Western Iowa loess records stronger last-glacial winds

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This article is part of the Fall 2014 issue of the Earth Science Matters Newsletter. 

 

Scientists have hypothesized that atmospheric dust concentrations may change under different climate scenarios, affecting planetary radiation balance, cloud dynamics, and dust inputs to oceans and terrestrial ecosystems. USGS CLU R&D scientists are exploring sources and concentrations of dust during past extreme climates to better understand causes of increased dustiness in the past. A recent study examines the hypothesis that stronger, more frequent winds increased dustiness during the last glacial period (28,000 – 10,000 years ago), when geologic records on land, in deep-sea cores, and in ice cores show that the Earth was a dustier planet. 

A site at Loveland, Iowa, along the Missouri River, contains one of the thickest deposits of last-glacial-age dust (loess) in the world. Based on several geochemical "fingerprinting" techniques, this loess was determined to have derived not only from glacial sources near the Missouri River, but also from non-glacial sources in Nebraska. Dating of these deposits using a technique known as Optically Stimulated Luminescence (OSL) has resulted in the first detailed chronology of last-glacial-age loess at Loveland. Deposition began after about 27,000 years ago and continued until about 17,000 years ago, when the initial glacial retreat began.

record of Laurentide ice sheet and western Iowa loess

Left to right: insolation from 31 ka to 17 ka at the top of the atmosphere at 65°N in July (from Berger and Loutre, 1991); time–distance diagram showing the southerly extent of the Laurentide ice sheet in the mid-continent of North America from ~31 cal ka BP to ~17 cal ka BP (redrawn from Johnson et al., 1997; their radiocarbon ages converted to calendar-year ages using Fairbanks et al. (2005)); stratigraphy, coarse/fine silt ratios in loess at Loveland, Iowa (particle size data from Muhs and Bettis (2000)); and loess mass accumulation rates (green line) at Loveland, calculated using a Bayesian reconstruction of OSL ages based on the 15 points (solid circles) shown.

(Credit: Dan Muhs, USGS, modified from Figure 9 in (Muhs et al. 2013) Public domain.)

OSL ages also indicate that accumulations rates of loess were not constant. Accumulation was highest and grain size was coarsest approximately 23,000 years ago, when about 10 m of loess accumulated in no more than 2,000 years and possibly much less. The timing of this accumulation period, indicating strongest winds, coincides with a period of low summer solar radiation reaching the earth’s surface at high latitudes in North America and the maximum southward extent of the last-glacial ice sheet. Such conditions would bring about a larger temperature contrast between the northern and southern regions in North America. An enhanced temperature contrast would result in more frequent strong winds. Many past climate models have not been able to produce a dustier last-glacial Earth. The results of this study show how geologic records can help to "fine-tune" climate models that are used for forecasting possible future climates. 

The paper was published in Quaternary Research and can be found at: https://pubs.er.usgs.gov/publication/70125300

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