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The Thebes Section in unglaciated southwestern Illinois contains a well preserved ∼500 kyr loess–paleosol sequence with four loesses and three interglacial soils. Various magnetic, mineralogical, and elemental properties were analyzed and compared over the thickness of soil sola. These proxies for soil development intensity have the following trend: Yarmouth Geosol>Sangamon Geosol>modern soil. Quartz/plagioclase, Zr/Sr, and TiO 2 /Na 2 O ratios were most sensitive to weathering. Frequency dependent magnetic susceptibility and anhysteretic remanent magnetization, greatest in A horizons, also correspond well with soil development intensity. Neoformed mixed-layered kaolinite/expandables, suggestive of a warm/humid climate, were detected in the Sangamon and Yarmouth soil sola. Clay illuviation in soils was among the least sensitive indicators of soil development. Differences in properties among interglacial soils are interpreted to primarily reflect soil development duration, with climatic effects being secondary. Assuming logarithmic decreases in weathering rates, the observed weathering in the Sangamon Geosol is consistent with 50 kyr of interglacial weathering (Oxygen Isotope Stage 5) compared to 10 kyr for the modern soil (Oxygen Isotope Stage 1). We propose that the Yarmouth Geosol in the central Midwest formed over 180 kyr of interglacial weathering (including oxygen isotope stages 7, 9, and 11).

In the 1870s, agricultural geologists (pioneer pedologists) in Germany, Denmark, and Russia conceived of the soil profile. In more than a century since, pedologists have generally agreed on the reasons and purpose for using symbols such as A-B-C for the designations, but not on the definitions themselves or the assigned significance of the designations. In this paper, we submit that two seemingly conflicting classes of profile concepts evolved in the USA from European roots. The conflict stems historically from arbitrarily defined thin and thick profile concepts, often referred to as the soil or geologic weathering profiles, respectively. The pedologic or thin profile concept is depth-restricted when compared with the geologic thick weathering profile. The geologic profile concept was developed as a homologue of the pedologic profile and is considered to be the full or complete profile of weathering. Throughout the 20th century many variations of the concept of profile appeared, and all seem to have pedo-geo conflicts, exemplified by the myriad C horizon definitions by soil scientists. Recent concepts, such as the pedoweathering profile, have integrated the terminology used by pedologists and geologists into a functional and useful classification for all horizons of complete profiles. Full 21st century understanding of soils beyond the historic 20th century needs of agriculture, increasingly requires a knowledge of soil properties to greater depth than merely the historic solum and upper C horizon, and makes understanding subsolum properties more critical than ever before.

The δ 13 C of soil carbonate in rhizoconcretions collected from aloess-paleosol sequence in the central United States indicates that grow-ing-season C 3 /C 4 plant ratio oscillated by 35% on a 900 ± 200 yr timescale during the late Wisconsinan glaciation. The pattern appears inphase with advance and retreat of the southern margin of the Lauren-tide ice sheet, suggesting influence by paleo–El Nino–Southern Oscilla-tion cycles. The δ 13 C of soil organic matter indicates that the annual av-erage C 3 /C 4 plant ratio oscillated only by 18%, with a periodicity of450 ± 100 yr, and closely matched the cyclic pattern of loess-paleosollayers. It suggests a periodic enhancement of the penetration of theGulf of Mexico air over the region during this time.Keywords: carbon isotope, loess-paleosol, glacial phase, paleo-ENSO cycle. INTRODUCTION The El Nino–Southern Oscillation (ENSO) causes global-scale climateanomalies. The El Nino and La Nina are two counterparts of ENSO, whichare associated with warming and cooling in surface water in the eastern trop-ical Pacific. On the basis of modeling studies, Cane (1998), Cane andClement (1999), and Clement and Cane (1999) proposed that frequency, in-tensity, and duration of long-term El Nino or La Nina intervals have oscil-lated on a millennial time scale during the past 150 k.y. They proposed thatduring glacial periods, these long-term ENSO cycles might alter the globalclimate via atmospheric teleconnections, in which El Nino–dominant inter-vals may cause retreat and La Nina–dominant intervals may cause advanceof the southern margin of the Laurentide ice sheet. The ENSO-sensitive re-gions currently in North America partially overlap the area that was coveredby the Laurentide ice sheet during the last glaciation (Fig. 1). ModernLa Nina conditions cause heavy snowfall, whereas El Nino conditions re-duce winter precipitation in these regions (http://www.ogp.noaa.gov/enso).It is reasonable to assume that La Nina– or El Nino–dominant intervals mighthave controlled advance and retreat of the southern margin of the Laurentideice sheet, if these intervals oscillate on a long-term time scale. In this paper,we use stable isotope ratios from a loess-paleosol exposure in the centralUnited States to seek evidence for millennial-scale paleo-ENSO cycles.

Narrow grabens displace Quaternary sediments near the northern edge of the Mississippi Embayment in extreme southern Illinois, east-central United States. Grabens are part of the Fluorspar Area Fault Complex (FAFC), which has been recurrently active throughout Phanerozoic time. The FAFC strikes directly toward the New Madrid Seismic Zone (NMSZ), scene of some of the largest intra-plate earthquakes in history. The NMSZ and FAFC share origin in a failed Cambrian rift (Reelfoot Rift). Every major fault zone of the FAFC in Illinois exhibits Quaternary displacement. The structures appear to be strike-slip pull-apart grabens, but the magnitude and direction of horizontal slip and their relationship to the current stress field are unknown. Upper Tertiary strata are vertically displaced more than 100 m, Illinoian and older Pleistocene strata 10 to 30 m, and Wisconsinan deposits 1 m or less. No Holocene deformation has been observed. Average vertical slip rates are estimated at 0.01 to 0.03 mm/year, and recurrence intervals for earthquakes of magnitude 6 to 7 are on the order of 10,000s of years for any given fault. Previous authors remarked that the small amount of surface deformation in the New Madrid area implies that the NMSZ is a young feature. Our findings show that tectonic activity has shifted around throughout the Quaternary in the central Mississippi Valley. In addition to the NMSZ and southern Illinois, the Wabash Valley (Illinois–Indiana), Benton Hills (Missouri), Crowley's Ridge (Arkansas–Missouri), and possibly other sites have experienced Quaternary tectonism. The NMSZ may be only the latest manifestation of seismicity in an intensely fractured intra-plate region.

Magnetic susceptibility (MS) patterns have proven useful for regional stratigraphic correlations of zones within thick, oxidized Peoria and Roxana Silts along the Illinois and Central Mississippi River valleys for more than 350 km. Variations in MS of C horizon loess are controlled by silt-sized magnetite content and are interpreted to reflect changes in sediment provenance due to fluctuations of the Superior and Lake Michigan glacier lobes and the diversion of the Mississippi River to its present course. Grain size distributions and scanning electron microscopic observations indicate that stratigraphic changes in MS are not significantly influenced by eolian sorting or diagenetic dissolution, respectively. Three compositional zones (lower, middle, and upper) are delineated within Peoria Silt which usually can be traced in the field by MS, the occurrence of clay beds, interstadial soils, and/or subtle color changes. These zones can be correlated with, but are generally of more practical use than, previously studied dolomite zones (McKay, 1977) or clay mineral zones (Frye et al.,1968). However, mineralogical analyses can help to substantiate zone boundaries when in question. MS and compositional zones may indirectly record a climatic signal, primarily through the effect that global cooling has had on ice lobe fluctuations in the Upper Mississippi drainage basin.

Hominids left Africa and occupied mainland Asia by 1.8 myr ago. About 1.15 myr agoHomo erectusand an associatedStegodon–Ailuropodafauna migrated from subtropical China across the Qinling Mountains into the temperate Loess Plateau. This migration may be an evolutionary milestone in human adaptability because it may represent the first occupation of a nontropical environment. Loess–paleosol stable isotope ratios from the last interglacial–glacial cycle provide comparative data for reconstructing the hominid paleoenvironments. The climate during Gongwangling hominid occupation about 1.15 myr ago was influenced by both Siberian–Mongolian winter and Indian summer monsoon systems characterized as a cold/cool, dry winter and warm/mild, semihumid summer and fall. The Gongwangling hominids preyed mainly on warm-climate-adapted animals such asStegodon–Ailuropodafauna, suggesting a warm season occupation. The stable isotope ratios also indicate that the Chenjiawo hominids occupied an environment similar to that of the Gongwangling about 650,000 yr ago. The associated fauna, with a mixture of forest and steppe, warm- and cold/cool-climate-adapted animal assemblage's, suggests a permanent occupation by this time. Thus, the reliable earliest and permanent occupation of temperate environments may have occurred 150,000 yr earlier in eastern Asia rather than in Europe.

Tertiary and/or Quaternary tectonic faulting is documented in three areas of southernmost Illinois: the Fluorspar Area Fault Complex (FAFC) in Pope and Massac Counties, the Ste. Genevieve Fault Zone (SGFZ) in Alexander and Union Counties, and the Commerce Fault Zone (CFZ) in Alexander County. In the FAFC, faults that strike NE and NNE displace Mounds Gravel (late Miocene to early Pleistocene) and, locally, the Metropolis terrace gravel (Pleistocene; pre-Woodfordian). No Woodfordian or younger deposits are deformed. Faults typically outline narrow, linear grabens that formed under tension with a component of strike slip. North-south to NW-trending vertical faults near the southeast end of the SGFZ displace Eocene sediments. Again, faults outline narrow grabens and show indications of strike slip. Deformed Quaternary sediments have not been observed. The CFZ, which trends northeast, displaces Mounds Gravel in Illinois and units as young as Peoria Silt (Woodfordian) in Missouri. Quaternary movement has been interpreted as right-lateral strike-slip. The CFZ coincides with a subtle gravity and magnetic lineament and seems to reflect a major feature in the basement. Surface expression in Illinois is subtle, but mafic and ultramafic intrusions, hydrothermal alteration and small faults align with the Commerce geophysical lineament. Earthquake foci in Missouri and Illinois lie on or close to the CFZ; some focal mechanisms fit the fault trend. Among these structures, only the CFZ exhibits slip that conforms to the current stress field (principal compressive stress axis E-W to ENE-WSW). Possibly, the stress field changed during Neogene time. Alternatively, high fluid pressures or local stress concentrations may have induced slip on less favorably oriented fractures. Tighter constraints are needed on timing, magnitude, and direction of Neogene displacement.

Few words in the realm of earth science have caused more debate than “loess”. It is a common term that was first used as a name of a silt deposit before it was defined in a scientific sense. Because this “loose” deposit is easily distinguished from other more coherent deposits, it was recognized as a matter of practical concern and later became the object of much scientific scrutiny. Loess was first recognized along the Rhine Valley in Germany in the 1830s and was first noted in the United States in 1846 along the lower Mississippi River where it later became the center of attention. The use of the name eventually spread around the world, but its use has not been consistently applied. Over the years some interpretations and stratigraphy correlations have been validated, but others have been hotly contested on conceptual grounds and semantic issues. The concept of loess evolved into a complex issue as loess and loess-like deposits were discovered in different parts of the US. The evolution of concepts in the central US developed in four indefinite stages: the eras of (1) discovery and development of hypotheses, (2) conditional acceptance of the eolian origin of loess, (3) “bandwagon” popularity of loess research, and (4) analytical inquiry on the nature of loess. Toward the end of the first era around 1900, the popular opinion on the meaning of the term loess shifted from a lithological sense of loose silt to a lithogenetic sense of eolian silt. However, the dual use of the term fostered a lingering skepticism during the second era that ended in 1944 with an explosion of interest that lasted for more than a decade. In 1944, R.J. Russell proposed and H.N. Fisk defended a new non-eolian, property-based, concept of loess. The eolian advocates reacted with surprise and enthusiasm. Each side used constrained arguments to show their view of the problem, but did not examine the fundamental problem, which was not in the proofs of their hypothesis, but in the definition of the term. Between 1944 and about 1950, the debates about loess reached a maximum level of complexity. The main semantic problem was submersed in peripheral arguments about physical properties and genetic interpretations. The scholarly treatment of the subject by Fisk and Russell stimulated quality responses from a diversity of earth scientists interested in academic and applied studies, particularly geo-history, pedology, soil mechanics and stratigraphy. The long-lasting popularity of loess studies during the bandwagon era lasted to about 1970. By that time, the analytical and technical interests had attracted the mainstream into the fourth era with a focus beyond the old arguments. Although Fisk and Russell found themselves defending an unpopular theory, they stimulated a scientific interest in the late Quaternary history of the Mississippi Valley that may never be exceeded.

This study examines and compares methods of dating pit/mound microtopography formed by tree uprooting, and provides 14 C evidence for the longevity of these landforms. Microtopography can often by dated by reference to known meteorological phenomena, or within certain age constraints, by dendrochronologic means. We used 14 C analysis of buried wood and charcoal in treethrow mounds in Michigan and Wisconsin, U.S.A. to arrive at estimates of the geochronometric ages of treethrow mounds. Results indicate that mounds in these areas often persist for more than 1000 years, which are two to five times longer than published estimates by less reliable methods. The longevity of treethrow mounds in these regions is ascribed to (l) sandy, porous soils which minimize runoff, (2) a continuous mat of forest litter and vegetation cover, (3) surface concentrations of gravel which may act as an “armor”, (4) large initial size of the features, and (5) soil freezing. Implications are that rates of mass movement due to uprooting may be substantially less than studies from other regions suggest.

Thick Roxana Silt (middle Wisconsinan) in central and southwestern Illinois traditionally has been interpreted as loess derived from valley-train deposits in the ancient Mississippi River valley. Winters et al. (H. A. Winters, J. J. Alford, and R. L. Rieck, Quaternary Research 29, 25–35, 1988) recently suggested that the Roxana was not directly related to glacial activity, but was derived from sediment produced by increased shoreline and spillway erosion associated with a fluctuating ancestral Lake Michigan. Because (1) paleoenvironmental and paleohydrologic conditions inferred in the hypothesis are unlikely for a loess depositional system and (2) loess did not accumulate during late Wisconsinan deglaciation under conditions similar to those hypothesized, we suggest the hypothesis should be rejected. Roxana distribution suggests the major source was drainage from the upper Mississippi River valley, and variations in loess thickness in Illinois can be explained by consideration of valley width, depth, orientation, and postdepositional erosion. Tills in the headwaters region of the ancient Mississippi drainage system in Minnesota and Wisconsin occur in the appropriate stratigraphic position and have colors and mineralogic compositions that suggest they could be the parent till of the Roxana. We believe a valley-train source for thick Roxana is most probable and urge continued consideration of middle Wisconsinan glaciation in the upper Great Lakes area.

An elutriator method has been developed to increase laboratory efficiency in making particle-size and mineralogical analyses of silt. The elutriator described quantitatively separates silts into three predetermined size fractions, which are clean and suitable for mineralogical analysis. To complete the particle-size analysis, sand fractions are determined by sieving and clay fractions are determined by the pipette method.