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Rates of soil development from four soil chronosequences in the southern Great Basin
QUATERNARY
RESEARCH
35, 383-399 (191)
Rates of Soil Development from Four Soil Chronosequences in the Southern Great Basin JENNIFER W. HARDEN,* EMILY M. TAYLOR,? CINDY HILL,* ROBERT K. MARK,* LESLIE D. MCFADDEN,+ MARITH C. Rmrnrs,t JANET M. SOWER&§ AND STEVEN G. WELL& *U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025; fU.S. Geological Survey, 613 Federal Center, Denver, Colorado 80225; SGeology Department, Universiry of New Mexico, Albuquerque, New Mexico 87131; and 55844 San Jose Avenue, Richmond, CA 94804 Received March 21, 1990 Four soil chronosequences in the southern Great Basin were examined in order to study and quantify soil development during the Quatemary. Soils of all four areas are developed in gravelly alluvial fans in semiarid climates with 8 to 40 cm mean annual precipitation. Lithologies of alluvium are granite-gneiss at Silver Lake, granite and basalt at Cima Volcanic Field, limestone at Kyle Canyon, and siliceous volcanic rocks at Fortymile Wash. Ages of the soils are approximated from several radiometric and experimental techniques, and rates are assessed using a conservative mathematical approach. Average rates for Holocene soils at Silver Lake are about 10 times higher than for Pleistocene soils at Kyle Canyon and Fortymile Wash, based on limited age control. Holocene soils in all four areas appear to develop at similar rates, and Pleistocene soils at Kyle Canyon and Fortymile Wash may differ by only a factor of 2 to 4. Over time spans of several millennia, a preferred model for the age curves is not linear but may be exponential or parabolic, in which rates decrease with increasing age. These preliminary results imply that the geographical variation in rates within the southern Great Basin-Mojave region may be much less significant than temporal variation in rates of soil development. The reasons for temporal variation in rates and processes of soil development are complexly linked to climatic change and related changes in water and dust, erosional history, and internally driven chemical and physical processes. 0 1991 University of Washington.
INTRODUCTION There are few numeric (radiometric or calibrated) ages for Quaternary geomorphic surfaces in the southern Great Basin. Techniques are needed to date surfaces reliably and correlate them to undated localities. Soils are potentially a useful correlation and dating tool because (1) soils develop on geomorphic surfaces at the onset of surface stabilization and (2) soil development increases systematically with increasing surface age, as demonstrated in arid/semiarid regions by several workers (Gile et al., 1966; Machette, 1985; Chadwick et al., 1984; McFadden, 1988; Holliday, 1988; Reheis, 1987a,b). The goals of this study are to characterize soil development in the south-
ern Great Basin in locations with dated Quaternary surfaces, to estimate rates of soil development in such locations, and to evaluate the potential and the limitations of using soils to correlate and approximately date surfaces in the region. With parent detritus of such contrasting lithologies as granitic-gneissic, granitic-basaltic, siliceous volcanic, and limestone, we hypothesized that a wide range of types and rates of soil development would be represented in this study, and that such a range most probably would be characteristic of most soils found in the southern Great Basin. Comparative studies require careful and consistent sampling and analysis of soils (e.g., replication of sampling sites on a given dated surface), consistent treatment of data for estimating 383 0033-5894191 $3.00 Copyright 0 1991 by the University of Washington. All rights of reproduction in any form reserved.
384
HARDEN
degree of soil development (e.g., estimates of parent material, methods of soil description and analysis), examination of how radiametric or numeric ages relate to the age of the surface and soil, and mathematics for estimating rates of soil development that account for the many uncertainties of our field and interpretive models (uncertainties of calibration dates, soil variability, shape of soil-time curves). GEOCHRONOLOGY, GEOMORPHOLOGY, AND SOILS In each area (Fig. 1; Table l), we studied several surfaces and soils for which relative ages are well defined. In addition, some of the surfaces are dated numerically by methods that date either the deposit or the surface and soil. Our analysis attempts to bracket the age of the surface, which is the best estimate for inception of soil formation. Such brackets are typically quite wide, especially for Pleistocene surfaces P
ET AL.
(Table I), and vary considerably in both precision and accuracy. For example, the uranium-trend method of Rosholt (1985), used for Pleistocene soils at Fortymile Wash, is experimental. Uranium series ages of soil carbonate, used at Kyle Canyon, have considerable errors (Sowers et al., 1988). We carefully evaluated and represented such uncertainties in our data analysis but emphasize that in some cases even these uncertainties are not well anchored to known ages. The geomorphic setting, climate, and vegetation in all four of the study areas are generally similar. We attempted to study soils on flattest parts of alluvial fans or terraces, although the oldest surface at Kyle Canyon exists as an erosional, rounded remnant. The rainfall of the areas has similar seasonal distribution, with most of the 8 to 40 cm annual precipitation (Table 1) occurring in cool winters and the remaining in hot summers. Pleistocene climates were cooler and effectively wetter (e.g., see I 116”
I 11P
37”3\0 ‘.
\
‘.
\
/.-.-\
‘\ --.-._.
;
1
AREA OF MAP
.T\LYI
NATIONAL“*, . ..e...*..r.MUNUMtNl \ ‘.
0
r \
.j 2.h
I._
Mercury Kyle Canvon
!
Study area
O,,,, 35”
‘.
q
40
km I
FIG. 1. Map showing location of four study areas in the southern Great Basin.
GREAT
BASIN
SOIL
Spaulding, 1985). Vegetation is typical of the southern Great Basin and varies with elevation and rainfall (Table 1). Silver Lake Alluvial fans at Silver Lake playa, derived primarily from granitic and gneissic rocks, provide an excellent soil chronosequence for the Holocene through latest Pleistocene (Wells et al., 1987). The surfaces have gentle gradients (less than 10%) and are relatively undissected. The ages of the four youngest fans (Table 1) were constrained by Wells et al. (1987) using data of Ore and Warren (1971); the oldest fan is estimated to be late Pleistocene by Dorn (1984) and R. I. Dorn (unpublished data, 1986). In these soils, there is a systematic change in soil morphology (Reheis et al., 1989), pedogenic carbonate, and clay content over time; the latter two components are derived primarily from eolian dust and precipitation-borne solutes (Reheis et al., 1989; Wells et al., 1987). Detrital calcite derived from reworked, older soils is evident from calcic rinds on modern alluvium and from rinds that are chipped or oriented on sides or tops of otherwise clean clasts found in the soils. Cima Volcanic Field Alluvial fans and fluvial terraces near the Cima Volcanic Field are composed of granitic detritus with minor amounts of basalt from the Cima field concentrated in the desert pavement. Seven surfaces, two of which have ages constrained by K/Ar ages of basalt flows (Turrin et al., 1985 and Turrin and Champion, 1991) appear relatively flat (2 to 9%) and undissected. Three of the fans were also sampled by R. I. Dorn for rock varnish on surface clasts for study of radiocarbon and cation ratio trends (Tables 1 and 3; R. I. Dorn, written communication, 1986). Radiometric ages do not permit an independent rate of soil development at Cima, but in later sections we discuss the
CHRONOSEQUENCES
385
ages of the Cima fans based on calibration of soils to Silver Lake fans. Soil development is dominated by the accumulation of clay and carbonate, much of which appear to be eolian in origin, as in the soils formed directly on the basalt flows (McFadden et al., 1986). The soils are very similar to those at Silver Lake, except there is little evidence for reworked carbonate at Cima. Fortymile Wash Alluvial fans and fluvial terraces along Yucca Wash, near Fortymile Wash, are developed in alluvium derived from welded and nonwelded ash flow tuffs, with a minor component of basalt. The surfaces and soils were studied by Taylor (1986) and dated primarily by uranium-trend analysis, which is somewhat experimental (Rosholt, 1985). The soils display an impressive increase of secondary silica with time as a result of the influence of the glass and opaline-rich siliceous parent material (Taylor, 1986). Much like the stages of carbonate described by Gile et al. (1966) and Machette (1985), the stages of secondary, opaline silica progress from undercoatings on clasts to laminar caps (Taylor, 1986). Kyle Canyon Three alluvial surfaces at Kyle Canyon were studied in detail by Sowers (1985, 1986) and by Reheis et al. (in press) and are composed almost exclusively of limestone detritus. Ages are based on radiocarbon ages of detrital carbon, uranium series analysis of soil carbonate, and a magnetic reversal (Table 1). The oldest surface (surface 1) exists as an elongate, rounded balena topography indicative of erosion, which could be exposing old groundwater carbonates to the zone of soil formation as the soil is eroding. To be conservative, we dashed lines representing data for surface 1 in Figure 3. Sowers et al. (1988) discuss many problems with all of the dating techniques, but emphasize that surface 3 may be oldest
Cc correlation near Beaty , Nev.
Qlc
incision
Fortymile
20
10
Wash Alluvial 0.15
800
130
15
Profde characteristics
SURFACES
FOUR
X?SD
AvlBtklBk, stage II carbonate
AvlBtWBk, stage I-II carbonate
AvlBtjlBk, stage I carbonate
46.4 k 9.4
19.7 + 9.5
6.6 2 0.9
MAP;
3
4
2
4
I
Fans and Fluvial 0
750
18
4 AiBtlKmlBk; stage IV carbonate AlKmlBk; stage IV carbonate
A/Btk/Bt; stage III carbonate
50 + 15
34 e 13
34 -c 1.9
Terraces? looO- to 1100-m elevation; 15°C MAT; 10 v. thin A/C; v. weak stage I carbonate 30 AlBtjlBtWCkn; 8.6 + 4.8 stage I-II carbonate
3ooo
750
80
2
12-cm MAP;
3
5
3
detritus
SD,
133 + 18
siliceous
DEVIATION)
30 -t 15
detritus
147 2 48
81 + 45
42 f 40
X + SD g - cm
-ClayC
STANDARD
volcanic
5245 ? (1544 est)
3830 * (1149 est)
1302 I (390 estjp
limestone
362 f 187
3% + 97
49 ? 18
61 2 2
detritus 6 k 10
taco,= X?SD g-cm
(x, MEAN;
granitic-gneissic 3
No. profiles
index”,’
AREAS
Alluvial Fans:‘840to 1670-m elevation; 12 to 18°C MAT; 16 to 41-m MAP; 0 15 A/Btk/Bk; stage II 6 + 1.5 6 carbonate
70
20
8
STUDY
Soil development
IN THE
to 300-m elevation; 21°C MAT; 8.3-cm Open gravel matrix, 2.3 k 1.2 very thin A@ 3.3 AvlBwk, stage I 1.9 t 2.7 carbonate 1
15
Fans$275-
Maximum
8
3.3
6
11
0
Alluvial 0
Silver Lake 0.20
2.5
Minimum
Bestestimate
lo3
OF GEOMORPHIC
Ages used for calibration,
CHARACTERISTICS
Kyle Canyon 5
SOIL
>4.4cco;ll to 22 cco; 75Ucr; 10 to 1SCcr >18cco; 130Ucr; 34Ccr >73OPr?; 70 to >35OUcr
~3.4pR;~ (2.6V14Ce Beta 375!443 >3.4PR;8.0 Css; ~15.5 css; (13 1vw A“G71) ’ >15.5css; (20 1VW~ Beta 1753:)
arroyo
upper
lower
AND
Dating information age in ld yr
1. AGES
Qlb
1
2
3
3
Qfl
w
QD
Qf4
Qf5
surface
Geomorphic
TABLE
2
3
3
No. profdes
>llOOA; <2lOOA
(l.2V’4C;’ Beta 20573; o-5 Ssl) (ll-l4V’4C;= Beta 17537; 3-5 Ssl) (2-6 Ssl)
(Sl3
(M-20
Qta
Qfs
Qf6
Qf5
Qf4
Cima Alluvial
loo0
350
150
Fans;’
900
270
145
AlBtWBtqlCkq; stage I-II carbonate; II silica A/BtWKqmlK; stage II-III carbonate; HI silica A/BtWKqm/Kqy; stage IV carbonate; IV silica
A/Bt/Bk; stage I carbonate A/Bt/Bk; stage II carbonate A/Btk/K; stage III carbonate A/Btk/Bk; stage III carbonate 39.0 + 11.6
19.2 ? 2.0
12.4 + 1.7
6.1 + 1.4
6.5 + .9
detritus
39.5 + (11.9 est)
30.7 + 1.4
22.8 + (6.8 est)
12-cm MAP; granitic 3.6 + 3.2
AEtwlBk; stage I carbonate
1200-m elevation; 15°C MAT; AlBk; stage I carbonate
2000
440
190
4
3
3
2
4
clasts
1656 ? (497 est)
487 + 199
86 + (26 est)
with basaltic 3
1
2
1
3072 2 (1536 est)
735 k 367
548 f (274 est)
1
2
1
No?e. (CSS) 14C shell, shoreline; (V14C) varnish radiocarbon; (PR)14C on packrat midden; (Cc) i4C on charcoal at depth;(date)not usedfor c&b&ion; (b’r)p&omqn&c reversal; (Ucr) Uranium-series on calcite rinds; (Cco) i4C on organics from calcite rinds; (Ccr) 14C on calcite rinds; (Ut) Uranium-trend on soil; (A) KAr on ash; (KAb) KAr on basalt; (Ssl) soil development calibrated to Silver Lake (this paper). a Harden, 1982; Harden and Taylor, 1983; modifications of this paper all properties included. b Reheis et al., 1989 c Parent material basal minimum; d Wells et al., 1987; S. B. Wells, written communication, 1989 e R. I. Dam, written communication, 1%9 f Sowers et al., 1988 a Average SD/X of other units h Taylor, 1986 i S. G. Wells, written communication, 1989 j Tunin et al., 1985 and Turrin and Champion, 1991.
(
Ssl)
Ssl)
24&34Out; <44out
Q2c
cx7
120-170ut
Q2b
388
HARDEN
at its upper end. Thus, four sites, surfaces 1, 2, upper 3, and lower 3, are considered separately for this study, with considerable age uncertainties that overlap among surfaces (Table 1, Fig. 3). In addition, the climate at lower surface 3 is significantly warmer and drier than at other sites (Table 1). The soils at Kyle Canyon are dominated by secondary carbonate, which in advanced stages appears to overwhelm other soil constituents such as silicate clay or iron oxides (Reheis et al., in press). The soils on fans 2 and 1 have abundant secondary carbonate to depths of 3 m. FIELD AND LABORATORY
METHODS
At Silver Lake, Cima, and Kyle Canyon, several pits were excavated to depths of 2 to 4 m on each geomorphic surface in order to estimate soil variability on each surface. Sites were selected at similar elevations where possible, so that orographic effects of precipitation are similar at each field area. Analyses that were performed include an array of physical, chemical, and mineralogical techniques, the descriptions and data of which are available in Taylor (1986); Sowers et al. (1988); Reheis et al. (1989); and Reheis et al. (in press). CALCULATIONS OF SOIL DEVELOPMENT AND RATES OF DEVELOPMENT
Three main measures of soil development considered in this study are the soil development index, pedogenic clay, and pedogenic carbonate. For all three measures, we define levee and eolian deposits as pedogenic components, and we use basal fan deposits at the base of each profile to estimate the composition of the original parent material for all horizons of the profile. In reality, admixtures of eolian and levee deposits are in some respects parent material that is then subject to processes such as dissolution, fractionation, and
ET
AL.
translocation. Ideally, isotopic signatures of dust and alluvium might help to track the complexity of influx and weathering (Muhs et al., 1990). For the present study, however, we define dust (and levee) admixtures as pedogenic attributes. The soil development index of Harden (1982) uses field descriptions of soils with additions of color-paling and lightening (Harden and Taylor, 1983), pH increase of Reheis (1987a), and a new carbonate index (described in Harden et al., 1991). For the carbonate index, horizon values of colorpaling and color-lightening are summed (a), multiplied by carbonate stage (Gile et al., 1966; Machette, 1985) (b), divided by the current maximum of 240 (c), and included in further sums or indices (d). Disseminated carbonate is assigned a stage of 0.5 for step (b). We used the spreadsheet program of Taylor (1988) for calculations. Pedogenic clay and carbonate are determined by subtracting estimated amounts of clay and carbonate in parent material from those in horizons, then multiplying by horizon thickness (and bulk density where possible) before summing the products through the profile. We used three approaches to estimate amounts of clay and carbonate in parent materials in order to calculate pedogenic clay and carbonate: (1) Parent material has no clay or carbonate, which maximizes profile sums. (2) Parent material can be estimated from a reasonable minimum value in each protile. These parent material estimates are different for each profile but are roughly similar within each chronosequence. (3) A maximum estimate for parentmaterial clay or carbonate was derived from time plots of the weighted mean. Such a mean, in percentage by weight, is obtained by multiplying percentage carbonate or clay by horizon thickness, summing the products through the profile, and dividing by profile thickness. Another way of interpreting the weighted mean is as a redistribution of all clay or carbonate in the devel-
GREAT
BASIN
SOIL
oped profile, wherein all horizons have equal amounts (in weight percent) of clay or carbonate. The increase over time reflects the accumulation of clay or carbonate by eolian admixtures and mineral weathering; the intercept at time = 0 years should reflect the original content in weight percent (assuming, of course, that the rates are linear; see Discussion). Because this third approach is used to maximize the estimate of clay or carbonate in the parent material, the intercept plus one standard deviation is used rather than just the intercept of the weighted mean versus time. Profile sums or accumulation indices (horizon sums of horizon carbonate minus parent carbonate) usually require bulk density for meaningful units of pedogenic clay or carbonate, but the high gravel content of these soils makes density measurements imprecise. For comparison of rates, we opted to calculate profile sums that exclude the density factor, resulting in units of gcm. Profile sums using estimated bulk densities also were calculated for soils with density data (used in Fig. 6), and results were consistent with the units of g-cm. The methods of Switzer et al. (1988) were used to assess estimates and uncertainties of rates of soil development and to estimate ages of soils at Cima. The purpose of the Switzer method is to represent mathematically the field and sampling limitations of chronosequences. There are two major flaws in using simple regression for chronosequences: (1) Both the x and y axes of soil vs age functions have considerable uncertainties, whereas simple regression fits the line with only errors in y. (2) Error estimates of even a two-error regression are inappropriate, because sample numbers are pathetically small and soil variability contributes significantly to errors in the soil development value. Thus, Switzer et al. (1988) employed a two-error-type regression (maximum likelihood estimate line below) and, most significantly, designed a Monte-Carlo approach of refitting the
389
CHRONOSEQUENCES
“regression” line to various data combinations. The Monte-Carlo-like approach results in much higher uncertainties for slopes, intercepts, and predictive ages as compared to simple regressions (Harden and Matti, 1989). Such a Monte Carlo approach is also being proposed for other calibrated dating techniques (Bierman and Gillespie, 1989). In the Switzer approach (Switzer et al., 1988)) a maximum likelihood line (similar to a two-error or weighted regression) is calculated from data with uncertainties in calibration dates and soil variability (dotted line, Figs. 2-5). The slope of the line represents rate of soil development, and the intercept represents the state of the soil at time zero. The uncertainty of the slope and intercept is obtained by iterating 100 data sets from a sampling of the means and standard deviations of the data (insert in Fig. 5). One hundred lines are calculated from the iterative data sets (10 lines plotted in Fig. 5), and the standard deviation of the 100 slopes represents uncertainty in the rate of soil development. For age estimates of a soil, the mean and standard deviation of the soil of unknown age are used to generate a range of 100 postulated soil development values, which are used in combination with the 100 lines above to compute 100 possible age estimates by the curves of soil development vs age. Figures 24 show the area that includes these 100 lines, which encompass a wide range of slopes and intercepts. The uncertainty in rates and intercepts (Table 2) is derived from the standard deviation of the 100 iterative lines. RESULTS
Average Rates of Soil Development For all measures of soil development (index, clay, and carbonate), time plots may appear to be roughly linear within each Holocene and Pleistocene time span, but are not linear for the longer time span of Holocene through Pleistocene (Figs. 2-4; Ta-
390
HARDEN
0
20
40
60
80
ET
AL.
0 0
20
Age (lo3 yr B.P.)
-$600]
f
40 Age (lo3
60
I 80
60
80
yr B.P.)
1:';
300
- r---------Iif- i I
0 lb n ”
ZO
40
fi0
SO
Age (IO3 yr B.P.)
0
20
40 Age (lo3
yr B.P.)
FIG. 2. Four measures of soil development versus time for soils at Silver Lake. Soil development index and total-texture calculated from Harden (1982) and Harden and Taylor (1983) with field index of carbonate as described in text. Profile sums of carbonate and <2 pm clay calculated from percentage of <2 mm times horizon thickness, summed in profile from dataof Tables l-3 (Reheis et al. (1989), and unpublished data). Vertical bars = 1 standard deviation from mean; horizontal bars = age uncertainty. Solid line = maximum likelihood estimate as described by Switzer et al. (1988) for solid points only; dotted line extends to dashed points excluded from line-fit. Outer dashed lines in d = range of uncertainty of line-tits determined by 100 iterative lines.
ble 2). For example, soils at Silver Lake appear to develop as linear functions over the 10,000-yr age span, but the Pleistocene point falls off the Holocene curve if the older age is used. Pleistocene soils at Kyle Canyon also appear to develop as linear functions, but the Holocene point requires a steeper curve toward the intercept (Fig. 3). The data at Mercury appear roughly linear for all soils between 10,000 and about 1.5 million yr old, with the possible exception of the index plot (Fig. 4). Age uncertainty allows alternate interpretations for the time curves, including very poor corre-
lation of soil with age, but the contrast between Holocene and Pleistocene curves is notable from best-estimate ages in Figures 2-4. For further comparison and exploration of the data, linear rates are calculated for the Holocene and separately for the Pleistocene by selecting either Holocene or Pleistocene soils within each study area (Table 2). In general, average rates of most soil-development parameters are precise to about a factor of 2 (e.g., in Table 2, a slope of 0.99 ? 0.46). Notably large uncertainties in slopes or rates include total texture at
GREAT 200H -2 5 3 2 8 loo2 u 9 i.E
BASIN
SOIL
391
CHRONOSEQUENCES
A
80007
B
h f7 3
c
-
(1) O-l 0
500 Age (103yrB.P.)
f 1000
500 Age (10'y-rB.P.)
100-c
a’ D 5 -3
H5 50-
,’
/’ A’
/’
.
a 2
Ott) 0
500
t 1000
Age (103yrB.P.)
0
(1) 0
500
1 1000
Age (10'yrB.P.)
FIG. 3. Four measures of soil development versus time for soils at Kyle Canyon. As described for Figure 2, but maximum likelihood line for Pleistocene points only. Holocene unit in parentheses (also drier climate and lowest elevation). Profile sum of CaCO, using volumetric percent CaCO, from Sowers et al. (1988) and correcting for bulk density from unpublished data.
Kyle and Mercury. The uncertainty in slope or rate appears to be due largely to calibration dates, which also are uncertain by a factor of about 2 (Table 1). The curve fit, however, must also contribute significantly to the slope error, because in some cases, the slopes have a smaller percentage error than the calibration dates. Rates of pedogenic accumulation of clay and carbonate are very sensitive to parent material assignments, as can be seen in the large variation in intercepts (e.g., in Table 2, intercepts of 47, 22, and 9 g-cm clay at Silver Lake); the rates, however, using the various calculations, vary by a factor of about 2x or less within a study area (e.g., 46 to 106 g-cm clay per millennium at Silver Lake).
Temporal and Geographical in Rates
Variation
The most significant result of this study is that despite the uncertainties in soil measurements and calibration ages, Holocene rates of soil development at Silver Lake are at least an order of magnitude higher than Pleistocene rates at Kyle and Fortymile Wash, and the latter two are not significantly different (Table 2; Figs. 3d and 4d). With the exception of some carbonate calculations, the diverse measures of index, profile clay, and profile carbonate still show that Holocene rates at Silver Lake are significantly higher than Pleistocene rates at Kyle and Mercury. For example, index rates at Silver Lake are 0.76 to 1.6; at Kyle
392
HARDEN
ET
AL.
t 760 Age(10’yrB.P.)
Age (10’yrB.P.)
Age (103yrB.P.)
Age(103yrB.P.)
1500
750
FIG. 4. Four measures of soil development versus time for soils at Fortymile Wash, Nevada. As described in Figure 2 with maximum likelihood line for all data, using data of Taylor (1986).
and Fortymile Wash, rates are 0.02 to 0.06 for properties other than carbonate. The high rates at Silver Lake probably are a result of the Holocene age of the soils rather than proximity to the playa, based on several interpretations. First, regional rates of dust flux today vary by only a factor of 2; rates range from 4.0 to 9.3 g clay/m’/yr and 1.4 to 3.3 g carbonate/m’/yr among the study areas (4 yr of dust collection by M. C. Reheis, unpublished data). Second, Holocene soils in the other field areas are very similar in development to Holocene soils at Silver Lake. Holocene soils in all of the areas are characterized by infilling of pores by fine-grained dust, development of structure, increased hardness in the upper profile, carbonate redistribution to depths of less than about a meter, and carbonate morphologies of stage I or II in a given horizon.
Third, when Silver Lake soils and ages are used as calibration for Holocene soils in the other three areas, the predictive ages agree reasonably with independent radiometric ages (Table 3, using methods of Switzer et al. (1988) described above), The similarity of Pleistocene rates at Kyle Canyon and Fortymile Wash is more tenuous than for Holocene rates, because dates are experimental. The soils are markedly different, with carbonate and siliceous morphologies, respectively. If, however, rates of comparative properties and indices vary by a factor of 4 as suggested in Table 2, then geographical variation in rates of soil development among these study areas are much less significant than temporal variation in rates between the Holocene and Pleistocene. In other words, data from (1) the exercise of age estimates for Ho-
GREAT
BASIN
SOIL
heis (1987a) from Wyoming show signiticant differences between Holocene and middle Pleistocene rates. Machette (1985) emphasized that rates of carbonate flux vary significantly throughout the western U.S., at least for Pleistocene soils. A future study should identify boundaries of regions in which dust or carbonate fluxes are generally similar. In turn, we should ask whether these boundaries changed in the past and whether the fluxes varied over time, especially in response to major glacial or pluvial periods.
60
0
Age W 60
6
Holocene and Pleistocene Soil Development
0
500 Age (103yr
393
CHRONOSEQUENCES
1000 B.P.)
1500
FIG. 5. Soil development index (Harden, 1982; see text for modifications) versus time for soils of three study areas. A = log time showing S, Silver Lake; M, Fortymile Wash; K, Kyle Canyon. B = linear time.
locene soils, (2) the comparison of Pleistocene rates between Fortymile Wash and Kyle Canyon (qualifying that the calibration ages have considerable uncertainties), (3) dust collection suggests that geographical variation in rates appears to vary by a factor of 2 or 3, whereas temporal variation in rates between the Holocene and Pleistocene is on the order of 10 or more. DISCUSSION Machette (1985) estimated that temporal variations of carbonate flux varied only by about a factor of about 2 between Holocene and middle Pleistocene soils within the Las Cruces, New Mexico basin, although data of Harden et al. (1985) from Utah and Re-
Rates of
The scope of this study only permits us to speculate as to why Holocene rates of soil development at Silver Lake are significantly higher than Pleistocene rates in the other areas. Several factors, however, should be discussed. First, many studies in moist regions have reported exponential rates of weathering over long time-spans (for example, Colman, 1981; Harden, 1988) and have attributed such trends to kinetics of mineral weathering. Two arguments against the importance of this factor in the southern Great Basin are (a) dissolution kinetics may not be exponential (Holdren and Bemer, 1979), and (b) Holocene soils in these study areas show little evidence for mineral weathering other than carbonate dissolution and iron oxide formation (Taylor, 1986; Reheis et al., in press; Reheis et al., 1989). In addition, young soils of this study are very porous and do not hold much water to allow primary mineral dissolution. Second, soil processes that dominate pedogenesis may change, causing an apparent decline in rate of overall development. Most of the clay and carbonate that build up rapidly in coarse-grained Holocene soils are probably eohan or atmospheric (wet and dry) in origin (Machette, 1985; McFadden, 1988; McFadden and Tinsley, 1985; Reheis et al., 1989), but accumulation may
AND INTERCEPTS
WITH
0.76 0.23 0.52 0.30
0.022 0.018 24.29 8.74
0.04 0.021 10.74 4.17
Slope per lo3 yr SD Intercept SD
Slope per l@ yr SD Intercept SD
Total texture
0.061 0.02 9.85 2.73
3.82 0.034 12.56 18.64
0.99 0.46 3.74 1.73
Field carbonate
0.019 0.013 21.71 7.33
0.022 0.0089 32.59 4.68
I.59 0.27 1.99 0.52
Index all properties
Profile minimum
2.42 0.76 5.85 87
yr) -
0.0031 0.004 0.6 0.6
0.207 0.0062 8.67 1.58
0.15 0.03 2.48 0.11
USING
0.85 0.53 81.8 63.6
7 -
40.36 20.9 161 21
=O%
% CaCO,
DATA
0.57 0.54 62.57 53.9
-
14.36 3.9 3.1 5.52
Maximum weighted mean
used for parent
Profile sum of carbonate intercepts in g-cm slopes in g-cm/lo’ yr
VERSUS TIME,
Weighted mean percent <2 mm
PARAMETERS
lower fans age range 200 to 11,000 yr (no 20,OOtl yr) 0.49 105.94 79.17 45.73 0.08 47 22 9 - 36.84 -3.62 0.74 - 113.78 0.15 256.3 49.76 19.66
=O%
Maximum weighted mean
% clay used for parent material
Kyle Canyon age range 15,000 to 800,000 yr (no 5000 0.019 0.009 24.75 4.81 Fortymile Wash age range 10,000 to 1,000,000 yr 0.041 0.0073 2.38 0.99 0.88 0.58 0.008 0.007 10.24 4.54 5.44 171 2.67 1.52 156 88 ___.
Silver Lake 1.51 0.38 1.68 0.85
Index no carbonate
OF SOIL DEVELOPMENT OF SWITZER ET AL., 1988
Profile sum of <2 pm clay intercepts in g-cm slopes in g-cm/IO’ yr
DEVIATIONS (SD) 1 AND METHODS
Weighted mean percent <2 mm
STANDARD TABLE
Soil development index intercepts in units of profile sum slopes in units of profile sum/103 yr
2. SLOPES
Slope per 10’ yr SD Intercept SD
TABLE
1.09 0.41 40. I5 100.00
5.13 0.95 12.25 3.47
22.67 6.9 - 13.55 10.6
Profile minimum
material
FROM
395
GREAT BASIN SOIL CHRONOSEQUENCES TABLE
3. PREDICTIVE
INDICES
FOR HOLOCENE
Surlicial unit Cima-QD Cima-Qf4 Cima-QfS Cima-Qf6 Cima-Qf7 Cima-QtIl FM Wash-lb FM Wash-lc Kyle-lower fan 3
AGES OF GEOMORPHIC SURFACES, BASED ON CALIBRATION OF SOIL SOILS AT SILVER LAKE (SOIL DEVELOPMENT INDEX OF HARDEN MODIFICATIONS DESCRIBED IN TEXT)
Equivalent fan surface at Silver Lake, based on soil age
Maximum likelihood estimate age,Y (10’ yr)
Error term0 at 95% level, (lo3 yr)
(37) (17)
(13) (3)
(35)
Qf 1
(18)
Qt?!-Qf 1
11 4 4 2
2 1 3 1 (7) 1
50 18 25 150 30 (30) 10
$-Qf4 QD-Qf4 QfS-Qf4 QD-Qf4
(2:) 10
Precision m
QQ-
DEVELOPMENT (1982) WITH
Independent age@ from Table 1 (IO3 yd 43-48 VCR: < 130KAB
1 l-14 VCR V14C 1.16 I?I 0.90’VL4C 20.15?? 310? 5-15
0 Switzer et al., 1988; parentheses used for pre-Holocene estimates, indicating ages >lO,OOO yr. b VCR and Vr4C varnish collection sites approximately located near soil pits.
reach a maximum once large pores are filled with fines. In addition, as the fines build up in the soil, water-holding capacity increases (Harden, 1988), which likely enhances in situ mineral transformations such as primary mineral weathering (Reheis et al., in press), formation of silicate clays (Taylor, 1986; Reheis et al., in press), and iron oxidation (Taylor, 1986; Reheis et al., in press). Concomitant with increased water retention, permeability eventually declines, causing less infiltration of protonated water, more runoff, and possibly more erosion. Such changes in pedogenic processes were discussed by McFadden and Weldon (1986) for soils in the dust-rich Cajon Pass area in California. Third, the importance of past climates and climatic change to rates of soil development is irrefutable (McFadden, 1988). Eolian dust may have accumulated at dramatically different rates during the Holocene and Pleistocene (Machette, 1985) both as a result of changes in source (e.g., lakes vs playas) and effective wetting (Spaulding, 1985; Quade er al., 1989) (see discussion below). In addition, the depth to which carbonate accumulated was greater in the Pleistocene than in the Holocene (McFadden and Tinsley, 1985; Mayer et al., 1988), an attribute that affects retention
of eolian carbonate and other pedogenic attributes. Fourth, the role of erosion in soil development is critical, although difficult to quantify in this type of study. For example, slow continual erosion (e.g., sheet erosion) might maintain the apparent flatness of a geomorphic surface, yet allow considerable amounts of material to erode. If erosion were systematically greater for each older surface, then a systematic progression of soil development with time would be found, but the rate of soil development would decrease with time. Alternatively, if erosion were quite variable from unit to unit, we would not observe systematic trends in soil development with time. Exponential and Step-Functions Age Curves
for
Although curve shapes over certain time spans generally might average out to linear functions (e.g., over the Holocene or Pleistocene), alternative models include exponential and step functions. Over a million years or more, the exponential function may best describe the decrease in rates over time (Fig. 6), as has been found by many studies of soils in moister regions (e.g., Harden, 1986; Dethier, 1988; and Reheis, 1987a,b).
396
HARDEN
01
ET
AL.
I I I I 6 9 12 15 Age (lo3 yr B.P.) FIG. 6. Maximum likelihood estimate (MLE) lines of index versus age. Inset = vertical area of mean and standard deviation of soil index represented by replicate soils on dated surface; horizontal area of age constraints over best-estimate age. Dotted line is MLE from mean index and best-estimate age, represented in inset where horizontal and vertical lines meet. Ten solid lines = MLE lines iterated from 10 data sets in Monte Carlo fashion from area inside shapes in insert. 0
I 3
Curve shapes of Silver Lake data over much shorter periods allude to a third alternative, the step-function model. Although better age control (more precise ages and more numerous units) would better resolve the curves, clay, and carbonate curves for Silver Lake (Fig. 7) are somewhat step like. High rates of increase are suggested at about 11,000 to 6,000 yr B.P. (using bestestimate ages in Table 1) and from about 2500 yr B.P. to the present (Fig. 7a). Given that much of the clay and carbonate are derived from eolian dust, these intervals of time could represent times of high dust flux, with rates about 10 times those before or after that period (Fig. 7a,b). This concept of dust “events” recently has received attention by Machette (1985), Chadwick and Davis (1990), and Reheis et al. (1989). As Chadwick and Davis emphasized, this concept may explain soil-
forming intervals proposed by Morrison (1978), whose studies were made in an area with significant dust sources. A most succinct perspective of the concept was presented by Machette (1985), who demonstrated that over long time spans, dust flux could be viewed as very rapid, nearly instantaneous events that are averaged to long-term rates. Because dust and carbonate accumulation is so complexly related to innumerable variables, however, we think many studies on arid soils should be conducted before soils can be linked directly to dust events. For example, an important alternative to high rates of dust flux invokes plugging of the Qfl soil before 8000 to 12,000 yr B.P. The close similarity between carbonate sums on Qfl and Qf2 suggests such a possibility. According to the model of McFadden and Weldon (1987), in which water-holding capacity reaches an intrinsic
GREAT
0
10
20
BASIN
SOIL
30
data. Adequate records would probably show dust-flux cycles on scales of decades correlated to drought years or of seasons correlated to dry seasons. Longer-term processes also occur throughout these dustflux cycles, such as chemical and mineralogical transformations. Soil processes such as iron oxidation, clay formation, or mineral dissolution may be intrinsically linked to dust as a source of relatively fresh material, but they probably occur more steadily over time than dust flux. Implications
I 10
20
I 30
AgeU03yrB.P.)
7. Profile carbonate and ~2 urn clay versus time for soils at Silver Lake. Profile sums of percentage of <2 mm times thickness and bulk density. For Silver Lake, bulk density was assumed to be between 1.6 and 2.0 g/cc; for each age group, minima and maxima of g-cm data (mean ? 1 SD from Table 1) were multiplied by 1.6 or 2, and the range was used for units of g/cc. Lines = step function if line drawn from mean values; straight function if maximum likelihood line (Switzer er al., 1988) drawn from all means. FIG.
threshold for chemical alteration, if the Qfl soil had been plugged with dust for very long, there might also be evidence for significant chemical and mineralogical transformations not seen in Qf2 soils. The appropriateness of the curve shape-whether step, separate linear, or exponential-may vary according to the time span being considered and the processes being quantified. Short-term processes related to dust flux may have variable rates on short time scales, for example a millennia as suggested by the Silver Lake
397
CHRONOSEQUENCES
for Correlation
and Dating
For the Holocene soils, the implications for correlation and approximate dating using soils is that calibration in one area might be appropriate for other sites within the general region and might be accurate within about a factor of 2 or 3 (Table 3); precision of the predictive ages averages about 40% (error of 2 standard deviations divided by best estimate age), as compared to about 18% error associated with Holocene radiometric ages (maximum minus minimum ages divided by best-estimate ages, divided by 2 for 2 standard deviation error). Using soils to estimate ages approximately doubles the age uncertainty associated with the t4C ages used by Wells et al. (1987). For Pleistocene soils, uncertainties in calibration ages and rates of soil development are considerable, and more work is needed before we can reliably correlate among study areas. If we can establish the causes of declining rates with time and if the causes are similar throughout the region with its diverse rock types, vegetation, dust fluxes, and climates, then semilog or exponential curves might conceivably serve as a model for long-term soil development. CONCLUSIONS Rates of soil development over the Holocene in four study areas in the southern Great Basin appear to vary by about a factor of 2 or 3. Rates appear to be broadly similar within the Holocene, probably because influx of dust is the predominant pro-
398
HARDEN
cess of soil formation and rates of dust flux vary geographically by such amounts. Rates of soil development over the Pleistocene are not significantly different at Kyle Canyon and Fortymile Wash and are about 10 times lower than Holocene rates at Silver Lake. Thus, the geographical variation in rates within the region is much less significant then temporal variation in rates of soil development. Over long time spans such as the Quaternary, curve shapes are best fitted to exponential functions, with decreasing rates of soil development over time. Changes in rates of development are probably related to climatic change and related changes in water and dust, erosional history, and/or internally driven chemical processes. Holocene soils can probably be correlated and approximately dated throughout the southem Great Basin with anticipated uncertainties of about a factor of 2 or 3. Pleistocene soils in our study areas lack the age control needed to characterize and understand age trends reliably. ACKNOWLEDGMENTS Many thanks to Ron Dom and Brent Turrin for collaboration on varnish and KAr dating, respectively, and to Kathy Parrish for time and expertise in illustrations, UNM students and R. Kyle at CU for soil analyses, and P. Birkeland, D. Muhs, 0. Chadwick, and D. Miller for helpful discussions. Thanks also to NSFPYI for support for some of the varnish data.
REFERENCES Bierman, P. R., and Gillespie, A. (1989). Rock varnish, alluvial fans, and tectonism in the southern Owens Valley, California. Geological Society of America Abstracts 21(6), A343. Chadwick, 0. A., Hecker, S., and Fonesca, J. (1984). A soils chronosequence at Terrace Creek: Studies of late Quatemary tectonism in Dixie Valley, Nevada. U.S. Geological Survey Open-File Report, 84-90. Chadwick, 0. A., and Davis, J. 0. (1990). Soilforming intervals caused by eolian sediment pulses in the Lahontan Basin, Northwestern Nevada. Geology 18, 243-246. Colman, S. M. (1981). Rock-weathering rates as functions of time. Quaternary Research 15, 250-264. Dethier, D. P. (1988). The soil chronosequence along the Cowlitz River, Washington. U.S. Geological Survey Bulletin, 1590-F. Dom, R. I. (1984). Geomorphological interpretation of
ET AL.
varnish in the Mojave Desert. In “Surficial Geology of the Eastern Mojave Desert, California” (J. C. Dohrenwend, Ed.). Geological Society of America Annual Meeting Field Trip Guidebook 14, Reno. NV. Gile, L. H., Peterson, F. F., and Grossman, R. B. (1966). Morphological and genetic sequences of carbonate accumulation in desert soils. Soil Science rock
101, 347-360.
Harden, D. R., Biggar, N. E., and Gillam, M. L. (1985). Quatemary deposits and soils in and around Spanish Valley, Utah. In “Quaternary Soils and Geomorphology of the Southwestern United States” (D. L. Weide, Ed.), pp. 43-64. Geological Society of America Special Paper 203, Reno, NV. Harden, J. W. (1982). A quantitative index of soil development from field descriptions: Examples from a chronosequence in central California. Geoderma 28, l-28. Harden, J. W. (1986). Soils developed in granitic alluvium near Merced, California. U.S. Geological Survey Bulletin, 1590-A. Harden, J. W. (1988). Genetic interpretations of elemental and chemical differences in a soil chronosequence, California. Geoderma 43, 17%193. Harden, J. W., and Matti, J. C. (1989). Holocene and late Pleistocene slip rates on the San Andreas fault in Yucaipa, California, using displaced alluvial-fan deposits and soil chronology. Geological Society of America Bulletin 101, 1107-l 117. Harden, J. W., and Taylor, E. M. (1983). A quantitative comparison of soil development in four climatic regimes. Quaternary Research 20, 342-359. Harden, J. W., Taylor, E. M., McFadden, L. D., and Reheis, M. C. (1991). “Calcic, Gypsic, and Siliceous Soil Chronosequences in Arid and Semiarid Environments.” Soil Science Society of America Special Paper 26, Chapter 1. Holdren, G. R., and Bemer, R. A. (1979). Mechanisms of feldspar weathering I. Experimental studies. Geochemica et Cosmochemica Acta 43, 1161-l 171. Holhday, V. T. (1988). Genesis of a late-Holocene soil chronosequence at the Lubbock Lake archeological site. Annual Association of American Geographers 78, 549-610. Machette, M. N. (1985). Calcic soils of the southwestem United States. In “Soils and Quatemary Geomorphology of the Southwestern United States” (D. L. Weide, Ed.). Geological Society of America Special Paper 203. Mayer, Larry, McFadden, L. D., and Harden, J. W. (1988). The distribution of calcium carbonate in desert soils: A model. Geology 16 (4), 303-306. McFadden, L. D. (1988). Climatic influences on rates and processes of soil development in Quatemary deposits of southern California. Geological Society of America Special Paper 216, 153-177. McFadden, L. D., and Tinsley, J. C. (1985). Rate and
GREAT
BASIN
SOIL CHRONOSEQUENCES
depth of pedogenic-carbonate accumulation in soils: Formation and testing of a compartment model. In “Quaternary Soils and Geomorphology of the Southwestern United States” (D. L. Weide, Ed.), pp. 23-24. Geological Society of America Special Paper 203, Reno, NV. McFadden, L. D., and Weldon, R. J. (1986). Rates and processes of soil development on Quaternary terraces in Cajon Pass, southern California. Geological Society of America Bulletin 98, 280-293. McFadden, L. D., Wells, S. G., and Dohrenwend, J. C. (1986). Influences of Quaternary climatic changes on processes of soil development on desert loess deposits of the Cima volcanic field, California. Catena 13, 361-389. Morrison, R. B. (1978). Quatemary soil stratigraphyConcepts, methods and problems. In “Quatemary Soils,” (W. C. Mahaney, Ed.), pp. 77-108. GeoAbstracts, Norwich. Muhs, D. R., Bush, C. A., Stewart, K. C., Rowland, T. R., and Crittenden, R. C. (1990). Geochemical evidence of Saharan dust parent material for soils developed on Quatemary limestones of Caribbean and western Atlantic Islands. Quaternary Research 33, 157-177. Ore, H. T., and Warren, C. N. (1971). Late Pleistocene-early Holocene geomorphic history of Lake Mojave, California. Geological Society of America Bulletin 82, 2553-2562. Quade, Jay, Cerling, T. E., and Bowman, J. R. (1989). Systematic variations in the carbon and oxygen isotopic composition of pedogenic carbonate along elevation transects in the southern Great Basin, United States. Geological Society of America Bulletin 101, 464-475. Reheis, M. C. (1987a). Gypsic soils on the Kane alluvial fans, Big Horn County, Wyoming. U.S. Geological Survey Bulletin, 1590-C. Reheis, M. C. (1987b). Soils in granitic alluvium in humid and semi-arid climates along Rock Creek, Carbon County, Montana. U.S. Geological Survey Bulletin, 1590-D. Reheis, M. C., Harden, J. W., McFadden, L. D., and Shroba, R. R. (1989). Development rates of late Quatemary soils, Silver Lake Playa, California. Soil Science Society of America Journal 53, 1127-l 140. Reheis, M. C., Sowers, J. M., Taylor, E. M., McFadden, L. D., and Harden, J. W., (in press). Morphology and genesis of calcic soils on Kyle Canyon Fan, Spring Mountains, Nevada. Geoderma. Rosholt, J. N. (1985). U-trend systematics for dating
399
sediment. U.S. Geological Survey Open-File Report, 85-298. Sowers, J. M. (1985). “Pedogenic Calcretes of the Kyle Canyon Alluvial Fan, Southern Nevada: Morphology and Development.” Unpublished PhD thesis, University of California, Berkeley. Sowers, J. M. (1986). Geomorphic map of the Kyle Canyon alluvial fan, southern Nevada. U.S. Geological Survey Open-File Report, 86-210. Sowers, J. M., Amudson, R. G., Chadwick, 0. A., Harden, J. W., Jull, A. J. T., Ku, T. L., McFadden, L. D., Reheis, M. C., Robinson, S. W., Szabo, B. J., and Taylor, E. M. (1988). Geomorphology and Peology on the Kyle Canyon alluvial fan, southern Nevada. In “Geological Society of America Field Trip Guidebook” (D. L. Weide and M. L. Faber, Eds.), pp. 137-158. Cordilleran Section Meeting, Las Vegas, NV. Spaulding, W. G. (1985). Vegetation and climates of the last 45,000 years in the vicinity of the Nevada Test Site, south-central Neveda. U.S. Geological Survey Professional Paper 1329, 83. Switzer, P. S., Harden, J. W., and Mark, R. K. (1988). A statistical method for estimating rates of soil development and ages of geologic deposits: A design for soil-chronosequence studies. Mathematical Geology 20, 49-61. Taylor, E. M. (1986). “Impact of Time and Climate on Quatemary Soils in the Yucca Mountain Area of the Nevada Test Site.” Unpublished manuscript, University of Colorado, Boulder, CO. Taylor, E. M. (1988). Instructions for the soil development index template-Lotus l-2-3. U.S. Geological Survey Open-File Report, 233A. Turrin, B. D., and Champion, D. E. (1991). 40Ar/39Ar Laser Fusion and K-Ar ages from the Quatemary Basaltic Volcanic Centers at Crater Flat, Nevada and Cima, California: Implications for the Age of the Youngest Volcanic Activity in the Yucca Mountain Area: Proceedings of the International High-Level Nuclear Waste Management Conference, Las Vegas, Nevada, (American Society of Civil Engineers and American Nuclear Society). Turrin, B. D., Dohrenwend, J. C., Drake, R. E., and Curtis, G. H. (1985). K-Ar ages from the Cima volcanic field, eastern Mojave desert, California. Isoch.lWest 44, 9-16. Wells, S. G., McFadden, L. D., and Dohrenwend, J. C. (1987). Influences of late Quatemary climatic changes on geomorphic and pedogenic processes on a desert peidmont, eastern Mojave Desert, California. Quaternary Research 27, 130-146.
RESEARCH
35, 383-399 (191)
Rates of Soil Development from Four Soil Chronosequences in the Southern Great Basin JENNIFER W. HARDEN,* EMILY M. TAYLOR,? CINDY HILL,* ROBERT K. MARK,* LESLIE D. MCFADDEN,+ MARITH C. Rmrnrs,t JANET M. SOWER&§ AND STEVEN G. WELL& *U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025; fU.S. Geological Survey, 613 Federal Center, Denver, Colorado 80225; SGeology Department, Universiry of New Mexico, Albuquerque, New Mexico 87131; and 55844 San Jose Avenue, Richmond, CA 94804 Received March 21, 1990 Four soil chronosequences in the southern Great Basin were examined in order to study and quantify soil development during the Quatemary. Soils of all four areas are developed in gravelly alluvial fans in semiarid climates with 8 to 40 cm mean annual precipitation. Lithologies of alluvium are granite-gneiss at Silver Lake, granite and basalt at Cima Volcanic Field, limestone at Kyle Canyon, and siliceous volcanic rocks at Fortymile Wash. Ages of the soils are approximated from several radiometric and experimental techniques, and rates are assessed using a conservative mathematical approach. Average rates for Holocene soils at Silver Lake are about 10 times higher than for Pleistocene soils at Kyle Canyon and Fortymile Wash, based on limited age control. Holocene soils in all four areas appear to develop at similar rates, and Pleistocene soils at Kyle Canyon and Fortymile Wash may differ by only a factor of 2 to 4. Over time spans of several millennia, a preferred model for the age curves is not linear but may be exponential or parabolic, in which rates decrease with increasing age. These preliminary results imply that the geographical variation in rates within the southern Great Basin-Mojave region may be much less significant than temporal variation in rates of soil development. The reasons for temporal variation in rates and processes of soil development are complexly linked to climatic change and related changes in water and dust, erosional history, and internally driven chemical and physical processes. 0 1991 University of Washington.
INTRODUCTION There are few numeric (radiometric or calibrated) ages for Quaternary geomorphic surfaces in the southern Great Basin. Techniques are needed to date surfaces reliably and correlate them to undated localities. Soils are potentially a useful correlation and dating tool because (1) soils develop on geomorphic surfaces at the onset of surface stabilization and (2) soil development increases systematically with increasing surface age, as demonstrated in arid/semiarid regions by several workers (Gile et al., 1966; Machette, 1985; Chadwick et al., 1984; McFadden, 1988; Holliday, 1988; Reheis, 1987a,b). The goals of this study are to characterize soil development in the south-
ern Great Basin in locations with dated Quaternary surfaces, to estimate rates of soil development in such locations, and to evaluate the potential and the limitations of using soils to correlate and approximately date surfaces in the region. With parent detritus of such contrasting lithologies as granitic-gneissic, granitic-basaltic, siliceous volcanic, and limestone, we hypothesized that a wide range of types and rates of soil development would be represented in this study, and that such a range most probably would be characteristic of most soils found in the southern Great Basin. Comparative studies require careful and consistent sampling and analysis of soils (e.g., replication of sampling sites on a given dated surface), consistent treatment of data for estimating 383 0033-5894191 $3.00 Copyright 0 1991 by the University of Washington. All rights of reproduction in any form reserved.
384
HARDEN
degree of soil development (e.g., estimates of parent material, methods of soil description and analysis), examination of how radiametric or numeric ages relate to the age of the surface and soil, and mathematics for estimating rates of soil development that account for the many uncertainties of our field and interpretive models (uncertainties of calibration dates, soil variability, shape of soil-time curves). GEOCHRONOLOGY, GEOMORPHOLOGY, AND SOILS In each area (Fig. 1; Table l), we studied several surfaces and soils for which relative ages are well defined. In addition, some of the surfaces are dated numerically by methods that date either the deposit or the surface and soil. Our analysis attempts to bracket the age of the surface, which is the best estimate for inception of soil formation. Such brackets are typically quite wide, especially for Pleistocene surfaces P
ET AL.
(Table I), and vary considerably in both precision and accuracy. For example, the uranium-trend method of Rosholt (1985), used for Pleistocene soils at Fortymile Wash, is experimental. Uranium series ages of soil carbonate, used at Kyle Canyon, have considerable errors (Sowers et al., 1988). We carefully evaluated and represented such uncertainties in our data analysis but emphasize that in some cases even these uncertainties are not well anchored to known ages. The geomorphic setting, climate, and vegetation in all four of the study areas are generally similar. We attempted to study soils on flattest parts of alluvial fans or terraces, although the oldest surface at Kyle Canyon exists as an erosional, rounded remnant. The rainfall of the areas has similar seasonal distribution, with most of the 8 to 40 cm annual precipitation (Table 1) occurring in cool winters and the remaining in hot summers. Pleistocene climates were cooler and effectively wetter (e.g., see I 116”
I 11P
37”3\0 ‘.
\
‘.
\
/.-.-\
‘\ --.-._.
;
1
AREA OF MAP
.T\LYI
NATIONAL“*, . ..e...*..r.MUNUMtNl \ ‘.
0
r \
.j 2.h
I._
Mercury Kyle Canvon
!
Study area
O,,,, 35”
‘.
q
40
km I
FIG. 1. Map showing location of four study areas in the southern Great Basin.
GREAT
BASIN
SOIL
Spaulding, 1985). Vegetation is typical of the southern Great Basin and varies with elevation and rainfall (Table 1). Silver Lake Alluvial fans at Silver Lake playa, derived primarily from granitic and gneissic rocks, provide an excellent soil chronosequence for the Holocene through latest Pleistocene (Wells et al., 1987). The surfaces have gentle gradients (less than 10%) and are relatively undissected. The ages of the four youngest fans (Table 1) were constrained by Wells et al. (1987) using data of Ore and Warren (1971); the oldest fan is estimated to be late Pleistocene by Dorn (1984) and R. I. Dorn (unpublished data, 1986). In these soils, there is a systematic change in soil morphology (Reheis et al., 1989), pedogenic carbonate, and clay content over time; the latter two components are derived primarily from eolian dust and precipitation-borne solutes (Reheis et al., 1989; Wells et al., 1987). Detrital calcite derived from reworked, older soils is evident from calcic rinds on modern alluvium and from rinds that are chipped or oriented on sides or tops of otherwise clean clasts found in the soils. Cima Volcanic Field Alluvial fans and fluvial terraces near the Cima Volcanic Field are composed of granitic detritus with minor amounts of basalt from the Cima field concentrated in the desert pavement. Seven surfaces, two of which have ages constrained by K/Ar ages of basalt flows (Turrin et al., 1985 and Turrin and Champion, 1991) appear relatively flat (2 to 9%) and undissected. Three of the fans were also sampled by R. I. Dorn for rock varnish on surface clasts for study of radiocarbon and cation ratio trends (Tables 1 and 3; R. I. Dorn, written communication, 1986). Radiometric ages do not permit an independent rate of soil development at Cima, but in later sections we discuss the
CHRONOSEQUENCES
385
ages of the Cima fans based on calibration of soils to Silver Lake fans. Soil development is dominated by the accumulation of clay and carbonate, much of which appear to be eolian in origin, as in the soils formed directly on the basalt flows (McFadden et al., 1986). The soils are very similar to those at Silver Lake, except there is little evidence for reworked carbonate at Cima. Fortymile Wash Alluvial fans and fluvial terraces along Yucca Wash, near Fortymile Wash, are developed in alluvium derived from welded and nonwelded ash flow tuffs, with a minor component of basalt. The surfaces and soils were studied by Taylor (1986) and dated primarily by uranium-trend analysis, which is somewhat experimental (Rosholt, 1985). The soils display an impressive increase of secondary silica with time as a result of the influence of the glass and opaline-rich siliceous parent material (Taylor, 1986). Much like the stages of carbonate described by Gile et al. (1966) and Machette (1985), the stages of secondary, opaline silica progress from undercoatings on clasts to laminar caps (Taylor, 1986). Kyle Canyon Three alluvial surfaces at Kyle Canyon were studied in detail by Sowers (1985, 1986) and by Reheis et al. (in press) and are composed almost exclusively of limestone detritus. Ages are based on radiocarbon ages of detrital carbon, uranium series analysis of soil carbonate, and a magnetic reversal (Table 1). The oldest surface (surface 1) exists as an elongate, rounded balena topography indicative of erosion, which could be exposing old groundwater carbonates to the zone of soil formation as the soil is eroding. To be conservative, we dashed lines representing data for surface 1 in Figure 3. Sowers et al. (1988) discuss many problems with all of the dating techniques, but emphasize that surface 3 may be oldest
Cc correlation near Beaty , Nev.
Qlc
incision
Fortymile
20
10
Wash Alluvial 0.15
800
130
15
Profde characteristics
SURFACES
FOUR
X?SD
AvlBtklBk, stage II carbonate
AvlBtWBk, stage I-II carbonate
AvlBtjlBk, stage I carbonate
46.4 k 9.4
19.7 + 9.5
6.6 2 0.9
MAP;
3
4
2
4
I
Fans and Fluvial 0
750
18
4 AiBtlKmlBk; stage IV carbonate AlKmlBk; stage IV carbonate
A/Btk/Bt; stage III carbonate
50 + 15
34 e 13
34 -c 1.9
Terraces? looO- to 1100-m elevation; 15°C MAT; 10 v. thin A/C; v. weak stage I carbonate 30 AlBtjlBtWCkn; 8.6 + 4.8 stage I-II carbonate
3ooo
750
80
2
12-cm MAP;
3
5
3
detritus
SD,
133 + 18
siliceous
DEVIATION)
30 -t 15
detritus
147 2 48
81 + 45
42 f 40
X + SD g - cm
-ClayC
STANDARD
volcanic
5245 ? (1544 est)
3830 * (1149 est)
1302 I (390 estjp
limestone
362 f 187
3% + 97
49 ? 18
61 2 2
detritus 6 k 10
taco,= X?SD g-cm
(x, MEAN;
granitic-gneissic 3
No. profiles
index”,’
AREAS
Alluvial Fans:‘840to 1670-m elevation; 12 to 18°C MAT; 16 to 41-m MAP; 0 15 A/Btk/Bk; stage II 6 + 1.5 6 carbonate
70
20
8
STUDY
Soil development
IN THE
to 300-m elevation; 21°C MAT; 8.3-cm Open gravel matrix, 2.3 k 1.2 very thin A@ 3.3 AvlBwk, stage I 1.9 t 2.7 carbonate 1
15
Fans$275-
Maximum
8
3.3
6
11
0
Alluvial 0
Silver Lake 0.20
2.5
Minimum
Bestestimate
lo3
OF GEOMORPHIC
Ages used for calibration,
CHARACTERISTICS
Kyle Canyon 5
SOIL
>4.4cco;
~3.4pR;~ (2.6V14Ce Beta 375!443 >3.4PR;
arroyo
upper
lower
AND
Dating information age in ld yr
1. AGES
Qlb
1
2
3
3
Qfl
w
QD
Qf4
Qf5
surface
Geomorphic
TABLE
2
3
3
No. profdes
>llOOA; <2lOOA
(l.2V’4C;’ Beta 20573; o-5 Ssl) (ll-l4V’4C;= Beta 17537; 3-5 Ssl) (2-6 Ssl)
(Sl3
(M-20
Qta
Qfs
Qf6
Qf5
Qf4
Cima Alluvial
loo0
350
150
Fans;’
900
270
145
AlBtWBtqlCkq; stage I-II carbonate; II silica A/BtWKqmlK; stage II-III carbonate; HI silica A/BtWKqm/Kqy; stage IV carbonate; IV silica
A/Bt/Bk; stage I carbonate A/Bt/Bk; stage II carbonate A/Btk/K; stage III carbonate A/Btk/Bk; stage III carbonate 39.0 + 11.6
19.2 ? 2.0
12.4 + 1.7
6.1 + 1.4
6.5 + .9
detritus
39.5 + (11.9 est)
30.7 + 1.4
22.8 + (6.8 est)
12-cm MAP; granitic 3.6 + 3.2
AEtwlBk; stage I carbonate
1200-m elevation; 15°C MAT; AlBk; stage I carbonate
2000
440
190
4
3
3
2
4
clasts
1656 ? (497 est)
487 + 199
86 + (26 est)
with basaltic 3
1
2
1
3072 2 (1536 est)
735 k 367
548 f (274 est)
1
2
1
No?e. (CSS) 14C shell, shoreline; (V14C) varnish radiocarbon; (PR)14C on packrat midden; (Cc) i4C on charcoal at depth;(date)not usedfor c&b&ion; (b’r)p&omqn&c reversal; (Ucr) Uranium-series on calcite rinds; (Cco) i4C on organics from calcite rinds; (Ccr) 14C on calcite rinds; (Ut) Uranium-trend on soil; (A) KAr on ash; (KAb) KAr on basalt; (Ssl) soil development calibrated to Silver Lake (this paper). a Harden, 1982; Harden and Taylor, 1983; modifications of this paper all properties included. b Reheis et al., 1989 c Parent material basal minimum; d Wells et al., 1987; S. B. Wells, written communication, 1989 e R. I. Dam, written communication, 1%9 f Sowers et al., 1988 a Average SD/X of other units h Taylor, 1986 i S. G. Wells, written communication, 1989 j Tunin et al., 1985 and Turrin and Champion, 1991.
(
Ssl)
Ssl)
24&34Out; <44out
Q2c
cx7
120-170ut
Q2b
388
HARDEN
at its upper end. Thus, four sites, surfaces 1, 2, upper 3, and lower 3, are considered separately for this study, with considerable age uncertainties that overlap among surfaces (Table 1, Fig. 3). In addition, the climate at lower surface 3 is significantly warmer and drier than at other sites (Table 1). The soils at Kyle Canyon are dominated by secondary carbonate, which in advanced stages appears to overwhelm other soil constituents such as silicate clay or iron oxides (Reheis et al., in press). The soils on fans 2 and 1 have abundant secondary carbonate to depths of 3 m. FIELD AND LABORATORY
METHODS
At Silver Lake, Cima, and Kyle Canyon, several pits were excavated to depths of 2 to 4 m on each geomorphic surface in order to estimate soil variability on each surface. Sites were selected at similar elevations where possible, so that orographic effects of precipitation are similar at each field area. Analyses that were performed include an array of physical, chemical, and mineralogical techniques, the descriptions and data of which are available in Taylor (1986); Sowers et al. (1988); Reheis et al. (1989); and Reheis et al. (in press). CALCULATIONS OF SOIL DEVELOPMENT AND RATES OF DEVELOPMENT
Three main measures of soil development considered in this study are the soil development index, pedogenic clay, and pedogenic carbonate. For all three measures, we define levee and eolian deposits as pedogenic components, and we use basal fan deposits at the base of each profile to estimate the composition of the original parent material for all horizons of the profile. In reality, admixtures of eolian and levee deposits are in some respects parent material that is then subject to processes such as dissolution, fractionation, and
ET
AL.
translocation. Ideally, isotopic signatures of dust and alluvium might help to track the complexity of influx and weathering (Muhs et al., 1990). For the present study, however, we define dust (and levee) admixtures as pedogenic attributes. The soil development index of Harden (1982) uses field descriptions of soils with additions of color-paling and lightening (Harden and Taylor, 1983), pH increase of Reheis (1987a), and a new carbonate index (described in Harden et al., 1991). For the carbonate index, horizon values of colorpaling and color-lightening are summed (a), multiplied by carbonate stage (Gile et al., 1966; Machette, 1985) (b), divided by the current maximum of 240 (c), and included in further sums or indices (d). Disseminated carbonate is assigned a stage of 0.5 for step (b). We used the spreadsheet program of Taylor (1988) for calculations. Pedogenic clay and carbonate are determined by subtracting estimated amounts of clay and carbonate in parent material from those in horizons, then multiplying by horizon thickness (and bulk density where possible) before summing the products through the profile. We used three approaches to estimate amounts of clay and carbonate in parent materials in order to calculate pedogenic clay and carbonate: (1) Parent material has no clay or carbonate, which maximizes profile sums. (2) Parent material can be estimated from a reasonable minimum value in each protile. These parent material estimates are different for each profile but are roughly similar within each chronosequence. (3) A maximum estimate for parentmaterial clay or carbonate was derived from time plots of the weighted mean. Such a mean, in percentage by weight, is obtained by multiplying percentage carbonate or clay by horizon thickness, summing the products through the profile, and dividing by profile thickness. Another way of interpreting the weighted mean is as a redistribution of all clay or carbonate in the devel-
GREAT
BASIN
SOIL
oped profile, wherein all horizons have equal amounts (in weight percent) of clay or carbonate. The increase over time reflects the accumulation of clay or carbonate by eolian admixtures and mineral weathering; the intercept at time = 0 years should reflect the original content in weight percent (assuming, of course, that the rates are linear; see Discussion). Because this third approach is used to maximize the estimate of clay or carbonate in the parent material, the intercept plus one standard deviation is used rather than just the intercept of the weighted mean versus time. Profile sums or accumulation indices (horizon sums of horizon carbonate minus parent carbonate) usually require bulk density for meaningful units of pedogenic clay or carbonate, but the high gravel content of these soils makes density measurements imprecise. For comparison of rates, we opted to calculate profile sums that exclude the density factor, resulting in units of gcm. Profile sums using estimated bulk densities also were calculated for soils with density data (used in Fig. 6), and results were consistent with the units of g-cm. The methods of Switzer et al. (1988) were used to assess estimates and uncertainties of rates of soil development and to estimate ages of soils at Cima. The purpose of the Switzer method is to represent mathematically the field and sampling limitations of chronosequences. There are two major flaws in using simple regression for chronosequences: (1) Both the x and y axes of soil vs age functions have considerable uncertainties, whereas simple regression fits the line with only errors in y. (2) Error estimates of even a two-error regression are inappropriate, because sample numbers are pathetically small and soil variability contributes significantly to errors in the soil development value. Thus, Switzer et al. (1988) employed a two-error-type regression (maximum likelihood estimate line below) and, most significantly, designed a Monte-Carlo approach of refitting the
389
CHRONOSEQUENCES
“regression” line to various data combinations. The Monte-Carlo-like approach results in much higher uncertainties for slopes, intercepts, and predictive ages as compared to simple regressions (Harden and Matti, 1989). Such a Monte Carlo approach is also being proposed for other calibrated dating techniques (Bierman and Gillespie, 1989). In the Switzer approach (Switzer et al., 1988)) a maximum likelihood line (similar to a two-error or weighted regression) is calculated from data with uncertainties in calibration dates and soil variability (dotted line, Figs. 2-5). The slope of the line represents rate of soil development, and the intercept represents the state of the soil at time zero. The uncertainty of the slope and intercept is obtained by iterating 100 data sets from a sampling of the means and standard deviations of the data (insert in Fig. 5). One hundred lines are calculated from the iterative data sets (10 lines plotted in Fig. 5), and the standard deviation of the 100 slopes represents uncertainty in the rate of soil development. For age estimates of a soil, the mean and standard deviation of the soil of unknown age are used to generate a range of 100 postulated soil development values, which are used in combination with the 100 lines above to compute 100 possible age estimates by the curves of soil development vs age. Figures 24 show the area that includes these 100 lines, which encompass a wide range of slopes and intercepts. The uncertainty in rates and intercepts (Table 2) is derived from the standard deviation of the 100 iterative lines. RESULTS
Average Rates of Soil Development For all measures of soil development (index, clay, and carbonate), time plots may appear to be roughly linear within each Holocene and Pleistocene time span, but are not linear for the longer time span of Holocene through Pleistocene (Figs. 2-4; Ta-
390
HARDEN
0
20
40
60
80
ET
AL.
0 0
20
Age (lo3 yr B.P.)
-$600]
f
40 Age (lo3
60
I 80
60
80
yr B.P.)
1:';
300
- r---------Iif- i I
0 lb n ”
ZO
40
fi0
SO
Age (IO3 yr B.P.)
0
20
40 Age (lo3
yr B.P.)
FIG. 2. Four measures of soil development versus time for soils at Silver Lake. Soil development index and total-texture calculated from Harden (1982) and Harden and Taylor (1983) with field index of carbonate as described in text. Profile sums of carbonate and <2 pm clay calculated from percentage of <2 mm times horizon thickness, summed in profile from dataof Tables l-3 (Reheis et al. (1989), and unpublished data). Vertical bars = 1 standard deviation from mean; horizontal bars = age uncertainty. Solid line = maximum likelihood estimate as described by Switzer et al. (1988) for solid points only; dotted line extends to dashed points excluded from line-fit. Outer dashed lines in d = range of uncertainty of line-tits determined by 100 iterative lines.
ble 2). For example, soils at Silver Lake appear to develop as linear functions over the 10,000-yr age span, but the Pleistocene point falls off the Holocene curve if the older age is used. Pleistocene soils at Kyle Canyon also appear to develop as linear functions, but the Holocene point requires a steeper curve toward the intercept (Fig. 3). The data at Mercury appear roughly linear for all soils between 10,000 and about 1.5 million yr old, with the possible exception of the index plot (Fig. 4). Age uncertainty allows alternate interpretations for the time curves, including very poor corre-
lation of soil with age, but the contrast between Holocene and Pleistocene curves is notable from best-estimate ages in Figures 2-4. For further comparison and exploration of the data, linear rates are calculated for the Holocene and separately for the Pleistocene by selecting either Holocene or Pleistocene soils within each study area (Table 2). In general, average rates of most soil-development parameters are precise to about a factor of 2 (e.g., in Table 2, a slope of 0.99 ? 0.46). Notably large uncertainties in slopes or rates include total texture at
GREAT 200H -2 5 3 2 8 loo2 u 9 i.E
BASIN
SOIL
391
CHRONOSEQUENCES
A
80007
B
h f7 3
c
-
(1) O-l 0
500 Age (103yrB.P.)
f 1000
500 Age (10'y-rB.P.)
100-c
a’ D 5 -3
H5 50-
,’
/’ A’
/’
.
a 2
Ott) 0
500
t 1000
Age (103yrB.P.)
0
(1) 0
500
1 1000
Age (10'yrB.P.)
FIG. 3. Four measures of soil development versus time for soils at Kyle Canyon. As described for Figure 2, but maximum likelihood line for Pleistocene points only. Holocene unit in parentheses (also drier climate and lowest elevation). Profile sum of CaCO, using volumetric percent CaCO, from Sowers et al. (1988) and correcting for bulk density from unpublished data.
Kyle and Mercury. The uncertainty in slope or rate appears to be due largely to calibration dates, which also are uncertain by a factor of about 2 (Table 1). The curve fit, however, must also contribute significantly to the slope error, because in some cases, the slopes have a smaller percentage error than the calibration dates. Rates of pedogenic accumulation of clay and carbonate are very sensitive to parent material assignments, as can be seen in the large variation in intercepts (e.g., in Table 2, intercepts of 47, 22, and 9 g-cm clay at Silver Lake); the rates, however, using the various calculations, vary by a factor of about 2x or less within a study area (e.g., 46 to 106 g-cm clay per millennium at Silver Lake).
Temporal and Geographical in Rates
Variation
The most significant result of this study is that despite the uncertainties in soil measurements and calibration ages, Holocene rates of soil development at Silver Lake are at least an order of magnitude higher than Pleistocene rates at Kyle and Fortymile Wash, and the latter two are not significantly different (Table 2; Figs. 3d and 4d). With the exception of some carbonate calculations, the diverse measures of index, profile clay, and profile carbonate still show that Holocene rates at Silver Lake are significantly higher than Pleistocene rates at Kyle and Mercury. For example, index rates at Silver Lake are 0.76 to 1.6; at Kyle
392
HARDEN
ET
AL.
t 760 Age(10’yrB.P.)
Age (10’yrB.P.)
Age (103yrB.P.)
Age(103yrB.P.)
1500
750
FIG. 4. Four measures of soil development versus time for soils at Fortymile Wash, Nevada. As described in Figure 2 with maximum likelihood line for all data, using data of Taylor (1986).
and Fortymile Wash, rates are 0.02 to 0.06 for properties other than carbonate. The high rates at Silver Lake probably are a result of the Holocene age of the soils rather than proximity to the playa, based on several interpretations. First, regional rates of dust flux today vary by only a factor of 2; rates range from 4.0 to 9.3 g clay/m’/yr and 1.4 to 3.3 g carbonate/m’/yr among the study areas (4 yr of dust collection by M. C. Reheis, unpublished data). Second, Holocene soils in the other field areas are very similar in development to Holocene soils at Silver Lake. Holocene soils in all of the areas are characterized by infilling of pores by fine-grained dust, development of structure, increased hardness in the upper profile, carbonate redistribution to depths of less than about a meter, and carbonate morphologies of stage I or II in a given horizon.
Third, when Silver Lake soils and ages are used as calibration for Holocene soils in the other three areas, the predictive ages agree reasonably with independent radiometric ages (Table 3, using methods of Switzer et al. (1988) described above), The similarity of Pleistocene rates at Kyle Canyon and Fortymile Wash is more tenuous than for Holocene rates, because dates are experimental. The soils are markedly different, with carbonate and siliceous morphologies, respectively. If, however, rates of comparative properties and indices vary by a factor of 4 as suggested in Table 2, then geographical variation in rates of soil development among these study areas are much less significant than temporal variation in rates between the Holocene and Pleistocene. In other words, data from (1) the exercise of age estimates for Ho-
GREAT
BASIN
SOIL
heis (1987a) from Wyoming show signiticant differences between Holocene and middle Pleistocene rates. Machette (1985) emphasized that rates of carbonate flux vary significantly throughout the western U.S., at least for Pleistocene soils. A future study should identify boundaries of regions in which dust or carbonate fluxes are generally similar. In turn, we should ask whether these boundaries changed in the past and whether the fluxes varied over time, especially in response to major glacial or pluvial periods.
60
0
Age W 60
6
Holocene and Pleistocene Soil Development
0
500 Age (103yr
393
CHRONOSEQUENCES
1000 B.P.)
1500
FIG. 5. Soil development index (Harden, 1982; see text for modifications) versus time for soils of three study areas. A = log time showing S, Silver Lake; M, Fortymile Wash; K, Kyle Canyon. B = linear time.
locene soils, (2) the comparison of Pleistocene rates between Fortymile Wash and Kyle Canyon (qualifying that the calibration ages have considerable uncertainties), (3) dust collection suggests that geographical variation in rates appears to vary by a factor of 2 or 3, whereas temporal variation in rates between the Holocene and Pleistocene is on the order of 10 or more. DISCUSSION Machette (1985) estimated that temporal variations of carbonate flux varied only by about a factor of about 2 between Holocene and middle Pleistocene soils within the Las Cruces, New Mexico basin, although data of Harden et al. (1985) from Utah and Re-
Rates of
The scope of this study only permits us to speculate as to why Holocene rates of soil development at Silver Lake are significantly higher than Pleistocene rates in the other areas. Several factors, however, should be discussed. First, many studies in moist regions have reported exponential rates of weathering over long time-spans (for example, Colman, 1981; Harden, 1988) and have attributed such trends to kinetics of mineral weathering. Two arguments against the importance of this factor in the southern Great Basin are (a) dissolution kinetics may not be exponential (Holdren and Bemer, 1979), and (b) Holocene soils in these study areas show little evidence for mineral weathering other than carbonate dissolution and iron oxide formation (Taylor, 1986; Reheis et al., in press; Reheis et al., 1989). In addition, young soils of this study are very porous and do not hold much water to allow primary mineral dissolution. Second, soil processes that dominate pedogenesis may change, causing an apparent decline in rate of overall development. Most of the clay and carbonate that build up rapidly in coarse-grained Holocene soils are probably eohan or atmospheric (wet and dry) in origin (Machette, 1985; McFadden, 1988; McFadden and Tinsley, 1985; Reheis et al., 1989), but accumulation may
AND INTERCEPTS
WITH
0.76 0.23 0.52 0.30
0.022 0.018 24.29 8.74
0.04 0.021 10.74 4.17
Slope per lo3 yr SD Intercept SD
Slope per l@ yr SD Intercept SD
Total texture
0.061 0.02 9.85 2.73
3.82 0.034 12.56 18.64
0.99 0.46 3.74 1.73
Field carbonate
0.019 0.013 21.71 7.33
0.022 0.0089 32.59 4.68
I.59 0.27 1.99 0.52
Index all properties
Profile minimum
2.42 0.76 5.85 87
yr) -
0.0031 0.004 0.6 0.6
0.207 0.0062 8.67 1.58
0.15 0.03 2.48 0.11
USING
0.85 0.53 81.8 63.6
7 -
40.36 20.9 161 21
=O%
% CaCO,
DATA
0.57 0.54 62.57 53.9
-
14.36 3.9 3.1 5.52
Maximum weighted mean
used for parent
Profile sum of carbonate intercepts in g-cm slopes in g-cm/lo’ yr
VERSUS TIME,
Weighted mean percent <2 mm
PARAMETERS
lower fans age range 200 to 11,000 yr (no 20,OOtl yr) 0.49 105.94 79.17 45.73 0.08 47 22 9 - 36.84 -3.62 0.74 - 113.78 0.15 256.3 49.76 19.66
=O%
Maximum weighted mean
% clay used for parent material
Kyle Canyon age range 15,000 to 800,000 yr (no 5000 0.019 0.009 24.75 4.81 Fortymile Wash age range 10,000 to 1,000,000 yr 0.041 0.0073 2.38 0.99 0.88 0.58 0.008 0.007 10.24 4.54 5.44 171 2.67 1.52 156 88 ___.
Silver Lake 1.51 0.38 1.68 0.85
Index no carbonate
OF SOIL DEVELOPMENT OF SWITZER ET AL., 1988
Profile sum of <2 pm clay intercepts in g-cm slopes in g-cm/IO’ yr
DEVIATIONS (SD) 1 AND METHODS
Weighted mean percent <2 mm
STANDARD TABLE
Soil development index intercepts in units of profile sum slopes in units of profile sum/103 yr
2. SLOPES
Slope per 10’ yr SD Intercept SD
TABLE
1.09 0.41 40. I5 100.00
5.13 0.95 12.25 3.47
22.67 6.9 - 13.55 10.6
Profile minimum
material
FROM
395
GREAT BASIN SOIL CHRONOSEQUENCES TABLE
3. PREDICTIVE
INDICES
FOR HOLOCENE
Surlicial unit Cima-QD Cima-Qf4 Cima-QfS Cima-Qf6 Cima-Qf7 Cima-QtIl FM Wash-lb FM Wash-lc Kyle-lower fan 3
AGES OF GEOMORPHIC SURFACES, BASED ON CALIBRATION OF SOIL SOILS AT SILVER LAKE (SOIL DEVELOPMENT INDEX OF HARDEN MODIFICATIONS DESCRIBED IN TEXT)
Equivalent fan surface at Silver Lake, based on soil age
Maximum likelihood estimate age,Y (10’ yr)
Error term0 at 95% level, (lo3 yr)
(37) (17)
(13) (3)
(35)
Qf 1
(18)
Qt?!-Qf 1
11 4 4 2
2 1 3 1 (7) 1
50 18 25 150 30 (30) 10
$-Qf4 QD-Qf4 QfS-Qf4 QD-Qf4
(2:) 10
Precision m
QQ-
DEVELOPMENT (1982) WITH
Independent age@ from Table 1 (IO3 yd 43-48 VCR: < 130KAB
1 l-14 VCR V14C 1.16 I?I 0.90’VL4C 20.15?? 310? 5-15
0 Switzer et al., 1988; parentheses used for pre-Holocene estimates, indicating ages >lO,OOO yr. b VCR and Vr4C varnish collection sites approximately located near soil pits.
reach a maximum once large pores are filled with fines. In addition, as the fines build up in the soil, water-holding capacity increases (Harden, 1988), which likely enhances in situ mineral transformations such as primary mineral weathering (Reheis et al., in press), formation of silicate clays (Taylor, 1986; Reheis et al., in press), and iron oxidation (Taylor, 1986; Reheis et al., in press). Concomitant with increased water retention, permeability eventually declines, causing less infiltration of protonated water, more runoff, and possibly more erosion. Such changes in pedogenic processes were discussed by McFadden and Weldon (1986) for soils in the dust-rich Cajon Pass area in California. Third, the importance of past climates and climatic change to rates of soil development is irrefutable (McFadden, 1988). Eolian dust may have accumulated at dramatically different rates during the Holocene and Pleistocene (Machette, 1985) both as a result of changes in source (e.g., lakes vs playas) and effective wetting (Spaulding, 1985; Quade er al., 1989) (see discussion below). In addition, the depth to which carbonate accumulated was greater in the Pleistocene than in the Holocene (McFadden and Tinsley, 1985; Mayer et al., 1988), an attribute that affects retention
of eolian carbonate and other pedogenic attributes. Fourth, the role of erosion in soil development is critical, although difficult to quantify in this type of study. For example, slow continual erosion (e.g., sheet erosion) might maintain the apparent flatness of a geomorphic surface, yet allow considerable amounts of material to erode. If erosion were systematically greater for each older surface, then a systematic progression of soil development with time would be found, but the rate of soil development would decrease with time. Alternatively, if erosion were quite variable from unit to unit, we would not observe systematic trends in soil development with time. Exponential and Step-Functions Age Curves
for
Although curve shapes over certain time spans generally might average out to linear functions (e.g., over the Holocene or Pleistocene), alternative models include exponential and step functions. Over a million years or more, the exponential function may best describe the decrease in rates over time (Fig. 6), as has been found by many studies of soils in moister regions (e.g., Harden, 1986; Dethier, 1988; and Reheis, 1987a,b).
396
HARDEN
01
ET
AL.
I I I I 6 9 12 15 Age (lo3 yr B.P.) FIG. 6. Maximum likelihood estimate (MLE) lines of index versus age. Inset = vertical area of mean and standard deviation of soil index represented by replicate soils on dated surface; horizontal area of age constraints over best-estimate age. Dotted line is MLE from mean index and best-estimate age, represented in inset where horizontal and vertical lines meet. Ten solid lines = MLE lines iterated from 10 data sets in Monte Carlo fashion from area inside shapes in insert. 0
I 3
Curve shapes of Silver Lake data over much shorter periods allude to a third alternative, the step-function model. Although better age control (more precise ages and more numerous units) would better resolve the curves, clay, and carbonate curves for Silver Lake (Fig. 7) are somewhat step like. High rates of increase are suggested at about 11,000 to 6,000 yr B.P. (using bestestimate ages in Table 1) and from about 2500 yr B.P. to the present (Fig. 7a). Given that much of the clay and carbonate are derived from eolian dust, these intervals of time could represent times of high dust flux, with rates about 10 times those before or after that period (Fig. 7a,b). This concept of dust “events” recently has received attention by Machette (1985), Chadwick and Davis (1990), and Reheis et al. (1989). As Chadwick and Davis emphasized, this concept may explain soil-
forming intervals proposed by Morrison (1978), whose studies were made in an area with significant dust sources. A most succinct perspective of the concept was presented by Machette (1985), who demonstrated that over long time spans, dust flux could be viewed as very rapid, nearly instantaneous events that are averaged to long-term rates. Because dust and carbonate accumulation is so complexly related to innumerable variables, however, we think many studies on arid soils should be conducted before soils can be linked directly to dust events. For example, an important alternative to high rates of dust flux invokes plugging of the Qfl soil before 8000 to 12,000 yr B.P. The close similarity between carbonate sums on Qfl and Qf2 suggests such a possibility. According to the model of McFadden and Weldon (1987), in which water-holding capacity reaches an intrinsic
GREAT
0
10
20
BASIN
SOIL
30
data. Adequate records would probably show dust-flux cycles on scales of decades correlated to drought years or of seasons correlated to dry seasons. Longer-term processes also occur throughout these dustflux cycles, such as chemical and mineralogical transformations. Soil processes such as iron oxidation, clay formation, or mineral dissolution may be intrinsically linked to dust as a source of relatively fresh material, but they probably occur more steadily over time than dust flux. Implications
I 10
20
I 30
AgeU03yrB.P.)
7. Profile carbonate and ~2 urn clay versus time for soils at Silver Lake. Profile sums of percentage of <2 mm times thickness and bulk density. For Silver Lake, bulk density was assumed to be between 1.6 and 2.0 g/cc; for each age group, minima and maxima of g-cm data (mean ? 1 SD from Table 1) were multiplied by 1.6 or 2, and the range was used for units of g/cc. Lines = step function if line drawn from mean values; straight function if maximum likelihood line (Switzer er al., 1988) drawn from all means. FIG.
threshold for chemical alteration, if the Qfl soil had been plugged with dust for very long, there might also be evidence for significant chemical and mineralogical transformations not seen in Qf2 soils. The appropriateness of the curve shape-whether step, separate linear, or exponential-may vary according to the time span being considered and the processes being quantified. Short-term processes related to dust flux may have variable rates on short time scales, for example a millennia as suggested by the Silver Lake
397
CHRONOSEQUENCES
for Correlation
and Dating
For the Holocene soils, the implications for correlation and approximate dating using soils is that calibration in one area might be appropriate for other sites within the general region and might be accurate within about a factor of 2 or 3 (Table 3); precision of the predictive ages averages about 40% (error of 2 standard deviations divided by best estimate age), as compared to about 18% error associated with Holocene radiometric ages (maximum minus minimum ages divided by best-estimate ages, divided by 2 for 2 standard deviation error). Using soils to estimate ages approximately doubles the age uncertainty associated with the t4C ages used by Wells et al. (1987). For Pleistocene soils, uncertainties in calibration ages and rates of soil development are considerable, and more work is needed before we can reliably correlate among study areas. If we can establish the causes of declining rates with time and if the causes are similar throughout the region with its diverse rock types, vegetation, dust fluxes, and climates, then semilog or exponential curves might conceivably serve as a model for long-term soil development. CONCLUSIONS Rates of soil development over the Holocene in four study areas in the southern Great Basin appear to vary by about a factor of 2 or 3. Rates appear to be broadly similar within the Holocene, probably because influx of dust is the predominant pro-
398
HARDEN
cess of soil formation and rates of dust flux vary geographically by such amounts. Rates of soil development over the Pleistocene are not significantly different at Kyle Canyon and Fortymile Wash and are about 10 times lower than Holocene rates at Silver Lake. Thus, the geographical variation in rates within the region is much less significant then temporal variation in rates of soil development. Over long time spans such as the Quaternary, curve shapes are best fitted to exponential functions, with decreasing rates of soil development over time. Changes in rates of development are probably related to climatic change and related changes in water and dust, erosional history, and/or internally driven chemical processes. Holocene soils can probably be correlated and approximately dated throughout the southem Great Basin with anticipated uncertainties of about a factor of 2 or 3. Pleistocene soils in our study areas lack the age control needed to characterize and understand age trends reliably. ACKNOWLEDGMENTS Many thanks to Ron Dom and Brent Turrin for collaboration on varnish and KAr dating, respectively, and to Kathy Parrish for time and expertise in illustrations, UNM students and R. Kyle at CU for soil analyses, and P. Birkeland, D. Muhs, 0. Chadwick, and D. Miller for helpful discussions. Thanks also to NSFPYI for support for some of the varnish data.
REFERENCES Bierman, P. R., and Gillespie, A. (1989). Rock varnish, alluvial fans, and tectonism in the southern Owens Valley, California. Geological Society of America Abstracts 21(6), A343. Chadwick, 0. A., Hecker, S., and Fonesca, J. (1984). A soils chronosequence at Terrace Creek: Studies of late Quatemary tectonism in Dixie Valley, Nevada. U.S. Geological Survey Open-File Report, 84-90. Chadwick, 0. A., and Davis, J. 0. (1990). Soilforming intervals caused by eolian sediment pulses in the Lahontan Basin, Northwestern Nevada. Geology 18, 243-246. Colman, S. M. (1981). Rock-weathering rates as functions of time. Quaternary Research 15, 250-264. Dethier, D. P. (1988). The soil chronosequence along the Cowlitz River, Washington. U.S. Geological Survey Bulletin, 1590-F. Dom, R. I. (1984). Geomorphological interpretation of
ET AL.
varnish in the Mojave Desert. In “Surficial Geology of the Eastern Mojave Desert, California” (J. C. Dohrenwend, Ed.). Geological Society of America Annual Meeting Field Trip Guidebook 14, Reno. NV. Gile, L. H., Peterson, F. F., and Grossman, R. B. (1966). Morphological and genetic sequences of carbonate accumulation in desert soils. Soil Science rock
101, 347-360.
Harden, D. R., Biggar, N. E., and Gillam, M. L. (1985). Quatemary deposits and soils in and around Spanish Valley, Utah. In “Quaternary Soils and Geomorphology of the Southwestern United States” (D. L. Weide, Ed.), pp. 43-64. Geological Society of America Special Paper 203, Reno, NV. Harden, J. W. (1982). A quantitative index of soil development from field descriptions: Examples from a chronosequence in central California. Geoderma 28, l-28. Harden, J. W. (1986). Soils developed in granitic alluvium near Merced, California. U.S. Geological Survey Bulletin, 1590-A. Harden, J. W. (1988). Genetic interpretations of elemental and chemical differences in a soil chronosequence, California. Geoderma 43, 17%193. Harden, J. W., and Matti, J. C. (1989). Holocene and late Pleistocene slip rates on the San Andreas fault in Yucaipa, California, using displaced alluvial-fan deposits and soil chronology. Geological Society of America Bulletin 101, 1107-l 117. Harden, J. W., and Taylor, E. M. (1983). A quantitative comparison of soil development in four climatic regimes. Quaternary Research 20, 342-359. Harden, J. W., Taylor, E. M., McFadden, L. D., and Reheis, M. C. (1991). “Calcic, Gypsic, and Siliceous Soil Chronosequences in Arid and Semiarid Environments.” Soil Science Society of America Special Paper 26, Chapter 1. Holdren, G. R., and Bemer, R. A. (1979). Mechanisms of feldspar weathering I. Experimental studies. Geochemica et Cosmochemica Acta 43, 1161-l 171. Holhday, V. T. (1988). Genesis of a late-Holocene soil chronosequence at the Lubbock Lake archeological site. Annual Association of American Geographers 78, 549-610. Machette, M. N. (1985). Calcic soils of the southwestem United States. In “Soils and Quatemary Geomorphology of the Southwestern United States” (D. L. Weide, Ed.). Geological Society of America Special Paper 203. Mayer, Larry, McFadden, L. D., and Harden, J. W. (1988). The distribution of calcium carbonate in desert soils: A model. Geology 16 (4), 303-306. McFadden, L. D. (1988). Climatic influences on rates and processes of soil development in Quatemary deposits of southern California. Geological Society of America Special Paper 216, 153-177. McFadden, L. D., and Tinsley, J. C. (1985). Rate and
GREAT
BASIN
SOIL CHRONOSEQUENCES
depth of pedogenic-carbonate accumulation in soils: Formation and testing of a compartment model. In “Quaternary Soils and Geomorphology of the Southwestern United States” (D. L. Weide, Ed.), pp. 23-24. Geological Society of America Special Paper 203, Reno, NV. McFadden, L. D., and Weldon, R. J. (1986). Rates and processes of soil development on Quaternary terraces in Cajon Pass, southern California. Geological Society of America Bulletin 98, 280-293. McFadden, L. D., Wells, S. G., and Dohrenwend, J. C. (1986). Influences of Quaternary climatic changes on processes of soil development on desert loess deposits of the Cima volcanic field, California. Catena 13, 361-389. Morrison, R. B. (1978). Quatemary soil stratigraphyConcepts, methods and problems. In “Quatemary Soils,” (W. C. Mahaney, Ed.), pp. 77-108. GeoAbstracts, Norwich. Muhs, D. R., Bush, C. A., Stewart, K. C., Rowland, T. R., and Crittenden, R. C. (1990). Geochemical evidence of Saharan dust parent material for soils developed on Quatemary limestones of Caribbean and western Atlantic Islands. Quaternary Research 33, 157-177. Ore, H. T., and Warren, C. N. (1971). Late Pleistocene-early Holocene geomorphic history of Lake Mojave, California. Geological Society of America Bulletin 82, 2553-2562. Quade, Jay, Cerling, T. E., and Bowman, J. R. (1989). Systematic variations in the carbon and oxygen isotopic composition of pedogenic carbonate along elevation transects in the southern Great Basin, United States. Geological Society of America Bulletin 101, 464-475. Reheis, M. C. (1987a). Gypsic soils on the Kane alluvial fans, Big Horn County, Wyoming. U.S. Geological Survey Bulletin, 1590-C. Reheis, M. C. (1987b). Soils in granitic alluvium in humid and semi-arid climates along Rock Creek, Carbon County, Montana. U.S. Geological Survey Bulletin, 1590-D. Reheis, M. C., Harden, J. W., McFadden, L. D., and Shroba, R. R. (1989). Development rates of late Quatemary soils, Silver Lake Playa, California. Soil Science Society of America Journal 53, 1127-l 140. Reheis, M. C., Sowers, J. M., Taylor, E. M., McFadden, L. D., and Harden, J. W., (in press). Morphology and genesis of calcic soils on Kyle Canyon Fan, Spring Mountains, Nevada. Geoderma. Rosholt, J. N. (1985). U-trend systematics for dating
399
sediment. U.S. Geological Survey Open-File Report, 85-298. Sowers, J. M. (1985). “Pedogenic Calcretes of the Kyle Canyon Alluvial Fan, Southern Nevada: Morphology and Development.” Unpublished PhD thesis, University of California, Berkeley. Sowers, J. M. (1986). Geomorphic map of the Kyle Canyon alluvial fan, southern Nevada. U.S. Geological Survey Open-File Report, 86-210. Sowers, J. M., Amudson, R. G., Chadwick, 0. A., Harden, J. W., Jull, A. J. T., Ku, T. L., McFadden, L. D., Reheis, M. C., Robinson, S. W., Szabo, B. J., and Taylor, E. M. (1988). Geomorphology and Peology on the Kyle Canyon alluvial fan, southern Nevada. In “Geological Society of America Field Trip Guidebook” (D. L. Weide and M. L. Faber, Eds.), pp. 137-158. Cordilleran Section Meeting, Las Vegas, NV. Spaulding, W. G. (1985). Vegetation and climates of the last 45,000 years in the vicinity of the Nevada Test Site, south-central Neveda. U.S. Geological Survey Professional Paper 1329, 83. Switzer, P. S., Harden, J. W., and Mark, R. K. (1988). A statistical method for estimating rates of soil development and ages of geologic deposits: A design for soil-chronosequence studies. Mathematical Geology 20, 49-61. Taylor, E. M. (1986). “Impact of Time and Climate on Quatemary Soils in the Yucca Mountain Area of the Nevada Test Site.” Unpublished manuscript, University of Colorado, Boulder, CO. Taylor, E. M. (1988). Instructions for the soil development index template-Lotus l-2-3. U.S. Geological Survey Open-File Report, 233A. Turrin, B. D., and Champion, D. E. (1991). 40Ar/39Ar Laser Fusion and K-Ar ages from the Quatemary Basaltic Volcanic Centers at Crater Flat, Nevada and Cima, California: Implications for the Age of the Youngest Volcanic Activity in the Yucca Mountain Area: Proceedings of the International High-Level Nuclear Waste Management Conference, Las Vegas, Nevada, (American Society of Civil Engineers and American Nuclear Society). Turrin, B. D., Dohrenwend, J. C., Drake, R. E., and Curtis, G. H. (1985). K-Ar ages from the Cima volcanic field, eastern Mojave desert, California. Isoch.lWest 44, 9-16. Wells, S. G., McFadden, L. D., and Dohrenwend, J. C. (1987). Influences of late Quatemary climatic changes on geomorphic and pedogenic processes on a desert peidmont, eastern Mojave Desert, California. Quaternary Research 27, 130-146.