Isotopic constraints on the source of Argentinian loess - With implications for atmospheric circulation and the provenance of Antarctic dust during recent glacial maxima

Abstract

We present rare-earth element (REE) and Sr-Nd isotopic data for Argentinian loess (28-38°S) with two aims: (1) to examine the source regions of Argentinian loess and the constraints that these put on palaeo-wind directions; (2) to further investigate the source of Antarctic ice-core dust and to test the hypothesis that some of it could be derived from a region to the north of Patagonia - into which the dry, dusty, westerly dominated Patagonian climate expanded during Quaternary glacial maxima. Sr-Nd isotopic data for Argentinian loess from north of 37°S are distinct from Patagonian loess compositions in that they have more radiogenic Sr (87Sr/86Sr=0.7059-0.7123) and less radiogenic Nd (ε Nd =-0.8 to -6.4). REE patterns and Sr-Nd isotopic values are relatively homogeneous for multiple samples taken from single loess sections but show significant differences between sections. In general, there is a northward change from Patagonia-like REE patterns and isotopic values away from volcanogenic signatures and towards those that are more like the continental crust. The latitudinal Nd isotopic pattern is remarkably similar to that for Andean volcanic rocks and suggests derivation of loess from the Andes by more-or-less direct westerly transport. For loess sections in the north, the data imply a contribution from Palaeozoic gneisses to the northwest in the Chilean Altiplano. Sr-Nd data for extra-Patagonian Argentinian loess north of 37°S do not support a significant role for this source region in supplying dust to Antarctica at the last glacial maximum. This conclusion contrasts with previous studies that suggest a significant northward shift in the climatic belts - and in particular the westerlies and the Antarctic Polar Front - during Quaternary glacial maxima. Very systematic relations between the Sr-Nd isotopic composition of loess and their Andean source highlights shifts in the Sr isotopic composition of loess to more radiogenic values and strongly suggests that the slight offset between Patagonian loess and ice-core dust identified by previous workers is due to grain-size differentiation effects.