Vignettes > Löess, Soil Development, and Glaciation of the Mississippi Valley
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Löess, Soil Development, and Glaciation of the Mississippi Valley

Donald T. Rodbell
Union College


Location

Continent: North America
Country: United States of America
State/Province: Tennessee
City/Town: various
UTM coordinates and datum: none

Setting

Climate Setting: Humid
Tectonic setting: Craton
Type: Process


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Distribution of major loess deposits on Earth. From Pye, 1995.


Southern margin of the Laurentide Ice Sheet during the maximum of the last Ice Age, and three loess exposure studied (red dots) along the lower Mississippi River Valley. Rodbell et al., 1997.


An ~11m thick exposure of loess near the village of Hornbeak in western Tennessee. The color variations visible reflect various states of soil development; 4 discrete löess layers were documented at this site. Rodbell et al., 1997.


Soil properties with depth below the ground surface at the Hornbeak, Tennessee loess exposure (Fig. 3). Rodbell et al., 1997.


Description

Deposits of fine-grained (silt-fine sand) sediment that date to recent Ice Ages can be found on nearly every major land mass on Earth (Pye, 1995). Deposition of this sediment, which is known as löess, is attributed to the climatic conditions that prevailed during the Ice Ages, and to the abundant supply of silt-sized particles generated by rock-to-rock abrasion within alpine and continental glaciers. This glacially-derived sediment added to the heightened supply of fine grained-sediment from non-glacial sources such as soil erosion from the desertification of continental interiors, dessication of some lake basins, and frost shattering of rocks. Increased wind speed, especially in the middle latitudes entrained these sediments depositing the silt and fine sand fractions within several hundred kilometers of their source while dispersing the finest clay-sized particles around the globe. The dusty Ice Ages are thus recorded in many parts of the world by deposits of löess; these range in thickness from centimeters to more than one hundred meters for the non-glacial löess deposits of central China.

Löess deposits are among the best preserved and most continuous of the land-based archives of global climate. Unlike glacial deposits, for example, which may be erased from the landscape by each successive ice advance, löess deposited beyond the extent of glaciers - and vegetated rapidly upon deposition - can record remarkably long and detailed records of climate change. Fundamental to reading the record of climatic change from loess deposits is discerning discrete löess deposits from among a stratigraphic sequence of löess units. Fortunately for those who study past climatic change, chemical weathering and soil development provide the essential evidence needed for distinguishing löess of one Ice Age from its stratigraphic neighbors.

For more than 100 years geologists have endeavored to decipher the record preserved in the löess deposits of the Mississippi Valley. During intervals when the Laurentide Ice Sheet (LIS) extended far enough south to enter the Mississippi drainage basin, the Mississippi became the principal meltwater channel for the southern margin of the LIS. During these times the Mississippi was a massive braided river that was likely marked by abrupt changes in discharge that carried the silt and fine sand produced by glacial abrasion southward. This fine-grained sediment was deposited along the edges of the broad, braided Mississippi channel during brief intervals of reduced meltwater flow, such as might occur during winter months. Subsequent drying of this sediment left it in a prime position to be entrained by westerly winds and deposited in thick piles, especially to the east of the Mississippi. Loess deposition abruptly ceased and the Mississippi adopted a meandering pattern whenever the southern margin of the LIS retreated northward out of the Mississippi drainage basin. The sequence of loess deposits along the banks of the Mississippi Valley thus generally records advances and retreats of the LIS, and the recognition of buried soil profiles - or their eroded remnants - is the key.

Soil profiles that are sufficiently buried to be isolated from modern soil forming processes are termed paleosols. Commonly only subsurface horizons are preserved, and though strongly developed paleosols are easily recognized in the field, more subtle paleosols require detailed field observations and even laboratory data to discern. Field observations and laboratory properties that document the relative abundance of organic matter, pedogenic iron, aluminum, clay, chemically resistant minerals, and a variety of magnetic properties are especially useful for the recognition of paleosols. The degree of soil rubification ("redness") is especially useful in many Mississippi Valley sequences. The extent to which the products of soil formation accumulate reflects the duration and/or intensity of the interval of soil formation. In addition, because the species of some products of soil formation, such as the species of clay or iron oxide, reflect, in part, the specific soil forming factors that prevailed during the interval of soil formation, some specific paleoenvironmental interpretations may be made from the soil data itself. For example, the Sangamon paleosol, which is clearly visible in the field, is a deep red (5YR-7.5YR Munsell hues) that reflects a protracted interval (perhaps 104-105 years) of soil formation under warm, well-drained soil conditions during which time little fresh löess was added to the landscape. In contrast, the Farmdale paleosol is dark and organic-rich with relatively little pedogenic iron reflecting a shorter interval in which soil formation and limited löess depositon were occurring simultaneously. Thus, though the major paleosols do record stratigraphic disconformities, some paleosols do not require the complete cessation of loess deposition. Recognition of the former is an important guide for efforts to date loess by luminescence geochronology or by radiocarbon dating, which are best applied to fresh, unaltered loess.

The mantling of many parts of the Earth's surface by loess has profoundly affected land use and geologic hazards in some regions. The silt loam texture of loess provides a near perfect balance of particle size between water logged clay-rich soils and sandy soils that hold very little water. Indeed the "bread basket" of North America owes much of its remarkable agricultural productivity to the mantle of löess that covers it. In contrast, though löess can be stable with nearly vertical slope, seismic shaking of loess can produce catastrophic landslides with characteristically long runout distances. The 1920 Haiyuan earthquake (Ms=8.5) demonstrated the high landslide hazard posed by löess on hillslopes in seismically active regions.

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