Grain size effect on Sr and Nd isotopic compositions in eolian dust: Implications for tracing dust provenance and Nd model age

Abstract

Strontium (Sr) and neodymium (Nd) isotopic compositions enable identification of dust sources and reconstruction of atmospheric dispersal pathways. The Sr and Nd isotopic compositions in eolian dust change systematically with grain size in ways not yet fully understood. This study demonstrates the grain size effect on the Sr and Nd isotopic compositions in loess and 2006 dust fall, based on analyses of seven separated grain size fractions. The analytical results indicate that Sr isotopic ratios strongly depend on the grain size fractions in samples from all types of eolian dust. In contrast, the Nd isotopic ratios exhibit little variation in loess, although they vary significantly with grain size in samples from a 2006 dust fall. Furthermore, Nd model ages tend to increase with increasing grain size in samples from all types of eolian dust. Comparatively, Sr isotopic compositions exhibit high sensitively to wind sorting, while Nd isotopic compositions show greater sensitively to dust origin. The principal cause for the different patterns of Sr and Nd isotopic composition variability with grain size appears related to the different geochemical behaviors between rubidium (Rb) and Sr, and the similar geochemical behaviors between samarium (Sm) and Nd. The Nd isotope data indicate that the various grain size fractions in loess have similar origins for each sample. In contrast, various provenance components may separate into different grain size fractions for the studied 2006 dust fall. The Sr and Nd isotope compositions further confirm that the 2006 dust fall and Pleistocene loess in Beijing have different sources. The loess deposits found in Beijing and those found on the Chinese Loess Plateau also derive from different sources. Variations between Sr and Nd isotopic compositions and Nd model ages with grain size need to be considered when directly comparing analyses of eolian dust of different grain size.

Sr–Nd isotopic constraints on detrital sediment provenance and paleoenvironmental change in the northern Okinawa Trough during the late Quaternary

“…Grain size may influence Sr isotope ratios of detrital materials due to varying mineralogy, and hence isotopic compositions of fractions of different sizes. Fine-grained fractions tend to be enriched in radiogenic 87 Sr because of high Rb/Sr ratios in micas in general and biotite in particular (Dasch, 1969;Feng et al, 2009). In contrast, fractions of different sizes from the same source have similar ε Nd values (i.e., Nd isotope composition is almost independent of grain size), which allows for reconstruction of particle provenance in deep-sea sediments (Goldstein et al, 1984;Jiang et al, 2013;Tütken et al, 2002).…”

Tracking potential source areas of Central European loess:examples from Tokaj (HU), Nussloch (D) and Grub (AT)

“…This is probably due to the presence of secondary carbonates. Several studies have shown that strontium isotope ratios in aeolian sediments are grain-size dependent and increase with decreasing grain size, since more radiogenic Sr is present in the ne fraction [42]. This coupling between grain size and Sr/ Sr ratio is usually not observed for Nd/ Nd.…”

“…This coupling between grain size and Sr/ Sr ratio is usually not observed for Nd/ Nd. The authors conclude that isotopic compositions should be measured on speci c grain size fractions of the samples instead of using bulk material [5,12,42]. However, there is some debate on this topic and other authors have argued that it is only the bulk fraction that contains the entire source area information of a sample [9,10,15].…”

Step heating of 40Ar/39Ar standard mineral mixtures: Investigation of a fine-grained bulk sediment provenance tool

“…Bulk Nd and Sr isotopic techniques are generally considered more reliable source tracers in fine-grained sediments Goldstein et al, 1984;Hemming et al, 1998;Goldstein and Jacobsen, 1988;McLennan et al, 1990;Clift et al, 2008;McLennan et al, 1990) but even these isotopic systems can vary with grain size within the fine-grained fraction (Feng et al, 2009). Sr isotopes can also be influenced by low-temperature alteration and variations in chemical vs. physical weathering (Clauer and Chaudhuri, 1995).…”