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Eolian History of North America

TASK 2: Understand the paleoclimatic significance of loess

Why is loess important to study?

Loess deposits contain valuable records of paleoclimates, or climates of the past. This task seeks understanding of the paleoclimatic role of loess deposits, which cover approximately 10% of the Earth's surface. Loess deposits can yield valuable information about past wind direction, past wind strength, moisture balance, and degree and type of vegetation cover. In between loess deposits, we often find ancient, buried soils that mark periods when loess was not being deposited. These buried soils, called paleosols, provide valuable information about past climate and vegetation.

	Figure 11: Diagram showing the nature of the loess stratigraphic record. In most regions, including much of North America, Europe, and China, loess was deposited during glacial periods and soils were formed during interglacial periods. Soils that become buried by younger loess are called "paleosols."
	Figure 11a: Photograph showing an unusually complete, long-term loess-paleosol record near Elba, Nebraska; the last interglacial-glacial cycle is represented by the Sangamon paleosol (last interglacial) and Peoria loess (last glacial). The modern soil at top began forming at the beginning of the present interglacial period.

A major concern in the world today is climate of the future. Forecasts about future climates are based largely on computer-generated models of the atmosphere and how it may change due to increases in greenhouse gases or other changes in the Earth's systems. These computer models of climate are becoming more and more sophisticated, but they still require testing. An effective way to test climate models for the future is to have the same computer models generate climates of the past and see how well they work. To do this, however, one must understand what the climate of the past was like. Loess deposits, with their paleosols, are often well suited for reconstructing past climates, because they span extremes of climates, full-glacial to full-interglacial climates.

Where are we studying loess?

• Mississippi River valley
• Great Plains of the United States
• Alaska

Figures 12-13 combined: Map showing the distribution of loess (orange) in North America and regions being studied in our project (in red).

What have we learned so far?

• Based on thickness and particle-size trends, loess deposits in the Great Plains and Midwest appear to have been deposited by winds from the northwest during the last glacial period. However, climate models seem to show winds from the northeast during this period. Thus, the loess record indicates that climate models may need to be tuned to match this wind pattern of the last ice age.

Figure 14: Map showing the Laurentide ice sheet during the last glacial period and the winds that are modeled to have developed from the high pressure cell over it. Note that these winds differ from the past winds that have been inferred from loess deposits. See Muhs & Bettis, 2000, Quaternary Research.

• Optically stimulated luminescence (OSL) dating of loess in Nebraska shows that loess deposition rates here during the last glacial period may have been higher than anywhere else in the world. The fine-grained portion of this loess, while still in transit, may have altered the overall solar radiation balance in the region, making the last-glacial cold period last longer than it otherwise would have.

Figure 15: Stratigraphic sections of thick loess in Nebraska and optically stimulated luminescence (OSL) ages in thousands of years (ka). The loess deposition rates here are among the highest in the world for the last glacial period and the dusty atmosphere may have cooled the region even after the last glacial climate had ended elsewhere. See Roberts et al., 2003, Quaternary Research.







• Isotopic and geochemical studies show that much of the loess in the Great Plains region of Nebraska, Colorado, and Kansas is not derived from glacial flour, but rather from silt-rich, but poorly consolidated bedrock called the White River Formation. This rock forms the famous Badlands National Park area of South Dakota. This finding is important because it means that during the last glacial period, conditions on the northern Great Plains were very dry and windy, with even less vegetation cover than is the case today.

Figure 16: Using lead (Pb) isotopes in K-feldspar as "fingerprints" for determining the sources of last-glacial (Peoria) loess in eastern Colorado. Pb isotopes define distinct compositions for South Platte River silts (possibly of glacial origin) and siltstones of the White River Formation (non-glacial origin). Loess silts plot in both fields and between the two fields, indicating that both source sediments were important. See Aleinikoff et al., 1999, Geological Society of America Bulletin.

• Unlike the Great Plains and Midwest, stratigraphic studies in Alaska show that there is little loess that dates to the last glacial period, even though glaciers were more extensive then and there should have been abundant loess. We interpret this to mean that during the last glacial period, Alaska was cold, dry, windy and had very little vegetation cover. There may have been little ability for the landscape to trap loess at that time. This reconstruction of climate and vegetation is consistent with fossil pollen records for this time period, based on studies by the USGS Ecosystem and Climate History of Alaska project (http://esp.cr.usgs.gov/research/alaska/).
• The last interglacial period, about 120,000 years ago, was a period of greater warmth and perhaps greater moisture than present in Alaska. This is based on fossil pollen and chemical properties of a paleosol found in loess in central Alaska.

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Task 2 Related Products

Muhs, D.R., Aleinikoff, J.N., Stafford, T.W., Jr., Kihl, R., Been, J., Mahan, S.A., and Cowherd, S., 1999, Late Quaternary loess in northeastern Colorado: Part I--Age and paleoclimatic significance: Geological Society of America Bulletin, v. 111, p. 1861-1875. [DOWNLOAD PDF]

Aleinikoff, J.N., Muhs, D.R., Sauer, R.R., and Fanning, C.M., 1999, Late Quaternary loess in northeastern Colorado: Part II--Pb isotopic evidence for the variability of loess sources: Geological Society of America Bulletin, v. 111, p. 1876-1883. [DOWNLOAD PDF]

Muhs, D.R., Ager, T.A., Been, J.M., Rosenbaum, J.G., and Reynolds, R.L., 2000, An evaluation of methods for identifying and interpreting buried soils in late Quaternary loess in Alaska: U.S. Geological Survey Professional Paper 1615, p. 127-146.

Muhs, D.R., and Bettis, E.A., III, 2000, Geochemical variations in Peoria loess of western Iowa indicate paleowinds of midcontinental North America during last glaciation: Quaternary Research, v. 53, p. 49-61. [DOWNLOAD PDF]

Muhs, D.R., and Zarate, M., 2001, Eolian records of the Americas and their paleoclimatic significance, in Markgraf, V., ed., Interhemispheric Climate Linkages, San Diego, Academic Press, p. 183-216.

Muhs, D.R., Ager, T.A., and Begét, J.B., 2001, Vegetation and paleoclimate of the last interglacial period, central Alaska: Quaternary Science Reviews, v. 20, p. 41-61. [DOWNLOAD PDF]

Muhs, D.R., Bettis, E.A., III, Been, J., and McGeehin, J., 2001, Impact of climate and parent material on chemical weathering in loess-derived soils of the Mississippi River Valley: Soil Science Society of America Journal, v. 65, p. 1761-1777. [DOWNLOAD PDF]

McGeehin, J., Burr, G.S., Jull, A.J.T., Reines, D., Gosse, J., Davis, P.T., Muhs, D., and Southon, J.R., 2001, Stepped-combustion 14C dating of sediment: A comparison with established techniques: Radiocarbon, v. 43, p. 255-261. [DOWNLOAD PDF]

Muhs, D.R., Ager, T.A., Bettis, E.A., III, McGeehin, J., Been, J.M., Begét, J.E., Pavich, M.J., Stafford, T.W., Jr., and Stevens, D.S.P., 2003, Stratigraphy and paleoclimatic significance of late Quaternary loess-paleosol sequences of the last interglacial-glacial cycle in central Alaska: Quaternary Science Reviews, v. 22, p. 1947-1986. [DOWNLOAD PDF]

Bettis, E.A., III, Muhs, D.R., Roberts, H.M., and Wintle, A.G., 2003, Last glacial loess in the conterminous U.S.A.: Quaternary Science Reviews, v. 22, p. 1907-1946. [DOWNLOAD PDF]

Muhs, D.R., and Bettis, E.A., III, 2003, Quaternary loess-paleosol sequences as examples of climate-driven sedimentary extremes: Geological Society of America Special Paper 370, p. 53-74. [DOWNLOAD PDF]

Roberts, H.M., Muhs, D.R., Wintle, A.G., Duller, G.A.T., and Bettis, E.A., III, 2003, Unprecedented last glacial mass accumulation rates determined by luminescence dating of loess from western Nebraska: Quaternary Research, v. 59, p. 411-419. [DOWNLOAD PDF]

Busacca, A.J., Begét, J.E., Markewich, H.W., Muhs, D.R., Lancaster, N., and Sweeney, M.R., 2004, Eolian Sediments, in: Gillespie, A.R., Porter, S.C., and Atwater, B.F, eds., The Quaternary Period in the United States: Amsterdam, Elsevier, p.275-309.

Muhs, D.R., McGeehin, J.P., Beann, J., and Fisher, E., 2004, Holocene loess deposition and soil formation as competing processes, Matanuska Valley, southern Alaska: Quaternary Research, v. 61, p. 265-276. [DOWNLOAD PDF]

Muhs, D.R., and Budahn, J.R., 2006, Geochemical evidence for the origin of late Quaternary loess in central Alaska: Canadian Journal of Earth Sciences, v. 43, p. 323-337. [DOWNLOAD PDF]

Muhs, D.R., 2006, The nature of the Quaternary loess record, in: Elias, S. (ed.), The Encyclopedia of Quaternary Sciences: Amsterdam: Elsevier, in press.

Muhs, D.R., 2006, Eolian sediments, soils, and paleosols in the Quaternary: An overview, in: Elias, S. (ed.), The Encyclopedia of Quaternary Sciences: Amsterdam: Elsevier, in press.

Roberts, H.M., Muhs, D.R., and Bettis, E.A. III, 2006, Loess records of North America, in: Elias, S. (ed.), The Encyclopedia of Quaternary Sciences: Amsterdam: Elsevier, in press.

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