Variable Hf-Sr-Nd Radiogenic Isotopic Compositions in a Saharan Dust Storm over the Atlantic: Implications for Dust Flux to Oceans, Ice Sheets and the Terrestrial Biosphere

Abstract

Isotopic characterization of aerosol mineral particles (atmospheric dust) of varying sizes is essential in classifying source areas, and for determining the source of dust deposited over oceans and icesheets. However, the e_ect of atmospheric transport on radiogenic isotopic compositions is not well constrained, making provenance interpretation di_cult. In order to investigate the isotopic variability of 176Hf/177Hf, 87Sr/86Sr and 143Nd/144Nd we analyzed eight airborne dust samples in two size fractions collected along a cross-Atlantic transect through a dust storm originating in the Sahara in late 2002. Past measurements of 176Hf/177Hf, 87Sr/86Sr and 143Nd/144Nd of dust have focused primarily on coarse sized particles (<30 _m), whereas far field deposition is primarily finer particles (<2 _m). Strontium or neodymium isotopic sorting based on distance is not evident in our dataset; however, the combined isotopic ratios of the dust collected suggests a Saharan origin. Hafnium isotopic compositions show an east to west trend towards more radiogenic compositions across the Atlantic, suggesting grain and mineral sorting during dust transport along the _4000 km transect. Transport models with variable dust particle diameter and wind speed demonstrate that the Hf isotopic compositions can be explained by the loss of the high-density mineral zircon during transport of dust from the source area. Modeling of this “zircon e_ect” in the Hf isotopic composition of marine, terrestrial and glacial dust deposits can reveal additional information concerning dust transport and sources in the geologic past.