Impact of Climate and Parent Material on Chemical Weathering in Loess-derived Soils of the Mississippi River Valley

Peoria Loess-derived soils on uplands east of the Mississippi River the midcontinent of North America (Ruhe, 1969; Hall valley were studied from Louisiana to Iowa, along a south-to-north and Anderson, 2000), Alaska (Muhs et al., 2001), and gradient of decreasing precipitation and temperature. Major element China (Maher et al., 1994). Critical to paleoclimatic interanalyses of deep loess in Mississippi and Illinois show that the composi- pretations, using paleosols, are reliable climofunctions tion of the parent material is similar in the northern and southern for modern soils, which give an understanding of soil parts of the valley. We hypothesized that in the warmer, wetter parts formation as a function of climate, all other factors beof the transect,mineral weathering should be greater than in the cooler, ing equal. drier parts of the transect. Profile average values of CaO/TiO2, MgO/ The importance of climate as a soil-forming factor has TiO2, K2O/TiO2 and Na2O/TiO2, Sr/Zr, Ba/Zr, and Rb/Zr represent proxies for depletion of loess minerals such as calcite, dolomite, horn- been appreciated by pedologists for more than 100 yr, blende, mica, and plagioclase. All ratios show increases from south to startingwithDokuchaev (1883) and emphasized by Jenny north, supporting the hypothesis of greater chemical weathering in (1941, p. 281) 60 yr ago. Since that time, there have been the southern part of the valley. An unexpected result is that profile many soil climosequence studies. Much of the work of average values of Al2O3/TiO2 and Fe2O3/TiO2 (proxies for the relative the past two decades has been summarized recently by abundance of clay minerals) show increases from south to north. This Birkeland (1999, p. 430).One of the most commonly used finding, while contrary to the evidence of greater chemical weathering geologic settings for soil climosequence studies is loess in the southern part of the transect, is consistent with an earlier study that covers a large region and spans gradients of temperwhich showed higher clay contents in Bt horizons of loess-derived ature and precipitation. Such studies have been consoils in the northern part of the transect. We hypothesize that soils ducted in loess-dominated landscapes in China (Maher in the northern part of the valley received fine-grained loess from sources to the west of the Mississippi River valley either late in the et al., 1994), Argentina (Alvarez and Lavado, 1998), last glacial period, during the Holocene or both. In contrast, soils in and New Zealand (Webb et al., 1986). In the USA, soil the southern part of the valley were unaffected by such additions