Isotopic and elemental tracers in ice and snow as indicators of source regions of aerosols and changing environmental conditions

Abstract

Pioneering studies of lead (Pb) concentrations in polar ice by Clair C. Patterson and co-workers (e.g. Murozumi et al., 1969; Boutron and Patterson, 1983, 1986) revealed important information on climatic changes dating many thousands of years in the past and the effect that humans have had on these pristine environments. Rosman and co-workers (e.g. Rosman et al., 1993, 1994a) extended this research utilising the isotopic composition of Pb preserved in the ice to identify the source regions of both natural and anthropogenic Pb transported to the polar regions, thereby adding significantly to the understanding of atmospheric transport mechanisms and the impact of continental anthropogenic activity on the pristine polar environments. This thesis extends this area of research by investigating seasonal and short-term variability in Pb, Ba and In concentrations and Pb isotopes through the accurate sectioning of decontaminated glacial ice cores at high spatial resolution. This improves our understanding of the processes impacting on the deep ice core records, which, in turn, expands the interpretation and application of these records.A new sample preparation technique, incorporating a mechanised circular saw blade arrangement into the lathe design, first described by Candelone et al. (1994), has been developed to sample glacial ice cores at high spatial resolution with minimal contamination. This technique was extensively tested using artificially made ice cores, prepared from ultra-pure water, to quantify and minimise the Pb, Ba and In contamination associated with the process. The residual contamination in each inner core slice from the procedure amounted to 0.2 ± 0.2 pg Pb with [superscript]206Pb/[superscript]207Pb, [superscript]208Pb/[superscript]207Pb and [superscript]206Pb/[superscript]204Pb ratios of 1.16 ± 0.12, 2.35 ± 0.16 and 15.3 ± 6.7 respectively, 1.5 ± 0.4 pg Ba and 0.6 ± 2.0 fg In. This technique was then utilised to obtain high resolution analyses of Pb, Ba and In concentrations, and Pb isotopes in ice core sections recovered from Law Dome, East Antarctica and from the East Rongbuk Glacier on the northern slope of Mount Everest in the Himalaya.Law Dome, East Antarctic Holocene ice cores provide strong evidence for the transport of anthropogenic Pb from Australia to the Law Dome environment during the late 19th century, with [superscript]206Pb/[superscript]207Pb and [superscript]208Pb/[superscript]207Pb ratios as low as 1.061 ± 0.001 and 2.339 ± 0.002 respectively, in close agreement with the Pb isotopic characteristics of Australian Broken Hill Pb ores. The onset of anthropogenic pollution of the Law Dome environment occurred in spring 1889 AD, associated with the onset of Pb ore smelting activities in Port Pirie, South Australia. Seasonal variability is most clearly seen in anthropogenic Pb and Ba (as a proxy for mineral dust) concentrations where peaks in concentrations generally occur in the autumn and spring periods, with consistently low concentrations during winter. Lead isotopic [superscript]206Pb/[superscript]207Pb ratios are inversely correlated with residual (anthropogenic + crustal) Pb concentrations with the lowest ratios occurring during periods of high Pb concentrations. The observed seasonal variability is associated with the annual cycle in position and strength of the Antarctic Circumpolar Trough, a region of low pressure situated off the coast of Antarctica, which moves closest to the Antarctic coast and increases in strength during the autumn and spring seasons. Concurrently, a strengthening of the Southern Ocean westerlies entrains and transports more atmospheric impurities from continental regions such as Australia. The low Pb concentrations and relatively more radiogenic [superscript]206Pb/[superscript]207Pb ratios that appear in the winter layers are indicators of aerosol inputs into the Law Dome environment from a well mixed Southern Hemisphere atmospheric background.Law Dome deep ice core sections, sampled at short-term resolution, show variability in all measured species over time indicating a general instability in temperature (as indicated by δ18O compositions) and environmental conditions (as indicated by elemental concentrations) prevailing during the periods of time integrated by the ice. Based on Pb isotopic [superscript]206Pb/[superscript]207Pb ratios, which reach values of 1.225 ± 0.004 and 1.230 ± 0.003 in glacial, and last glacial – Holocene transition ice respectively, noticeable volcanic Pb contributions are found to occur in both time periods, reaching as high as ~39% and ~46% respectively. These increases are associated with a decrease in mineral dust concentrations due to relatively warmer climatic conditions prevailing within the times integrated by the ice core sections. A link between temperature and environmental conditions in glacial ice is observed when changes in δ18O compositions (as a proxy for temperature) exceed 0.24‰. In contrast, there is no link between temperature and environmental conditions during the last glacial – Holocene transition period, demonstrating a complicated climatic and environmental state existing at Law Dome at this time that incorporates aspects of both cold and warm climates.The high spatial resolution sectioning of continental glacial ice cores recovered from the East Rongbuk Glacier on the northern slopes of Mount Everest, and dated to the 18th and 20th centuries, has similarly shown variability in all measured species over time. Elemental Pb, Ba and In concentrations are very well correlated in all ice core sections confirming the environment of the East Rongbuk Glacier, within the investigated time periods, to be strongly controlled by the input of mineral dust. Isotopic [superscript]206Pb/[superscript]207Pb ratios in 18th century ice range from 1.18 – 1.21 which are attributed to a natural mineral dust background devoid of any influences of anthropogenic [or volcanic] activity. During 18th century monsoon periods (when mineral dust concentrations are generally low), local Himalayan material plays an important role on crustal inputs into the sampling region, with some inputs of Indian derived material possibly sourced to the Indian Peninsula, and Trans-Himalayan belt/Lhasa block material sourced to locations slightly north of Mount Everest. During non-monsoon periods (when mineral dust concentrations are generally high), it is likely that mineral dust from regions in northern Africa and central Asia are impacting on the high altitude Himalaya, with some possible contributions from the arid regions in north-western India and local Himalayan material.Based on elemental (Pb/Ba) and Pb isotopic ratios, there is no indication ofanthropogenic [or volcanic] Pb contributions in the 20th century East RongbukGlacier ice core samples indicating that, prior to 1952 AD, the high altitude samplingsite was isolated from the effects of anthropogenic Pb pollution. Consequently, inthe 20th century, Pb transported to the East Rongbuk Glacier site is sourced tomineral dust. An increase in elemental concentrations is observed in 20th century ice,indicating increased mineral dust deposition relative to the 18th century.Concurrently, a marked change in the isotopic characteristics of 20th century ice coredata, relative to 18th century ice core data, is observed, with a clear segregation ofmonsoon and non-monsoon data and the transition of 20th century data to lessradiogenic values. This adjustment likely reflects environmental changes that haveoccurred from the 18th to the 20th centuries.