The dissolved consti-tuents were collected from these columns with 50 per-cent or more efficiency.

Chemical and mineralogical description. The par-ticulates consist mainly of fine-grained clay, illite, andmontmorillonite, but include small amounts ofmagnetite, quartz, and feldspar (Marvin et al.,1967). The material collected from the settling tankis quite different; it contains an appreciable amountof magnetite and hematite. Certain heavy minerals,such as magnetite, would have concentrated on thebottom of the settling tank; however, the increasein concentration is so marked that it is difficult toexplain on the basis of the amount of water storedin the tank, unless there was a higher concentrationof "heavies" in the ice layers above a depth of 70 m(less than 200-year-old ice) compared with the 80-m layer (250-year-old ice). The illite appears to bewind-blown dust from the continents; the "heavies"appear to have an extraterrestrial component. Theconcentrations of oxides collected on the filter papersare SiO2 (51 percent), AI203 (23 percent), Fe2O3(14 percent), MgO (4 percent), K 20 (4 percent),Na20 (2 percent), CaO (2 percent), and NiO (<0.1percent). (These concentrations are similar to thoseof illite except for higher Fe and Na contents.)

The most interesting feature of the dissolvedmaterial is that it contains a much higher concen-tration of Ni than the particulates and has a Ni/Coratio of 17, which is close to that in meteorites. Thepositive ion concentrations in 10 mole/l are: Al,37; Na, 200; K, 21; Ca, 22; Mg, 31; Fe, 3.5; Ni,0.14; Co, 0.0064; Mn, 0.3; Cr, <0.05; and Ti,<0.13. The Ni/Fe and the Ni/Co ratios are muchhigher than in common materials of the Earth's crust.The dissolved material does not contain unusualamounts of chromium or manganese; hence, contam-ination by steel seems unlikely. To explain the nickel

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concentration on the basis of an influx of carbona-ceous chondritic-type material would require an influxof one million tons per year over the Earth. Recentresults (Barker and Anders, 1968; Hanappe et al.,1968) indicate smaller influx rates for this type ofmaterial. If the high nickel concentration in thissample of melted ice is due to extraterrestrial material,then a single large event, such as the Tunguska event,or a few large events during the past 300 years raisedthe nickel concentration of the sample. This sampleintegrates the precipitation from approximately 50 to300 years ago. There is excess Na over what would beexpected from dissolved illite and chondritic material.The probable source is wind-blown sea spray.

The finely divided clay, mainly illite, was carriedby winds from the continents; approximately 10 g/lwas deposited at Camp Century. The approximately10 g/l of NaCl was probably carried by winds fromsea spray.

A1 6, Be'°, and Ar39. Sufficient amounts of materialwere collected to make searches for isotope anomaliesprofitable. If an extraterrestrial component of thematerial originated from small particles (less than 5cm in diameter) of chondritic composition, it wouldcontain appreciable amounts of Al"" and Ar39 pro-

duced by action of solar flares and cosmic rays (Was-son, 1963). Since some Al 26 is produced in the atmo-sphere, only an excess Al 26 over that expected fromthe atmosphere can be attributed to this extrater-restrial source. Another radioactive isotope, BOO,which is produced in the Earth's atmosphere bycosmic , rays, gives a measure of the amount of Al26and Be1 ° in the dissolved material and in the par-ticulates. In the former, (3.2O-9) >( 10 Al26 and(18.4±6) X 10 Be"' dmp/l were found (McCork-ell et al., 1967). Only upper limits for Al26 and Be"were found in the particulates (McCorkell et al.,1967; Fireman and Langway, 1965); their additionsto the dissolved activities are insignificant. By com-bining the activities with the precipitation rate,which is known to be 30 cm 3 of water/cm2/year,the Al26 and Be'° production rates are calculated tobe (1.7 ±0.5) X 10' and (3.6±1.1) >< 10- 2 atoms/sec/crn 2 . The low Al26 to Be" ratio is consistent withthe production of these isotopes in the Earth's atmo-sphere (Lal, 1963). The result disagrees with thoseof Lal and Venkatavaradan (1966) but agrees withthose of Tanaka et al. (1968) on ocean sediment.

If the Ni and Co contents indicate an influxrate of chondritic-type material of one million tons/year, the low Al26 content indicates that the extrater-restrial material arrived in the form of bodies thatwere greater than 5 cm in diameter and that wereshielded from solar flares.

An upper limit for the radioactive isotope Ar 39 of0.04 dpm was observed in a 15-g sample of materialtaken from the storage tanks. This limit correspondsto 2 dmp/kg. The low Ar39 could be explained eitherby shielding of the extraterrestrial material or by dis-solution of the component containing Ar39.

Rare-gas anomalies. Rare-gas anomalies exist inmeteorites and serve as a test for material of extrater-restrial origin. The two common types of rare-gasanomalies are (1) spallation, evidenced by high He 3/He4, Ne21/Ne22, and Ar38/Ar36 ratios, and (2) pri-mordial, evidenced by He3/He 4 ratios of 10-3 to10 and Ar°/Ar36 ratios of less than 300. Searchesfor spallation anomalies were made in the samplescollected from the filter papers and storage tanks byexamining the release pattern of the rare gases as afunction of temperature. There was no evidence forHe3/He4, Ne21/Ne22, or Ar38/Ar36 ratios indicative ofspallation. There was, however, evidence for primor-dial anomalies.

In a dense fraction (>3.2 g/cm3 ) from the stor-age-tank collection, the Ar 40/Ar36 ratio was 230±5 in10 6 cm3/g of Ar released ats000l,200°C. after 1-hourheating at 500, 800, and 1,000°C. (Tilles, 1967).In another sample of this dense fraction, the Ar40/Ar36 ratio was 240±5 in 2 X 10 6 cm3/g of Ar re-leased in 1-hour heatings at 1,200°C. After 1-hour

Figure 2. Water wellfor collection of dis-solved material and

particulates.

November-December 1968 251

heating at 700°C., the Ar36/38 ratio in this argonwas 5.2±0.1. This argon is similar to that first foundby Merrihue (1964) in ocean sediments and to theprimordial argon in meteorites. This argon is con-clusive evidence for extraterrestrial material. In twosamples from the filter-paper collections, the amountof primordial argon was less than one-tenth theamount in the dense fraction. The primordial argon isconcentrated in the dense fraction that consists large-ly of magnetite; the magnetite is probably extrater-restrial.

Approximately 10-10 cm 3/g of He was found inthe second sample of the dense fraction. At the tem-perature at which the He' was released, 700 0 C., theHe'/He ratio was 1.5 >< 10. This helium couldhave a component similar to the primordial heliumin gas-rich meteorites.

Acknowledgments. Parts of the work reported inthis article were done in collaboration with R. H.McCorkell, C. C. Langway, Jr., and U. B. Marvin.This work was supported in part by National ScienceFoundation grant GA-855.

References

Barker, J. L. and E. Anders. 1968. Accretion rate of cosmicmatter from iridium and osmium contents of deep seasediments. Geochimica et Cosmochimica Acta, 32: 627-645.

Fireman, E. L. and C. C. Langway, Jr. 1965. Search foraluminum-26 in dust from the Greenland ice sheet.Geochimica et Cosmochimica Acta, 29: 21-27.

Hanappe, F., M. Vosters, E. Picciotto, and S. Deutsch. 1968.Chimie des neiges antarctiques et taux de deposition dematière extraterrestre. Earth and Planetary Science Let-ters, 4: 487-496.

Lal, D. 1963. On the investigations of geophysical processesusing cosmic ray produced radioactivity. In: EarthScience and Meteoritics, p. 115-140. Amsterdam, NorthHolland Publishing Co.

Lal, D. and V. S. Venkatavaradan. 1966. Low-energyprotons: Average flux in interplanetary space during thelast 100,000 years. Science, 151: 1381-1383.

Marvin, U. B., W. H. Pinson, Jr., and R. H. McCorkell.1967. Mineralogy and Chemical Composition of Dustfrom the Greenland Ice Cap. Paper presented at the30th Annual Meeting of the Meteoritical Society.

McCorkell, R. H., E. L. Fireman, and C. C. Langway, Jr.1967. Aluminum-26 and beryllium-10 in Greenland ice.Science, 158: 1690-1692.

Merrihue, C. 1964. Rare gas evidence for cosmic dust inmodern Pacific red clay. New York Academy of Sciences.Annals, 119: 351-367.

Tanaka, S., K. Sakamoto, J . Takagi, and M. Tsuchimoto.1968. Search for Al in deep sea sediment. Science, 160:1348-1349.

Tilles, D. 1967. Some studies of separated fractions of lowaccumulation-rate "dust." Smithsonian Contributions toAstrophysics, 11 NASA SP-135: 399-412.

Wasson, J. 1963. Radioactivity in interplanetary dust.Icarus, 2: 54-87.

Whipple, F. L. 1961. Particulate contents of space. In:Medical and Biological Aspects of the Energies of Space,p. 49-70. Columbia University Press.

Investigations of Cosmic Ray IntensityVariations in Antarctica

MARTIN A. POMERANTZ

Bartol Research Foundation ofThe Franklin Institute

Spatial anisotropies are studied with ground-basedcosmic ray instruments by relating observed timevariations in the intensity to directions in space bymeans of highly sophisticated analytical procedures.This technique provides the only available means forprobing the gross features of the interplanetary medi-um, as well as the electromagnetic conditions in re-gions of space that are not accessible to spacecraft.Portions of the celestial sphere that lie appreciably faroutside the ecliptic plane can be viewed only by in-struments located at very high latitudes, hence polarstations are crucial for investigating the three-dimen-sional spatial characteristics of the interplanetarymedium.

Last year, the discovery of a new type of cosmic rayintensity variation, characterized by a north-southasymmetry, was reported. This discovery stimulated aconcerted search for other events manifesting an-isotropies perpendicular to the ecliptic plane. Severalhave been identified and are now being studied ingreat detail. Although there are notable differences inthe characteristics of the individual events, the ob-served intensity fluctuations at the different stationsappear to be in general accord with the predictions ofa theoretical model based on a diffusion mechanism.

An unusual event followed the sequence of solardisturbances in September 1966 (see figure). Thecosmic ray flux at the antarctic stations started to de-crease on September 14 and, on September 15,reached a minimum value about 4 percent belowthe pre-event level. However, the intensity reductionat the arctic stations Thule and Alert did not beginuntil September 15. In an effort to understand the

SEPTEMBER 966

Nucleonic intensity recorded at Mc-Murdo and at Thule, Greenland, fol-lowing solar disturbances on Septem-

ber 14, 1966.

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