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Absence of Glaciation in Illinois during Marine Isotope Stages 3 through 5
QUATERNARY RESEARCH ARTICLE NO.
46, 19–26 (1996)
0040
Absence of Glaciation in Illinois during Marine Isotope Stages 3 through 5 B. BRANDON CURRY Illinois State Geological Survey, 615 E. Peabody Dr., Champaign, Illinois 61820 AND
MILAN J. PAVICH United States Geological Survey, Mail Stop 955, National Center, Reston, Virginia 22092 Received October 25, 1995
tope record (and its proxy record of global ice volume) therefore do not necessarily coincide with ages from the sediment record from Illinois (Fig. 1). The discordance between marine isotope events and Illinois terrestrial records appears to correlate with volumes of global glacial ice smaller than those of large glacial events such as stages 2 and 6 (Shackleton, 1987). For example, late Altonian loess (Roxana Silt), deposited chiefly from 45,000 to 30,000 years ago (McKay, 1979; Curry and Follmer, 1992; Leigh and Knox, 1993) has no distinct deep sea cold-corollary (Fig. 1) and occurred when there was at least 35% less global ice than during maximum stage 2 conditions (Shackleton, 1987). The Sangamonian interglacial stage has long been correlated presumptively with marine isotope stage 5 (Follmer, 1983). However, Richmond and Fullerton (1986) restricted the Sangamonian Stage (and implicitly, development of the Sangamon Geosol) to substage 5e (130,000 to 123,000 yr ago; Martinson et al., 1987). Currently, the only numerical ages of Sangamonian material in the type area of central Illinois have been determined by electron spin resonance methods (ESR) from the Hopwood Farm locality (Fig. 2). Ages of a mastodont molar include 71,000 { 8000 yr, 91,000 { 12,000 yr, and 120,000 { 15,000 yr depending on the uranium-uptake model used to calculate the age (Blackwell et al., 1990). Although the ESR ages fall largely in the range of stage 5, their uncertainty precludes correlation with any substage of stage 5. Hence, the age of the Sangamonian Stage in its type area remains poorly known. Critical to assessing the age of the Sangamon Geosol is determination of the age of its Illinoian loessial or other glacigenic parent material. Widely assumed to have been deposited during marine isotope stage 6 (Follmer, 1983), Illinoian sediments have been dated numerically for only two sites in Illinois. At the Hopwood Farm locality (Fig. 2), ESR ages on Sangamonian enameloid gar scales 0.1 to 1.3 m below the ESR-dated mastodont molar discussed above
A 10Be inventory and 14C ages of material from a core from northernmost Illinois support previous interpretations that this area was ice free from ca. 155,000 to 25,000 yr ago. During much of this period, from about 155,000 to 55,000 yr ago, 10Be accumulated in the argillic horizon of the Sangamon Geosol. Wisconsinan loess, containing inherited 10Be, was deposited above the Sangamon Geosol from ca. 55,000 to 25,000 yr ago and was subsequently buried by late Wisconsinan till deposited by the Lake Michigan Lobe of the Laurentide Ice Sheet. The Sangamonian interglacial stage has been correlated narrowly to marine oxygen isotope substage 5e; our data indicate instead that the Sangamon Geosol developed during late stage 6, all of stages 5 and 4, and early stage 3. q 1996 University of Washington.
INTRODUCTION
The history of Quaternary climatic change interpreted from the sediment record of glaciated midwestern North America has evolved as understanding of sedimentation processes, pedology, and chronology has improved (Leverett, 1899; Frye and Willman, 1973; Ruhe, 1976; Clark et al., 1993). For deposits beyond the range of reliable radiocarbon dating (ca. 35,000 yr ago; Curry and Follmer, 1992), lack of numerical ages has resulted in poorly tested correlations among glaciated midwestern North America successions (Clark and Lea, 1986; Kempton et al., 1985; Clark et al., 1993). Confirmation of correlations is now possible, however, as ages determined from relatively recent techniques (e.g., thermoluminescence (TL), aminostratigraphy, electron spin resonance, U-series, and 10Be inventories) are critically evaluated and compared (e.g., Markewich et al., in press). With the exception of the last glaciation, the southern margin of the Laurentide Ice Sheet did not necessarily wax and wane in sync with other sectors of the ice sheet during the last two glacial–interglacial cycles (Clark et al., 1993). Ages of climatic shifts indicated by the marine oxygen iso19
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FIG. 1. Correlation of Illinois chronostratigraphy with the normalized d18O curve and geochronology of Martinson et al. (1987).
implies an age of at least 113,000 yr for underlying Illinoian till. However, additional work needs to be done to test the nature and stability of the ESR signal measured in the gar scales (Blackwell et al., 1990; in preparation). A single TL age of 77,000 { 8000 ka by Forman et al. (1992) on late Illinoian loess near the Pleasant Grove section (Fig. 2) implies that deposition of the subjacent till is related to global cooling during stage 4 of the marine isotope record, and further, that the Sangamon Geosol at that site was developed during stage 3. The surficial tills at Hopwood Farm and at Pleasant Grove, however, are correlated with members of the Glasford Formation (McKay, 1979) thought to be facies of one depositional unit (McKay, personal communication). Hence, the age of Illinoian sediment in its type region remains unresolved. Here we report a 10Be inventory on a profile of the Sangamon Geosol identified in boring MC-8 located in northern Illinois (Fig. 2) that sets temporal constraints on advances of the Lake Michigan Lobe during the early Wisconsinan, Sangamonian, and Illinoian stages. 10Be inventories of paleosols or sediment sequences enable the determination of the duration of surface exposure, and by inference, the age of the parent material. Our inventory was made on samples from one of the northeasternmost occurrences in Illinois of the Sangamon Geosol developed in glacigenic sediment de-
posited by the Lake Michigan Lobe (Fig. 2). Absence of ‘‘stage 4’’ glacigenic sediment suggested by the 10Be inventory from this boring reflects absence of the Lake Michigan Lobe in Illinois from at least 130,000 to 25,000 yr ago, bolstering previous interpretations based on 14C ages and paleosol macromorphology (Curry, 1989; Curry and Follmer, 1992).
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METHODS
Cores were collected with a Central Mining Equipment (CME) continuous core sampler and Mobil B30-S drill rig. Samples were logged and subsampled for analyses of mineral phases, particle size, organic carbon (Table 1), bulk density, 14C, and 10Be (Tables 1–3). Particle-size distribution of 12 samples was analyzed with a Sedigraph after dispersion with sodium hexametaphosphate. The gravel and sand fractions were analyzed by wet sieving (Table 1). Textural classes are in USDA terminology. Phase analyses of the õ2 mm fraction of eleven samples were quantitatively determined using X-ray diffraction of oriented, ethylene-glycol solvated samples (Hughes et al., 1994). Feldspar, carbonate, and quartz contents also were estimated by using X-ray diffraction of unoriented smears of the fine silt fraction (4 to 16 mm). Analyses were made
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Samples for 10Be analyses were scraped from core segments representative of pedogenic horizons. After samples were mixed and ground to a fine powder, aliquots were separated and weighed. A calibrated, dissolved spike of 9Be was added to each of the dried samples at room temperature. The spiked samples were mixed with NaCO3 and heated to 12007C in quartz crucibles. The resulting glass beads were processed chemically to extract a purified BeO powder. The 10 Be/9Be ratios of the BeO samples were measured by AMS at the Lawrence Livermore Center for Accelerator Mass Spectrometry. The AMS measurements have a 2-sigma precision of 5%. Analyzed ratios were calibrated against standards of known 10Be/9Be. Typical 10Be/9Be ratios for argillic horizons in soils are about 10011. These are converted to 10 Be concentrations in units of 106 atoms of 10Be per gram of soil based on sample weight and spike weight (Table 3). STRATIGRAPHY
FIG. 2. Location of boring MC-8 and other sites discussed in text.
using a theta–theta Scintag X-ray diffractometer. The relative percentages of several phases were calculated using a proprietary program of Scintag including: expandable clay ˚ (angstrom), illite, kaolinite, minerals that swelled to ca. 17 A chlorite, and a randomly interstratified kaolinite/expandable phase (Table 2). The latter phase commonly is found in the argillic horizons of the Sangamon Geosol and other paleosols (Hughes et al., 1994).
Below 14.1 m of diamicton correlated with the Tiskilwa Formation are several units characterized by their distinctive color, stratification, pedogenic features, and mineral phases (Fig. 4). Pedostratigraphic units include the Farmdale and Sangamon Geosols (Curry and Follmer, 1992) which are associated with Morton Tongue and Robein Member of the Mason Group (Hansel and Johnson, in press) and Pearl Formation (Willman and Frye, 1970), respectively. Unit A is 27 cm thick and composed of wood-bearing, stratified, calcareous silt loam. As with underlying unit B, a loessial origin is implied by the silt content and by ratios of greater than three of the medium silt (16–31 mm) and coarse silt (31–63 mm) (Follmer, 1983; Curry, 1989). The
FIG. 3. Lithofacies log, interpreted environment of deposition, and lithostratigraphy of boring MC-8 (Curry, 1995).
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TABLE 1 Particle-Size Distribution and Grain-Size Ratios Particle-size distribution
Grain-size ratios
% of õ2 mm fraction (%) Unit
Depth (m)
% Gravel
Sand
Silt
Clay
õ 1 mm
(16–32 mm)/ (32–64 mm)
õ1 mm/ (1–4 mm)
Tiskilwa Fm. Tiskilwa Fm. A B B C C D D D E E F
13.7 14.0 14.3 14.7 14.8 15.0 15.4 15.7 16.1 16.9 18.1 18.3 18.9
2.6 2.9 0.0 0.0 0.0 0.0 1.7 2.5 1.8 10.0 8.0 10.2 8.7
37.9 32.6 0.7 5.8 7.8 40.0 54.9 39.6 50.0 63.8 61.8 89.6 69.7
36.2 38.4 76.2 61.7 61.6 39.1 21.7 11.8 12.8 12.4 27.3 10.4 20.0
25.9 29.0 23.1 32.5 30.6 20.9 23.4 48.6 37.2 23.8 10.9 0.0 10.3
15.5 17.8 12.0 18.6 16.7 9.5 14.1 43.3 32.1 19.2 6.2 — 6.0
1.6 1.5 5.7 5.8 5.8 3.7 1.7 1.2 1.4 0.9 0.9 — 1.4
1.5 1.6 1.1 1.3 1.2 0.8 1.5 8.1 6.4 4.3 1.3 — 1.4
unit contains chlorite, which implies limited or no weathering (Droste, 1956; Willman et al., 1966). A radiocarbon age on wood from this unit of 24,780 { 360 yr B.P. (ISGS2601) is among the youngest from this stratigraphic horizon in the area and approximates the time of onset of the last glaciation. The 10Be concentration in unit A is 346 1 106 atom/g (Table 3). We correlate the unit with the Morton Tongue of the Mason Group on the basis of stratigraphic position and composition that includes abundant silt, calcite, and dolomite. Unit B, about 61 cm thick, is composed of banded, leached, organic-rich silt loam and silty clay loam. The unit has weak, medium granular structure with white silt coatings (silans), which are pedogenic features typical of
˚ , ethylene glycol solvated); I, Note. E, expandable clay minerals (17A illite; K/E, randomly interstratified kaolinite and smectite; K, kaolinite; C, chlorite.
paleo-A-horizons formed in loess in the region. The bands, from 2 to 5 cm thick, are imparted by slight changes in hue. Organic silt from the darkest band at the top of the unit yielded a radiocarbon age of 26,030 { 450 yr B.P. (ISGS-2602; Fig. 4). The 10Be concentration in unit B is significantly higher than in unit A. Concentrations increase upward from 908 1 106 atoms/g to 992 1 106 atoms/g and are correlated positively with the pedogenic fine clay content (õ1 mm; Table 3). The 10Be concentrations in unit B also are significantly higher than concentrations of about 400 1 106 atoms/g measured in the Farmdale Geosol developed in Roxana Silt in the lower Mississippi River Valley (Pavich et al., 1994). We correlate unit B with the Robein Member of the Roxana Silt (also of the Mason Group) and the Farmdale Geosol because of the 14C age, pedogenic characteristics, grain-size distribution, and stratigraphic position. The Robein Member in this region was deposited from ca. 55,000 to 25,000 yr ago (McKay, 1979; Curry, 1989; Curry and Follmer, 1992; Leigh and Knox, 1993). Unit C is composed of somewhat friable and stratified sandy clay loam to sandy loam 40 cm thick. The upper part also contains a medium-silt-to-coarse-silt ratio between that of units B and D, implying mixing of fartraveled loess with local silt derived from glacigenic sediment. The unit contains common biopores and a weak, medium granular structure, pedogenic features common to A-horizons developed in sandy parent material. Small organic flecks in unit C yielded an AMS 14C age of 38,500 { 5000 yr B.P. (CAMS-7591; WW-172). Because of the pedogenic features and change in parent material texture and mineralogy, we correlate unit C with the A-horizon
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TABLE 2 Semi-Quantitative Analysis of Phases in the õ2 mm Fraction Unit
Depth (m)
E
I
K/E
K
C
A B B C D D D D E
14.3 14.7 14.8 15.0 15.4 15.7 16.1 16.8 18.0
13 22 29 81 62 59 61 81 38
56 34 36 7 8 3 5 9 52
0 0 0 0 26 37 32 7 1
5 13 11 4 4 1 2 3 2
26 31 24 8 0 0 0 0 7
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TABLE 3 Results of 10Be Analyses of Selected Samples from Core MC-8 Unit
Mid-depth (cm)
Thickness (cm)
A B B C C D D D D D
1420 1437 1463 1493 1518 1539 1550 1585 1637 1676
17 22 28 25 23 16 23 44 46 39
10
Be (106 atoms/gm) observed
10
Be (106 atoms/gm) correcteda
346 992 908 626 561 814 1074 792 261 164
— 431 347 65 0 650 910 628 97 0
Density gm/cm3
10 Be inventory (1010 atoms/cm2)
% Clay (õ4 mm)
1.6 1.8 1.9 1.9 2.1 2.0 2.0 2.0 2.0 2.0
— 1.43 1.46 0.24 0.00 1.56 3.14 4.14 0.67 0.00
23.1 25.0 32.5 21.0 20.9 23.4 48.6 37.2 30.0 23.8
a Concentration is corrected by subtracting the inherited concentration, assumed to be the lowest concentration measured in the C-horizon of the paleosol. The assumed depositional flux is 1.3 1 106 atoms/cm2 yr.
of the Sangamon Geosol developed in middle and early Wisconsin an alluvium of the Mason Group. Concentrations of 10Be in unit C range from 561 1 106 to 626 1 106 atoms/g, significantly less than those in either the overlying Robein Member and Farmdale Geosol (unit B) or underlying Pearl Formation and Sangamon Geosol (unit D). The stratification and lower 10Be concentration imply that unit C was deposited rapidly relative to either units
FIG. 4. Radiocarbon ages, particle-size distribution, grain size ratios, pedostratigraphy of the units of interest identified in boring MC-8.
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10
B or D. Furthermore, because unit C was likely a pedogenic A-horizon, 10Be likely was chemically mobilized or removed on elluviated clay. Unit D, interpreted here as the argillic horizon of the Sangamon Geosol, is composed of leached, hard sandy clay to sandy clay loam 143 cm thick. Pedogenic features of argillic horizons are prevalent such as thick, continuous argillans and abundant soft pebbles of sedimentary and
Be inventory, mineralogy of the õ2 mm fraction (%), lithostratigraphy, and
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plutonic rocks (‘‘ghosts’’). The matrix was very dark gray (10YR 3/1; Munsell, 1988) when fresh, but oxidized within minutes to reddish brown (5YR/4/4) and brown (7.5YR 4/4). Unit D 10Be concentrations range from 164 1 106 to 1074 1 106 atoms/g. These values are similar to the range of concentrations measured from argillic horizons of Sangamon Geosol profiles elsewhere in the mid-continental United States (Pavich et al., 1994). Another common characteristic, also observed in the MC-8 core, is the positive correlation between the fine clay content (õ1 mm) and 10Be concentration. The low 10Be concentration from the base of unit D (164 1 106 atoms/g) approximates values normally measured in unaltered glacigenic diamictons. The range of 10Be concentrations in unit D is consistent with chemical precipitation or adsorption of most of the meteoric 10Be during pedogenesis in the near-surface argillic horizon as pedogenic clay was forming. The low solubility of 10Be at near neutral pH (Pavich et al., 1986) probably limited the amount of 10Be that was transported to greater than 1 m below the surface of the Sangamon Geosol. Quantitative phase analysis of unit D reveals abundant randomly interstratified kaolinite and smectite, no chlorite, and much less illite than in other units. Detection of the interstratified kaolinite and smectite requires analysis of diffractograms of samples that underwent several heat treatments (Hughes et al., 1994). The origin of the kaolinitesmectite is not understood, but may be related to weathering of feldspars, especially plagioclase, in the fine silt fraction. The 10Be concentration was not measured in units below unit D possessing only C-horizon pedogenic features (Fig. 4). Little or no chemical weathering in these units is further implied by abundant, easily weathered chlorite.
is subtracted from the meteoric inventory delivered during pedogenesis. Because 10Be has low chemical mobility at near neutral pH but may be lost by erosion from the soil surface (Pavich et al., 1986), the observed inventory may be less than the 10Be delivered to the soil during pedogenesis. For the time of interest, the past 200,000 yr, radioactive decay (t1/2 Å É1.5 1 106 years) is negligible. Depositional Flux
We estimated how long the Farmdale and Sangamon Geosols were exposed to meteoric input of 10Be prior to deposition of unit A (the Morton Tongue of the Mason Group) by using the 10Be inventories in Table 3 and an assumed constant deposition flux of 1.3 1 106 atom/cm2 r yr discussed below. The inventories (atom/cm2) in each sampled layer are calculated by multiplying the corrected concentration by the layer thickness and bulk density. The concentrations are corrected by subtracting the background concentration measured in the parent material (Table 3; Fig. 4). The horizon inventories are summed for the pedon. Minimum surface exposure duration is the pedon inventory divided by the deposition flux. This calculation assumes the following: (1) meteoric 10Be remained in the solum upon adsorbtion or precipitation, (2) little or no 10Be was lost in solution or by erosion, and (3) the inherited inventory (i.e., the 10Be transported to the site adsorbed on dust or loess)
Meteoric 10Be flux is controlled by rainfall amount (Monaghan et al., 1986; Brown et al., 1989). For temperate northern latitudes, a deposition flux of about 1.3 1 106 atom/ cm2 yr for 100 cm/yr rainfall fit the data collected by Brown et al. (1989) and a production model presented by Monaghan et al. (1986). This depositional flux produced reasonable minimum age estimates for a series of fluvioglacial terraces in Slovenia (Pavich and Vidic, 1993), and scaled for a dryer climate produced reasonable minimum age estimates for terraces along the Merced River, California (Pavich et al., 1986). Estimates are uncertain for the long-term variations of depositional flux of 10Be due to fluctuations in production rate. Monaghan et al. (1986) estimate a 20% global production rate variation. There is evidence from marine cores that a constant mean delivery rate has existed over the past 400,000 yr. Southoun et al. (1987) measured 10Be deposition to a North Atlantic site. They found that the 10Be concentration varied by no more than {25%, and that there was no correlation of high or low production rates with glacial/interglacial cycles. Assuming that the delivery rate to marine sediments is a proxy for the magnitude of production rate variation, we infer that for the period of interest that the production rate varied no more than {25%. For our calculations, we assume that the deposition flux of meteoric 10Be during pedogenesis was 1.3 1 106 atom/cm2 yr under humid conditions with rainfall of about 100 cm/yr. The present day mean annual precipitation in north-central Illinois is about 91 cm/yr. Inherited concentrations of 10Be must be subtracted prior to calculation of post-depositional 10Be inventories. Inherited concentrations in till are lower than for silty loess (ca. 100 – 150 1 106 vs ca. 200 1 106 atom/g; Shen et al., 1992; Pavich and Markewich, 1994; Pavich et al., 1994). The lower concentration in glacigenic sediment in this region, compared to loess, is because the till contains more redeposited Paleozoic and pre-Paleozoic bedrock, and less reworked, pedogenically altered late Quaternary sediment. Here we assume that the lowest concentration in the least weathered horizons of the Robein Member and Pearl Formation are the concentrations of inherited 10 Be in those respective units. The inherited concentrations are subtracted from the inventories in the Sangamon and Farmdale Geosols (Fig. 4; Table 3).
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CALCULATION OF SURFACE EXPOSURE DURATION
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Age Calculations Our estimates of the 10Be inventory and depositional flux suggest a minimum duration of exposure of the Farmdale Geosol of about 30,000 yr. This duration, added to the 14C age of 25,000 yr for the Tiskilwa Formation, implies that the age of the base of the Roxana Silt is about 55,000 yr. This is consistent with the regional 14C age estimate of 45,000 to 55,000 yr (McKay, 1979; Curry and Follmer, 1992; Leigh and Knox, 1993). Without correction for the inherited 10Be concentration of 561 1 106 atom/g, the minimum age of the base of the Roxana Silt is 150,000 yr, about 95,000 yr older than the accepted 14C-based age of about 55,000 yr stated above. The duration of exposure of the Sangamon Geosol in MC8 based on the corrected 10Be inventory is 100,000 yr. This is a significantly longer period than calculated for the Farmdale Geosol in accordance with their relative soil development (Follmer, 1983). According to the 14C ages and 10Be inventory from MC-8, the age of the parent material of the Sangamon Geosol, the Pearl Formation, is about 155,000 yr. The age is the sum of the exposure age of 100,000 yr for the Sangamon Geosol (assuming an inherited 10Be concentration of 164 1 106 atoms/g), the duration of Wisconsinan loessial sedimentation discussed above (30,000 yr), and the age of the late Wisconsinan diamicton (25,000 yr). An age of 155,000 yr for the Pearl Formation falls within the range of ages for marine isotope stage 6 (about 130,000 to 200,000 yr ago; Martinson et al., 1987). Evidence for widespread erosion of the Sangamon and Farmdale Geosols in northern Illinois (Kempton et al., 1985) supports our interpretation of local reworking and concentration of 10Be-rich material in the Farmdale Geosol and Robein Member of the Roxana Silt in core MC-8. While there is regional evidence for redeposition and concentration of 10Be in unit B (Roxana Silt; Markewich et al., in press), there is no interstratified kaolinite-smectite (Fig. 4). This observation suggests that 10Be was associated with some other phase in the soil clay fraction, that remobilization of 10Be was chemical, or that there was postdepositional alteration of interstratified kaolinite-smectite in the Farmdale Geosol.
about 100,000 yr. Hence, it is inappropriate to restrict the Sangamonian Stage in Illinois to marine isotope substage 5e. This restriction was proposed by Richmond and Fullerton (1986), who implied that the Sangamon Geosol and lastinterglacial Eemian sediments in Europe were temporally equivalent in their type regions—a view not supported by our results. ACKNOWLEDGMENTS We thank Randall Hughes for X-ray diffraction data, Robert Vaiden for field assistance, and William Dey for particle-size analyses. Richard Berg, E. Donald McKay, Ardith Hansel, Leon Follmer, Peter Clark, and Paul Bierman reviewed the manuscript.
REFERENCES
The 10Be inventory of the Farmdale and Sangamon Geosols in boring MC-8 indicates Illinois was glacier-free from ca. 155,000 to 25,000 yr ago. Previous to this study, the hiatus of glaciation was based on 14C ages, paleosol morphology, and presumptive correlation of the Sangamonian and Illinioan stages with marine isotope stages 5 and 6, respectively (Follmer, 1983; Kempton et al., 1985; McKenna, 1985; Curry, 1989; Curry and Follmer, 1992). Our results also imply that meteoric 10Be accumulated in the argillic horizon of the Sangamon Geosol in northern Illinois for
Blackwell, B. B., Curry, B. B., Saunders, J. J., Schwarcz, H. P., and Woodman, N. Dating the Sangamon interglacial interval near the type locality: Electron spin resonance (ESR) dating mammal teeth and gar scales from Hopwood Farm, southern Illinois, in preparation. Blackwell, B. B., Schwarcz, H. P., Saunders, J. J., Woodman, N., and Curry, B. B. (1990). Dating the Sangamon: Electron Spin Resonance (ESR) dating mammal teeth and gar scales from Hopwood Farm, Montgomery County, Illinois. Geological Society of America, Abstracts with Programs 22, A85. Brown, L, Stensland, G. I., Klein J., and Middleton, R. (1989). Atmospheric deposition of 9Be and 10Be. Geochimica et Cosmochimica Acta 53, 135– 142. Clark, P. U., Clague, J. J., Curry, B. B., Dreimanis, A., Hicock, S. R., Miller, G. H., Berger, G. W., Eyles, N., Lamothe, M., Miller, B. B., Mott, R. J., Oldale, R. N. Stea, R. R., Szabo, J. P., Thorleifson L. H., and Vincent, J.-S. (1993). Initiation and development of the Laurentide and Cordilleran Ice Sheets following the last interglaciation. Quaternary Science Reviews 12, 79–114. Clark, P. U., and Lea, P. (1986). Reappraisal of early Wisconsin glaciation in North America. Geological Society of America Abstracts With Programs 18(6), 565. Curry, B. B. (1989). Absence of Altonian glaciation in Illinois. Quaternary Research 31, 1–13. Curry, B. B. (1995). ‘‘Groundwater Protection Mapping for McHenry County, Illinois: Drilling Report.’’ Illinois State Geological Survey Open File Series 1995-1, 123 p. Curry, B. B., and Follmer, L. R. (1992). The last glacial/interglacial transition in Illinois: 122–25 ka. In ‘‘The Last Interglacial-Glacial Transition in North America’’ (P. U. Clark and P. D. Lea, Eds.), pp. 71–88. Geological Society of America Special Paper 270. Droste, J. B. (1956). Clay minerals in calcareous till in northeastern Ohio. Journal of Geology 64, 187–190. Follmer, L. R. (1983). Sangamonian and Wisconsinan Pedogenesis in the midwestern United States, In ‘‘Late Quaternary Environments of the United States, Vol. I, The Late Pleistocene’’ (S.C. Porter, Ed.), pp. 138– 144, The University of Minnesota Press. Forman, S. L., Bettis, E. A. III, Kemmis, T. J., and Miller, B. B. (1992). Chronologic evidence for multiple periods of loess deposition during the Late Pleistocene in the Missouri and Mississippi River Valley, United States: Implications for the activity of the Laurentide Ice Sheet. Palaeogeography, Palaeoclimatology, Palaeoecology 93, 71–83. Frye, J. C., and Willman, H. B. (1973). Climatic history as interpreted from the Lake Michigan Lobe, In ‘‘The Wisconsinan Stage’’ (R. F. Black,
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R. P. Goldthwait and H. B. Willman, Eds.), pp. 135–152. Geological Society of America Memoir 136.
Monaghan, M. C., Krishnaswami, S., and Turekian, K. K. (1986). The global production rate of 10Be. Earth and Planetary Science Letters 76, 279–287.
Pavich, M. J., Brown, L., Harden, J. W., Klein, J., and Middleton, R. (1986). 10 Be distribution in soils from Merced River terraces. Geochemica et Cosmochemica Acta 50, 1727–1735. Pavich, M. J., and Markewich, H. W. (1994). Beryllium-10 In ‘‘Second Progress Report on Chronostratigraphic and Paleoclimatic Studies, Middle Mississippi River Valley, Eastern Arkansas and Western Tennessee’’ (H. W. Markewich, Ed.). United States Geological Survey Open File Report 94-208, 55 p. Pavich, M. J., and Vidic, N. (1993). Applications of paleomagnetic and 10 Be analyses to chronostratigraphy of Alpine Glaciofluvial terraces, Sava River, Slovenia. Geophysical Monographs 78, 263–275. Pavich, M. J., Markewich, H. W., and Millard, H. T. Jr. (1994). Applications of 10Be to loess stratigraphy of the lower Mississippi Valley, U.S.A. In ‘‘8th International Conference on Geochronology, Cosmochronology and Isotope Geology,’’ abstract. Richmond, G. M., and Fullerton, D. S. (1986). Introduction to Quaternary glaciations in the United States of America, In, ‘‘Quaternary Glaciations in the United States of America’’ (G. M. Richmond and D.S. Fullerton, Eds.), Quaternary Science Reviews 5, 3–10. Ruhe, R. V. (1976). Stratigraphy of mid-continent loess, U.S.A. In ‘‘Quaternary Environments’’ (W. C. Mahaney, Ed.), pp. 153–167. York University Geographical Monographs 5. Shackleton, N. J., (1987). Oxygen isotopes, ice volume and sea level. Quaternary Science Reviews 6, 183–190. Shen, C., Beer, L. Liu, T. Oeshger, H., Suter, M., and Wolfli, W. (1992). 10 Be in Chinese loess. Earth and Planetary Science Letters 109, 169– 177. Southoun, J. R., Ku, T. L., Nelson, D. E., Reyss, J. L., Duplessy, J. C., and Vogel, J. S. (1987). 10Be in a deep-sea core: implications regarding 10Be production changes over the past 420 ka. Earth and Planetary Science Letters 85, 356–364. Willman, H. B., Glass, H. D., and Frye, J. C. (1966). ‘‘Mineralogy of glacial tills and their weathering profiles in Illinois. Part II—Weathering profiles.’’ Illinois State Geological Survey Circular 400, 76 p.
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Hansel, A. K., and Johnson, W. H. (in press). ‘‘Wedron and Mason Groups: A lithostratigraphic reclassification of deposits of the Wisconsin Episode, Lake Michigan Lobe area.’’ Illinois State Geological Survey Bulletin. Hughes, R. E., Moore, D. M., and Glass, H. D. (1994). Qualitative and Quantitative Analysis of Clay Minerals in Soils. In ‘‘Quantitative Methods in Soil Mineralogy,’’ pp. 330–359. Soil Science Society of America Miscellaneous Publications, Madison, WI. Kempton, J. P., Berg, R. C. and Follmer, L. R. (1985). ‘‘Revision of the Stratigraphy and Nomenclature of Glacial Deposits in Central Northern Illinois.’’ Illinois State Geological Survey Guidebook 19, pp. 1–19. Leigh, D. S. (1994). Roxana silt of the Upper Mississippi Valley: Lithology, source and paleoenvironment. Geological Society of America Bulletin 106, 430–442. Leigh, D. S., and Knox, J. C. (1993). AMS radiocarbon age of the Upper Mississippi River Roxana Silt. Quaternary Research 39, 282–289. Leverett, F. (1899). ‘‘The Illinois Glacial Lobe.’’ U.S. Geological Survey Monograph 38, 817 p. Markewich, H. W., Wysocki, D. A., Pavich, M. J., Rutledge, E. M., Millard, H. T., Jr., Rich, F. J., Maat, P. B., Rubin, M., and McGeehin, J. P. (in press). Paleopedology plus TL, 10Be, and 14C dating as tools in stratigraphic and paleoclimatic investigations, Mississippi River Valley, U. S. A. Quaternary International. Martinson, D. G., Pisias, N. G., Hays, J. D., Imbrie, J., Moore, T. C., Jr., and Shackleton, N. J. (1987). Age dating and the orbital theory of the ice ages: Development of a high-resolution 0 to 300,000 - year chronostratigraphy. Quaternary Research 27, 1–29. McKay, E. D. III (1979). ‘‘Wisconsinan Loess Stratigraphy of Illinois.’’ Illinois State Geological Survey Guidebook 13, pp. 95–108.
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Absence of Glaciation in Illinois during Marine Isotope Stages 3 through 5 B. BRANDON CURRY Illinois State Geological Survey, 615 E. Peabody Dr., Champaign, Illinois 61820 AND
MILAN J. PAVICH United States Geological Survey, Mail Stop 955, National Center, Reston, Virginia 22092 Received October 25, 1995
tope record (and its proxy record of global ice volume) therefore do not necessarily coincide with ages from the sediment record from Illinois (Fig. 1). The discordance between marine isotope events and Illinois terrestrial records appears to correlate with volumes of global glacial ice smaller than those of large glacial events such as stages 2 and 6 (Shackleton, 1987). For example, late Altonian loess (Roxana Silt), deposited chiefly from 45,000 to 30,000 years ago (McKay, 1979; Curry and Follmer, 1992; Leigh and Knox, 1993) has no distinct deep sea cold-corollary (Fig. 1) and occurred when there was at least 35% less global ice than during maximum stage 2 conditions (Shackleton, 1987). The Sangamonian interglacial stage has long been correlated presumptively with marine isotope stage 5 (Follmer, 1983). However, Richmond and Fullerton (1986) restricted the Sangamonian Stage (and implicitly, development of the Sangamon Geosol) to substage 5e (130,000 to 123,000 yr ago; Martinson et al., 1987). Currently, the only numerical ages of Sangamonian material in the type area of central Illinois have been determined by electron spin resonance methods (ESR) from the Hopwood Farm locality (Fig. 2). Ages of a mastodont molar include 71,000 { 8000 yr, 91,000 { 12,000 yr, and 120,000 { 15,000 yr depending on the uranium-uptake model used to calculate the age (Blackwell et al., 1990). Although the ESR ages fall largely in the range of stage 5, their uncertainty precludes correlation with any substage of stage 5. Hence, the age of the Sangamonian Stage in its type area remains poorly known. Critical to assessing the age of the Sangamon Geosol is determination of the age of its Illinoian loessial or other glacigenic parent material. Widely assumed to have been deposited during marine isotope stage 6 (Follmer, 1983), Illinoian sediments have been dated numerically for only two sites in Illinois. At the Hopwood Farm locality (Fig. 2), ESR ages on Sangamonian enameloid gar scales 0.1 to 1.3 m below the ESR-dated mastodont molar discussed above
A 10Be inventory and 14C ages of material from a core from northernmost Illinois support previous interpretations that this area was ice free from ca. 155,000 to 25,000 yr ago. During much of this period, from about 155,000 to 55,000 yr ago, 10Be accumulated in the argillic horizon of the Sangamon Geosol. Wisconsinan loess, containing inherited 10Be, was deposited above the Sangamon Geosol from ca. 55,000 to 25,000 yr ago and was subsequently buried by late Wisconsinan till deposited by the Lake Michigan Lobe of the Laurentide Ice Sheet. The Sangamonian interglacial stage has been correlated narrowly to marine oxygen isotope substage 5e; our data indicate instead that the Sangamon Geosol developed during late stage 6, all of stages 5 and 4, and early stage 3. q 1996 University of Washington.
INTRODUCTION
The history of Quaternary climatic change interpreted from the sediment record of glaciated midwestern North America has evolved as understanding of sedimentation processes, pedology, and chronology has improved (Leverett, 1899; Frye and Willman, 1973; Ruhe, 1976; Clark et al., 1993). For deposits beyond the range of reliable radiocarbon dating (ca. 35,000 yr ago; Curry and Follmer, 1992), lack of numerical ages has resulted in poorly tested correlations among glaciated midwestern North America successions (Clark and Lea, 1986; Kempton et al., 1985; Clark et al., 1993). Confirmation of correlations is now possible, however, as ages determined from relatively recent techniques (e.g., thermoluminescence (TL), aminostratigraphy, electron spin resonance, U-series, and 10Be inventories) are critically evaluated and compared (e.g., Markewich et al., in press). With the exception of the last glaciation, the southern margin of the Laurentide Ice Sheet did not necessarily wax and wane in sync with other sectors of the ice sheet during the last two glacial–interglacial cycles (Clark et al., 1993). Ages of climatic shifts indicated by the marine oxygen iso19
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FIG. 1. Correlation of Illinois chronostratigraphy with the normalized d18O curve and geochronology of Martinson et al. (1987).
implies an age of at least 113,000 yr for underlying Illinoian till. However, additional work needs to be done to test the nature and stability of the ESR signal measured in the gar scales (Blackwell et al., 1990; in preparation). A single TL age of 77,000 { 8000 ka by Forman et al. (1992) on late Illinoian loess near the Pleasant Grove section (Fig. 2) implies that deposition of the subjacent till is related to global cooling during stage 4 of the marine isotope record, and further, that the Sangamon Geosol at that site was developed during stage 3. The surficial tills at Hopwood Farm and at Pleasant Grove, however, are correlated with members of the Glasford Formation (McKay, 1979) thought to be facies of one depositional unit (McKay, personal communication). Hence, the age of Illinoian sediment in its type region remains unresolved. Here we report a 10Be inventory on a profile of the Sangamon Geosol identified in boring MC-8 located in northern Illinois (Fig. 2) that sets temporal constraints on advances of the Lake Michigan Lobe during the early Wisconsinan, Sangamonian, and Illinoian stages. 10Be inventories of paleosols or sediment sequences enable the determination of the duration of surface exposure, and by inference, the age of the parent material. Our inventory was made on samples from one of the northeasternmost occurrences in Illinois of the Sangamon Geosol developed in glacigenic sediment de-
posited by the Lake Michigan Lobe (Fig. 2). Absence of ‘‘stage 4’’ glacigenic sediment suggested by the 10Be inventory from this boring reflects absence of the Lake Michigan Lobe in Illinois from at least 130,000 to 25,000 yr ago, bolstering previous interpretations based on 14C ages and paleosol macromorphology (Curry, 1989; Curry and Follmer, 1992).
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METHODS
Cores were collected with a Central Mining Equipment (CME) continuous core sampler and Mobil B30-S drill rig. Samples were logged and subsampled for analyses of mineral phases, particle size, organic carbon (Table 1), bulk density, 14C, and 10Be (Tables 1–3). Particle-size distribution of 12 samples was analyzed with a Sedigraph after dispersion with sodium hexametaphosphate. The gravel and sand fractions were analyzed by wet sieving (Table 1). Textural classes are in USDA terminology. Phase analyses of the õ2 mm fraction of eleven samples were quantitatively determined using X-ray diffraction of oriented, ethylene-glycol solvated samples (Hughes et al., 1994). Feldspar, carbonate, and quartz contents also were estimated by using X-ray diffraction of unoriented smears of the fine silt fraction (4 to 16 mm). Analyses were made
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Samples for 10Be analyses were scraped from core segments representative of pedogenic horizons. After samples were mixed and ground to a fine powder, aliquots were separated and weighed. A calibrated, dissolved spike of 9Be was added to each of the dried samples at room temperature. The spiked samples were mixed with NaCO3 and heated to 12007C in quartz crucibles. The resulting glass beads were processed chemically to extract a purified BeO powder. The 10 Be/9Be ratios of the BeO samples were measured by AMS at the Lawrence Livermore Center for Accelerator Mass Spectrometry. The AMS measurements have a 2-sigma precision of 5%. Analyzed ratios were calibrated against standards of known 10Be/9Be. Typical 10Be/9Be ratios for argillic horizons in soils are about 10011. These are converted to 10 Be concentrations in units of 106 atoms of 10Be per gram of soil based on sample weight and spike weight (Table 3). STRATIGRAPHY
FIG. 2. Location of boring MC-8 and other sites discussed in text.
using a theta–theta Scintag X-ray diffractometer. The relative percentages of several phases were calculated using a proprietary program of Scintag including: expandable clay ˚ (angstrom), illite, kaolinite, minerals that swelled to ca. 17 A chlorite, and a randomly interstratified kaolinite/expandable phase (Table 2). The latter phase commonly is found in the argillic horizons of the Sangamon Geosol and other paleosols (Hughes et al., 1994).
Below 14.1 m of diamicton correlated with the Tiskilwa Formation are several units characterized by their distinctive color, stratification, pedogenic features, and mineral phases (Fig. 4). Pedostratigraphic units include the Farmdale and Sangamon Geosols (Curry and Follmer, 1992) which are associated with Morton Tongue and Robein Member of the Mason Group (Hansel and Johnson, in press) and Pearl Formation (Willman and Frye, 1970), respectively. Unit A is 27 cm thick and composed of wood-bearing, stratified, calcareous silt loam. As with underlying unit B, a loessial origin is implied by the silt content and by ratios of greater than three of the medium silt (16–31 mm) and coarse silt (31–63 mm) (Follmer, 1983; Curry, 1989). The
FIG. 3. Lithofacies log, interpreted environment of deposition, and lithostratigraphy of boring MC-8 (Curry, 1995).
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TABLE 1 Particle-Size Distribution and Grain-Size Ratios Particle-size distribution
Grain-size ratios
% of õ2 mm fraction (%) Unit
Depth (m)
% Gravel
Sand
Silt
Clay
õ 1 mm
(16–32 mm)/ (32–64 mm)
õ1 mm/ (1–4 mm)
Tiskilwa Fm. Tiskilwa Fm. A B B C C D D D E E F
13.7 14.0 14.3 14.7 14.8 15.0 15.4 15.7 16.1 16.9 18.1 18.3 18.9
2.6 2.9 0.0 0.0 0.0 0.0 1.7 2.5 1.8 10.0 8.0 10.2 8.7
37.9 32.6 0.7 5.8 7.8 40.0 54.9 39.6 50.0 63.8 61.8 89.6 69.7
36.2 38.4 76.2 61.7 61.6 39.1 21.7 11.8 12.8 12.4 27.3 10.4 20.0
25.9 29.0 23.1 32.5 30.6 20.9 23.4 48.6 37.2 23.8 10.9 0.0 10.3
15.5 17.8 12.0 18.6 16.7 9.5 14.1 43.3 32.1 19.2 6.2 — 6.0
1.6 1.5 5.7 5.8 5.8 3.7 1.7 1.2 1.4 0.9 0.9 — 1.4
1.5 1.6 1.1 1.3 1.2 0.8 1.5 8.1 6.4 4.3 1.3 — 1.4
unit contains chlorite, which implies limited or no weathering (Droste, 1956; Willman et al., 1966). A radiocarbon age on wood from this unit of 24,780 { 360 yr B.P. (ISGS2601) is among the youngest from this stratigraphic horizon in the area and approximates the time of onset of the last glaciation. The 10Be concentration in unit A is 346 1 106 atom/g (Table 3). We correlate the unit with the Morton Tongue of the Mason Group on the basis of stratigraphic position and composition that includes abundant silt, calcite, and dolomite. Unit B, about 61 cm thick, is composed of banded, leached, organic-rich silt loam and silty clay loam. The unit has weak, medium granular structure with white silt coatings (silans), which are pedogenic features typical of
˚ , ethylene glycol solvated); I, Note. E, expandable clay minerals (17A illite; K/E, randomly interstratified kaolinite and smectite; K, kaolinite; C, chlorite.
paleo-A-horizons formed in loess in the region. The bands, from 2 to 5 cm thick, are imparted by slight changes in hue. Organic silt from the darkest band at the top of the unit yielded a radiocarbon age of 26,030 { 450 yr B.P. (ISGS-2602; Fig. 4). The 10Be concentration in unit B is significantly higher than in unit A. Concentrations increase upward from 908 1 106 atoms/g to 992 1 106 atoms/g and are correlated positively with the pedogenic fine clay content (õ1 mm; Table 3). The 10Be concentrations in unit B also are significantly higher than concentrations of about 400 1 106 atoms/g measured in the Farmdale Geosol developed in Roxana Silt in the lower Mississippi River Valley (Pavich et al., 1994). We correlate unit B with the Robein Member of the Roxana Silt (also of the Mason Group) and the Farmdale Geosol because of the 14C age, pedogenic characteristics, grain-size distribution, and stratigraphic position. The Robein Member in this region was deposited from ca. 55,000 to 25,000 yr ago (McKay, 1979; Curry, 1989; Curry and Follmer, 1992; Leigh and Knox, 1993). Unit C is composed of somewhat friable and stratified sandy clay loam to sandy loam 40 cm thick. The upper part also contains a medium-silt-to-coarse-silt ratio between that of units B and D, implying mixing of fartraveled loess with local silt derived from glacigenic sediment. The unit contains common biopores and a weak, medium granular structure, pedogenic features common to A-horizons developed in sandy parent material. Small organic flecks in unit C yielded an AMS 14C age of 38,500 { 5000 yr B.P. (CAMS-7591; WW-172). Because of the pedogenic features and change in parent material texture and mineralogy, we correlate unit C with the A-horizon
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TABLE 2 Semi-Quantitative Analysis of Phases in the õ2 mm Fraction Unit
Depth (m)
E
I
K/E
K
C
A B B C D D D D E
14.3 14.7 14.8 15.0 15.4 15.7 16.1 16.8 18.0
13 22 29 81 62 59 61 81 38
56 34 36 7 8 3 5 9 52
0 0 0 0 26 37 32 7 1
5 13 11 4 4 1 2 3 2
26 31 24 8 0 0 0 0 7
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TABLE 3 Results of 10Be Analyses of Selected Samples from Core MC-8 Unit
Mid-depth (cm)
Thickness (cm)
A B B C C D D D D D
1420 1437 1463 1493 1518 1539 1550 1585 1637 1676
17 22 28 25 23 16 23 44 46 39
10
Be (106 atoms/gm) observed
10
Be (106 atoms/gm) correcteda
346 992 908 626 561 814 1074 792 261 164
— 431 347 65 0 650 910 628 97 0
Density gm/cm3
10 Be inventory (1010 atoms/cm2)
% Clay (õ4 mm)
1.6 1.8 1.9 1.9 2.1 2.0 2.0 2.0 2.0 2.0
— 1.43 1.46 0.24 0.00 1.56 3.14 4.14 0.67 0.00
23.1 25.0 32.5 21.0 20.9 23.4 48.6 37.2 30.0 23.8
a Concentration is corrected by subtracting the inherited concentration, assumed to be the lowest concentration measured in the C-horizon of the paleosol. The assumed depositional flux is 1.3 1 106 atoms/cm2 yr.
of the Sangamon Geosol developed in middle and early Wisconsin an alluvium of the Mason Group. Concentrations of 10Be in unit C range from 561 1 106 to 626 1 106 atoms/g, significantly less than those in either the overlying Robein Member and Farmdale Geosol (unit B) or underlying Pearl Formation and Sangamon Geosol (unit D). The stratification and lower 10Be concentration imply that unit C was deposited rapidly relative to either units
FIG. 4. Radiocarbon ages, particle-size distribution, grain size ratios, pedostratigraphy of the units of interest identified in boring MC-8.
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B or D. Furthermore, because unit C was likely a pedogenic A-horizon, 10Be likely was chemically mobilized or removed on elluviated clay. Unit D, interpreted here as the argillic horizon of the Sangamon Geosol, is composed of leached, hard sandy clay to sandy clay loam 143 cm thick. Pedogenic features of argillic horizons are prevalent such as thick, continuous argillans and abundant soft pebbles of sedimentary and
Be inventory, mineralogy of the õ2 mm fraction (%), lithostratigraphy, and
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plutonic rocks (‘‘ghosts’’). The matrix was very dark gray (10YR 3/1; Munsell, 1988) when fresh, but oxidized within minutes to reddish brown (5YR/4/4) and brown (7.5YR 4/4). Unit D 10Be concentrations range from 164 1 106 to 1074 1 106 atoms/g. These values are similar to the range of concentrations measured from argillic horizons of Sangamon Geosol profiles elsewhere in the mid-continental United States (Pavich et al., 1994). Another common characteristic, also observed in the MC-8 core, is the positive correlation between the fine clay content (õ1 mm) and 10Be concentration. The low 10Be concentration from the base of unit D (164 1 106 atoms/g) approximates values normally measured in unaltered glacigenic diamictons. The range of 10Be concentrations in unit D is consistent with chemical precipitation or adsorption of most of the meteoric 10Be during pedogenesis in the near-surface argillic horizon as pedogenic clay was forming. The low solubility of 10Be at near neutral pH (Pavich et al., 1986) probably limited the amount of 10Be that was transported to greater than 1 m below the surface of the Sangamon Geosol. Quantitative phase analysis of unit D reveals abundant randomly interstratified kaolinite and smectite, no chlorite, and much less illite than in other units. Detection of the interstratified kaolinite and smectite requires analysis of diffractograms of samples that underwent several heat treatments (Hughes et al., 1994). The origin of the kaolinitesmectite is not understood, but may be related to weathering of feldspars, especially plagioclase, in the fine silt fraction. The 10Be concentration was not measured in units below unit D possessing only C-horizon pedogenic features (Fig. 4). Little or no chemical weathering in these units is further implied by abundant, easily weathered chlorite.
is subtracted from the meteoric inventory delivered during pedogenesis. Because 10Be has low chemical mobility at near neutral pH but may be lost by erosion from the soil surface (Pavich et al., 1986), the observed inventory may be less than the 10Be delivered to the soil during pedogenesis. For the time of interest, the past 200,000 yr, radioactive decay (t1/2 Å É1.5 1 106 years) is negligible. Depositional Flux
We estimated how long the Farmdale and Sangamon Geosols were exposed to meteoric input of 10Be prior to deposition of unit A (the Morton Tongue of the Mason Group) by using the 10Be inventories in Table 3 and an assumed constant deposition flux of 1.3 1 106 atom/cm2 r yr discussed below. The inventories (atom/cm2) in each sampled layer are calculated by multiplying the corrected concentration by the layer thickness and bulk density. The concentrations are corrected by subtracting the background concentration measured in the parent material (Table 3; Fig. 4). The horizon inventories are summed for the pedon. Minimum surface exposure duration is the pedon inventory divided by the deposition flux. This calculation assumes the following: (1) meteoric 10Be remained in the solum upon adsorbtion or precipitation, (2) little or no 10Be was lost in solution or by erosion, and (3) the inherited inventory (i.e., the 10Be transported to the site adsorbed on dust or loess)
Meteoric 10Be flux is controlled by rainfall amount (Monaghan et al., 1986; Brown et al., 1989). For temperate northern latitudes, a deposition flux of about 1.3 1 106 atom/ cm2 yr for 100 cm/yr rainfall fit the data collected by Brown et al. (1989) and a production model presented by Monaghan et al. (1986). This depositional flux produced reasonable minimum age estimates for a series of fluvioglacial terraces in Slovenia (Pavich and Vidic, 1993), and scaled for a dryer climate produced reasonable minimum age estimates for terraces along the Merced River, California (Pavich et al., 1986). Estimates are uncertain for the long-term variations of depositional flux of 10Be due to fluctuations in production rate. Monaghan et al. (1986) estimate a 20% global production rate variation. There is evidence from marine cores that a constant mean delivery rate has existed over the past 400,000 yr. Southoun et al. (1987) measured 10Be deposition to a North Atlantic site. They found that the 10Be concentration varied by no more than {25%, and that there was no correlation of high or low production rates with glacial/interglacial cycles. Assuming that the delivery rate to marine sediments is a proxy for the magnitude of production rate variation, we infer that for the period of interest that the production rate varied no more than {25%. For our calculations, we assume that the deposition flux of meteoric 10Be during pedogenesis was 1.3 1 106 atom/cm2 yr under humid conditions with rainfall of about 100 cm/yr. The present day mean annual precipitation in north-central Illinois is about 91 cm/yr. Inherited concentrations of 10Be must be subtracted prior to calculation of post-depositional 10Be inventories. Inherited concentrations in till are lower than for silty loess (ca. 100 – 150 1 106 vs ca. 200 1 106 atom/g; Shen et al., 1992; Pavich and Markewich, 1994; Pavich et al., 1994). The lower concentration in glacigenic sediment in this region, compared to loess, is because the till contains more redeposited Paleozoic and pre-Paleozoic bedrock, and less reworked, pedogenically altered late Quaternary sediment. Here we assume that the lowest concentration in the least weathered horizons of the Robein Member and Pearl Formation are the concentrations of inherited 10 Be in those respective units. The inherited concentrations are subtracted from the inventories in the Sangamon and Farmdale Geosols (Fig. 4; Table 3).
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CALCULATION OF SURFACE EXPOSURE DURATION
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Age Calculations Our estimates of the 10Be inventory and depositional flux suggest a minimum duration of exposure of the Farmdale Geosol of about 30,000 yr. This duration, added to the 14C age of 25,000 yr for the Tiskilwa Formation, implies that the age of the base of the Roxana Silt is about 55,000 yr. This is consistent with the regional 14C age estimate of 45,000 to 55,000 yr (McKay, 1979; Curry and Follmer, 1992; Leigh and Knox, 1993). Without correction for the inherited 10Be concentration of 561 1 106 atom/g, the minimum age of the base of the Roxana Silt is 150,000 yr, about 95,000 yr older than the accepted 14C-based age of about 55,000 yr stated above. The duration of exposure of the Sangamon Geosol in MC8 based on the corrected 10Be inventory is 100,000 yr. This is a significantly longer period than calculated for the Farmdale Geosol in accordance with their relative soil development (Follmer, 1983). According to the 14C ages and 10Be inventory from MC-8, the age of the parent material of the Sangamon Geosol, the Pearl Formation, is about 155,000 yr. The age is the sum of the exposure age of 100,000 yr for the Sangamon Geosol (assuming an inherited 10Be concentration of 164 1 106 atoms/g), the duration of Wisconsinan loessial sedimentation discussed above (30,000 yr), and the age of the late Wisconsinan diamicton (25,000 yr). An age of 155,000 yr for the Pearl Formation falls within the range of ages for marine isotope stage 6 (about 130,000 to 200,000 yr ago; Martinson et al., 1987). Evidence for widespread erosion of the Sangamon and Farmdale Geosols in northern Illinois (Kempton et al., 1985) supports our interpretation of local reworking and concentration of 10Be-rich material in the Farmdale Geosol and Robein Member of the Roxana Silt in core MC-8. While there is regional evidence for redeposition and concentration of 10Be in unit B (Roxana Silt; Markewich et al., in press), there is no interstratified kaolinite-smectite (Fig. 4). This observation suggests that 10Be was associated with some other phase in the soil clay fraction, that remobilization of 10Be was chemical, or that there was postdepositional alteration of interstratified kaolinite-smectite in the Farmdale Geosol.
about 100,000 yr. Hence, it is inappropriate to restrict the Sangamonian Stage in Illinois to marine isotope substage 5e. This restriction was proposed by Richmond and Fullerton (1986), who implied that the Sangamon Geosol and lastinterglacial Eemian sediments in Europe were temporally equivalent in their type regions—a view not supported by our results. ACKNOWLEDGMENTS We thank Randall Hughes for X-ray diffraction data, Robert Vaiden for field assistance, and William Dey for particle-size analyses. Richard Berg, E. Donald McKay, Ardith Hansel, Leon Follmer, Peter Clark, and Paul Bierman reviewed the manuscript.
REFERENCES
The 10Be inventory of the Farmdale and Sangamon Geosols in boring MC-8 indicates Illinois was glacier-free from ca. 155,000 to 25,000 yr ago. Previous to this study, the hiatus of glaciation was based on 14C ages, paleosol morphology, and presumptive correlation of the Sangamonian and Illinioan stages with marine isotope stages 5 and 6, respectively (Follmer, 1983; Kempton et al., 1985; McKenna, 1985; Curry, 1989; Curry and Follmer, 1992). Our results also imply that meteoric 10Be accumulated in the argillic horizon of the Sangamon Geosol in northern Illinois for
Blackwell, B. B., Curry, B. B., Saunders, J. J., Schwarcz, H. P., and Woodman, N. Dating the Sangamon interglacial interval near the type locality: Electron spin resonance (ESR) dating mammal teeth and gar scales from Hopwood Farm, southern Illinois, in preparation. Blackwell, B. B., Schwarcz, H. P., Saunders, J. J., Woodman, N., and Curry, B. B. (1990). Dating the Sangamon: Electron Spin Resonance (ESR) dating mammal teeth and gar scales from Hopwood Farm, Montgomery County, Illinois. Geological Society of America, Abstracts with Programs 22, A85. Brown, L, Stensland, G. I., Klein J., and Middleton, R. (1989). Atmospheric deposition of 9Be and 10Be. Geochimica et Cosmochimica Acta 53, 135– 142. Clark, P. U., Clague, J. J., Curry, B. B., Dreimanis, A., Hicock, S. R., Miller, G. H., Berger, G. W., Eyles, N., Lamothe, M., Miller, B. B., Mott, R. J., Oldale, R. N. Stea, R. R., Szabo, J. P., Thorleifson L. H., and Vincent, J.-S. (1993). Initiation and development of the Laurentide and Cordilleran Ice Sheets following the last interglaciation. Quaternary Science Reviews 12, 79–114. Clark, P. U., and Lea, P. (1986). Reappraisal of early Wisconsin glaciation in North America. Geological Society of America Abstracts With Programs 18(6), 565. Curry, B. B. (1989). Absence of Altonian glaciation in Illinois. Quaternary Research 31, 1–13. Curry, B. B. (1995). ‘‘Groundwater Protection Mapping for McHenry County, Illinois: Drilling Report.’’ Illinois State Geological Survey Open File Series 1995-1, 123 p. Curry, B. B., and Follmer, L. R. (1992). The last glacial/interglacial transition in Illinois: 122–25 ka. In ‘‘The Last Interglacial-Glacial Transition in North America’’ (P. U. Clark and P. D. Lea, Eds.), pp. 71–88. Geological Society of America Special Paper 270. Droste, J. B. (1956). Clay minerals in calcareous till in northeastern Ohio. Journal of Geology 64, 187–190. Follmer, L. R. (1983). Sangamonian and Wisconsinan Pedogenesis in the midwestern United States, In ‘‘Late Quaternary Environments of the United States, Vol. I, The Late Pleistocene’’ (S.C. Porter, Ed.), pp. 138– 144, The University of Minnesota Press. Forman, S. L., Bettis, E. A. III, Kemmis, T. J., and Miller, B. B. (1992). Chronologic evidence for multiple periods of loess deposition during the Late Pleistocene in the Missouri and Mississippi River Valley, United States: Implications for the activity of the Laurentide Ice Sheet. Palaeogeography, Palaeoclimatology, Palaeoecology 93, 71–83. Frye, J. C., and Willman, H. B. (1973). Climatic history as interpreted from the Lake Michigan Lobe, In ‘‘The Wisconsinan Stage’’ (R. F. Black,
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