ORIGIN AND PALEOCLIMATIC SIGNIFICANCE OF LATE QUATERNARY LOESS IN NEBRASKA

We present new data on the provenance and paleoclimatic significance of last-glacial (Peoria) loess in Nebraska. Spatial variability of particle size indicates paleowinds from the northwest, in disagreement with climate models that generate northeasterly winds from a glacial anticyclone. Geochemical data show variability in the amount of carbonate leaching, temporally and spatially. Decreased syndepositional carbonate leaching upsection mirrors progressively increasing mass accumulation rates. Syndepositional leaching of carbonate was greatest at localities in eastern Nebraska, and lower westward into eastern Colorado. We hypothesize that this reflects an eastward-increasing moisture gradient, similar to that of today. The reconstructed moisture gradient is consistent with a zonal circulation pattern, in agreement with westerly or northwesterly paleowinds at the time of loess deposition. The origin of Peoria loess in the central Great Plains has been debated for almost six decades. Isotopic and geochemical data indicate that the most important source in Nebraska is fine-grained volcaniclastic siltstone of the White River Group, with smaller additions from other sediments. The Nebraska Sand Hills was also a source of loess, but probably only in areas proximal to this major dune field. It is likely, however, that fine-grained silts were transported episodically through the Nebraska Sand Hills from White River Group and other sources to the north. A new computer model of climate and dust generation [Mahowald et al., 2006, JGR] simulates the central Great Plains as a significant non-glacial dust source during the last glacial period, in good agreement with our results.