climatic implications of alternating clay and carbonate formation in semiarid soils of south-central...

QUATERNARY RESEARCH 27, 270-282 (1987)

Climatic Implications of Alternating Clay and Carbonate Formation in Semiarid Soils of South-Central Montana

MARITH C. REHEIS

U.S. Geological Survey, Mail Stop 913, Federal Center, Box 25046, Denver, Colorado 80225

Received July 8, 1986

Evidence for climatic change is found in petrographic thin sections from soils formed on glaciofluvial deposits of Rock Creek and the lower Clarks Fork, Montana. These soils, presently in a semiarid climate, range from late Pliocene to Holocene in age, and have undergone periodic fluctuations in soil moisture caused by climatic changes. In the lower parts of soil B horizons, accretion of illuvial layers of clay (argillans) occurs mainly during wet (glacial) climatic periods, whereas carbonate precipitates mainly during dry (interglacial) climatic periods. Thin-section studies of the a&Ian and carbonate layers show that: (1) post-Pinedale soils that have formed only in the present interglacial climate contain areas of secondary carbonate unrelated to argillans, (2) soils formed on outwash of successively older glaciations contain proportionately more alternating layers of argillans and carbonate, and (3) the maximum number and sequence of layers in a soil correspond to the number of local cycles of glacial-outwash deposition and subsequent stream incision that followed the beginning of soil formation. These cycles are inferred to correspond to local glacial-interglacial fluctuations. The correspondence between the microscopic record and the glacial-outwash record for Rock Creek suggests that some of the climatic changes seen in the marine oxygen-isotope record did not strongly affect the study area. 0 1987 University of Washington

INTRODUCTION

The relation between calcic soils and modern climates has long been studied; however, interpretation of pre-Holocene paleoclimate from calcic soils has been hin- dered by the absence of well-dated Pleisto- cene soils. Problems of interpretation also arise because some calcic-soil properties, such as depth to horizons of calcium-carbonate accumulation (calcic horizons), can be affected by other soil-forming factors, such as differences in parent material and soil age. In addition, soil properties that reflect a specific climate can be destroyed or obscured when the climate changes. The calcic soils near Rock Creek in south-central Montana seem particularly suitable for paleoclimatic study because they formed on a sequence of glaciofluvial terrace deposits of similar composition that record a 2,000,000-yr history of fluctuating climate.

The depth to a horizon of pedogenic cal-

cium carbonate* is related to soil moisture (Jenny and Leonard, 1939; Arkley, 1963; Gile, 1975; McFadden and Tinsley, 198.5), and thus to climate, as is the depth to which clay particles can be translocated (Rutter et al., 1978; Torrent et al., 1980; Mahaney, 1981). Carbonate generally precipitates at a greater depth than that to which clay can be translocated, probably because carbonate moves in solution rather than as particulate matter. A calcic horizon containing translocated clay is commonly interpreted to represent a climatic change; clay was translocated in a moister climate but was later engulfed by carbonate when the climate became drier (Gile et al., 19661. However, this effect can also be produced

1 Microprobe analyses indicate that the calcium carbonate in Rock Creek soils contains varying small proportions of magnesium. For brevity, the term “carbonate” is used here to refer to all of this material.

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270

CLIMATE RECORD IN TERRACE SOILS 271

by the increase in the available water- holding capacity of a soil as silt and clay accumulate over time (Birkeland, 1984, p. 313). Interpretations of wet-to-dry climatic changes have also been made in micromorphologic studies of argillans (translocated clay particles) that are engulfed by carbonate (Allen and Goss, 1974; El-Tezhani er al., 1984). The opposite relation, argillans coating carbonate masses or nodules representing a climatic change from dry to wet, has been reported only rarely (Yarilova, 1964). Clay particles can be translocated in a calcareous soil, despite the tendency of clay to flocculate in the presence of Ca2+ ions, if channels or pores are available and if precipitation is ade- quate (Goss et al., 1973; Holliday, 1985).

This report examines the micromorphologic relations between argillans and carbonate layers in soils of known age that have undergone periodic fluctuations in soil moisture caused by climatic fluctuations, and compares the climatic cycles suggested by these relations with the cycles indicated by the glacial-outwash stratigraphy. A total of 18 soils were described and sampled at 1 to 3 sites on each of 7 fluvial terraces. Oriented peds from a total of 33 calcic B horizons and K horizons from these and supplemental sites were impreg- nated with blue epoxy, and 85 thin sections were cut from them parallel to the ground surface. These thin sections were studied under plane and polarized light, and se- lected areas in three thin sections were further examined with microprobe techniques.

In order to relate changes in soil micromorphology to paleoclimate, (1) the soils must be dated, and (2) the expectable amount of local climatic change must be known or inferred. The following sections summarize information from Reheis (in press) on relict soil ages and paleoclimate in the study area, and describe the general character of the calcic soils.

Age of the Rock Creek Terraces

The calcic soils of Rock Creek and the

lower part of the Clarks Fork of the Yel- lowstone River have formed on fluvial outwash gravel (except for nonglacial Holo- cene alluvium) derived mainly from non- calcareous granites of the Beartooth Mountains and some andesites of the Ab- saroka Range, Wyoming and Montana (Fig. 1). The fluvial terraces are dated by correlation, tephrochronology, and the average incision rate of the lower Clarks Fork (ash sites are described by Reheis (in press)). Those terraces directly dated or correlated include the Pinedale, lower Roberts, and Mesa (Table 1). The lowest major terrace of Rock Creek grades to mo- raines correlated with the Pinedale glaciation. Pinedale glacial deposits in West Yel- lowstone were dated at 30,000-10,000 yr B.P. with obsidian hydration techniques by Pierce et al. (1976). Thus, the lowest major Rock Creek terrace is named the “Pine- dale” (Ritter, 1967) and is assigned an age of 20,000 + 10,000 yr (Table 1). The uppermost fluvial deposits of the lower Roberts terrace on the lower Clarks Fork contain

1W lob 4tq I / ” -

30km

M .I *‘Billings

FIG. I. Physiographic, geologic, and cultural fea- tures of the study area. HR, Huckleberry Ridge ash (2,010,ooO yr old); LC, Lava Creek ash (610,000 yr old); F, musk ox fossil site; YNP. Yellowstone Na- tional Park; WY, West Yellowstone. Specific soil sites (not shown) are given by Reheis (in press).

272 MARITH C. REHEIS

TABLE 1. AGEANDMEANHEIGHTABOVESTREAM LEVELOFROCKCREEKTERRACESIN

THE STUDKAREA

Height above Best estimate Rock Creek- and range for Clarks Fork

Terrace age (lo3 yr) confluence (m)

Holocene 7 (3-11) 5.4 ? 1.5 Pinedale 20 (10-30) 15.0 k 3.0 Bull Lake 140 (90- 190) 30.2 rt 1.9 Boyd 415 (305-525) 64.7 2 7.6 Lower Roberts 600 (590-610) 88.1 2 4.4 Upper Roberts 945 (78% I 105) 128.9 ?z 8.0 Mesa 2000 (1990-2010) 252.3 2 10.5

the 610,000-yr-old Lava Creek A and B volcanic ashes (Izett, 1981); thus, this terrace surface is slightly younger than the ash beds, or about 600,000 yr old (Table 1). The Mesa terrace is correlated with a fluvial terrace south of Billings (Fig. 1) using heights above stream level. The uppermost fluvial deposits of the terrace south of Billings contain the 2,010,000-yr-old Huck- leberry Ridge ash (Izett, 1981), and so the Mesa terrace is about 2,000,OOO yr old (Table 1). The ages of the other Rock Creek and lower Clarks Fork terraces are estimated from the average incision rates of the lower Clarks Fork, using the heights of the terraces above stream level and the known ages of the modern stream and the three dated terraces (Table 1). The estimated ages of the Boyd and upper Roberts terraces have large errors because of variations in their terrace heights; however, these errors are considered to be maximum because they include errors in both the ages and heights of the dated terraces. The estimated ages of relict soils formed on the terraces are as follows: Holocene, 7000 + 3000 yr; Pinedale, 20,000 + 10,000 yr; Bull Lake, 140,000 & 50,000 yr; Boyd, 415,000 + 110,000 yr; lower Roberts, 600,000 + 10,000 yr; upper Roberts, 945,000 + 160,000 yr; and Mesa, 2,000,OOO 2 10,000 yr. In summary, the Rock Creek and lower Clarks Fork terraces span Quaternary time and, because the associated terrace deposits are glaciofluvial, record changes in

the fluvial regime that reflect major climatic changes.

Climate and Distribution of Carbonate in the Calcic Soils near Rock Creek

The climate in the study area has alter- nated between warm during interglaciations and cool during glaciations. The modern climate is semiarid and considered representative of interglacial ages. Mean annual precipitation at Joliet , Montana (Fig. l), is 40 cm; mean annual temperature is 8”C, with extreme monthly means of 2 1°C in July and - 6°C in January. Mean annual temperature during the latest glaciation was at least 10°C lower, on the basis of the presence of ice- and sand-wedge polygons in intermontane basins of Wyo- ming (Mears, 1981), depression of snow lines (Porter et al., 1983), and the occurrence of late Pleistocene fossil vertebrate faunas with tundra affinities in the Wyo- ming basins (e.g., Walker, 1982). Gates (1976) estimated a decrease in July temperature of 13°C for the Yellowstone-Bighorn region from modeling of the global climate at 18,000 yr B.P. The occurrence of a musk ox vertebra in fluvial deposits older than 100,000 yr southeast of Rock Creek (Fig. 1) and the presence of frost-oriented stones in glaciofluvial deposits underlying the Lava Creek A and B ashes (Fig. 1) suggest that some pre-Wisconsin glacial climates were as cold as the Wisconsin climate (Reheis, in press).

If glacial temperatures decreased by lO”C, then the associated decrease in evapotranspiration must have caused soil moisture in the study area to increase, un- less precipitation also decreased proportionately. One measure of soil moisture is the leaching index, which is the yearly sum of the monthly excesses of precipitation over evapotranspiration (Arkley, 1963). The leaching index at present is 10 cm at Joliet (24-yr climatic record) but could have been as much as 22 cm during glaciations (Reheis, in press). This estimate could be too high, however, because the local mean


annual precipitation may have decreased during glacial maxima in comparison with the modern amount. Nevertheless, the modern climate probably is appreciably drier than the climate during glacial maxima.

The modern climatic gradient along Rock Creek is reflected in the soils. At the mountain front, about 50 km upstream from the calcic soils near Joliet (Fig. I), the mean annual precipitation is 25 cm higher than at Joliet. There, the soils contain abundant translocated clay but no pedo- genie carbonate (Reheis, in press). Down- stream, soils exhibit progressively less translocated clay and more pedogenic carbonate. In this valley, then, translocated clay is associated with higher precipitation, whereas pedogenic carbonate is associated with lower precipitation.

The calcic soils near Rock Creek are characterized by soft, porous pedogenic carbonate, which is eolian in origin (Reheis, in press). In the youngest soils (Holocene and post-Pinedale), carbonate typically coats the bottoms of clasts, but small amounts are found in the soil matrix; these concentrations constitute soil carbonate “stage I” and weak “stage II” of Gile er al. (1966). Further accumulation of carbonate over time results in the formation of “stage II” horizons in post-Bull Lake soils and of “stage III” horizons, with carbonate found throughout the soil matrix, in soils formed on deposits older than Bull Lake (120,000 yr old). The post- upper Roberts (~945,000 yr old) and post- Mesa (<2,000,000 yr old) soils contain large amounts of soft, porous carbonate, but even these soils do not have cemented (petrocalcic) horizons.

The pre-Holocene calcic soils near Rock Creek have argillic horizons with translocated clay that are progressively better developed in older soils. The argillic horizon in each soil coincides, especially in its lower part, with the uppermost zone of carbonate accumulation in that soil. These calcic-argillic horizons are potential re-

corders of soil moisture fluctuations. caused by climatic change, that result in overall changes in the depth at which argillans and carbonate preferentially accumulate.

EVIDENCE FOR CLIMATIC CHANGE IN THE CALCIC SOILS NEAR ROCK CREEK

Examination of both calcic-soil morphology in outcrop and of micromorphology in thin section indicates a record of climatic changes. For example, some calcic soils under a slightly moister climate on Rock Creek have calcic horizons with wavy upper boundaries (Fig. 2A), whereas such horizons in the drier climate farther downstream typically have smooth upper boundaries. The wavy boundaries are interpreted to indicate carbonate dissolution and suggest a climatic change from dry to wet (Reheis, in press). Thin sections of these soils reveal grain argillans (illuvial clay) engulfed by carbonate (Fig. 2B), fea- tures suggesting a climatic change from wet to dry (Allen and Goss, 1974; El-Tezhani ct al., 1984). Argillans are seen in polarized light as light-yellow bands with sharp outer boundaries on grains or lining pores; these bands become extinct upon rotation of the microscope stage (Brewer, 1972). Pedo- genie carbonate in thin sections of soils near Rock Creek is seen as irregular or roughly circular masses, or locally as curved fragments of former clast-bottom coatings (Fig. 2C). The masses are generally composed of microcrystalline calcite. whereas the pebble coats have somewhat coarser crystals. These masses of pedo- genie carbonate commonly have roughly concentric rings or crosscutting relations that display variations in texture and color. Many areas characterized by particular textures or colors are separated and out- lined by bands that strongly resemble argillans within the carbonate mass, similar to those seen by Mermut and Jongerius (1980) in a soil from Turkey. The micro- stratigraphic record of argillans and car-


FIG calcic 0% PI argiila tween carbon ate (c:

2. Typical appearance of carbonate and argillans in calcic soils. (A) Wavy upper bou lndary of horizon indicates dissolution of formerly more shallow carbonate. Diameter of lens cap ,50 mm. lotomicrograph of a thin section from similar calcic horizon under polarized light. TI hin grain .ns (a) along edges of grains (g) were emplaced before carbonate (c), which now fills area be- grains. (C) Calcic B horizon under polarized light. Reworked fragments of coarsely CI .ystalline

nate pebble coats (PC) enveloped in matrix of younger, less dense, finely crystalline carbon-


bonate can be interpreted as a paleoclimatic record.

However, microprobe analyses of Rock Creek calcic soils reveal that not all the argillans and carbonate layers, so clearly visible in thin sections, can be discriminated by chemical composition. For example, four textural layers (I-4) of pedogenic carbonate can be distinguished in the scanning electron micrograph shown in Figure 3A, especially along the bottom half of the photograph. In Figure 3B, however, these four layers are represented by only two obvious areas of different densities of calcium atoms. Under the petrographic microscope, the four carbonate layers are separated by argillans (Fig. 4D). In contrast, microprobe analysis shows that silicon ions are more dense within the two inner carbonate layers (2 and 3, Fig. 3C) but are not concentrated in layers that might represent argillans (compare the distribution of silica in Fig. 3C and of argillans in Fig. 4D). The magnesium component of the smectitic clays that compose most of the argillans does not discriminate the layers any better than silica: the magnesium content of the clays is probably obscured by that of the carbonate. The absence of correspondence between the morphology of the carbonate and clay layers in thin section and their chemical composition revealed by the microprobe is not well understood. One ex- planation may be that the silicate clays in the argillans have been replaced by carbonate (Millot et al., 1977; Reheis, 1986). For argillans to remain visible in thin section, however, either the replacement is only partial, or the replacing carbonate mimics the texture or crystallographic ori- entation of the argillans. In this report, bands that in thin section resemble argillans within carbonate masses are as- sumed to represent relicts of former accu- mulations of translocated clay, which may have been partly or completely replaced by carbonate.

Detailed study of the Rock Creek thin

sections reveals that the maximum number of carbonate and argillan bands forming a mass of pedogenic carbonate increases with soil age (Fig. 4). The frequency and type of these relations were quantified by counting the number of carbonate bands and argillans in all the carbonate masses found in each thin section (Table 2). Holo- cene and post-Pinedale soils (<20,000 ye old) contain small patches of carbonate that are separate from the thin, scattered grain argillans also present (Fig. 4A). Post-Bull Lake soils (< 120,000 yr old) contain masses of carbonate that have at most a three-phase stratigraphy: an inner core ot carbonate surrounded by an argillan, which in turn is surrounded by carbonate (Fig. 4B). The post-Boyd soils (<415,000 yr old) have at most a five-phase stratigraphy: three carbonate layers alternating with two argillans (Fig. 4C). Soils on the lower Roberts terrace (600,000 yr old) have as many as seven layers, consisting of four carbonate layers and three argillans (Fig. 4D). The maximum number of layers observed, consisting of five carbonate layers and four argillans, is preserved in the post- upper Roberts (<945,000 yr old) and post- Mesa (<2,000,000 yr old) soils.

The less complex patterns of layers are more common than the more complex patterns. For example, eight thin sections were examined from post-lower Roberts soils (<600,000 yr old, Table 2). All of these thin sections contained many examples of two carbonate layers and one argillan. Three thin sections, however, contained no examples of four carbonate layers and three argillans, and those thin sections that did show this relation contained only one or two examples of it. It is intuitively rea- sonable that the chance of observing a carbonate-argillan mass in thin section that perfectly reflects a paleoclimatic record de- creases with the age of the soil and the number of climatic fluctuations that affected the soil. Thus, the absence of sys- tematic increase in the complexity of layers


FIG. 4. Photomicrographs of thin sections from calcic soils showing progressive accretion of carbonate and argillan layers. Polarized light except Figure 4C which is under plane light. (A) Holocene soil (~7000 yr old) with finely crystalline carbonate (c) in voids. No argillans are present. (B) Cal- careous mass in post-Bull Lake soil (<120,000 yr old) contains two carbonate units (c,, c,) separated by argillan (a,). (C) Post-Boyd soil (<415,000 yr old) contains three carbonate units (c,-c,). Argillans, visible only under polarized light, separate the three units. Soil matrix and void space surround calcareous mass c,-c~. (D) Post-lower Roberts soil (<600,000 yr old) contains calcareous mass com- prising four carbonate units (c,-c,) separated by three argillans (a,-a,). Dashed line shows area of Figure 3 (except in reverse).

in soils older than 945,000 yr (Table 2) pedogenic carbonate, rather than to an ab- could be due either to nonpreservation of sence of climatic oscillations. layers or to inconsistent accretion of an ar- The pattern of maximum argillan-car- gillan or carbonate layer to each mass of bonate accretion closely corresponds to the

FIG. 3. Microprobe analysis of carbonate and argillan layers in calcic B horizon of <600,000-yr-old coil. Area of photographs is shown in Figure 4D. Carbonate layers 1-4 correspond to layers cl-c4 (Fig. 4D); numbers increase toward edge of carbonate mass and with decreasing age. Area 5 is outside main carbonate mass but contains small carbonate ooids. (A) Four carbonate layers shown by secondary electron image, best distinguished in lower half of photograph. Small bright spots are detrital grains (d); long lath in center is probably a mica grain (m). (B) Density of Ca atoms reflects at least two carbonate layers. Oldest carbonate layer on right (1) has the highest Ca density: layers to left (2-4) have a relatively low Ca density. (C) Density of Si atoms is most concentrated in carbonate layers 1 and 3.

278 MARITH ‘2. REHEIS

FIG. 4.-Continued.

local record of climatic change inferred from the glacial-outwash deposits in Rock Creek valley. Soils on Pinedale outwash and Holocene alluvium that formed under the present-day dry (interglacial) climate contain a few thin argillans that are physi- cally unrelated to the small masses of carbonate. Post-Bull Lake soils, however, have developed through three main climatic intervals: the relatively dry climates of both the pre-Pinedale and present-day interglaciations, and the relatively moist climate of the intervening Pinedale glaciation. Post-Bull Lake soils contain numerous carbonate masses that include an engulfed argillan which surrounds an in-

terior carbonate mass (3 layers, Table 2). These relations suggest that carbonate tends to precipitate in the lower parts of B horizons of these soils during dry, interglacial ages, whereas argillans accrete during more moist, glacial ages. Soils formed on each of the next three older outwash deposits (Boyd, lower Roberts, and upper Roberts terraces) correspondingly contain an additional argillan-carbonate layer pair for each older terrace. The exact match of micromorphologic stratigraphy and terrace stratigraphy suggests that no outwash deposits reflecting major intervals of cold climate are missing from the local record of the past one million years. Major climatic


TABLE 2. MICROSCOPIC RELATIONS BETWEEN CARBONATE BANDS AND ARGILLANS IN CALCAREOUS B HORIZONS OF ROCK CREEK SOILS

_-.. --

Number of Recognized patterns of band9 Terrace thin sections/

and estimated soil horizons/ Carb separate 2 carb. 3 carb, 4 carb, 5 cab. age” soil profiles from arg’ 1 arg 2 arg 3 arg 4 arg

___-._ Holocene. 41111 4 0 0 0 0

7,000 yr Few Pinedale. VI412 V 0 0 0 0

20,000 yr Few Bull Lake, VI412 V V 0 0 0

120,000 yr Common 2-21 Boyd, VI413 V V 7 0 0

41S,OOO yr Many lo-116 O-IV Lower Roberts, 81413 8 8 7 5 0

600,000 yr Many 8-60 o-17 o-2 Upper Roberts, 71211 7 7 7 5 I

945,000 yr Many 74-245 8-18 o-5 o-2 Mesa, 81312 8 8 8 5 1

2,OOO.OOO yr Many 20-152 2-19 O-8 o- I

a Estimated ages are considered to be the best approximation for the duration of soil development. Age ranges are given in the text.

b The first row for each terrace lists the number of petrographic thin sections exhibiting each relation: the second row lists the range in the number of occurrences per section of each relation.

c Carb. carbonate band; arg, argillan.

variations that caused the accretion of alternating layers of pedogenic argillans and carbonate also appear to have produced local mountain glaciations and interglaciations extensive enough to deposit and in- cise outwash terraces.

DISCUSSION

The climate record inferred from microscopic evidence of carbonate layers and argillans in Rock Creek soils is consistent with the preserved depositional record of outwash terraces for the past million years. Although the times of terrace incision coincide with some glacial terminations as recorded in the marine oxygen-isotope record (Reheis, in press), there are fewer terraces than terminations (Fig. 5). No terraces exist that correspond to the terminations of glacial stages 8, 10, 14, 18,20, or 22 of Shackleton and Opdyke (1976). Al- though the ages of the Boyd and upper Roberts terraces are not closely con- strained, each terrace appears to represent only one episode of outwash; no buried

soils or depositional unconformities have been observed in any of the Rock Creek terrace deposits.

Three main hypotheses can explain the absence of depositional units along Rock Creek for some glaciations and interglaciations interpreted from the marine record. First, some outwash terraces representing the missing glacial advances may not have been preserved. Second, some local glaciations may have been caused by a different combination of climatic factors that failed to produce the major fluctuations in available moisture which are required to gen- erate a climatic record of argillans and carbonate layers. For example, if both precipitation and temperature in the Rock Creek area decreased greatly during a glacial ad- vance, the amount of soil moisture could have remained relatively low, and so the soils could have continued to accumulate mainly carbonate rather than clay. Thus. such a glaciation would not be reflected in the soil climatic record. Third, some climatic changes indicated by fluctuations in


:- Terrace Heights UR

O&100- r-

CfZE B LR /

F- jr- GE P BL

,--- ’ Terrace Chronology <$ o-w’- c--c( -Nl I 4 I

0) j t-lI+llI- Searles Lake

IV---t----V-----t-VI- 0 4t

-I- MM /” vW V28-239

Age (ka)

FIG. 5. Schematic diagram comparing various types of long climatic record, from base up. (A) I80 fluctuations and numbered glacial stages, sketched from Pacific core V28-239 of Shackleton and Op- dyke (1976). (B) Searles Lake hydrologic regimes of Smith (1984). (C) Rock Creek terraces. Dots denote best estimates of times of surface stabilization and start of soil formation, corresponding to terminations of local glaciations (Reheis, in press); horizontal bars show age ranges inferred from correlations or incision rates. Terrace height above stream level and time of incision are shown by dashed profile-shaped lines above age ranges: P. Pinedale; BL, Bull Lake; B, Boyd; LR, lower Roberts; UR, upper Roberts. (D) Micromorphic relations in Rock Creek soils: accretion of argillans (white) and carbonate layers (dark) correspond to periods of relatively moist, glacial climates and dry, interglacial climates, respectively.

the marine oxygen-isotope record may not have strongly influenced the Rock Creek area, because the marine oxygen-isotope record appears more sensitive to ice volume than to climate. This third hy- pothesis is the simplest, and the correspondence of the depositional and soil climatic records for the past one million years sup- ports it. Glacial stages 6, 12, and 16 of Shackleton and Opdyke (1976), which are regarded as cot-relatives of the Bull Lake, Boyd, and lower Roberts terraces, respectively, show the largest deviations in 180 values toward glacial conditions (Fig. 5). The other glacial isotopic stages not represented by outwash along Rock Creek have smaller deviations in IsO value.

The absence of correspondence between oceanic and continental climatic records, as noted by other workers (e.g., Gibbons et al., 1984), is commonly attributed to poor preservation of depositional units. How- ever, some geographic areas characterized by specific types of geomorphic processes may respond to, and have records of, different magnitudes or frequencies of cli-

matic change than that recorded by the marine oxygen-isotope record. For example, speleothem records of western North America (Harmon, 1981) indicate that interglacial conditions prevailed from 400,000 to 275,000 yr B.P. with one short, 2.5,000- yr-long period of glacial conditions. This interval spans most of stages 9, 10, and 11 (two interglaciations and one glaciation) of the marine oxygen-isotope record. The stratigraphic record of outwash terraces along Rock Creek, the Clarks Fork, and the Yellowstone River in the study area is ex- ceptionally well preserved (Reheis, in press), yet it lacks an outwash terrace corresponding to stage 10 (Fig. 5). This suggests that local glacial conditions were not intense or long enough to change stream equilibrium significantly.

The changes in climate for the last million years recorded by Rock Creek deposits are similar to those recorded by Searles Lake, California (Fig. 5). Smith (1984) noted that changes in the major hydrologic regimes of Searles Lake correspond to only some of the changes in the


marine oxygen-isotope record. He dated these changes in regime at about 10,000, 130,000, 310,000, 570,000, and l,OOO,OOO yr. Four of these changes coincide with the ages of Rock Creek terraces (20,000, 120,000, 600,000, and 945,000 yr, Table 1). The Boyd terrace (415,000 yr old) may correspond to the end of two brief deep-lake cycles at about 400,000 yr B.P. that inter- rupt a long dry-lake period in the Searles core. However, the direction of climatic change in the Rock Creek and Searles records is not always compatible: for example, Searles Lake deposits record changes from wet to dry conditions at l,OOO,OOO, 570,000, and 10,000 yr, but the change in hydrologic regime at 130,000 yr was from drier to wetter. Smith (1984) observed several similar discrepancies in the direction of climatic change between several indicators of global climate and the Searles Lake core, and he emphasized that the timing of climatic change may be a more widely applicable component among paleoclimatic studies than the direction of change.

ACKNOWLEDGMENTS

1 thank T. R. Walker and P. W. Birkeland, of the University of Colorado, who sparked my interest in the microscopic study of soils. 1 am grateful to the numerous landowners along Rock Creek who per- mitted excavation and sampling of soils. R. P. Chris- tian of the U.S. Geological Survey assisted with microprobe studies. V. T. Holliday of the University of Wisconsin at Madison, R. Drees of Texas A&M Uni- versity, H. E. Doner of the University of California at Berkeley, and G. I. Smith, M. N. Machette, and D. E. Stuart-Alexander of the U.S. Geological Survey criti- cally read and commented on early versions of the manuscript.

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climatic implications of alternating clay and carbonate formation in semiarid soils of south-central...

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