VOL. 77, NO. 33 JOURNAL OF GEOPHYSICAL RESEARCH NOVEMBER 20, 1972

On the Use of StableIsotopes To Tracethe Originsof Ice in a FloatingIce Tongue

ANTHONY J. Gow

U.S. Army Cold Regions Research and Engineering Laboratory Hanover, New Hampshire 03755

SAMUEL EPSTEIN

Division o/ Geological and Planetary Sciences,California Institute o• Technology Pasadena, California 91109

Stable isotope analysis has been used successfullyto distinguish between several different ice types in an ice tongue floating on sea water in Antarctica. At one critical location this technique has provided the only means of discriminating unambiguously between glacial ice and fresh-water ice formed from desalinated sea water. This part of the ice tongue is now underlain by a layer of desalted •a water thick enough to prevent any further accretion of sea ice at this location.

The floating shelfiiketongue of the Koettlitz composedof sea ice for a distance of 26 km glacier, which extends for a distance of ap- back from the ice front. The thickness of ice proximately50 km into McMurdo Sound,Ant- varied from 9 to 15 meters, and the actual trans- arctica (Figure 1), has attracted considerable formation from a glacial ice tongue to a sea ice scientific attention mainly because of the fish shelf could be observed at a number of loca- and othermarine animals found on its ablating tions.The resultsof this exploratorydrilling not surface[Debenham, 1920, 1948, 1965; Swithin- only confirmedDebenham's long-standing hy- bank et al., 1961; Swithinbank, 1970; Gow pothesisof sea ice replacementof the Koettlitz et al., 1965; Gow, 1967]. Debenham [1920] glacierbut also disposedof any possibilitythat hypothesizedthat theseremains were originally the fresh-water ice near the Dailey Islands was incorporated into the bottom of the ice tongue glacial in origin. The exact nature of this fresh- by the freezing-onof sea water and were then water ice was not resolveduntil core samples subsequentlyexposed by ablation (melting) of were subjectedto stable isotopeanalysis. These the upper surfaceof the ice tongue. data are consideredsufficiently interesting to be The first test of Debenham's hypothesiswas published independently of the other studies conductedby Gow et al. [1965], who discovered conductedon the Koettlitz ice tongue.They not by core drilling that the ice tongue in the vi- only resolve a very interesting glaciological cinity of the Dailey Islands (located near the anomaly but with data obtained at other loca- ice front) was domposedof fresh-waterice not tions serve to demonstratethe great value of sea{ce, as wasrequired by the hypothesis.Sub- isotopicmethods in tracing the originsof the ice sequently,however, it wasdiscovered by Zotikov in a compositeglacier such as the Koettlitz. and Gow [1967] that the ice immediately up- The sametechn{que has sincebeen applied by stream of the Dailey Islands was saline. Addi- œyonset al. [1971] to help resolvethe internal tional drilling by Gow [1967] along the center structure of an arctic ice shelf. line of the ice tongue showedthat it was indeed ANALYTICAL TECI-INIQUES

x Contribution 2148, Publications of the Divi- The isotopiccomposition of sea water is rela- sion of Geological and Planetary Sciences, Cali- tively constant throughout the oceans of the fornia Institute of Technology, Pasadena, Cali- world, and only minor fractionation of the fornia 91109. stable isotopesoccurs when sea water freezes. Copyright (•) 1972 by the American Geophysical Union. Accordingto Friedman et al. [1964], sea ice

6552 DEUTEl•IU• ANI) OXYGEN 18 IN SEA ICE 6553

164ø 166 ø

, k1 "\ /l \ I s., o..,":..D •.;...... '.., I ),,.,,'" "..,...... • -•-I

, ,

/• •'KoelllilzGlacier and,• McMurdo•• ' , ••• • .... Sound• R, , • • - • •,,", ...... •i

I /' , • , • , Fig. 1. Map of McMurdo Sound region showinglocations of drill holes on the Koettlitz glacier tongue. Data in this report are restrictedto measurementsmade on coresfrom holes 1, 3, and 5 and from the hole markedD, drilleddirectly in front of the easternmostDailey Island. contains only about 2% more deuterium than RESULTS AND DISCUSSION the water from which it was frozen, and a Resultsof isotopicand salinitymeasurements parallel fractionation can be expectedfor •80. of samplesfrom the Koettlitz ice tongueare The sea water to sea ice fractionationis very presented in Tables 1 and 2. Core holes 1 and minor in comparisonwith fractionationproc- 3 (seeFigure I for the locationsof drill holes) essesin the atmosphere,which ultimately lead were drilled in ice provisionallyidentified as to the precipitationin the polar regionsof snow seaice on the basisof salinityand petrographic very much depleted in deuterium and •80. The studies of the cores. Results of studies at hole principlesinvolved in the fractionationof oxy- I are presentedby Zotikov and Gow [1967], gen isotopes are discussedby Epstein and and identical relationshipswere subsequently Mayeda [1953], Dansgaard [1953], and Ep- observed in cores from hole 3. Ice from core stein [1956]. Essentially,the samerelationships hole 5 was provisionallyidentified as glacialice, apply to the fr•ctionation of the hydrogen which in terms of texture (Figure 2) and salt isotopes[Friedman et al., 1964]. Isotopevaria- content differs radically from sea ice. Stable tions are expressedhere as a deviation 3 from isotopedata in Table 1 confirmthese identifica- standardmean oceanwater (SMOW), a posi- tions completely. tive value indicatingenrichment and a negative The 3 values at the locations of holes I and 3 value indicating depletion with respect to show little if any significant variation with SMOW. depthin the ice,but they all showa slightposi- Isotopic data were supplementedby meas- tive deviation or enrichment in deuterium and urementsof electrolytic conductivityof melted •O. The only exceptionis the sampleof sea core samples. These data were then converted water obtained from the bottom of hole 3. This to salinitieson the assumptionthat the ratios water was enriched slightly in salt (41%o as of salts in the ice sampleswere the same as compared with 33%0for sea water in McMurdo those in standard sea water. Sound), probably as, a result of rejectionof 6554 Gow AND EPSTEIN TABLE1. Stable Isotope and Salinity Variations of the Koettlitz Glacier Tongue, McMurdo Sound, Antarctica

Samp1e Sample •D, • 180 Salinity Ice Location Depth,meters ø/oo ø/oo ø/oo Type

Hole 1 0.1 +18.5 +2.51 0.2 Sea 1 +11.8 +1.57 1.00 Sea 4 +15.2 +1.67 1.44 Sea 8 +16.1 +1.80 2.59 Sea 11 +12.9 +1.51 5.15 Sea 12 +14.6 +1.57 5.19 Sea 12.8 +15.4 +1.61 5.76 Sea

Hole 3 2 +14.4 +1.76 2.10 Sea 6 +15.8 +1.74 5.26 Sea 9 +15.2 +1.90 1.75 Sea 11 +15.6 +1.66 5.82 Sea 12 +12.8 +1.77 2.88 Sea 12.9 +15.1 +1.85 5.51 Sea 15 +15.7 +1.85 5.26 Sea * =05.6 =1.12 41.0 Sea

Hole Sñ 1 -288.8 -58.17 O.OS Glacial 7 -2S7.5 -55.46 0.01 Glacial

For locations of samples see Figure 1. , *Sea water from bottom. ñDid not penetrate to bottom of ice tongue at this location; other two holes penetrated ice bottom. salts during freezing,and the deuteriumvalue with the theoretical and observed values re- is very closeto that reportedfor antarcticdeep portedby Friedmanet al. [1964] and with the water [Friedman et al., 1964]. The approximate valuesreported by O'Neil [1968] for fractiona- 2% enrichmentin deuteriumof the seaice over tion between ice and fresh water. that of the sea water also agreesvery closely It had been suggestedby Zotilcovand Gow TABLE2. Stable Isotope Variations in Ice from [1967] on the basis of temperature measure- the Vicinity of the Dailey Islands, Koettlitz mentsin a drill holethat brackishwater, formed Glacier Tongue, McMurdoSound, Antarctica from the mixing of sea water and glacialmelt, Sample 6D, 6180, mightbe involved'in the freezingprocess. This Depth, meters O/oo O/oo Ice Type* possibilitywas heightenedby the knowledgeof the. existenceof a body of fresh water located 4 -7.3 -0.34 FW directly beneaththe ice in the vicinity of the 6 -6.5 -0.48 FW 7 +0.4 -0.58 FW Dailey Islands[Gow et al., 1965]. However,the 7.7 -4.3 -0.62 FW 8 values at core holesI and 3 clearly showthat * -21.1 -3.16 UFW only straight sea water is freezingonto the bottom of the ice tongueat thesetwo locations. Location D in Figure 1. This freezing has producedexceptional thick- of*F•, desalinated fresh-watersea iceice' derivedUFW, unfrozen from thefresh melt water.water nesses ofsea ice. At hole4, for example, theice *Sub-icewater sample. measurednearly 15 meters thick. This thick- DEUTERIUM AND OXYGEN 18 IN SEA ICE 6555

• DAILEY ABLATING I' .... ?•. .,.,;,._•'.•.'--:?'.".:'.:'.'-'•{•'-•.","CE'-:.-:S,•L'•:-'i'-'.'-'.'-:.{:.:-:.-::Y"//•lro ..•-.".--'•i",".•----:lllllll•__-_ / •, •,.--•',':•'" ', '" : ßß ." ..... ' .' '." ...... ' .... ' ':.:"::" .':.!:'::'' '. :'1 ----

Fig. 2. Schematic crosssection of Koettlitz glacier tongue depicting (top) crystal structure characteristicsof the major ice types and (bottom) processesinvolved in its formation. The scale bar on the section of sea ice from hole 1 measures1 cm. This same scale also applies to the other three sections. Glacier ice from hole 5 is composedof equidimensional crystals and contains numerous bubbles of air. Sea ice from holes I and 3 features elongated inter- locked crystals. Parallel trains of inclusions are. brine. pockets. Very fine grained crystal structure of ice from near the tide crack at D is typical of fresh-water (F.W.) frazil. ness is. for actively growing sea ice. and cer- about. 90 km to the northwest of hole 5 on the tainly exceedsthe 12-meter maximum thick- Koettlitz ice tongue, and the low deuterium nessesof sea ice reported from the Arctic by concentrationsin Lake Vanda are attributed by Cherepaaov [1966]. Ragotzkie and Friedman [1965] to the inflow Salinities of cores from hole 5 are. less than of melt water from the. Upper Wright and thoseat locations1 and 3 by 2-3 ordersof mag- Lower Wright glaciers. Together these two nitude, and the isotopic compositionof ice at glaciersspan much the samerange of elevations location 5 clearly demonstratesthat it is of as the Koettlitz glacier, but. only the Lower glacial origin. The very low concentrations.of Wright glacieris contributingmelt water (SD = deuterium and •80 would indicate that this ice --259%o) at the presenttime. originatedin the upper reachesof the Koettlitz The most interesting data were obtained in glacier.On the.basis. of presentday rdationships the vicinity of the Dailey Islands, where the between isotopic compositionand temperature first of two holes drilled in November 1963 [Lo.rius,1968] and temperature and elevation [Gow et al., 1965] showedthat this part of the [Bentley et al., 1964] the ice at location 5 prob- Koettlitz ice tongue was actually composedof ably formed in a region of mean annual air salt-free ice.. It was also discovered that the temperature of --28 ø to --30øC at an devation ice was underlain by a body of fresh water, the of about 1000 meters on the Koettlitz glacier. existence of which was attributed to the drain- This locationwould be approximately40 km up- age down tide cracks.of surface melt water stream from hole 5. The deuterium concentra- formed during the ablation season.Tide cracks tions at. hole 5 are very similar to those meas- are formed as a result of the rise and fall of ured by Ragotzkie aad Friedman [1965] in the ice in contact with the islands.During the Lake Vanda, situated in the Wright Valley, Vic- summer, considerablequantities of water are toria Land, Antarctica. Wright Valley is located dischargedinto those cracks, and freezing of 6556 Gow AND EPSTEIN this water to the undersideof the ice tongue In a sense,this systemis a self-generatingone was believed to be a major source of nourish- apparently with little or no interaction with ment at this location. Friedman et al. [1961] other sources of water. This circumstance could have reported a similar situation involving the explain why the 8 valuesof the water and the freezingof surfacesnow melt to the undersideof overlying ice are practically the same, though arctic sea ice. However the ultimate source of other factors concernedwith freezing may be the fresh water at the •Dailey Islands (whether involved. For example, observationsby Gow derivedfrom glaciers • or fromseasonal snow et al. [1965] indicate that freezing could be melt) could not be determined for certain. Ice occurringat the interfacebetween the sea water from a secondhole, located approximately400 and the fresh water rather than the water's meters from the first, was tentatively identified being frozen directly to the undersideof the ice. as glaciali•e on the basisof petrographicstud- Crystals formed at the interface would then ies [Gow et al., 1965]. The isotopedata (Table float up and attach themselvesto the under- 2) disprove both the glacial and snow melt surface of the ice. Such a process has been origins. Instead, it would appear that both the advocatedby. Untersteinerand Badgley[1958] ice and the underlying melt water are of direct to explain the freezing of fresh water to the marine origin. Though salt concentrationsin undersides of ice floes in the Arctic Ocean. The this ice varied from 15 to 90 ppm [Gow et al., operationof a similar mechanismin the vicinity 1965], very much less than the variation in sea of the Dailey Islands would also be compatible ice and actually approaching polar glacial ice with the frazillike textures that characterize the in purity, 8 values of both the ice and the ice at this location, as is illustrated in Figure 2. underlying fresh water approximate closelythe Whatever the mechanismof ice accretionmight isotopic compositionof antarctic sea water. be at this location, it would appear that an es- The simplestexplanation of •the above ob- sentially nonfractionatingprocess of freezingis servationsis that the ice in the vicinity of the occurringin the fresh-waterlayer. The inferred Dailey Islands was originally as saline as that compositional structure of the Koettlitz ice in other parts of the ice tongue but that at tongue is demonstratedschematically in Fig- some stage in the history of ice growth the sea ure 2. water per se was replacedby desaltedsea water. Desalinationis a characteristicphenomenon of CONCLUSIONS aging sea ice, a fact that is clearly indicated in Stable isotope studies definitely confirm the the salinity data obtainedon samplesat holes fact that the lower half of the 50-km-long i and 3. At both locationsthe near-surface(or Koettlitz ice tongue,McMurdo Sound, Ant- older) sea ice is appreciably depleted in salts. arctica, is composedof sea ice up to 15 meters Fresh-water melt pools and streams occur over thick. This transformation is accomplishedby much of the surfaceof the Koettlitz ice tongue, the combinedprocesses of abla.tionof the origi- but it is only in the vicinity of the •Dailey nal glacial ice at the upper surfaceand freezing Islands, where tide cracks penetrate to the of seawater onto the bottom. Straight seawater bottomof the icetongue [Gow et al., 1965],that only appears to have been involved in the surface melt water is able to drain down to freezing process.No evidencewas obtained for the underside of the ice. Here it becomes sand- brackish-water ice, not even in the anomalous wichedbetween the ice bottom and the slightly region of salt-free ice near the Dailey Islands, densersea water, which it now replacesin the where the ice tongue is underlain directly by accreting process.This body of fresh water is fresh water. This fresh water is shownby stable estimated to be at least 3 meters thick at hole 1. isotope analysis to be of direct marine origin When this water freezes,it doesso with all the and is derived most probably from the melting petrographic and salinity characteristics of of old desalinatedsea ice on the upper surface fresh-waterice; yet, it still retainsthe isotopic of the ice tongue. This water drains down tide identity of sea water. Measurementsof surface cracks to the underside of the ice, where it ablation and ice thickness at this location indi- forms a stable body of fresh water, which cate that it would take 15-20 years for bottom naturally gives rise to salt-free ice when it ice to reach the surface. freezes. It would appear that enough fresh DEUTERIUM AND OXYGEN 18 IN SEA IcE 6557 water is producedduring the ablation seasonto tent of natural water in the hydrologic cycle, sustain the fresh-water layer and thus prevent Rev. Geophys. Space Phys., 2, 177-224, 1964. Gow, A. J., Antarctic glaciological studies, A•t- any further accretionof sea ice at this location. arctic J. U.S., 2, 121-122, 1967. The formation of fresh-water (nonglacial) ice Gow, A. J., W. F. Weeks, G. Hendrickson, and in this way is almost certainly limited to areas R. Rowland, New light on the mode of uplift within the immediate vicinity of tide cracks. of the fish and fossiliferous moraines of the McMurdo ice shelf, Antarctica, J. Glaciol., 5, Acknowledgment. This research was supported 813-828, 1965. by National Science Foundation grant GA-12945. Lorius, C., Calottes polaires, Sciences, 56, 40-53, Sept.-Oct. 1968. (Also Rep. 305, Expeditions REFERENCES Polaires Francaises, Missions Paul-Emile Vic- tor.) Bentley, C. R., R. L. Cameron, C. Bull, K. Lyons, J. B., S. M. Savin, and A. J. Tamburi, Kojima, and A. J. Gow, Physical characteristics Basement ice, Ward Hunt ice shelf, Ellesmere of the antarctic ice sheet, Antarctic Map Folio Island, Canada, J. Glaciol., 10, 93-100, 1971. Set., folio 2, Amer. Geogr. Sot., New York, O'Neil, J. R., Hydrogen and oxygen isotope frac- 1964. tionation between ice and water, J. Phys. Chem., Cherepanov, N. V., Structure of sea ice of great 72, 3683-3684, 1968. thickness,Voprory ArkticheskogoLedovedeniya, Ragotzkie, R. A., and I. Friedman, Low deuterium Tr. 267, pp. 13-18, Arkt. i Antarki N.-I. In- content of Lake Vanda, Antarctica, Science,148, stituta, 1964. (English translation, Rep. T448R, 1226-1227, 1965. Defens• Res. Board, Ottawa, Ontario, Jan. 1966.) Swithinbank, C. W. M., Ice movement in the Dansgaard, W., The abundanceof 018 in atmos- McMurdo Sound area of Antarctica, I•t. Symp. pheric water and water vapor, Tellus, 5, 461- Antarctic Glaciol. Explor., Hanover, New Hamp- 469, 1953. shire, Int. Ass. Sci. Hydrol., Publ. 86, 472-487, Debenham, F., A new mode of transportation by 1970. ice: The raised marine muds of South Victoria Swithinbank, C. W. M., D. G. Darby, and D. E. Land (Antarctica), Quart. J. Geol. Soc. London, Wohlschlag, Faunal remains on an antarctic ice 75, 51-76, 1920. shelf, Science, 133, 764-766, 1961. Debenham, F., The problem of the Great Ross Untersteiner, N., and F. I. Badgley, Preliminary Barrier, Geogr. J., 112, 198-218, 1948. results of thermal budget studies on arctic pack Debenham, F., The genesisof the McMurdo ice ice during summer and autumn, in Arctic Sea shelf, Antarctica, J. Glaciol., 5, 829-832, 1965. Ice, Publ. 598, pp. 85-92, National Academy of Epstein, S., Variation of the 0-18/0-16 ratio of Sciences,Washington, D.C., 1958. fresh water and ice, in Nuclear Processesin Zotikov, I. A., and A. J. Gow, The thermal and Geologic Settings, Publ. 400, pp. 20-24, National compositional structure of the Koettlitz ice Academy of Sciences,Washington, D.C., 1956. tongue, McMurdo Sound, Antarctica, in Physics Epstein, S., and T. Mayeda, Variation of 0-18 o] S•ow a•d Ice: International Con]erence on content of water from natural sources,Geochim. Low Temperature Science, Proceedings,vol. 1, Cosmochim. Acta, 4, 213-224, 1953. edited by H. Oura, pp. 469-478, Hokkaido Uni- Friedman, I., B. Schoen, and J. Harris, The deu- versity, Sapporo, Japan, 1967. terium concentration in arctic sea ice, J. Geo- phys. Res., 66, 1861-1864, 1961. Friedman, I., A. C. Redfield, B. Schoen, and (l•eceived May 22, 1972; J. Harris, The variation of the deuterium con- revised July 11, 1972.)