Summertime formation of Depth Hoar in central Greenland

.Abstract. Summertime solar heating of near-surface snow in central Greenland causes mass loss and grain growth. These depth hoar layers become seasonal markers which are observed in ice cores and snow pits. Mass redistribution associated with depth-hoar formation can change concentrations of immobile chemicals by as much as a factor of two in the depth hoar, altering atmospheric signals prior to archival in ice. For methanesulfonic acid (MSA) this effect is not significant because the summer maximum does not coincide with the density minimum, and the amplitude of the annual (MSA) signal is more than a factor of ten.

from snow originally of higher density).Depositional depth hoars can form at any time of the year, but exhibit certain characteristics (general thinness, tendency to fill low spots in the underlying snow surface) that allow them to be distinguished from dingenetic depth hoars [Alley,!988].Benson [1962] observed that in Greenland a stratigraphic discontinuity forms in late summer or autumn, with a coarse-grained, low-density layer often containing depth hoar overlain by a finer, denser layer.Traditionally, formation of the depth hoar in such a sequence on ice sheets has been linked to diagenesis during the autumn in the seasonal temperature cycle, when summer-warmed snow loses vapor to colder overlying air, by diffusion, convection, and wind pumping [e.g.Bader, 1939;Benson, 1962;Gow, 1968; although summertime formation of depth hoar has been recognized, e.g.Benson, 1962].
Co!beck [1989b] modeled the effects of annual and diurnal temperature cycles and solar radiation on grain growth in polar, alpine, and seasonal snows.He calculated that diurnal radiative and temperature forcing should cause rapid grain growth near the surface of seasonal and alpine snows; however, he assumed a low value for radiative heating in polar snow, and estimated that depth-hoar formation on ice sheets must be linked to the annual temperature ,,wave.
A potential problem with forcing polar depth-hoar formation by seasonal temperature changes is that we typically observe depth hoars to be a few centimeters thick, with sharp basal contacts, but the 1/e depth for penetration of the warmth of a summer is several meters; The seasonali :ty of MSA in recent snow •d ice reflectcs its distribution in the aerosol, as demonstrated by the relationship between MSA and oxygen isotopes in ice from southern and central Greenland [Whung et al., 1989].In Greenland, MSA may originate from either regional emissions from the surrounding Greenland and Norwegian Seas, or from long-distance transport from lower-latitude regions in the North Atlantic.MSA is an excellent seasonal tracer because it is chemically stable, and is not subject to contamination by sampling or polarcamp operations [Saltzman, unpublished data].

Methods
We undertook a series of observations of snow in the top few meters at the GISP2 site.Detailed observations were undertaken in one two-meter pit which was back-lit to highlight stratification, and in a series of shallower pits dug every few hours to few days.Less-detailed observations were conducted in other pits as deep as six meters.Measurements included qualitative observations, volume-mass density sampling using box samplers or standard SIPRE snow-density tubes, and thin sections prepared using dimethyl phthalate as a pore filler.
Samples were collected for MSA analysis using stainlesssteel box samplers and were stored frozen in sealed plastic bags prior to analysis.
We also conducted high-resolution temperature measurements in our pits to monitor diurnal forcing.We used a digital thermometer with a single thermocouple probe (K-type) having a time constant of a few seconds.
To avoid radiative heating of the probe, air temperatures Samples were injected using an artion preconcentration column; volumes ranged from 5 to 8 mls.The detection limit of the analysis under these conditions is 0.2 ppb.

Near-Surface
photo•e•stry causes atmospheric OH •ncentrafion.s m be low and thus DMS conversion to MSA to be slow.
were measured in the shadow of the observer.For shallow snow, we inserted the probe into a layer, shaded it, and observed the temperature evolution.Rapid cooling of the probe was followed by a break in slope and slower cooling of the snow; we extrapolated the snowcooling curve back to time zero to obtain the snow temperature.Measurements of MSA were made on-site with a modular anion chromatograph with chemical suppression, using AG4/AS4 and AMMS columns (Dionex).The eluant used for the analysis was 5.5 mM NaOH, with 11.25 mM HzSO 4 as the regenerant.

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Fig. 1.MSA concentrations and densities measured in a shallow snowpit on a dune on June 21.Mass loss and resulting increase in MSA concentration are evident near 1 cm depth on the sunny side of the dune where a depth hoar has formed, but not in deeper snow or on the shady side of the dune where no hoar formation has occurred.