SUMMERTIME FORMATION of DEPTH HOAR in CENTRAL GREENLAND R.B.Alley1, E.S
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GEOPHYSICALRESEARCH LETTERS, VOL. 17, NO. 12, PAGES2393-2396, DECEMBER 1990 SUMMERTIME FORMATION OF DEPTH HOAR IN CENTRAL GREENLAND R.B.Alley1, E.S. Saltzman 2, K.M. Cuffey !, J.J. Fitzpatrick 3 .Abstract.Summertime solar heating of near-surface from snoworiginally of higherdensity). Depositional snowin centralGreenland causes mass loss and grain depthhoars can form at anytime of theyear, but exhibit growth. These depth hoar layersbecome seasonal certaincharacteristics (general thinness, tendency to fill markerswhich are observedin ice coresand snow pits. lowspots in theunderlying snow surface) that allow them Massredistribution associated with depth-hoarformation to be distinguishedfrom dingeneticdepth hoars [Alley, canchange concentrations of immobilechemicals by as !988]. muchas a factor of two in the depth hoar, altering Benson [1962] observed that in Greenland a atmospheric signals prior to archival in ice. For stratigraphicdiscontinuity forms in late summer or methanesulfonicacid (MSA) thiseffect is notsignificant autumn,with a coarse-grained,low-density layer often because the summer maximum does not coincide with the containingdepth hoar overlainby a finer, denserlayer. densityminimum, and the amplitude of the annual Traditionally, formation of the depth hoar in sucha (MSA)signal is more than a factorof ten. sequenceon ice sheetshas been linked to diagenesis duringthe autumnin the seasonaltemperature cycle, Introduction when summer-warmedsnow loses vapor to colder overlyingair, by diffusion,convection, and wind pumping Ice cores contain a paleoenvironmentalrecord of [e.g.Bader, 1939;Benson, 1962; Gow, 1968;although many atmospheric chemicals with seasonal time summertime formation of depth hoar has been resolution.Seasonal variations in the physicalproperties recognized,e.g. Benson, 1962]. of snowand ice provide layeringwhich can be usedto Co!beck[1989b] modeledthe effectsof annual and time changesin chemical inputs. Here we use data diurnaltemperature cycles and solarradiation on grain collectedduring June and July, 1990 at the GISP2 site growth in polar, alpine, and seasonal snows. He [38.5oWlong., 72.6oN lat., 3210 m elev; Hodge eta!., calculatedthat diurnalradiative and temperatureforcing 1990],near the Summitof the Greenlandice cap,to show should cause rapid grain growth near the surface of thatlow-density, coarse-grained layers called depth hoar seasonaland alpine snows;however, he assumeda low arecaused in thisregion by summerinsolation. valuefor radiativeheating in polar snow,and estimated The large crystalsof depth hoar form when steep that depth-hoarformation on ice sheetsmust be linked to temperature gradients cause rapid vapor flux past the annualtemperature ,,wave. relatively few nucleation sites (crystals or grains). A potentialproblem with forcingpolar depth-hoar Becauselow-density snow has fewer grainsand lower formationby seasonaltemperature changes is that we thermalconductiv/ty than high-densitysnow, depth hoar typicallyobserve depth hoarsto be a few centimeters formswhen low-density snow is subjectedto temperature thick,with sharpbasal contacts, but the 1/e depthfor gradients [Colbeck, 1983; 1989b; Sommerfeld, 1983, penetrationof the warmthof a summeris severalmeters; Perlaand Ommanney, 1985; Alley, 1988]. there does not seem to be a good mechanism for concentrating the mass loss from such a broad Background temperaturegradient into a thinlayer with a sharpbase, asobserved. Below we showythat the radiativeforcing on Low-densitysnow can be produceddepositionally summerdays in centralGreenland is sufficientto cause (e.g.by snowfallwithout wind packing, or by burialof strongnear-surface warming and surface-temperature low-densitysurface hoar) or diagenetically(via mass loss gradients,localizing depth-hoar formation near the surface. EarthSyste m Science Center and Department of Methanesu!fonicacid (MSA;CH3SO3-) is .ansulfur- Geosciences,The PennsylvaniaState University. bearinganion formed from the atmosphericreaction of RosenstielSchool of Marine and Atmospheric dimethylsulfide(DMS) with OH radicals. In high- Science, University of Miami. latituderegions, aerosol MSA •bits a stronglyseasonal Branchof SedimentaryProcesses, United States signal with high concentrationsduring the summer Geological Survey. months[Saltzman eta!.,, 1986; Watts et al., 1'987;Ayers et al., 1986]. This se•ona!ity stemsfrom 'twofactors: !) spring/summerincreases in oceanicproductivity cause increasedDMS emissions;and 2) lack of wintertime Copyright 1990 by the American Geophysical Union. photo•e•stry causesatmospheric OH •ncentrafion.s m ?aper number 90GL02443 be low and thus DMS conversion to MSA to be slow. 0094-8534 / 9'0/ 90GL- 02443503. O0 The seasonali:ty of MSA in recentsnow •d ice reflectcsits 2393 2394 Alley et al.: Depth Hoar in CemralGreenland distribution in the aerosol, as demonstratedby the mmhigh and 1-2 m wide)of relativelyhomogeneous, relationshipbetween MSA and oxygenisotopes in ice fine-grainedsnow. A surfacehoar about5 to !0 mm from southernand central Greenland[Whung et al., thickwas deposited that night. 1989]. In Greenland,MSA may originatefrom either Over the nextweek, clear skies, high temperatures regionalemissions from the surroundingGreenland and (afternoons greater than -10 C), and light winds NorwegianSeas, or from long-distancetransport from prevailed.After only24 hoursof thisweather pattern, a lower-latituderegions in the North Atlantic. MSA is an 5 to 10mm thick depth hoar developed in theslopes of excellentseasonal tracer because it is chemicallystable, older dunesfacing the afternoonsun. On the dunewe and is not subjectto contaminationby samplingor polar- studiedin greatestdetail, the depthhoar developed campoperations [Saltzman, unpublished data]. larger grains(mean interceptmore than doubledin three days) and lower densitythan the snowjust beneathit Methods (lessthan 220 kg/m3, comparedto 388kg/m3 after four days); the depth hoar thickenedduring the days We undertook a series of observations of snow in the followingour initial observations,but did not appearto top few metersat the GISP2 site. Detailed observations changedensity greatly. The snowat the samedepth as were undertakenin one two-meterpit whichwas back-lit the depth hoar but on the shady side of the dune to highlightstratification, and in a seriesof shallowerpits appearedsimilar to the snowbeneath it and to the snow dug every few hours to few days. Less-detailed beneath the depth hoar when first observed, and observationswere conductedin other pits as deep as six maintainedthis appearanceuntil a shady-sidedepth hoar meters. Measurementsincluded qualitative observations, beganforming two to three dayslater. volume-massdensity samplingusing box samplers or MSA concentrations were measured on material standard SIPRE snow-densitytubes, and thin sections from within, above, and below the depth hoar, andfrom prepared using dimethyl phthalate as a pore filler. the equivalentdepth 10 to 20 cm awayon the shadyside Sampleswere collected for MSAanalysis using stainless- of the dune. These data, shown in Figure 1, are steel box samplersand were storedfrozen in sealed consistant with vapor loss causing increased MSA plasticbags prior to analysis. concentrations in the depth hoar. Based on MSA We also conducted high-resolutiontemperature concentrations and densities in the material below the measurementsin our pits to monitordiurnal forcing. We depth hoar, it appears that the vapor loss occurred useda digitalthermometer with a singlethermocouple primarily upward,causing surface-hoar growth or lossto probe(K-type) having a timeconstant of a fewseconds. the free atmosphere. To avoidradiative heating of the probe,air temperatures Over the few daysfollowing 6/21, depth hoar first were measured in the shadow of the observer. For developedbeneath flat areasand beneaththe sidesof shallowsnow, we insertedthe probeinto a layer,shaded dunesthat were shadedduring afternoons,and thenon it, and observedthe temperatureevolution. Rapid the new, fine-grained duneslacking wind crusts. This coolingof the probe was followed by a breakin slopeand order may suggestsome role for wind crustsin depth- slowercooling of the snow;we extrapolatedthe snow- hoar formation, or that the very fine grains and high coolingcurve back to time zero to obtainthe snow density of the new dunes reduced solar heatingby temperature. conductingheat awayefficiently, or that densificationby Measurements of MSA were made on-site with a sinteringof the fine-graineddunes kept pacewith the modular anion chromatograph with chemical mass loss. suppression,using AG4/AS4 and AMMS columns (Dionex).The eluant used for the analysis was 5.5 mM Temperature.Measurem .ents.. NaOH, with 11.25 mM HzSO4 as the regenerant. Sampleswere injected using an artion preconcentration Basedon Colbeck[1986b], we hypothesizedthat the column;volumes ranged from 5 to 8 mls. The detection observednear-surface mass loss was causedby solar limitof theanalysis under these conditions is 0.2 ppb. heating.We thencollected detailed temperature profiles beneatha flat regionat one- to two-hourlyintervals Results (Figure2), whichshow steep temperature gradients •at wouldcause net vapormotion out of the depthhoar Near-Surface Observations formingnear 10 mm depth. Vapor motion would be by diffusion,and possibly by wind pumping and therm• A series of wind- and snowstorms at the GiSP2 site convection,and could involve direct transfer or repeated ended duringthe eveningof 19 June 1990,leaving a condensationand sublimationfrom grain