Chemical concentrations of pollutant lead aerosols, terrestrial dusts and sea salts in Greenland and Antarctic snow strata☆

Changes in sea level

An empirical model of firn densification from the surface to the zone of pore close-off has been constructed. Fundamental rate equations have been derived for the first two stages of densification. In the first stage, for densities less than 0.55 Mg m−3, the densification rate is proportional to the mean annual accumulation times the term (ρ i − ρ), where ρ is the density of the snow and ρ i is the density of pure ice. The densification rate in the second stage, where 0.55 Mg m −3 < ρ < 0.8 Mg m−3, is proportional to the square root of the accumulation rate and to (ρi− ρ). Depth–density and depth–age calculations from this model are compared with observation. Model accumulation rates are within about 20% of values obtained by other techniques. It is suggested that depth intervals of constant density in some Antarctic cores may represent a synchronous event in the 1880 ’s when ten times the normal accumulation fell within a year or two.

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Firn Densification: An Empirical Model

[1] The discovery of the 2012 extreme melt event across almost the entire surface of the Greenland ice sheet is presented. Data from three different satellite sensors – including the Oceansat-2 scatterometer, the Moderate-resolution Imaging Spectroradiometer, and the Special Sensor Microwave Imager/Sounder – are combined to obtain composite melt maps, representing the most complete melt conditions detectable across the ice sheet. Satellite observations reveal that melt occurred at or near the surface of the Greenland ice sheet across 98.6% of its entire extent on 12 July 2012, including the usually cold polar areas at high altitudes like Summit in the dry snow facies of the ice sheet. This melt event coincided with an anomalous ridge of warm air that became stagnant over Greenland. As seen in melt occurrences from multiple ice core records at Summit reported in the published literature, such a melt event is rare with the last significant one occurring in 1889 and the next previous one around seven centuries earlier in the Medieval Warm Period. Given its rarity, the 2012 extreme melt across Greenland provides an exceptional opportunity for new studies in broad interdisciplinary geophysical research.

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The extreme melt across the Greenland ice sheet in 2012

Changes in ice mass are estimated from elevation changes derived from 10.5 years (Greenland) and 9 years (Antarctica) of satellite radar altimetry data from the European Remote-sensing Satellites ERS-1 and -2. For the first time, the dH/dt values are adjusted for changes in surface elevation resulting from temperature-driven variations in the rate of firn compaction. The Greenland ice sheet is thinning at the margins (-42 � 2G t a -1 below the equilibrium-line altitude (ELA)) and growing inland (+53 � 2G t a -1 above the ELA) with a small overall mass gain (+11 � 3G t a -1 ; -0.03 mm a -1 SLE (sea-level equivalent)). The ice sheet in West Antarctica (WA) is losing mass (-47 � 4G t a -1 ) and the ice sheet in East Antarctica (EA) shows a small mass gain (+16 � 11 Gt a -1 ) for a combined net change of -31 � 12 Gt a -1 (+0.08 mm a -1 SLE). The contribution of the three ice sheets to sea level is +0.05 � 0.03 mm a -1 .T he Antarctic ice shelves show corresponding mass changes of -95 � 11 Gt a -1 in WA and +142 � 10 Gt a -1 in EA. Thinning at the margins of the Greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate. The marked thinnings in the Pine Island and Thwaites Glacier basins of WA and the Totten Glacier basin in EA are probably ice- dynamic responses to long-term climate change and perhaps past removal of their adjacent ice shelves. The ice growth in the southern Antarctic Peninsula and parts of EA may be due to increasing precipitation during the last century.

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Mass Changes of the Greenland and Antarctic Ice Sheets and Shelves and Contributions to Sea-level Rise: 1992-2002

: The Greenland ice sheet is treated as a monomineralic rock formation, primarily metamorphic, but with a sedimentary veneer of snow and firn. The sedimentary part is perennial above the firn line, and the classical methods of stratigraphy and sedimentation can be profitably applied to it. During a 4-yr period 146 pit studies and 288 supplementary Rammsonde profiles were made along 1100 miles of over snow traverse (Fig. I). Temperature, density, ram hardness, and grain size were measured in the strata exposed in each pit. Stratification of snow results from variations in the conditions of deposition and is emphasized by subsequent diagenesis. Summer layers are coarser grained and have generally lower density and hardness values than winter layers; they may also show evidence of surface melt. The onset of fall is usually identified by an abrupt increase in density and hard ness accompanied by a decrease in grain size. This stratigraphic discontinuity is used as the annual reference plane.

Carl S. Benson

Papers

Stratigraphic studies in the snow and firn of the Greenland ice sheet