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New Information Gained From Middle to Low Latitude Ice Coring
The Tibetan/Himalayan regions are dominated by monsoonal circulation, while the Tien Shan/Pamir in central Asia and the Altai/Sayani in the Siberian mountains lie on the upwind side of the Tibetan Plateau and act as an initial barrier to the westerly jet stream and Siberian High. Asian glaciers contain records documenting the internal and external hydrological cycles over Eurasia and on the advection of fresh water to and from the Indian, Atlantic, Pacific, and Arctic Oceans. The Siberian Altai is the only mountain system in the Asian Arctic Basin that nourishes the great Siberian Rivers from alpine glaciers and thus has an impact on global thermohaline circulation. Asian and Siberian glaciers are ideal natural archives to extract environmental data relating: to past climate, de- and reforestation, biomass burning, permafrost freezing and thawing, and atmospheric pollution related to aeolian erosion of surrounding deserts and increasing anthropogenic emissions (Adhikary et al., 1995). Many potential ice core sites have yet to be investigated from the European Alps to Kamchatka and southward into Tibet and the Himalayas.
Several studies demonstrate that seasonal variations in glaciochemistry over the Himalayas/Tibetan Plateau display detailed monsoon signals characterized by δ18O variability related to strong removal of the heavy isotopic component during the intense monsoon rains that spread from the Indian coast to the Himalayas (Wushiki, 1977; Wake and Stievenard, 1995; Kang et al., 2000) and dramatic changes in ion concentration as a result of monsoon driven changes in source region (Mayewski et al., 1983; Wake and Mayewski, 1993; Kang et al., 2000). The δ18O signals from ice cores obtained on the Tien Shan and Siberian Altai, like many Asian ice core sites, have very distinct annual and seasonal variations up to -38â (Aizen and others, 2001; 2002; 2004a,b; Kreutz and others, 2001, 2003; Olivier and others, 2003). δ18O and δ3OH relationships reveal diverse moisture source contributions, for example, Siberian Altai glaciers receive moisture from the Atlantic and Pacific Oceans and the AralñCaspian closed drainage basin, that together account for two-thirds of the annual accumulation to these glaciers (Aizen and others 2004b). In central Asian and Siberian glaciers, temperature is the first order constraint on δ18O (Yao et. al., 1999; Aizen et al., 2004 a,b,c), with minimum values associated with coldest winter temperatures. In Southeast Asia, however, the amount of precipitation appears to be the first order constraint on δ18O. Under the monsoon conditions present in southeastern Asia, the largest precipitation amounts and highest summer temperatures are associated with the lowest stable isotope values (He, et. al, 2002; Aragual-Araguas and others, 1999; Aizen and others 2004c). A significant portion of the snow deposited in the Tien Shan is related to water vapor originating over the region around the Aral and Caspian Seas and is associated with development of the southeastern periphery of the Siberian High (Aizen and others 2004a).
Several Asian cores provide evidence of past patterns in atmospheric circulation and composition. A recent core from Dasuopu Glacier, Mt. Xixiabangma in the central Himalayas reveals that this site is sensitive to fluctuations in the intensity of the South Asian monsoon (Thompson et al., 2000) validating previous ice core results from a survey of Himalayan glacier fluctuations and an ice core from the Ladakh Himalaya (Mayewski et al., 1980, 1983, 1984). New results from ice cores to bedrock on Mt. Everest reveal instrumentally calibrated proxies for the Mongolian High, Southeast Asian Low (Qin et al., 2000; Zhang et al., 2003; Hou et al., 1999; Ren et al., 2004; Kang et al., 2002a, 2003) and anthropogenic impact on the chemistry of the atmosphere in the form of increased levels of ammonium and oxalate (Kang et al., 2001a, 2001b, 2002b).
South America
Ice cores recovered from South American glaciers and ice caps represent the only high-resolution (annual to decadal-scale) proxy records available from the inter-tropical Americas. In particular, ENSO variability, movement of the inter tropical convergence zone (ITCZ), and the interaction of the zonal westerly system over the past few decades to centuries can be investigated using the archive of atmospheric information preserved in Andean glaciers and ice caps.
Previous work in South America has focused largely on tropical latitudes. To fully resolve spatial and temporal climate variability in South America, including both physical (temperature, precipitation, atmospheric circulation) and chemical changes, an array of high resolved ice core records stretching from the southern tip of South America to the equator is required. Most glaciated peaks in South America contain glaciers on the order of 50-200 meters thick. Accumulation rates range from approximately 0.5-1 meter H2O eq./year in northern sites to perhaps 2-3+ meters H2 O eq./year in Patagonia. Thus, record lengths are variable, but data spanning at least the last 200-500+ years should be available at all sites. Much longer (millennial-scale) records are possible in places, as demonstrated by previous work (Thompson et al., 1998). Recent work by Swiss teams (Ginot et al., 2002) at Cerro Tapado, Illimani, and Chimborazo reveals good signal preservation at high elevation sites (above 5500 meters). A method to check signal preservation that can be utilized more productively is to identify tephra layers (via glass, trace element, and sulfur isotopic measurements) that should be abundant based on Andean volcanic activity (i.e., Cerro Hudson event in 1991). Previous studies in the southern reaches of South America suggest that well-preserved records with annual layering are contained within glaciers located higher than 2300 m (Schwikowski et al., 2002). Interpretation of ice core records from southern reaches of South America will be greatly strengthened by comparison with existing ice core records to the north (eg., Knusel et al., 2002; Thompson et al., 1998) and the extensive spatial array of ice cores collected and analyzed by the International Trans Antarctic Expedition (ITASE, Mayewski and Goodwin, 1997) to the south.