A Mid–Late Quaternary loess–paleosol record in Simmons Farm in southern Illinois, USA

Quaternary Science Reviews 28 (2009) 93–106

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A Mid–Late Quaternary loess–paleosol record in Simmons Farm in southern Illinois, USA Hong Wang a, *, Craig C. Lundstrom b, Zhaofeng Zhang b, David A. Grimley a, William L. Balsam c a

Illinois State Geological Survey, 615 E. Peabody Drive, Champaign, IL 61820, USA Department of Geology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA c Department of Geology, University of Texas at Arlington, Arlington, TX 76019, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 November 2007 Received in revised form 15 September 2008 Accepted 25 September 2008

In unglaciated areas of the Mississippi Valley region, the typical full loess–paleosol succession contains the Modern Soil developed in Peoria Silt, weakly developed Farmdale Geosol developed in Roxana Silt, Sangamon Geosol developed in Loveland Silt, and Yarmouth Geosol developed in Crowley’s Ridge Silt. Although a fifth loess called the Marianna Silt is reported at one area, the paleosol that separates the Crowley Ridge and Marianna Silts is not well defined. Previous thermoluminescence (TL) and optical stimulated luminescence (OSL) age chronology has suggested multiple phases of Sangamon Geosol developed in Loveland Silt, but clear morphological evidence of polygenetic Sangamon Geosol profiles have not been found. Recently, a thick loess–paleosol sequence has been studied in the middle Mississippi Valley in unglaciated southern Illinois, USA. Soil morphology and analytical results revealed five loesses and associated paleosol units. Two Sangamon Bt horizons were found separated by a thick ACtk horizon, interpreted to indicate two phases of Sangamon Geosol development. This well-preserved loess–paleosol succession provides one of the most complete mid–late Quaternary loess records in the middle Mississippi Valley to date, and is important for studying the stratigraphic framework and paleoclimate and environment changes. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Loess–paleosol records are an important proxy for paleoclimate and environment studies in the Midwest USA. Loess deposits reflect complex climate and landscape responses to glacial and fluvial system aggradation and sediment availability. Major paleosols reflect a stabilized landscape with climatic conditions suitable for biological activities. In the Mississippi Valley, the typical loess– paleosol succession contains, from top to bottom, the Modern Soil developed in Peoria Silt (silt: lithostratigraphic term used to nongenetically describe loess and related deposits), weakly developed Farmdale Geosol (geosol: paleosol that can be traced regionally) developed in Roxana Silt, Sangamon Geosol developed in Loveland Silt, and Yarmouth Geosol developed in Crowley’s Ridge Silt (Leighton and Willman, 1950; Thorp et al., 1951; Ruhe, 1965, 1974; Willman and Frye, 1970; Follmer, 1978, 1996; Markewich et al., 1998; Forman et al., 1992; Forman and Pierson, 2002; Grimley et al., 1998, 2003; Rutter et al., 2006). A fifth loess unit known as the Marianna Silt is rarely seen and only reported in Arkansas (Rutledge et al., 1990, 1996). However, the paleosol that separates the Crowley

* Corresponding author. Tel.: þ1 217 244 7692; fax: þ1 217 244 7004. E-mail address: [email protected] (H. Wang). 0277-3791/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2008.09.021

Ridge and Marianna Silts has not been well defined and is not observed in other Arkansas loess sites (Markewich et al., 1998). Another loess unit that is older than the Loveland and younger than the Crowley’s Ridge Silts is the Wyalusing Silt that has been reported in Wisconsin (Leigh and Knox, 1994) but not elsewhere in the Mississippi Valley. In Iowa, the Pisgah Loess is a silt unit that is stratigraphically and chronologically equivalent to Roxana Silt (Bettis, 1990). Although TL and improved OSL dating techniques suggested multiple phases of loess deposition and Sangamon Geosol found in some areas (Forman et al., 1992; Rodbell et al., 1997; Forman and Pierson, 2002), clear soil morphological evidence of polygenetic Sangamon Geosol formation is not yet known in the Mississippi Valley loess record. At a recently cored thick loess bluff at the Simmons Farm in unglaciated southern Illinois, USA, we identified five loess units all with associated paleosols including two phases of Sangamon Geosol development. This unusual loess–paleosol succession is located in a karst landscape in uplands along the northeast bank of the Mississippi River. This site provides excellent information for studying loess–paleosol stratigraphy, paleoclimate, and past environments. In this paper, we present soil morphology evidence and analytical results to define loess and paleosol units for a more detailed stratigraphic framework than most loess records in the Mississippi Valley. The loess–paleosol stratigraphy and geochemical

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H. Wang et al. / Quaternary Science Reviews 28 (2009) 93–106

and physical parameters obtained at this site provide useful proxies for reconstructing paleoclimate and environment changes since the middle Quaternary Period. 2. Background on Midwest loess–paleosol stratigraphy and paleoenvironment records The Peoria Silt was deposited during the late Wisconsinan glaciation mainly from about 25 14C ka (thousand of years ago) to about 10 14C ka in the Mississippi Valley (Leigh and Knox, 1994; Follmer, 1996; Hansel and Johnson, 1996; Muhs and Bettis, 2000; Mason, 2001; Wang et al., 2000, 2003a, b). The glacigenic Peoria Silt in the Midwest reflects an active ice front of the Laurentide Ice Sheet (LIS) during this time interval (Wang et al., 2000, 2003a). In some of thick loess sections in Illinois the Peoria Silt contains several weakly developed paleosol A horizon complexes, each of which reflects slightly warmer and wetter interstadial and semiinterstadial conditions. These intervals likely correlate with the retreat of the LIS southern front in Illinois (Wang et al., 2003a; 2004; Wang, 2002). These paleosol A horizon complexes are slightly redder and darker than less pedogenically-altered Peoria Silt, and the reddish hue becomes progressively stronger toward the basal Peoria. The Farmdale Geosol A horizon complex separates the Peoria and Roxana Silts and the Farmdale Geosol is a wellknown pedostratigraphic unit in the region. At some sites in southern Illinois, the lightness (L*), total iron content (Fe %), and matrix carbonate content (MCC%) have been used to define weak interstadial paleosol A horizon complexes within the Peoria Silt, along with the Farmdale Geosol (Wang et al., 2003a). The Roxana Silt is a pinkish to tan silt unit and in areas of thick loess can be divided into the upper Meadow, middle McDonough, and basal Markham silt members in Illinois (Willman and Frye, 1970; McKay, 1979). The McDonough member has not been found practical and its use has been abandoned (McKay, 1979). The Roxana Silt equivalent in Iowa of Missouri River source is named as the Pisgah Silt (Bettis, 1990). The Meadow Member, predominantly loess, typically displays upper pink, middle tan (and/or gray), and lower pink zonation, while the Markham Member is a tan silt unit of colluvial or eolian origin (Willman and Frye, 1970; McKay, 1979). The Markham Silt Member is the parent material of the Chapin Geosol, a moderately developed interstadial-class soil with a B horizon in many cases (Willman and Frye, 1970). The Roxana Silt reflects catchment processes with aggraded, braided rivers associated with an increased sediment supply from glacial, periglacial, as well as nonglacial sources (Leigh and Knox, 1994; Grimley, 2000). Willman and Frye (1970) identified the Chapin Geosol as a soil with a B horizon developed in the Markham Silt Member of the basal Roxana Silt, which conformably overlies the Sangamon Geosol; they assigned an age of 75 ka to define the Sangamonian and Wisconsinan boundary. Follmer (1982, 1983) suggested that the Chapin Geosol is a continuous extension of Sangamon Geosol development because of gradational changes in soil morphology. This implies that the thin incrementally deposited early Wisconsinan Markham Silt was incorporated into the upper horizon of the Sangamon Geosol as an over-thickened upper solum. The age of 45–50 ka was reassigned for the base of the Roxana Silt. Curry and Follmer (1992) also stated that the Roxana Silt deposition initiated at about 50 ka based on radiocarbon age extrapolation and that this time also coincides with a significant climate cooling to periglacial conditions. Pollen studies confirmed that the early Wisconsinan in southern Illinois was a prairie environment with scattered Picea trees; a colder boreal forest environment did not occur in southern Illinois until mid-Wisconsinan (Zhu and Baker, 1995). A Missouri speleothem record also confirms that the cold periglacial climates did not happen until about 55 ka (Dorale et al., 1998). Consequently, Curry and Baker (2000) reinterpreted the pollen record in southern

Illinois of Zhu and Baker (1995) reclassifying more than 1 m of early Wisconsinan sediments into the Sangamonian interval, suggesting that the Wisconsinan Stage began as the Roxana Silt (Meadow Silt here) was deposited. Grimley et al. (2003) included the Markham Silt of the basal Roxana at many loess sections (Grimley, 1996; Grimley et al., 1998) into the upper solum of the Sangamon Geosol, suggesting that Sangamon Geosol formation ended at the middle Wisconsinan. The 55 ka age of initial deposition of the Roxana Silt is supported by the majority of OSL dates obtained from loess sections in the middle Mississippi Valley (Forman et al., 1992; Forman and Pierson, 2002; Rodbell et al., 1997). The record of glacial meltwater pulses in Gulf of Mexico sediments also indicate that 55 ka was the important interval for active Laurentide ice front (Tripsanas et al. 2007). However, the Markham Silt Member as the basal Roxana Silt has been observed in dozens of loess sections in Illinois (Willman and Frye, 1970; Grimley, 1996; Grimley et al., 1998) and at least one site in Indiana (Hall and Anderson, 2001). The pollen record in southern Illinois indicates that deciduous forest existed during the late Sangamonian (Zhu and Baker, 1995). Some OSL dates obtained from the basal Roxana Silt at Bonfil Quarry in Missouri averaged ca. 66 ka, suggesting Roxana Silt deposited before 55 ka, though interpreted as pedo-mixing of older Sangamon Geosol with younger Roxana Silt (Forman and Pierson, 2002). The Sangamon Geosol, because of its thick, well-expressed, reddish brown Bt horizon in most loess bluffs, is an important marker horizon for loess stratigraphic study in the Midwest USA. In Illinois, the Sangamon Geosol has been described as consisting of two types: (1) an in-situ soil profile developed into Illinoian till, outwash, and silt; and (2) an accretionary profile developed into Sangamonian colluvium/alluvium sediments (Willman and Frye, 1970). Ruhe (1969, 1974) and Ruhe et al. (1974) recognized two separate Sangamon B horizons in Iowa and Indiana, suggesting two phases of Sangamon Geosol development. However, Follmer (1982, 1983) suggested that two phase development of the Sangamon Geosol did not exist and so called ‘‘upper Sangamon’’ was correlative to the Chapin Geosol that was in fact the upper solum of the Sangamon Geosol (Curry and Follmer, 1992). The Loveland Silt is a brownish to tan silt unit where unweathered. The Loveland Silt is the parent material of the Sangamon Geosol and is underlain by the Yarmouth Geosol. The Loveland Silt at the Loveland type section, Iowa, and Teneriffe Silt at the Pleasant Grove School site, Illinois, thought to be correlative units (Willman and Frye, 1970), were dated using TL and OSL methods. The results indicate that the unweathered Loveland Silt is ca. 157  18 ka (n ¼ 7), while unweathered Teneriffe Silt is only ca. 93  5 ka (n ¼ 4) (Forman and Pierson, 2002), suggesting that the Loveland Silt was deposited during the Illinoian and Teneriffe Silt during the Sangamonian intervals (Forman et al., 1992; Forman and Pierson, 2002). The OSL dates for Loveland Silt at Tennessee sites likely agree with the difference observed in Iowa and Illinois (Rodbell et al., 1997). The TL and OSL ages imply that a model of multiple phase development may be applied to the Sangamon Geosol because an interglacial-class soil unit is observed on the top of both silt units. The Yarmouth Geosol in well-drained loess bluffs typically expresses a bright reddish brown color with strong soil fabric and is an important marker bed for loess stratigraphy in the Midwest. Its reddish brown color and pedogenic fabric are much stronger than those found in the Modern Soil, the Farmdale Geosol, the Chapin Geosol, and Sangamon Geosol, and can be easily recognized where preserved in loess outcrops and cores. There are no reliable radiometric ages for the Yarmouth Geosol so far. The TL ages in Arkansas suggest that the Yarmouth Geosol is correlated with MIS 7 (Markewich et al., 1998; Rutter et al., 2006). Because OSL ages in Missouri yielded minimal ages, the Yarmouth Geosol could be older than MIS 7 (Forman and Pierson, 2002). On the basis of mineral

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weathering and neoformation, a long duration of Yarmouth Geosol development has historically been envisioned (Willman and Frye, 1970). Magnetic properties, mineralogy, and geochemistry data obtained in Illinois were used to suggest correlating the Yarmouth Geosol to MIS 7–11 (Grimley et al., 2003). A fourth loess unit in southern Illinois (Grimley et al., 2003) has been correlated with the type Crowley’s Ridge Silt in Arkansas of the lower Mississippi Valley (Porter and Bishop, 1990). The TL dates obtained from the Phillips Bayou section, another Arkansas site, suggested that the fourth Silt was deposited between 200 and 250 ka and likely near 250 ka (Markewich et al., 1998). However, a strong paleosol was not developed into its top at this site and therefore it may not be clear if it is Crowley’s Ridge Silt. On the other hand, TL and OSL ages of 160 to 274 ka obtained at Bonfil Quarry in Missouri were considered minimum ages for the Crowley’s Ridge Silt (Forman and Pierson, 2002). Based on these ages, the Crowley’s Ridge Silt could be correlated to MIS 8 (Markewich et al., 1998; Rutter et al., 2006) or older (Forman et al. 1992; Forman and Pierson, 2002). Based on weathering intensity of the Yarmouth Geosol, the Crowley’s Ridge Silt at Thebes section, southern Illinois, was correlated to MIS 12 (Grimley et al., 2003). The fifth loess unit is rarely seen and only reported as the Marianna Silt in Arkansas (Rutledge et al., 1990, 1996). It could be the first deposit of alluvium and loess on Tertiary sand and gravel (Follmer, 1996). Due to lack of an interglacial-class paleosol, correlation to Marianna Silt in Illinois has not been made so far, though units below Crowley’s Ridge Silt have been noted (Grimley, 1996). 3. Study site, material, analytical background and methods The study site is located at the Simmons Farm in Monroe County, Illinois, in the unglaciated part of southwestern Illinois. It is at the top of a loess-covered bluff in a karstic landscape along northeast

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side of the Mississippi River Valley (38 70 3000 N, 90 70 3000 W; Fig. 1). The highest point of this bluff is about 214 m above sea level. A large diameter core (7.6 cm) was drilled from the top of the bluff down to limestone bedrock. The loess sediments were 25.6 m thick, one of the thickest loess cores documented in southern Illinois. Because of the thickness of the loess sediments, a second and smaller diameter core (3.8 cm) was immediately planned and drilled in parallel with the first one within 5 m distance to increase the recovery rate for high resolution geophysical and geochemical analyses. The larger diameter core was split in half manually using a 20cm long metal blade and a hammer. One half of the core was laid out in 10  120-cm metal trays for detailed description and stratigraphic correlation. The entire other half of the large core and the smaller diameter core were both sampled at 3.25-cm intervals. There were 783 loess samples collected and the total missing interval was estimated to be less than 20 cm. All samples were gently ground and dry sieved through a 63-mm sieve to remove secondary carbonate rhizoliths and concretions for geochemical and physical analyses of matrix materials of eolian silt. 3.1. Soil morphology Soil color, texture, structure, and characteristics of clay skins and iron/manganese stains on soil peds were used to identify soil B horizons of paleosol units. Soil macro (>2 mm) to meso (0.5–2 mm) morphologies were described in characterizing the soil fabric strength of paleosol B horizons. The Modern Soil developed on forested loess bluffs in southern Illinois commonly is comprised of thin A and E horizons and a thick, reddish brown Bt horizon. By comparing the strength of the modern Bt horizon, the paleosol units with well-developed Bt or weakly developed Bw horizons were classified as either interglacial- or interstadial-scale paleosol units. The meso morphology analysis was performed under a 6–10 magnification microscope.

Fig. 1. Location of Simmons Farm loess cores. (A) The extensive loess–paleosol record was found on the east bluff of Mississippi River Valley (38 70 3000 N, 90 70 3000 W). The site is located in an unglaciated region of southwest Illinois. (B) The location and loess thickness in southwestern Illinois.

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3.2.

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C and U-series isochron dating

Two soil samples were taken from the middle of the lower pink member of the Roxana Silt at a depth of 12.2 m for AMS 14C analysis of total soil organic carbon. About 5 g of sieved soil sample was treated with 2 M HCl for at least 15 h at 25  C to remove carbonates and fulvic acids, and then rinsed to pH  5 with distilled water. The residue was oven-dried and 500–750-mg samples were loaded into quartz tubes with 1 g Cu, 1 g CuO, and 50 mg Ag foil. The tubes were evacuated for at least 2 h and sealed with a torch. The organic carbon was converted to CO2 by sealed quartz tube combustion at 800  C for 3 h in a programmable muffle furnace. The tubes were cooled to 25  C over 20 h and the CO2 in the tube was purified and collected by cryogenic distillation for AMS 14C and d13C analyses. Twelve samples were taken from the Loveland Silt including the likely multiple phases of Sangamon Geosol for U-series dating of pedogenic carbonate. Because the samples contain fine grained, intergrown carbonates within previously deposited loess sediments, any sampling of the carbonate will contain a high proportion of detrital material. Therefore, the use of a U-series isochron technique, which corrects for detrital contamination (Bischoff and Fitzpatrick, 1991; Ludwig and Titterington, 1994; Candy et al., 2005)

is the most appropriate method for dating open-system carbonates. Because of worries about fractionation of nuclides during partial dissolution methods, we used the method of multiple separate samples, which uses differences in the amount of carbonate materials vs non-carbonate detrital materials to control the isochron. We selected two soil carbonate nodules collected from the Loveland Silt at depths 15.9 and 16.2 m, and one carbonate nodule also collected from the Loveland Silt at depth 17.0 m for the U-series isochron analysis (see Fig. 2). Soil carbonate nodules are formed in middle or lower soil solum when dissolved biogenic CO2 leached downward. The secondary carbonate nodules should reflect minimal soil ages of the paleosol development from above but not the depositional age of their parent materials. Either three, four, or five subsamples were taken from each of the carbonate nodules. Each subsample was dissolved completely using HCl, HNO3, HF, and HClO4 acids. The sample was then dried down and H3BO3 was added to remove any excess F mostly bound in CaF2 precipitates. Once a given sample was completely dissolved in 8 N HCl, it was loaded onto standard columns containing AG 1-X8 100–200 mesh resins. Each subsample went through 22 and 10.25-ml columns for purification of the U and Th. Samples were measured for U and Th isotopic ratios by multi-collector ICP-MS using the Mu Plasma HR

Fig. 2. Simmons loess core showing five interglacial-class, two moderately-expressed interstadial-class soils, and five loess units. Two Sangamon (Sang.) Bts are separated by the ACtk horizon. Yarmouth (Yarm.) Bt shows the strongest reddish hue. Trays are about 1.22 m long. The missing intervals were sampled from the second core for analyses.

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located in the Department of Geology, University of Illinois at Urbana-Champaign. Briefly, samples were run using a DSN-100 desolvating nebulizer with a sensitivity of 400 V/ppm at a 0.1-ml/ min uptake rate. Analyses used standard bracketing using UCSC ThA (Th) and U960 (U) for calibrating the ion counter gain and the mass fractionation. Abundance sensitivity during analysis was <400 ppb at 1 AMU and U and Th standards were run concurrently during each analytical session to assess accuracy. Isotope ratios were then converted to activity ratios and U to Th concentration ratios calculated. Isoplot (Ludwig and Titterington, 1994) was then used for the U-series isochron age estimation. 3.3. Magnetic susceptibility Magnetic susceptibility (c) has been widely used as a paleoclimatic proxy in Chinese loess studies since Heller and Liu (1982) found maximum c values in paleosol horizons and minimum c values in less pedogenically-altered loess layers. The c variations in Chinese loess deposits are generally controlled by the in-situ pedogenically-formed ultrafine ferrimagnetic minerals (Zhou et al., 1990; Maher and Thompson, 1991, 1992; Zheng et al., 1991; Evans and Heller, 1994). The frequency dependent magnetic susceptibility (cfd), the percentage difference at high and low frequency, reflects the concentration of superparamagnetic minerals in the 0.018–0.020 mm size range (Banerjee and Hunt, 1993) formed mainly during pedogenesis. Using these c and cfd proxies, the Chinese loess pedostratigraphy has been widely correlated to paleoclimate records between continents and oceans (Heller and Liu, 1982; Kukla et al., 1988, 1990; Kukla and An, 1989; Hovan et al, 1989; Petit et al., 1990). In the Midwestern USA, the c variations have been used for characterizing paleosol units (Grimley et al., 2003) and for correlation or provenance studies of loess units (Feng and Johnson, 1995; Grimley et al., 1998; Johnson and Willey, 2000). We measured c values in SI units normalized by mass using a Bartington (model MS2B) magnetic susceptibility meter at low frequency (430 Hz). The measurements were acquired on every other 3.25-cm interval sample from the Simmons loess core. A total of 381 samples were analyzed. Samples (<63 mm) were lightly remolded into 5.28-cm3 plastic cubes prior to the measurement. Pre- and post-measurement of background c values were averaged to provide a minor correction of each sample’s c value in avoiding background c value drift. Values of cfd were computed from the percentage difference between the c values measured at 430 Hz (low frequency) and 4300 Hz (high frequency). 3.4. Diffuse reflectance Diffuse reflectance spectra have been shown to be useful in loess–paleosol stratigraphic studies (Ji et al., 2001; Wang et al., 2003a). The reflectance spectra can be mathematically converted to color parameters L*a*b* defined by the Commission Internationale de l’Eclairage (CIE, 1978). A HunterLab MiniScan XE Plus spectrophotometer was used to obtain diffuse reflectance data from the large diameter Simmons loess core. This spectrophotometer automatically converts the reflectance data into the L*a*b* values. One half of the larger diameter core was shaved as flat as possible, and the diffuse reflectance measurements were completed continuously at 2.5-cm increments. Prior to and periodically during measuring of the Simmons Farm loess samples, the spectrophotometer was standardized by measuring manufacturer-provided black glass and white tile. The L*, a*, and b* values of the calibration remained within 1.0% of the nominal values. The L* value, a lightness parameter, is scaled from 0 for black to 100 for white and is approximately equivalent to grayscale reflectance (Chapman and Shackleton, 1998). The a* value measures red (positive values) to green (negative values) hues and the b* value

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measures yellow (positive values) to blue (negative values) hues. Variations in these soil color components are essentially controlled by soil minerals and organic content. Because soil formation generally removes light-colored carbonate minerals through the leaching process and accumulates dark-colored organic matter and iron oxides, most soil units are darker in color (lower L* values) than unweathered loess units. The L* parameter has been a useful indicator for stratigraphy and climate change studies (Balsam and Deaton, 1991; Chapman and Shackleton, 1998; Porter, 2000; Ji et al., 2001; Wang et al., 2003a, Wang et al., 2006). Red hue in soils results mostly from the presence of pedogenic hematite (Schwertmann and Taylor, 1989; Scheinost and Schwertmann, 1999; Cornell and Schwertmann, 2003). Hematite formation can be accelerated by a longer and warmer dry period following a shorter period of soil wetness (Maher, 1998). As hematite content increases, the soil becomes redder, which is indicated by increasing a* values. The yellow hue in loess sediments mainly results from the presence of goethite, which forms under a broad range of environmental conditions. Strong seasonal wetting and drying during soil formation, however, may play an important role in goethite formation (Macedo and Bryant, 1987, 1989). 3.5. Kaolinite content The concentration of kaolinite has been used to indicate weathering intensity in a Chinese loess–paleosol sequence (Kalm et al., 1990). The correlation between the L*a*b* values and changes in kaolinite contents obtained from the Simmons loess samples may provide a way to test whether the observed color variations of loess sediments are controlled more by pedogenic or provenance factors. The kaolinite content of the shaved Simmons Farm core was measured at 10-cm increments using a portable infrared mineral analyzer (PIMA). The PIMA is a Fourier-transform infrared (FTIR) reflectance spectrometer that operates in the short-infrared wavelength range, from 130 to 2500 nm. The PIMA ViewÔ software package includes features to display and analyze spectra, either one at a time, as multiple overlays, or as single or multiple stacks of spectra. The spectra were analyzed through comparison with the PIMA reference spectra stored in standard libraries. 4. Results and Discussion 4.1. Soil morphology and pedostratigraphy Through comparison of fabric characteristics of modern Bt horizon and intervals in the core, we identified five Bt and two Bw horizons from the Simmons Farm. The fabric characteristics include soil color, macro and meso morphology, development scale of clay coatings, and degree of Fe–Mn stains. Paleosols with Bt horizons indicate interglacial-class pedostratigraphic units, while paleosols with Bw horizons indicate interstadial-class pedostratigraphic units. Brief descriptions of the Modern Soil, all paleosols, and the five loesses are in Table 1. The following paragraphs describe these soils in order from the top of the succession. The modern Bt horizon is about 1 m thick, fine textured (silty clay loam), reddish brown color (7.5 YR 5/4), and underlies a 5-cm dark A and a 5-cm light-colored E horizon (Fig. 2). The macro and meso morphologies show that the modern Bt horizon has angular blocky structure, thick clay skins, and distinct Fe–Mn stains (Figs. 3 and 4). The thin upper solum of A and E horizons (~10 cm) and the strength of Bt horizon of the Modern Soil provide references for defining the upper boundary and development scale of other interglacial-class paleosols. Considering erosion commonly occurs on the upper solum of a paleosol unit before the deposition of fresh silt begins, the upper boundary of a paleo-B

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Table 1 Description of Simmons loess–paleosol stratigraphy. Stratigraphy

Depth (m)

Pedofeatures

Modern Soil

0–1.2

Peoria Silt

1.2–6.7

Roxana Silt

6.7–14.7

Chapin Geosol

13.4–14.0

Sangamon Geosol

14.7–17.0

Loveland Silt Yarmouth Geosol

17.0–18.3 18.3–20.1

Crowley’s Ridge Silt Prairie Du Rocher Geosol

20.1–22.6 21.3–21.9

Geosol 3

22.6–23.7

Marianna Silt Geosol 4

23.7–24.7 24.7–25.6

Bedrock

25.6–

A: 4-cm, dark, organic rich, silty loam; strong aggregated granular particles; root traces and worm channels; voids; leached E: 6-cm, pale (10 YR 7/4) silty loam; root traces; bio-aggregation; granular; some iron stains in matrix; leached Bt: 1.1-m, reddish (7.5 YR 5/4) silty clay loam; angular blocky, prismatic structures; Fe–Mn stains, thick clay skins; leached AC: Light grayish (10 YR 6/4); calcareous, course texture; uniform and massive, containing numerical reddish weathering bands, known as paleosol A horizon complexes AC: Subdivided into four zones: upper pink (7.5 YR 5/5), 6.7–9.1 m; middle tan (10 YR 6/4), 9.1–10.4 m; lower pink (7.5 YR 5/5), 10.4–13.4 m; bottom tan (10 YR 6/4), 13.4–14.6 m; finer texture, uniform and massive, some intervals leached Bw: yellowish brown (10 YR 6/4) silt loam; prismatic structure; large voids; secondary carbonate fillings. Developed in bottom tan (Markham Silt) unit Bt-1: dark reddish brown (7.5 YR 5/4) silty clay loam; angular blocky, subangular blocky; distinctive clay skins with Fe–Mn stains; root traces, animal burrows, voids; 14.7–15.5 m. ACtk: 15.5–16.2 m; Bt-2: as Bt-1; 16.2–17.0 m AC: brownish (10 YR 5/5) silt loam; partially leached; carbonate concretions; massive Bt: bright reddish brown (5 YR 5/6) silty clay; smooth rounded blocky, very thick clay skins with Fe–Mn concretions and stains; root traces, animal burrows, voids; hard when dry AC: light yellowish brown (10 YR 7/6) silt loam; leached; finer texture; weak aggregation; uniform and massive Bw: yellowish brown (10 YR 8/5) silt loam; subangular block; thin and sparse clay skins; carbonate rhizoliths; developed in the middle of the Crowley’s Ridge Loess Bt: ocher (7.5 YR 6/4) silty clay, smooth rounded blocky; thick clay skins; distinct Fe–Mn stains; root traces, animal burrows, voids; has a similar fabric strength to the Yarmouth Bt horizons AC: light yellowish brown (10 YR 7/4) silt loam; dense, leached; coarser texture; uniform and massive Bt: brown to reddish brown silty clay loam (5 YR 4/3), subangular blocky, clay skins, Fe–Mn stains, root traces, animal burrows, voids; highly leached; pebbles Limestone

horizon is used to define the closest upper boundary of the paleosol unit at this site. The Chapin Bw horizon is less than 1 m thick, yellowish (or tan) brown color (10 YR 6/4), and overlies the Sangamon Geosol (Fig. 2). The lithological contact between the Markham silt member and the Sangamon Geosol is abrupt and gradational features from one soil to another were not observed, and thus it is not a continuous extension of the Sangamon Geosol in the Simmons loess cores. The macro and meso morphologies show that the Chapin Bw horizon has coarse texture (silt loam), prismatic and angular blocky structure, large voids, and pedogenic carbonate in fillings, but no clay skins or Fe–Mn stains (Figs. 3 and 4). In comparison with the Modern Soil, the Chapin soil lacks in-situ-formed pedogenic

hematite (indicated by reddish hue), clay skins, and Fe–Mn stains, suggesting that the degree of pedogenesis, weathering intensity, and translocation of clay and other iron oxide minerals, is considerably weaker than in the Modern Soil. The Chapin soil was developed into >1-m-thick Markham silt member, and formed during the early Wisconsinan interval. The Sangamon Geosol unit in the Simmons cores contains two Bt horizons, which are separated by a 40 cm thick ACtk horizon. Large amounts of secondary carbonate minerals and uniform or massive structures were observed in the ACtk horizon (Fig. 2). Secondary carbonates including nodules and concretions transgress from the upper Sangamon Bt (Bt-1) horizon, indicating pedogenesis occurred during the formation of the upper Sangamon

Fig. 3. Specimens showing soil colors and macro-morphologies (>2 mm) of six Bt and two Bw horizons. All specimens contain strong aggregation structure including blocky and prismatic features, root traces, and disseminated carbonate rhizoliths. Clay skins and Fe and Mn stains are prominent in specimens of Bt horizons.

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Fig. 4. Soil color and meso-morphologies (<2 and >0.5 mm) of six Bt and two Bw horizons. Meso-morphologies showing strong relief (fabric strength), biopores and cracks, root traces and carbonate rhizoliths (bioturbation), and clay skins (red arrows) and Fe–Mn stains and/or concretions. Scale bar, 0.5 cm.

Bt horizon. The ACtk horizon was identified as showing less altered loess deposit. Because traces of clay coatings and Fe–Mn stains were observed along root channels and it contains large amounts of carbonates, we used the subscript (tk) to describe the C of this AC horizon. The Bt-1 (upper) and Bt-2 (lower) horizons are each about 1 m thick. They are dark reddish brown in color (7.5 YR 5/4), and have fine texture (silty clay loam), angular and subangular blocky structure, thick clay skins, distinct Fe–Mn stains, and abundant disseminated pedogenic carbonate rhizoliths (Figs. 3 and 4). Both Bt horizons in the Sangamon Geosol contain large amounts of secondary carbonates, which were not observed in either the modern or the Yarmouth Bt horizons (Fig. 2). The Yarmouth Bt horizon was easily identified by its distinctive 5 YR 5/6 bright reddish brown color and very fine grained texture (silty clay) (Fig. 2). Strong soil fabric morphology, such as rounded and smooth blocky structure, extremely thick clay skins, and distinct Fe–Mn stains, characterize the Yarmouth Bt horizon (Figs. 3 and 4). It shows stronger pedogenic features than either the Sangamon or modern Bt horizons. The Yarmouth Bt max is about 1.2 m

thick and the entire B horizon thickness is about 2.2 m thick. The Crowley’s Ridge Silt, the parent material of the Yarmouth Geosol, is approximately 4 m thick. The lower 1.8 m of relatively unweathered loess sediments shows more tan than reddish brown colors and has less soil structure than the overlying B horizons. The Simmons Farm core contains the thickest section of Crowley’s Ridge Silt reported so far in Illinois; this thickness is comparable to that of the type locality in eastern Arkansas (Rutledge et al., 1990; Porter and Bishop, 1990). The Prairie Du Rocher Geosol is a relatively strong interstadial paleosol, here named informally for a town near the Simmons Farm. Its texture (silt loam) is much coarser than the Modern Soil and similar to the Chapin interstadial-class soil unit. The Prairie Du Rocher Bw horizon is about 60 cm thick, light yellowish brown (10 YR 8/5), has angular blocky structure, a high density of pedogenic carbonate rhizoliths, thin/sparse clay skins, and no Fe–Mn stains on peds (Figs. 3 and 4). This interstadialclass paleosol occurs in the lower portion of the Crowley’s Ridge Silt (Fig. 2).

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4.2. Age constraints: middle Roxana Silt and Sangamon Geosol The AMS 14C dates of two soil samples from the upper middle Roxana Silt are 29.7  0.9 (ISGS A0706) and 27.9  0.7 (ISGS A0707) 14C ka BP. Although these dates appear to be younger than expected, they fall into typical range for Roxana Silt overall (Hansel and Johnson, 1996). The processes of soil organic matter accumulation in sediments indicates that the 14C dates are the minimum age of soil development. Conversion of the 14C ages to calendar years using the graphic model of Beck et al. (2001) suggests that the middle of the lower pink member of the Roxana Silt, at 1.2 m above the Chapin Geosol is at least older than 36.5 cal. ka, within the lower–middle Meadow Silt member of the Roxana Silt. The U-series isochron ages of the two pedogenic carbonate nodules, collected from beneath the Bt-1 horizon (depths: 15.9 and 16.2 m), are 80  8 (2s) ka and 93  19 (2s) ka (Fig. 5). The pedogenic carbonate nodule collected from the basal Sangamon Bt-2 horizon (depth 17.0 m), gave an age of 104  26 (2s) ka (Fig. 5). These U-series isochron ages of soil carbonates indicate the minimal age of the previous pedogenic process, and thus the deposition age of their parent loessial materials should be significantly older. These ages suggest that both Bt-1 and Bt-2 horizons are part of the Sangamon Geosol. 4.3. Magnetic susceptibility The variations of magnetic susceptibility (c) and frequency dependent magnetic susceptibility (cfd) are not fully in phase. For instance, high c peaks that range from 90 to 100 (103 m3/kg) were found in the Peoria–Roxana transition and lower portion of the Roxana Silt where high cfd values were absent. High cfd peaks found at Geosols 3 and 4 had no corresponding peaks in the c curve (Fig. 6). The c variations in <63-mm loess samples are likely controlled by a combination of pedogenic neoformation, chemical alteration, and provenance. High c peaks in the upper and lower

1.00

Data-point error crosses are 2σ Age (16.2 m) = 80 ± 8 ka, MSWD = 7.5

0.90

/ 238U

The name Geosol 3 refers herein to the third interglacial-class geosol unit from the top of the succession (the Sangamon is the first and the Yarmouth is the second) in the Simmons Farm loess cores. The Geosol 3 Bt horizon is less than 1 m thick, has light brown color (7.5 YR 6/4), and a very fine grained texture (silty clay) (Fig. 2). Strong fabric morphologies such as rounded and smooth blocky structures, thick clay skins, and distinct Fe–Mn stains are evident (Figs. 3 and 4). Geosol 3 shows stronger pedogenic features than the Modern Soil and Sangamon Geosol; the strength of the pedogenic features is similar to those in the Yarmouth Geosol. Based on correlation to the lower Mississippi River Valley (Rutledge et al., 1996), the parent loess for Geosol 3 is interpreted as the Marianna (fifth) loess unit, which is the oldest loess reported in the Mississippi Valley region. The Marianna Silt is about 2 m thick in Simmons Farm cores, and will provide valuable information for understanding sedimentary properties and environment conditions of the fifth loess unit in the future. The name Geosol 4 refers herein to the fourth interglacial-class paleosol unit in the Simmons Farm cores. The Geosol 4 Bt horizon is less than 20 cm thick, dark brown (7.5 YR 4/3), silty clay texture, subangular blocky structure, thick clay skins, large Fe–Mn concretions, and pebbles and chert fragments (Figs. 2, 3, and 4). Geosol 4 is immediately underlain by the limestone bedrock. The silty materials in Geosol 4 are likely of eolian origin, but the pebbles and chert fragments in Geosol 4 probably come from the underlying limestone bedrock or other nearby bedrock (Fig. 2). Geosol 4 appears to show stronger soil fabric such as thick clay skins and distinct Fe–Mn concretions than the Modern Soil. But the relatively weak fabric strength is not consistent with a residuum or Terra Rossa paleosol.

230Th

100

Age (17 m) = 104 ± 26 ka MSWD = 20

0.80

Age (15.9 m) = 93 ± 19 ka, MSWD = 2.4

0.70 1.00

1.20

1.40 232Th

1.60

1.80

2.00

/ 238U

Fig. 5. U-series isochron plot showing ages of pedogenic nodules of the Sangamon Geosol.

pink members of the Roxana Silt (Fig. 6) may reflect a greater contribution of magnetite-rich Lake Superior sediment in Meadow Silt Member, as has been reported by Grimley et al. (1998). High c peaks in the interglacial paleosols associated with high cfd % likely result from the neoformation of ultrafine ferromagnetic minerals. Low c value excursions in the Marianna and/or older loess sediments suggest the presence of a small amount of primary magnetic minerals, and lower c values in portions of the Loveland Silt probably reflect dilution of the magnetic signal by precipitation of large amounts of secondary carbonate minerals. In general, cfd values were found to be higher in the paleosol horizons and lower in the less weathered parts of the loess units, which suggests that the cfd values were controlled by ferimagnet or neoformation during pedogenesis. The pattern of cfd variations confirms the separation of the moderately-expressed Chapin and strongly-expressed Sangamon Geosols. The pattern of cfd variations also confirms the separation of the Sangamon Bt-1 and Bt-2 horizons by a much less pedogenically-altered ACtk horizon (Fig. 6). The cfd variations, which reflect ultrafine particles of magnetic minerals, indicate the concentration of biogenically and in-situ-formed magnetic minerals, and consequently the intensity of pedogenesis. Maximum cfd values are 6% in the Modern Soil. Comparison of cfd values of paleosol units to the Modern Soil value provides useful information for the assessment of paleosol development scales. The cfd values range from 3.5% in the Chapin, 5.5 and 5.8% in the Sangamon Bt-1 and Bt-2, 8.5% in the Yarmouth, 7.5% in the Prairie Du Rocher, 5.8% in Geosol 3, and 7% in Geosol 4 (Fig. 6). The cfd variations suggest that pedogenic neoformation of the Sangamon Bt-1, Bt-2, and Geosol 3 Bt horizons is similar to the modern Bt horizon, and pedogenic neoformation of the Yarmouth Bt, Prairie Du Rocher Bw, and Geosol 4 Bt horizons is stronger than the modern Bt horizon. Ferimagnetic neoformation in the Chapin Bw horizon is less than the modern Bt horizon. The results agree with soil development scales in terms of macro–meso morphology and fabric strength of the pedostratigraphy except for the Prairie Du Rocher Bw horizon. High cfd % values in the moderately-expressed Prairie Du Rocher Bw horizon could result from a high background value of ultrafine magnetite and maghemite derived from Geosol 3 or older paleosols.

H. Wang et al. / Quaternary Science Reviews 28 (2009) 93–106

Frequency dependent magnetic susceptibility

Stratigraphy

Magnetic susceptibility (10-8 m3/kg) 20

40

60

80

100

101

Litho-

Pedo-

Time-

120

0

Modern Bt Peoria Silt

2

4

6

Farmdale A 8

Upper pink

Wisconsinan

Roxana Silt

Depth (m)

10

12

14

Middle tan

Meadow member >36.5 ka * >34.6 ka *

Lower pink

Bottom tan

Chapin Bw

Markham member 93 ± 19 ka * 80 ± 8 ka *

16

104 ± 26 ka

Loveland Silt 18

Sangamon Bt-1 Sangamonian Sangamon Bt-2

Illinoian

*

Yarmouth Bt 20

pre-Illinoian

22

Crowley's Ridge Silt

Prairie Du Roucher Bw

Marianna Silt

Geosol 3 Bt

pre2-Illinoian

24

Geosol 4 Bt 26

Bedrock -2

0

2

4

6

8

10

Xfd (%) Fig. 6. Magnetic susceptibility (c) and frequency dependent magnetic susceptibility (cfd) in loess–paleosol stratigraphy in Simmons Farm.

4.4. Diffuse reflectance (L*a*b*) As expected, lower L* (darker) and higher a* (redder) values were found in the interglacial and moderately-expressed interstadial paleosol units except for the Chapin Bw horizon. The L* values range from 35.0 to 70.0, while a* values range from 4.18 to 16.15 (Fig. 7). The strong reddish brown color of the Yarmouth Geosol is clearly indicated by the broad peaks of low L* (45.0) and high a* (15.0) values (Fig. 7). Two separate peaks of low L* and high a* values are observed in the Sangamon Geosol, and the significant variation of L* and a* values are also found in Geosols 3, 4, and Prairie Du Rocher as well. The reddish hues could have resulted from in-situ hematite formation under the strong pedogenesis and/or longer weathering duration. Hematite commonly forms through dehydration of a ferrihydrite precursor in high temperature environments, e.g. in subtropical and tropical climate regimes

(Schwertmann and Murad, 1983; Schwertmann, 1971). Hematite formation is also accelerated by a longer warm–dry period following a shorter wet period (Maher, 1998). The broad peaks of relatively high a* values therefore suggest that short warm–wet seasons followed by long hot–dry periods prevailed during formation of the Yarmouth Geosol. A high background value of hematite is observed in the lower portion of the Crowley’s Ridge Silt (Fig. 7). As the size of hematite grains increases, the color of the minerals becomes purple and finally black (Schwertmann and Cornell, 1991). The presence of pedogenic hematite together with higher organic content generally makes soil horizons darker in color (low L* values) than loess layers, which contain abundant primary light-colored minerals such as calcite, dolomite, quartz, and illite. Thus, L* and a* values are useful proxies to identify major and moderately developed paleosol units in the Simmons loess record.

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L*

b*

a*

Darker

% Kaolinite

Stratigraphy

Yellower

Redder

Pedo-

Litho75

65

55

45

35

16

20

24

28

Time-

32

0

Modern Bt

2

4

Peoria Silt

6

Wisconsinan Upper pink

8 Roxana Silt

Middle tan

Depth (m)

10

Meadow

>36.5 ka >34.6 ka

Lower pink

12

14

16

Farmdale A

Bottom tan

* *

Markham

Chapin Bw

93 ± 19 ka 80 ± 8 ka

* * 104 ± 26 ka *

Loveland Silt

18

Sang. Bt1

Sangamonian

Sang. Bt2

Illinoian Yarm. Bt

20

Crowley's Ridge Silt

pre-Illinoian PDR Bw

22

24

Geosol 3 Bt

Marianna Silt

pre2-Illinoian Geosol 4 Bt

26

2

4

6

8 10 12 14 16

18

24

30

36

42

Bedrock

Fig. 7. L* (lightness), a* (redness), b* (yellowness), and kaolinite content in loess–paleosol stratigraphy in Simmons loess cores. PDR, Prairie Du Rocher.

The broad peak of high b* values ranging from 24 to 31 from the basal Marianna Silt to the top of the Sangamon Geosol suggests that the goethite content that causes the stronger yellowish hue is significantly higher throughout this zone than through the Roxana and Peoria Silt units (Fig. 7). Soil color variations that are controlled by the mineralogy indicate that sedimentary sources for the basal Roxana (Markham) and older units are different, and it provides another piece of evidence for the Sangamonian and Wisconsinan boundary (Fig. 7). Since hematite is such a strong coloring agent (Deaton and Balsam, 1991), it takes only a small amount of fine grained hematite to ‘‘stain’’ the yellowish goethite particles. However, the HunterLab MiniScan Plus spectrophotometer used in this study is able to detect the yellowish hue even in the presence of high hematite content (>0.02%). The high a* and b* values suggest that both hematite and goethite contents are high in the Yarmouth Geosol. This combination further suggests that soil forming conditions oscillated between hot–dry and warm–wet seasonal conditions. Because b* values in Geosol 3 are similar and a* values in Geosol 3 are lower than those in the Yarmouth Geosol, Geosol 3 contains relatively more goethite. This suggests that long warm– wet and short hot–dry seasonal conditions occurred frequently in

the Geosol 3 period, and the wetter soil conditions could partially dissolve the hematite and relatively enrich the goethite content (Kraus and Hasiotis, 2006) or the wet soil conditions could prevent hematite formation. Therefore, climatic conditions during Geosol 3 formation differ from Yarmouth Geosol development. In addition to the effect of pedogenesis, the L* and a* parameters are also related to variations in the provenance of the silt layers. For instance, high a* and low L* values were found in the upper and lower pink members, and low a* and high L* values were found in the middle and bottom tan members of the Roxana Silt unit (Fig. 7). These variations are an indicator of source changes. The Chapin Geosol is developed in the bottom tan, Markham Silt Member, and its matrix color overwhelms the moderately pedogenically induced reddish colors, explaining why lower L* and high a* values were not found in the Chapin Bw horizon (Fig. 7). 4.5. Kaolinite content Kaolinite content, measured using a PIMA, are generally higher in the interglacial and moderately-expressed interstadial paleosol units than loess units (Fig. 7). The highest kaolinite contents, greater than 30%, were

H. Wang et al. / Quaternary Science Reviews 28 (2009) 93–106

found in the Modern Soil, Yarmouth Geosol, Geosol 3, and Geosol 4. The low kaolinite contents in the Sangamon Geosol could result from dilution by secondary carbonates such as disseminated rhizocretions, rhizoliths, nodules, and concretions. The relatively high kaolinite content in Peoria and Roxana Silts are attributed to the glacial sediments derived from the western Superior lobe that incorporated Cretaceous shale and weathering profiles on pre-Cambrian granitic rocks (Grimley, 2000). The high kaolinite content in Modern Soil in the Mississippi Valley could also be attributed to pedogenesis superposed on provenance factors. The variation in kaolinite content is in phase with the L* (lightness) and a* (redness) variations and in trend with the b* (yellowness) parameters, especially in the loess units older than the Sangamon Geosol. The agreement of kaolinite content with the other proxies suggests that pedogenic alteration was the main cause of kaolinite formation in interglacial-class paleosols (Fig. 7). The high kaolinite content also

103

suggests that the Yarmouth Geosol and Geosol 3 are Ultisols, formed in warm temperate environments with a long duration of weathering. 5. Discussion The 25.6-m loess record from the Simmons Farm site in southern Illinois is one of the most complete and continuous loess– paleosol successions in the entire Midwest. Although the Oil City loess record in Wisconsin (Jacobs et al., 1996) contains similar numbers of paleosols and loess and colluvium units, the entire thickness is only 9 m. The superposed and welded paleo-Bt horizons at this Wisconsin site made boundaries unclear for some stratigraphic units especially older than Roxana Silt (Jacobs and Knox, 1994; Jacobs et al., 1996). The Bledsoe Core site in Arkansas contains five loesses and soils (Rutledge et al., 1990) but with

Fig. 8. Correlation of Simmons Farm loess to sites in the Midwest with somewhat reliable luminescence age controls. The ‘‘unnamed soil’’ on the Glasford Formation at the Pleasant Grove School section, was originally named as Pike Soil when the Teneriffe and Loveland Silts were thought correlative (Willman and Frye, 1970). The term ‘‘Pike Soil’’ is no longer in use in the Illinois stratigraphic code because the formation of such interglacial-class soil units during glacial periods has been questioned. The aim of this study was to correlate the ‘‘unnamed soil’’ with the Sangamon Bt-2 horizon of the Simmons loess record.

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minimal data available for correlation and assessment of pedogenic development. Thus the Simmons Farm loess record is used here to correlate with other specific loess records in the Midwest. 5.1. Chapin and Sangamon Geosol The soil color, texture, and fabric strength of the Sangamon Bt horizons are distinctively different from the overlying Chapin Bw horizon. The obvious sedimentary contact reflects the geological boundary between these Wisconsinan and Illinoian–Sangamonian silt units. In fact, a similar contact between the Markham and the Loveland Silts has been observed in many loess sections in Illinois (Grimley, 1996; Grimley et al., 1998). The loess–paleosol record at the Simmons Farm locality is perhaps the first site in the region to show the Chapin and the upper (Bt-1) and lower (Bt-2) Sangamon phases. Interstadial-class soils correlative to the Chapin may occur in the Phillips Bayou loess section in Arkansas, where it was called an unnamed paleosol (Fig. 8) unit (Markewich et al., 1998), and also in the Eustis Ash pit section in Nebraska and a few sections in Kansas (Great Plains), where a ‘‘reddish soil’’ has been noted (Feng et al., 1994). The TL date obtained above the unnamed soil at the Phillips Bayou site suggests that the unnamed soil was developed between about 65 and 55 ka (Markewich et al., 1998), and the TL dates obtained from the ‘‘reddish soil’’ at the Eustis site were reported to be about 69  6.4 kyr (Feng et al., 1994). The pedogenic unit of the top Sangamon Geosol, which was considered to occur during MIS 4 in eastern Nebraska (Mason et al., 2007), could be correlative to the Chapin Geosol. The Chapin Geosol was also identified in southern Indiana, which overlies the Sangamon Geosol (Hall and Anderson, 2001). Several OSL dates obtained from the basal Roxana Silt at Bonfils Quarry in Missouri had an average age of ca. 66 ka (Forman and Pierson, 2002). Although somewhat elevated values of magnetic susceptibility in basal Roxana Silt at this site could be used to interpret as pedo-mixing of Sangamon Geosol and basal Roxana Silt (Forman and Pierson, 2002), they could also be interpreted as a zone of weaker pedogenesis than that in the Sangamon Geosol (Fig. 8). Local ecosystems in southern Illinois dramatically changed from deciduous forest to prairies with scatted Picea trees (Zhu and Baker, 1995). We envision that the Markham Silt in the basal Roxana may have been deposited during this transition period when the landscape was unstable. The moderately well-expressed pedogenic features of the Chapin Geosol suggests that the early Wisconsinan was not a cold period, and the periglacial conditions did not begin until ca. 55 ka, when the deposition of the Meadow Silt Member of the Roxana Silt began. Our U-series isochron dates of soil carbonate nodules collected from beneath the Sangamon Bt-1 horizon, and from the basal Bt-2 horizon were 80  8 (2s), 93  19 (2s), and 104  26 (2s) ka, respectively. The U-series isochron dates of soil carbonate nodules indicate minimal age of soil formation but not the loess depositional age. Thus, the loessial materials above the Bt-2 horizon could be deposited between 80  8 ka and/or 93  19 and 104  26 ka. The thick ACtk horizon may reflect a dry soil with higher

evaporation condition, which we speculate to have formed during all or part of the MIS 5d substage. We correlate the Sangamon Bt-1, ACtk, and Bt-2 horizons at the Simmons Farm to the MIS 5a–c, 5d, and 5e substages, respectively. Thus, the Sangamon Geosol profile in the Simmons Farm fully agrees with the pollen record that the warm–moist deciduous forest environment occurred during early and late, and warm–dry woodland/savannah environments occurred during the middle phase of the Sangamonian age (Zhu and Baker, 1995). The U-series isochron dates suggest that the ACtk horizon may be correlative to the Teneriffe Silt at the Pleasant Grove School section in Illinois (Forman and Pierson, 2002) and to Loveland Silt 2 (Fig. 8) at Tennessee sites (Rodbell et al., 1997). 5.2. Yarmouth Geosol In previous Midwestern loess chronology studies, TL and OSL dates yielded minimal ages for the Crowley’s Ridge Silt (Forman et al., 1992; Forman and Pierson, 2002), and no consensus has been reached for correlating the Yarmouth Geosol to the MIS record. At this stage, we do not have age controls on the Yarmouth Geosol. Although weathering proxies, e.g. cfd, a*, and kaolinite content, show much greater values in the Yarmouth than any other geosol units at the Simmons Farm section, we cannot confidently correlate with the MIS record. Without reliable luminescence, age control correlation of the Yarmouth Geosol with other sites in the Midwest is also skeptical (Fig. 8). 5.3. Prairie Du Rocher Geosol, Geosol 3, and Geosol 4 It is difficult to correlate the Prairie Du Rocher, Geosol 3, and Geosol 4 with MIS and other loess records in the Midwest without numerical age chronology (Fig. 8). 5.4. Discussion of Midwest loess (litho-) stratigraphy The Chapin Geosol, developed in the Markham Silt, is a cool interstadial-class soil and the Sangamon Geosol, developed in the Loveland Silt, is a warm interglacial-class soil. Their contact is the boundary of the Sangamonian interglacial and Wisconsinan glacial intervals (Willman and Frye, 1970). A variably cool but not cold climate condition during the early Wisconsinan does not conflict with the evidence that cold periglacial climate started during the middle Wisconsinan at ca. 55 ka reported by Tripsanas et al. (2007), Dorale et al. (1998), Zhu and Baker (1995), Curry and Follmer (1992), and Forman and Pierson (2002). Our pedostratigraphy, analytical results, and paleoclimate and environment reconstruction suggest that the Roxana consists of the basal Markham Silt and upper Meadow Silt members. It is noticeable that in some stratigraphic studies, the term of the Roxana Silt is only restricted to the Meadow Silt member (see Table 2). The Simmons loess record does not support that the Chapin is an extension of the Sangamon Geosol and to extend the Sangamonian interval up to 55 ka, when ‘‘Roxana’’ (the Meadow Silt member here) formation and periglacial conditions started.

Table 2 Comparison of Midwest loess units. This study

Forman and Pierson, 2002

Rodbell et al., 1997

Leigh and Knox, 1994

Follmer, 1982

Willman and Frye, 1970

MIS

Peoria Roxana

Peoria Roxana/Pisgah

Peoria Roxana Loveland 3

Peoria Roxana

Peoria Roxana

2 3 4

Sangamonian Loveland Illinoian Loveland

Teneriffe Loveland

Loveland 2 Loveland 1

Loveland Wyalusing

Loveland

Peoria Meadow McDonough Markham ‘‘Berry Clay’’ Loveland

Crowley’s Ridge Marianna

Crowley’s Ridge

Meadow Markham

MIS, Marine Isotope Stages.

5 6 Not clear Not clear

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The observation of two separate Sangamon Bt horizons in Simmons loess cores suggests that loessial material in ACtk and upper Bt horizon was deposited during the Sangamonian Stage, while loessial material in the lower and beneath the lower Bt horizon was deposited during the Illinoian Stage (by default). The Sangamonian Loveland may be correlative to Teneriffe Silt (Forman and Pierson, 2002), Loveland 2 (Rodbell et al., 1997), and Berry Clay equivalence in local areas (Willman and Frye, 1970) (see Fig. 8, Table 2). The observation of two Sangamon Bt horizons separated by a zone enriched with secondary carbonate indicates that the Sangamonian warm–humid interval in the Midwest was interrupted by a dry environment. A loess–paleosol sequence older than Yarmouth Geosol is a relatively unusual occurrence in the Midwest. It is the first time in the middle Mississippi River Valley region that the Marianna Silt has been observed with the associated Geosol 3 on the top and underlain by the Geosol 4. 6. Conclusions The long loess–paleosol record found at the Simmons Farm site in southern Illinois on the east bluff of the Mississippi River Valley contains five interglacial-class soils – the Modern Soil, the Sangamon Geosol, the Yarmouth Geosol, Geosol 3, and Geosol 4, and two moderately-expressed interstadial-class soils – the Chapin and the Prairie Du Rocher Geosols. These soils are developed in five loess units, the Peoria, Roxana, Loveland, Crowley’s Ridge, and Marianna Silt units. The Sangamon Geosol contains two Bt horizons, separated by a 40 cm thick macro-massive ACtk horizon. The U-series isochron dates suggest that the loess material of ACtk and Sangamon Bt-1 horizons was deposited during the Sangamonian age. The basal Roxana Silt, known as Markham Silt Member is observed with clear upper and lower boundaries. The Peoria, Meadow Silt (middle Roxana Silt), Markham Silt (basal Roxana Silt) are correlated to MIS 2, 3, and 4 (Table 2). U-series isochron dates on pedogenic carbonates in the Simmons Farm cores suggest that the Sangamon Bt-1 and Bt-2 horizons are correlative to MIS 5a–c and 5e, respectively. Thus, the Sangamonian Loveland and Illinoian Loveland Silts are correlated to MIS 5 and 6 (Table 2). Without the numerical age chronology, we cannot confidently correlate the Yarmouth Geosol, Prairie Du Rocher Geosol, Geosol 3, and Geosol 4, Crowley’s Ridge Silt, and Marianna Silt to the MIS record. A definitive correlation of older paleosol units in the Simmons Farm succession with the MIS record will be made when OSL dates and magnetic property are available in the future. Acknowledgments We thank Z. Lasemi and R.D. Norby for locating the Simmons Farm site, the ISGS drilling team for collecting the Simmons Farm cores, J.M. Dexter for taking photographs, R.E. Hughes for PIMA analysis, L.R. Follmer, B.B. Curry, and E.D. McKay for discussions, K.C. Hackley, D.A. Keefer, and J.H. Goodwin for helpful reviews. We are grateful to Drs Steven L. Forman, Peter M. Jacobs, and the other anonymous reviewer for valuable suggestions and comments. This research was partially supported by a grant from the University of Illinois Campus Research Board. This publication was authorized by the Chief, Illinois State Geological Survey. References Balsam, W.L., Deaton, B.C., 1991. Sediment dispersal in the Atlantic Ocean: evaluation by visible light spectra. Reviews in Aquatic Sciences 4, 411–447. Banerjee, S.K., Hunt, C.P., 1993. Separation of local signals from the regional paleomonsoon record of the Chinese loess plateau: a rock magnetic approach. Geophysical Research Letters 20, 843–846.

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