Lithogenic, Marine and Anthropogenic Aerosols in an Ice Core from the Saint Elias Mountains, Yukon, Canada: Lead-Aerosol Provenance and Seasonal Variability
As of 2002, Degree of Master of Science (MS) Quaternary and Climate Studies published under the auspices of the Climate Change Institute.
Abstract
Soluble major ions (Na+), trace metals (La, Ce, Pr, Al, Ti, Pb, Bi, Co, Cu, Zn, Sb and Cd) and stable radiogenic Pb isotopes (208Pb, 207Pb and 206Pb) were measured in a 61 meter-long (1970-2001 CE.) ice core from Eclipse Icefield (60.51 °N, 139.47°W, 3017 masl) in the St. Elias Mountains, Yukon, Canada in order to investigate intraannual variability in the concentrations of lithogenic, marine and anthropogenic aerosols and the sources of Pb pollution. Seasonal dust concentration ([Al]) varies by a factor of 2.3 and is highest during late spring and early summer and 54% of extreme dust events (dust concentrations above the 95th percentile) also occur during these two seasons. Dust concentration maxima are concurrent with both maximum trans-Pacific transport of Asian aerosols and dust storm frequency in Asia and the St. Elias Mountains. Sea salt concentrations ([Na+]) are highest during late autumn and early winter suggesting that the strength of the Aleutian Low and storm frequency and intensity exert a strong control on landward transport of marine air masses in the North Pacific. Pb concentrations are highest during spring and early summer and are slightly elevated during early winter, however the magnitude of intra-annual variability of Pb is less than dust and sea-salt. The slight seasonal variations in Pb concentrations are likely controlled by dust-Pb interactions and atmospheric transport efficiency. An average Pb enrichment factor of 31.8 and Pb isotope ratios significantly less radiogenic (average 208Pb/207Pb = 2.448+0.007 and 206Pb/207Pb = 1.176±0.012) than those in upper continental crust indicate that Pb deposited at Eclipse Icefield is predominantly anthropogenic and is likely derived from coal combustion in Eurasia and China. The Pb at Eclipse Icefield is isotopically distinct from Pb measured polluted air ascribed to Arctic Haze at Barrow, Alaska. Our results suggest that Arctic Haze, as observed in the North American Arctic, does not penetrate as far south as the St. Elias Mountains, rather, polluted air masses in the St. Elias Mountains are transported across the North Pacific. The Pb-isotope ratios from Eclipse Icefield provide direct geochemical evidence of trans-Pacific pollutant transport and evidence of significant Asian aerosol deposition at 3000 meters above sea level. Furthermore, Pb of Chinese origin becomes increasingly prevalent at Eclipse Icefield from the 1970's to the 1990's, likely as a result of increased coal combustion due to increasing energy demands from China's developing populations.