DOI: 10.25921/v12k-k977
title: NOAA/WDS Paleoclimatology - Kohfeld and Harrison 2001 Mid-continental USA Gridded Maps of Loess Thickness

description: Atmospheric dust is an important feedback in the climate system, potentially affecting the radiative balance and chemical composition of the atmosphere and providing nutrients to terrestrial and marine ecosystems. Yet the potential impact of dust on the climate system, both in the anthropogenically disturbed future and the naturally varying past, remains to be quantified. The geologic record of dust provides the opportunity to test earth system models designed to simulate dust. Records of dust can be obtained from ice cores, marine sediments, and terrestrial (loess) deposits. Although rarely unequivocal, these records document a variety of processes (source, transport and deposition) in the dust cycle, stored in each archive as changes in clay mineralogy, isotopes, grain size, and concentration of terrigenous materials. Although the extraction of information from each type of archive is slightly different, the basic controls on these dust indicators are the same. Changes in the dust flux and particle size might be controlled by a combination of (a) source area extent, (b) dust emission efficiency (wind speed) and atmospheric transport, (c) atmospheric residence time of dust, and/or (d) relative contributions of dry settling and rainout of dust. Similarly, changes in mineralogy reflect (a) source area mineralogy and weathering and (b) shifts in atmospheric transport. The combination of these geological data with process-based, forward-modelling schemes in global earth system models provides an excellent means of achieving a comprehensive picture of the global pattern of dust accumulation rates, their controlling mechanisms, and how those mechanisms may vary regionally. The Dust Indicators and Records of Terrestrial and MArine Palaeoenvironments (DIRTMAP) data base has been established to provide a global palaeoenvironmental data set that can be used to validate earth system model simulations of the dust cycle over the past 150,000 years.
description: Late Wisconsin (Peoria) Loess of the mid-continental USA represents the 
thickest deposition of last glacial loess, ranging from trace amounts 
(< 1 m) to accumulations as thick as 40-50 m in parts of western Nebraska 
and western Iowa. Accumulation of Peoria Loess coincides primarily with 
the last glacial period, although accumulation was not synchronous across 
the entire mid-continental region. The maximum-limiting of Peoria Loess 
range from approximately 30-34 ka (organic matter from the uppermost 
Roxana Silt and Gilman Canyon Formation) to 20-22 ka in other regions.  
The actual time period of deposition varies from region to region 
(see e.g. Muhs and Bettis, 2000; Bettis et al., submitted).

Although a map of loess thickness across the mid-continental USA has 
been made (Thorp and Smith, 1952; republished in generalized form by 
Ruhe, 1983), this compilation is now more than 50 years old and has 
never been assembled as a digital data set, allowing for a 
representation of loess as a geomorphological feature that can be 
directly compared with regional model simulations over North America. 
The purpose of digitizing loess thickness is to provide this information 
as part of the DIRTMAP data base (Kohfeld and Harrison, 2001), 
to demonstrate spatial changes in accumulation across the 
mid-continental USA, and to make it available to the geomorphology, 
Quaternary and earth system modeling communities.