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Low-latitude Ice Cores and Freshwater Availability

  • Natalie Marie Kehrwald

Recent retreat of Tibetan Plateau glaciers affects at least half a billion people. Himalayan glaciers seasonally release meltwater into tributaries of the Indus, Ganges, and Brahmaputra Rivers and supply freshwater necessary to support agricultural and economic practices. Tibetan Plateau glaciers are retreating more rapidly than mountain glaciers elsewhere in the world, and this retreat is accelerating. The Naimona’nyi ice field (30°27 ’N; 81°91’E, 6050 masl) has not accumulated mass since at least 1950, as evidenced by the virtual lack of radiogenic isotopes (36Cl, 3H, and beta radioactivity) present in the ice core. These isotopes were produced by U.S. and Soviet atmospheric thermonuclear bomb tests conducted in the 1950s and 1960s and provide independent dating horizons for the ice cores. Lead-210 dates imply that the uppermost preserved glacial ice on Naimona’nyi formed during the 1940s. While this is the highest documented glacial thinning in the world other glaciers at elevations similar to that of Naimona’nyi, such as Kilimanjaro (3°4’S; 37°21’E, 5893 masl), are also losing mass at their summits. The global scope of high-elevation glacial thinning suggests that ablation on the Earth’s highest ice fields may be more prevalent as global mean temperatures continue to increase. Glacial thinning has not been taken into account in future projections of regional freshwater availability, and the net mass loss indicates that Himalayan glaciers currently store less freshwater than assumed in models. The acceleration of Tibetan Plateau glacial retreat has been hypothesized to be due in part to deposition of black carbon (BC) from biomass burning on to ice fields, thereby lowering the reflectivity of the glacier surface and melting the upper ice. Small (>0.1-1 mg) and ultra-small samples (< 0.05 mg) of the BC and total organic carbon (TOC) fractions of Naimona’nyi were measured and radiocarbon dated to determine a biomass burning and/or fossil fuel source. The uppermost sample (5 m) corresponds with a 210Pb date of ~1850 AD, and contains a BC content and radiocarbon age within the range of natural variability. BC from recent anthropogenic activity is unlikely to have migrated through the firn and ice into the upper ice core as BC is a hydrophobic substance. The high-elevation thinning on Naimona’nyi appears to be a response to increased mid-tropospheric (4-7 km) temperatures rather than primarily driven by changes in surface albedo. However, a marked increase in modern BC and TOC was measured since 1880 AD in the annually-dated Andean Quleccaya ice core (13°56’S; 70°50’W; 5670 masl). No increase in radiocarbon-dead (> 60,000 ka) BC or TOC was noted, suggesting that the source of the carbon was from biomass burning, with a possible contribution of Amazon slash and burn clearing, rather than the input of fossil fuel combustion. The age of the BC and TOC is thousands of years older than the age of the adjacent ice, and should not be used to date the ice core. Low-latitude ice cores place recent glacier retreat into a longer time perspective and quantify parameters such as BC which may accelerate retreat.

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    Ohio State University, Geological Sciences
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