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Accueil du site → Doctorat → Australie → 2007 → A HOLOCENE RECORD OF TRANS-TASMAN DUST TRANSPORT : QUANTIFYING DUST EMISSIONS FROM EASTERN AUSTRALIA USING GEOCHEMICAL PROXIES.

University of Queensland (2007)

A HOLOCENE RECORD OF TRANS-TASMAN DUST TRANSPORT : QUANTIFYING DUST EMISSIONS FROM EASTERN AUSTRALIA USING GEOCHEMICAL PROXIES.

Marx, Samuel Kenneth

Titre : A HOLOCENE RECORD OF TRANS-TASMAN DUST TRANSPORT : QUANTIFYING DUST EMISSIONS FROM EASTERN AUSTRALIA USING GEOCHEMICAL PROXIES.

Auteur : Marx, Samuel Kenneth

Grade : Doctor of Philosophy PhD 2007

Université de soutenance : University of Queensland. School of Geography, Planning and Architecture

Résumé
The transport of dust from the arid and semi-arid source regions of central and eastern Australia and its subsequent deposition in New Zealand is investigated using geochemical proxies of contemporary and geologic (Holocene) records of dust export. Australian dusts are characterized using a geochemical fingerprinting technique that allows their provenance to be established. Certain elements were found to be affected by pollution, mineral sorting, weathering/​solubility, or method constraints and were not useful for provenancing dusts. The relative abundance of 20 other elements were found to reflect the chemistry of source area sediments and can be used to provenance long-range Australian dust deposited in New Zealand. This is achieved by matching the trace element chemistry of dusts believed to have undergone long-range trans-Tasman transport with those of potential Australian source areas, allowing the source area of the dust to be established at geologic-catchment scale - a spatial resolution not previously achieved. To account for dust from multiple source areas being transported during trans-Tasman dust transport events, and the contamination of long-range Australian dust deposited in New Zealand by local material, a mixing model was developed to determine the relative contribution of up to three source areas in contributing dust to the deposited sample. The application of this model allowed the first continuous records of dust emissions from eastern Australian to be constructed in New Zealand. In addition, this model also allowed Australian dust deposition rates in New Zealand to be linked directly to changes in source area conditions for the first time. The variability in, and controls on contemporary rates of Australian dust transport over New Zealand were established using an 11 year record of weekly atmospheric concentrations of the radio-nuclide 210Pb from New Zealand. Lead-210 is the daughter product of 222Rn, a gas which escapes into the atmosphere from continental crust. Consequently, concentrations of 222Rn (and by analogy 210Pb) are high over Australia, while they are very low in oceanic settings such as New Zealand. Lead-210 is extremely particle reactive, so once formed in the atmosphere, attaches immediately to aerosols implying it could be used as a dust tracer. Lead-210 activity was measured on dusts collected from New Zealand that had previously been determined to be at least partly of Australian origin using the trace element provenance model. In addition, 210Pb activity was also measured on the source sediments of these samples (from both New Zealand and Australia), also established using the trace element model. Results showed that 210Pb activity in the dust samples was orders of magnitude higher than in source sediments. This is because dust scavenges 210Pb from the atmosphere, thereby showing that 210Pb is a highly effective proxy measure of dust. Lead-210 activity in the long-range dusts collected from New Zealand was found to be related to the percentage of Australia material in a linear fashion. As a result, atmospheric 210Pb concentration data from New Zealand was converted into the first high resolution, decadal record of Australian dust flux. Clear seasonality was apparent, with dust flux highest in autumn-winter following summer river flows in dust source which results in sediment being unavailable for aeolian transport. Inter-annual variability in dust concentrations also occurred and this was postulated to be associated with the El Niño – Southern Oscillation (ENSO) phenomenon and its relationship to the hydro-meteorology of dust source areas in Australia. Consequently, it was concluded that the condition and availability of sediment within dust source areas is a major control on Australian dust emission rates. A Holocene record of Australian dust deposition in New Zealand was constructed from a peat core extracted from an alpine ombrotrophic rain-fed mire in Central Otago, New Zealand. The dust content of the core was determined by combusting the organic component, leaving the mineral dust. Dust samples through the core were analysed by ICP-MS and their provenance was established. This showed that dust in the core was primarily of Australian origin. Four phases of significant Australian dust deposition were identified in the 8000 year old core ; 1) a period of low dust deposition (0.142 g m-2 yr-1) from 7000 to 5000 B.P corresponding to a lacusterine phase in Lake Eyre, the most important Australian dust source ; 2) a marked increase in dust deposition after 5000 B.P (0.577 g m-2 yr-1) coincident with Lake Eyre entering an ephemeral playa stage, and at the same time as global proxies suggest enhanced ENSO variability ; 3) a peak in the deposition of Australian dust between 2500-1700 B.P (1.558 g m-2 yr-1) corresponding with a period of heightened ENSO variability ; and 4) a return to lower dust deposition after 700 B.P (0.2984 g m-2 yr-1). The dust deposition record in the core is therefore an excellent proxy of climate variability and associated environmental change, with high dust deposition occurring during periods of marked climate instability. In particular, Australian dust was found to serve as an excellent proxy for ENSO intensity as it is a significant control on climate variability in the region. The dust transport and deposition rates measured by this study show that Australia has been a significnat source of dust in the Southern Hemisphere throughout the mid to late Holocene. Average contemporary Australian dust concentrations measured in New Zealand of 5.3 ?g m-3 are comparable to dust concentrations downwind of other major global dust sources such as the Sahara measured at Miami, Florida. In addition, Australian dust deposition rates in the peat core from New Zealand were also comparable to rates of long-range dust deposition in many other settings, such as the deposition of North African dust in the European Alps. As a result, Australian dust transport is likely to play a significant role in bio-geophysical systems in the region, such as soil formation, with Australian dust found to contribute between 50-100% of mineral content in some New Zealand settings.

Présentation (National Library of Australia)

Page publiée le 12 septembre 2007, mise à jour le 3 mai 2019