Lead, Nd and Sr isotope records of pelagic dust: Source indication versus the effects of dust extraction procedures and authigenic mineral growth

Abstract

Eolian silicate dust of the 328 cm long piston core KODOS 02-01-02 from the northeast equatorial Pacific was analyzed for chemical compositions and Nd, Sr, and Pb isotopic ratios to evaluate their use as hemispheric dust source discriminator and to examine the potential influence of authigenic smectite on these source signals. Neodymium and Sr isotope ratios of the core determine a change from southern to northern hemisphere dust deposition at about 7.6 Ma. Samples deposited between 15.5 and 7.6 Ma resemble southeast equatorial Pacific surface and down-core dust values, while younger samples correspond to signatures typical for dust derived from central Asia. The 15.5 to 7.6 Ma lower interval of the core is smecite-rich; however, previously proposed chemical fingerprints for authigenic smectite such as positive Eu anomalies or high Fe/Al ratios are not observed in the KODOS samples. The up to 40% smectite in the KODOS core dust are thus interpreted to represent largely detrital smectite, and no evidence is found for authigenic modification of Nd–Sr isotopic or element ratio source signals of the dust records. Lead isotope results of the core lack prominent down-core variation with changing source regimes as reported above. A compilation of Pacific Pb isotope dust records reveals that Pb isotope ratios vary as a function of silicate dust extraction procedures. The step of removing biogenic silica thereby appears critical. Use of NaOH appears most effective, while minimally attacking eolian silicate material. It is concluded that of the Pb isotopic dust records published so far, only a few are useful for dust source identification. However, once the silicate dust extraction procedure is further optimized for retrieving the pristine silicate dust Pb isotopic composition, specific source signatures should become well resolvable.