Re-evaluation of the composition of sediments from the Murray Darling Basin of Australia as a Potential Source Area for airborne dust to EPICA Dome C in Antarctica. Reply to Comment on “Lead isotopic evidence for an Australian source of aeolian dust to Antarctica at times over the last 170,000 years” by P. De Deckker, M. Norman, I.D. Goodwin, A. Wain

Abstract

We re-examined the data presented by De Deckker et al. (2010; Lead isotopic evidence for an Australian source of aeolian dust to Antarctica at times over the last 170, 000 years. Palaeogeography, Palaeoclimatology, Palaeoecology 285, 205–223), which argued for some parts of the Murray Darling Basin (MDB) in south-eastern Australia to have been potential source areas (PSA) for dust in the EPICA Dome C ice core. This was done in light of the comments tabled by Kamber et al. (Comment on “Lead isotopic evidence for an Australian source of aeolian dust to Antarctica at times over the last 170,000 years” by P. De Deckker, M. Norman, I.D. Goodwin, A. Wain and F.X. Gingele [Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 205–223], 2010-this issue) suggesting that the MDB samples are variably contaminated by anthropogenically-produced Pb that originated from the Broken Hill mine, from smelters, and from agricultural practices and coal burning. We argue, first of all, that the MDB samples are not as comprehensively contaminated with Pb as claimed by Kamber et al., and the Pb isotope data presented by De Deckker et al. (2010) do provide useful information about potential dust source. Following the approach of Kamber and colleagues (Marx et al., 2010), we compare the Pb isotopic compositions of MDB sediments against their Pb content and diagnostic elemental ratios such as Pb/Ta, and Pb/Ga to evaluate possible anthropogenic contamination. We find that 3 of these MDB samples have notably high Pb concentrations, anomalously high Pb/Ta and Pb/Ga ratios, and Pb isotopic compositions that are consistent with anthropogenic contamination from a Broken Hill-type Pb source. The remaining 20 samples show a wide range of isotopic compositions and no correlation with either Pb content or element ratios, consistent with natural signatures. The 3 contaminated samples are located in heavily used agricultural areas in the Murray sub-basin but, importantly, none of these 3 samples were considered by De Deckker et al. (2010) to represent potential source areas for dust in the Dome C ice core. Secondly, previous studies have shown that significant Pb contamination in Australian surficial sediments tends to be localised, and that sub-surface samples can be used reliably for comparison with deposits that contain airborne dust. Thirdly, we argue that valid evaluation of potential sedimentary dust sources can only be made on the same grain size fractions as those found in Antarctic ice cores, as it was recently demonstrated by Feng et al. (2010) and Vallelonga et al. (2010) that Pb isotopic ratios of aeolian deposits and Australian sediments vary with grain size. Our measurements were made on <2 μm fractions of MDB sediments, which are directly comparable to the particle size of ice core dust. We, therefore, maintain that the Darling sub-basin represents a potentially significant source area for dust delivered to the Dome C site through time.