Application of OSL dating to middle to late Holocene arroyo sediments in Kanab Creek, southern Utah, USA

Quaternary Geochronology 10 (2012) 167e174

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Research paper

Application of OSL dating to middle to late Holocene arroyo sediments in Kanab Creek, southern Utah, USA Michelle C. Summa-Nelson a, b, *, Tammy M. Rittenour a, b a b

Utah State University Luminescence Lab, 1770 N. Research Pkwy, Suite 123, North Logan, UT 84341, USA Utah State University, Department of Geology, 4505 Old Main Hill, Logan, UT 84322, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 October 2011 Received in revised form 1 May 2012 Accepted 7 May 2012 Available online 15 May 2012

Middle to late Holocene alluvium, identified as Quaternary alluvial unit 4 (Qa4), along Kanab Creek in southern Utah, USA was dated using optically stimulated luminescence (OSL) on quartz sand, and by radiocarbon dating of detrital charcoal. Entrenchment beginning in 1882 AD created arroyo walls that expose up to 35 m of the Qa4 alluvium. The stratigraphy and sedimentology suggest that fluvial aggradation along the study reach occurred rapidly. Due to the high sediment supply, short transport distances and semi-arid climate with flashy discharge, partial bleaching (zeroing) of the luminescence signal was expected to be a problem for OSL dating. We approached this problem by first using smallaliquot (w20 grains) and single-grain dating of quartz sand to reduce the number of grains contributing to the OSL signal. Second, we used statistical parameters based on single-grain and small-aliquot equivalent dose (De) distributions of bleached sediment to help identify partial bleaching and to inform if a minimum age model (MAM) should be used for age calculation. Comparison of results with radiocarbon ages demonstrates the success of OSL dating on Kanab Creek arroyo-fill deposits, although careful attention should be paid to the sedimentary facies and stratigraphy of the targeted sample horizon to minimize the effects of partial bleaching. Thin, decimeter-scale plane-bedded and ripple cross-bedded sandy lithofacies were found to be the best target for OSL dating, as these sediments showed minimal evidence for incomplete solar resetting. Additionally, results generally indicate that better-bleached sediments are found in downstream reaches. Age control from these arroyo-fill deposits was acquired in order to fulfill larger research goals of understanding regional arroyo incision and aggradation cycles. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: OSL, optically stimulated luminescence dating Partial bleaching Single-grain dating Arroyo Kanab Creek

1. Introduction Arroyos are entrenched channels with vertical walls and flat bottoms incised into fine-grained alluvium. These and other semiarid and arid fluvial systems are commonly subject to highmagnitude stream flow events with high sediment loads (Bull, 1991). These fluvial conditions can lead to incomplete zeroing of the luminescence signal of the sediment due to turbid water conditions and rapid burial (e.g. Porat et al., 2001). Application of optically stimulated luminescence (OSL) dating to arroyo deposits in southeastern Colorado was previously addressed by Arnold et al. (2007) and Bailey and Arnold (2006). They conclude that the large overdispersion in equivalent dose (De) distributions was due to

* Corresponding author. Utah State University, Department of Geology, 4505 Old Main Hill, Logan, UT 84322, USA. Tel.: þ1 435 797 9242; fax: þ1 435 797 1588. E-mail addresses: [email protected] (M.C. Summa-Nelson), [email protected] (T.M. Rittenour). 1871-1014/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2012.05.002

a diverse array of bleaching conditions in the arroyo environment (Arnold et al., 2007); we expect a similar result for Kanab Creek, an arroyo system in southern Utah, USA. Semi-arid fluvial systems containing arroyos are sensitive to disturbances within the watershed and can provide records of past hydroclimatic change. Historical accounts document rapid entrenchment of river systems in the southwestern US during the late 1800s to early 1900s following a series of large floods, and prehistoric arroyo cut-fill events are seen in the exposed arroyo walls (e.g. Hereford, 2002; Webb et al., 1991; Cooke and Reeves, 1976). Although previous work on arroyo systems suggests that changes in regional climate and hydrology are important factors in arroyo incision and aggradation (e.g. Hereford, 2002; Cooke and Reeves, 1976), there is no clear consensus on what drives arroyo cutting and filling events. Study of arroyo systems may provide insight into arid land fluvial adjustment to future climate change, given their sensitivity to minor hydrologic changes in the recent past. Research goals are to 1) determine the levels of partial bleaching in arroyo-fill sediments from Kanab Creek in southern

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Utah, 2) identify sedimentary facies best suited for OSL dating in this semi-arid environment and 3) to use a combination of OSL and radiocarbon dating to constrain the timing of aggradation of Quaternary alluvium 4 (Qa4), the most prominent alluvial fill in the study area and one of the thickest arroyo-fill deposits in the region. 2. Background Kanab Creek is a mixed alluvial and bedrock stream that flows from the high bedrock plateaus of southern Utah to the Grand Canyon in northern Arizona, USA (watershed area: 6013 km2). Its w200 km long catchment trends northesouth and drains part of the Grand Staircase province of the Colorado Plateau (Fig. S1). In the study area, Kanab Creek occupies a 30e35 m deep mature arroyo incised into fine-grained alluvial terraces in upstream reaches and pre-1880s floodplain sediments downstream. The decade-long period of entrenchment that formed this w25 km long arroyo was the result of a series of large-magnitude floods associated with heavy precipitation events and rapid snowmelt (see review by Webb et al., 1991). Kanab Creek abruptly transitioned from a shallow, perennial stream to a deeply incised ephemeral arroyo channel. The timing and nature of this incision is well documented by historic observations, however, the record of paleo-arroyo aggradation and entrenchment events has not been studied in detail. Our geomorphic and stratigraphic investigations indicate that the canyon reach, upstream of the broad basin-fill reach, is dominated by one alluvial package (Summa, 2009) (see Fig. S1 for locations of geomorphic reaches). This 30e35 m thick terrace fill is composed of Quaternary alluvial unit 4 (Qa4) and represents a fairly continuous period of aggradation with high sediment supply (Summa, 2009). At one key outcrop exposure (location 4 on Fig. S1; Fig. 1), four radiocarbon samples from previous research suggest that the age of the Qa4 alluvium is 5.4e3.5 cal kyr BP2010 (Smith, 1990) (Table 2). We revisited this site along with new locations along Kanab Creek to describe the sedimentology and stratigraphy, resample for radiocarbon dating, and to test the application of OSL dating on quartz sand in this semi-arid, high sediment supply environment where the deposits are dominated by near-channel overbank flood packages. 2.1. Description of Holocene alluvial stratigraphy The focus of this research was to constrain the timing of aggradation of the Qa4 fill along a 16 km reach of Kanab Creek upstream of the town of Kanab, UT (Fig. S1). This study area covers w8% of the entire length of the river, where Kanab Creek flows in

*

Salt Lake City

C dating

Semi-arid to arid drainages commonly experience flashy flows with high sediment loads, which reduces the potential for resetting of the luminescence signal during transport and can cause OSL age overestimation (e.g. Jain et al., 2004; Wallinga, 2002; Porat et al., 2001; Olley et al., 1999). The sedimentology and stratigraphy of Qa4 alluvium suggests sediment deposition under conditions of high sediment load and rapid aggradation. Additionally, sediment sources in Kanab Creek are derived from easily eroded sedimentary

Quaternary alluvium 2 (Qa2)

Quaternary alluvium 3 (Qa3)

Kanab Creek

Quaternary alluvium 4 (Qa4)

flow

Qat4

1565 Elevation in meters

14

Radiocarbon sample, cal yr BP2010 OSL sample

Quaternary alluvium 1 (Qa1)

Kanab Creek

~1880 AD incision

Qat2

1555 c-12, 380-590

c-10, 1340-1360

c-13, 380-720 Qat1 c-14, 580-740

1535

covered by slump and vegetation 1525

2.2. Potential problems with OSL and

N

UTAH

1545

a narrow (0.5 km wide) canyon cut into the Jurassic Navajo Sandstone bedrock before opening into a broader (3 þ km wide) basinfill reach. Four geomorphic terrace surfaces have been mapped in the study area: Quaternary alluvial terrace 4 (Qat4) is the highest off the modern channel (17e35 m). The three lower terraces, Qat3 (7e24 m), Qat2 (6e18 m), and Qat1 (3e5 m) (Fig. 1) will not be discussed in detail here (see Summa, 2009 for details). Due to the complexity of geomorphic terraces, map units represent terrace surfaces and do not always coincide with the alluvial fills underlying them. Where the stratigraphy can be seen, the Qa4 alluvium commonly underlies both the Qat4 and Qat3 terrace surfaces. Qat3 is a fill-cut, or fill-strath, terrace underlain by Qa4 alluvium, and in other instances it is a cut-fill terrace, underlain by Qa3 alluvium. We will refer to all cut-fill terraces as fill terraces hereafter. At location 4 (Fig. 1) the Qat3 and Qat2 geomorphic surfaces are nearly the same level above the modern channel and therefore cannot be identified based on geomorphic height alone. Here, the Qat2 geomorphic surface is a fill terrace as identified by Qa2 alluvium underlying this surface. The Qa4 alluvium is composed of decimeter to meter-thick tabular to broadly lenticular fine-grained, massive and fining upward silty sand and variegated clay beds (see Table S1 and Fig. S2 for details on facies). Common sandy facies within the Qa4 alluvium that were sampled for OSL dating include plane bed (Sh), low-angle (Sl), trough (St) and ripple (Sr) crossbeds, and massive sandy units (Sm); lithofacies nomenclature follow Miall (2000). Interbedded silt and clay facies (Fsm and Fsmv) are also present within the stratigraphy. While coarser-grained gravelly facies are present in other Kanab Creek alluvial assemblages, they are rare in the Qa4 alluvium. The dominantly fine-grained alluvial facies in the Qa4 fill represent overbank and near-channel deposition with intervals of backwater floodplain sedimentation. Multiple weak to moderately developed buried soils are seen within the stratigraphy representing short periods of reduced deposition and fluvial stability.

c-1, 3470-3760

Qat3

c-11, 1440-1580

c-3, 3770-4040

USU-364sg 6.61±0.73ka c-2, 3700-3900

c-6, 4920-5380 c-5, 4770-5360 USU-363sg 6.05±2.24ka

Fig. 1. Cutbank stratigraphy of the Qa4 fill deposit with inset Qa3, Qa2, and Qa1 deposits at site 4 (Fig. S1) Kanab Creek southern Utah, outcrop is north-facing. Based on OSL and radiocarbon results, the age of the Qa4 fill at this location is w6.1e3.5 ka. This stratigraphy represents very rapid aggradation, up to 35 m, during this time. Note that at this site and many others in the canyon reach, the geomorphic terraces are complex. The Qa4 alluvium commonly underlies both the Qat4 and Qat3 terrace surfaces. Therefore, Qat3 is both a fillcut terrace underlain by Qa4 alluvium and a cut-fill terrace underlain by Qa3 alluvium.

M.C. Summa-Nelson, T.M. Rittenour / Quaternary Geochronology 10 (2012) 167e174

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Table 1 Partial bleaching statistics and OSL age information. USU Lab number

Location Depth ODa (%) (Fig. S1) (below Qat4 surface, m)

USU-263 USU-285 USU-286 USU-290 USU-292 USU-292sg USU-359 USU-360 USU-360sg USU-363 USU-363sg USU-364 USU-364sg USU-423 USU-423sg USU-446 USU-446sg USU-520 USU-520sg

8 1 1 6 3 3 2 1 1 4 4 4 4 7 7 1 1 3 3

5 w14 w13 2.6 4.2 4.2 2.2 w16 w16 30 30 3.5 3.5 15 15 15 15 1.3 1.3

24.2 54.6 44.2 33.4 59.4 87.0 36.0 55.6 55.2 34.4 34.8 27.6 18.3 18.8 25.4 30.5 17.7 17.9 40.3

                  

Faciesb; bed Num. aliquots/ Dose rate thickness (m) grainsc (Gy/ka)

Skew

4.1e 0.76e 1.07e 10.1e 7.2e 0.32 5.0e 0.29 0.47 10.4e e 1.15e 11.9 e 0.21 6.6 0.46 9.7e 0.62 8.9e 4.4e 0.07 0.70e 4.5e 4.3e 1.04e 5.4 0.74 0.35 3.7e 1.29e 5.4e 1.37e 6.2e 6.0 0.23 5.0e 0.22 1.41e 5.9e

Sm; 0.4 Sm; 0.4 Sl; 0.6 Sm; 1.4 Sl; 0.5 Sl; 0.5 Sl; 1.0 Sh; 0.5 Sh; 0.5 St; 0.7 St; 0.7 Sl; 0.6 Sl; 0.6 Sr; 0.6 Sr; 0.6 Sh; 0.2 Sh; 0.2 Sl; 0.4 Sl; 0.4

26 21 23 28 23 74 21 20 66 38 133 34 92 34 82 20 64 18 92

(53) (43) (43) (61) (55) (2400) (80) (70) (3300) (81) (3000) (61) (2200) (61) (1700) (35) (1200) (34) (2300)

1.51 0.68 1.11 1.10 0.93 0.93 1.10 0.89 0.89 0.70 0.70 1.03 1.03 1.55 1.55 1.21 1.21 1.10 1.10

                  

0.07 0.04 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.04 0.04 0.05 0.05 0.07 0.07 0.06 0.06 0.05 0.05

Equivalent dosed, CAM aged (ka) MAM aged (ka) CAM/MAM De (Gy) 8.53 3.44 4.23 5.50 e 3.54 6.07 e 5.98 e 4.24 e 6.78 e 7.41 e 6.15 e 4.41

   

0.74 0.28 0.83 0.57

8.17 9.15 9.83 8.84 8.44 9.30 8.95 11.91 16.69 13.74 9.75 8.09 6.61 4.88 5.62 6.54 5.08 4.79 6.24

 1.40  1.63  2.90  1.54  0.59  0.83  0.62  1.07

                  

1.02 2.53 2.02 1.33 2.37 2.72 1.65 3.23 3.58 1.93 1.17 1.04 0.73 0.54 0.68 1.11 0.61 0.65 0.88

5.64 5.03 3.83 4.98 3.44 3.82 5.52 3.71 6.70 6.70 6.05 5.51 6.01 3.79 4.78 4.97 3.05 3.69 4.02

                  

0.61 0.55 0.80 0.62 1.08 1.53 1.53 0.94 3.28 1.56 2.24 0.56 0.65 0.55 0.63 0.65 0.95 0.88 1.01

1.45 1.82 2.57 1.78 2.45 2.43 1.62 3.21 2.50 2.05 1.61 1.47 1.10 1.29 1.18 1.32 1.67 1.30 1.55

a

Overdispersion (OD) calculated as part of central age model (CAM) (Galbraith et al., 1999). b See Table S1 for facies information. Facies codes are based on grain size (capital letter) and sedimentary structure (lower case letter), following Miall (2000) nomenclature. c Number of aliquots/grains used in age calculation and total number of aliquots/grains analyzed in parentheses. d De and ages calculated using either the CAM or minimum age model, MAM-3 for small-aliquot (SA) and MAM-4 for single-grain (SG) of Galbraith et al. (1999), bold age indicates model used. Error on De and age is 2-sigma standard error. e Critical skew and/or OD for partial bleaching, MAM used to calculate De. Critical skew based on standard error of the skew (Bailey and Arnold, 2006) at 1-sigma level for SA and 2-sigma for SG. Critical OD values are >15% for SA and >25% SG De distributions.

bedrock, local hillslopes, short-system tributaries, and sediment collapse from arroyo walls and cut banks, further limiting solar resetting. These proximal sediment-source conditions likely contribute a large portion of partially bleached grains to Kanab Creek alluvium. Radiocarbon ages from detrital charcoal can provide independent age control to test how well OSL dating works in this setting. However, similar to partial bleaching problems with OSL dating, radiocarbon ages can produce age overestimates if reworked organic material is sampled (Gillespie et al., 1992). In a semi-arid fluvial system like Kanab Creek, erosion of older alluvium containing charcoal and redeposition within the river channel is likely

due to undercutting and collapse of vertical arroyo walls. Age overestimation is also possible if charcoal from dead or ancient trees on the landscape is transported into the fluvial system (Baker, 1987). These problems will be amplified for bulk radiocarbon samples due to the large volume of material needed for analysis; we therefore regard some of the results from Smith (1990) as maximum ages. Our sampling has focused on collection of small angular charcoal fragments for AMS radiocarbon dating. Alternatively, both OSL and radiocarbon dating can produce age underestimates due to bioturbation and mixing of younger sand grains and charcoal into older deposits. This was avoided here through careful sample selection, observance of original sediment bedding,

Table 2 Radiocarbon sample information. Location (see Fig. S1)

Sample num. (this paper)

Lab IDa

Depth (m)

14

Qa4 4 4 4 5 4 4 7 7 7

c-1 c-2 c-3 c-4 c-5 c-6 c-7 c-8 c-9

e Beta-256844 e Beta-256845 e e e e e

w4 4.5 w4.5 5 w25 w20 w20 w27 w25

3320 3460 3560 3690 4360 4460 4460 4980 5345

Qa3 4 4

c-10 c-11

UCIAMS-105786 UCIAMS-105785

Qa2 4 4 4

c-12 c-13 c-14

Beta-256843 e e

1.4 10 8 w10 w12

Calibrated ageb (cal yr BP2010)

C age

        

60 40 50 40 90 90 390 85 90

1360  15 1545  15 420  40 485  95 625  70

3470e3760 3700e3900 3770e4040 3970e4210 4770e5360 4920e5380 4070e6230 5650e5970 6000e6350 1340e1360 1440e1580 380e590 380e720 580e740

a Beta-# refers to samples analyzed at Beta Analytic, Inc. (Summa, 2009), UCIAMS-# refers to samples analyzed at Keck Carbon Cycle AMS facility, Earth System Science Dept. Univ. of California Irvine, all others are from Smith (1990). b Using IntCal09 calibration curve (Reimer et al., 2009) and corrected to report ages in BP2010 by adding 60 years to calibrated age range. Calibrated ages are rounded to the nearest decade.

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and by not sampling soils or within the upper meter of the landform. 3. Methods Samples for OSL and AMS radiocarbon dating were collected from Qa4 alluvium underlying the Qat4 and Qat3 terraces and from basin-fill deposits downstream. Twelve OSL samples were collected and analyzed at the Utah State University Luminescence Laboratory for small-aliquot and single-grain single-aliquot regenerative dose (SAR) analysis of quartz sand (Murray and Wintle, 2000). Fourteen radiocarbon ages were compiled from the field area; nine older radiocarbon ages from Smith (1990), three AMS radiocarbon ages from Summa (2009), and two new AMS ages. Nine of the radiocarbon samples are from the Qa4 deposit, while two from the Qa3 alluvium and three from the Qa2 alluvium are presented for comparative purposes. Radiocarbon ages were calibrated to years before 2010 using the IntCal09 calibration curve (Reimer et al., 2009) and addition of 60 years to the calibrated results. Although radiocarbon samples generally provide maximum ages, they are important for comparison and validation of the OSL ages due to potential problems with partial bleaching in these arroyo deposits. 3.1. Sample collection and preparation OSL sample sites were chosen based on position within the stratigraphy and depth below mapped terrace surfaces, taking care to sample the Qa4 alluvium where contacts with the younger alluvial fills were not visible. OSL samples were collected in opaque metal tubes that were tightly packed with the target sand to minimize mixing. Sediment for dose rate calculation was collected within a 30-cm radius of the OSL sample tube. Samples were opened under dim amber light (w590 nm) and sieved to 90e150 mm (90e180 mm for USU-446). The quartz fraction was isolated by using 10% hydrochloric acid and bleach to dissolve carbonates and organic material, sodium polytungstate (2.7 g/cm3) to remove heavy minerals and concentrated hydrofluoric and hydrochloric acids to remove feldspars, etch the quartz, and prevent formation of fluorite precipitates (see Rittenour et al., 2005 for details). The samples were then re-sieved to remove the <75 mm grain-size fraction of etched quartz and any partially dissolved feldspars. Purity of the samples was checked using infrared (IR) stimulation for all aliquots. Five small, angular charcoal fragments were collected from Qa4, Qa3 and Qa2 deposits and individual pieces were sent to Beta Analytic, Inc. and the Keck Carbon Cycle AMS Lab for pre-treatment and AMS radiocarbon dating (Table 2). The remaining nine radiocarbon samples were collected by Smith (1990), details on analysis, the material submitted and locations within the stratigraphy are less certain for these samples. 3.2. Dose rate determination Representative subsamples were sent to the ALS CHEMEX Laboratory in Elko, Nevada to measure the concentrations of the K, Rb, Th, and U using ICPeMS and ICPeAES analyses (Table S2). In situ moisture content was measured from samples collected over the course of two years covering a broad range of temperature and moisture conditions. Most samples were dry upon collection (<1 wt% H2O, except for USU-363 containing 2.5% moisture content), thus an assumed value of 3  3% was used to represent the average moisture history of the samples. Dose rate calculations include cosmic contribution, influence of water attenuation, uncertainty in elemental measurements and dose rate conversion factors (see footnotes in Table S2 and Rittenour et al., 2005 for details).

3.3. Optical measurements The SAR technique of Murray and Wintle (2000) was used for small-aliquot (1-mm diameter, w20 grains per disk) and singlegrain measurements of quartz sand (1e3 grains per hole). Optical measurements were performed on Risø TL/OSL Model DA-20 readers with blueegreen light emitting diodes (LED) (470  30 nm) as the stimulation source for small-aliquot measurements. The luminescence signal was measured through 7.5-mm UV filters (U-340) over 40 s (250 channels) at 125  C with LED at 90% power (36 mW/cm2), and calculated by subtracting the average of the last 5 s (background signal) from the first 0.7 s (4 channels) of the signal decay curve. The luminescence signals for Kanab Creek quartz show rapid decay dominated by the fast component of the signal (Murray and Wintle, 2003) (Fig. S3); fast ratios range from 11 to 34 (following Durcan and Duller, 2011). Doseeresponse curves were fit within saturatingexponential and saturating-exponential plus linear fits to calculate De values. Single-grain OSL measurements were conducted using Risø single-grain disks, and grains were stimulated with a green laser (532 nm) at 90% power (135 mW/cm2) (Bøtter-Jensen et al., 2000) following IR stimulation. The luminescence signal was measured over 1 s (60 channels total) with a 0.1 s pause before and after stimulation, and calculated by subtracting the average of 0.7e0.9 s (background signal, channels 40e55 out of 60) from 0.1 to 0.14 s (peak signal, channels 6e8) of the signal decay curve. Based on the single-grain data, 13e23% of the grains produced a measurable luminescence signal in response to given doses (signal to background ratio > 3) (Table S3). A preheat-plateau dose-recovery (PP-DR) test was performed on three samples; USU-285, USU-446, and USU-520, to determine if the SAR protocol could recover a known applied dose for the Kanab Creek samples, as well as determining the proper preheat temperature as suggested by Murray and Wintle (2003). For the PPDR tests, five small (1-mm) aliquots were used at each temperature step and preheat temperatures were increased in 20  C increments from 180  C to 300  C, and held for 10 s. First, each non-heated and non-irradiated aliquot was optically bleached at room temperature by blueegreen LED at 90% power for 40 s twice, each followed by a 1000-s pause to let the thermally transferred charges decay (Li and Li, 2006). The aliquots were irradiated with 12.5e14.3 Gy, and then the recovered doses were measured using the SAR protocol with 160  C cut heat following test doses (held for 0 s), and optical measurements at 125  C for 40 s (90% diode power). Results from the PP-DR test suggest that dose recovery is not dependent on preheat temperature (Fig. S4). Recycling ratio results from repeat doses show no general trend with preheat temperature, and all results are within error of unity except for USU-285 at the highest preheat temperature (300  C). Recuperation of signal when no dose is given increased with preheat temperature, but all samples are below 10% of the given dose. Based on these PP-DR results a 240  C preheat (PH) was chosen for Kanab Creek samples. In addition to the PP-DR tests, a small subsample of USU-363 was placed in a clear snap-seal bag and bleached for 7 h in direct sunlight and then given a known dose of 14 Gy. Small-aliquot results from this experiment produced a dose recovery of 0.99 (240 C PH) with an overdispersion value of 7  3%, a value similar to Fuchs et al.’s (2007) artificially bleached reference sample. However, because some grains were likely shielded during the bleaching experiment, we loaded a subsample of the sunlight exposed sand onto disks (1-mm and single-grain) and exposed them to four additional hours of direct sunlight (11 h total), then dosed them with 12.4 Gy. For the small-aliquots the resulting ratio of given to recovered dose was 1.07 and overdispersion was 4  3% (Fig. 2c). For single-grain, the dose-recovery ratio was 0.92 and

M.C. Summa-Nelson, T.M. Rittenour / Quaternary Geochronology 10 (2012) 167e174

overdispersion was 14  6%. Results from this sun-bleached sample were used as a guide to identify overdispersion values of wellbleached single-grain and small-aliquot De distributions. However, well-bleached natural samples should show greater variability than results from dose-recovery experiments due to microdosimetry and post-burial mixing (e.g. Thomsen et al., 2007; Feathers, 2003). Therefore we chose higher overdispersion thresholds of 15% for small-aliquot and 25% for single-grain distributions as indicators of partial bleaching, similar to Olley et al. (2004) and Feathers et al. (2006). These criteria were used in part to assess the level of partial bleaching and when these overdispersion thresholds were exceeded, a minimum age model (MAM) was used to calculate ages. Samples with significant positive skew were also analyzed using the MAM. 3.4. Equivalent dose (De) and error calculation De values were calculated using the central age model (CAM) or MAM of Galbraith et al. (1999) of at least 18 accepted aliquots or at least 60 accepted grains of quartz sand. Aliquots were rejected if they had evidence of feldspar contamination, recycling ratio <0.85 or >1.15, recuperation >15% of the natural signal, or natural De greater than the highest regenerative dose given (similar to Rittenour et al., 2005 rejection criteria). In addition, the few negative De values present in small-aliquot data were rejected due to a number of problems including: [100% De error and very poor doseeresponse curve fitting. Due to reduced luminescence signals, single-grain rejection criteria were loosened in comparison to small-aliquot data. Grains were rejected if they had low signal response to given doses (signal peak less than three times the background), De greater than the highest regenerative dose given, poor recycling ratios >2, recuperation >30% of the natural signal, negative De values, and/or a poor doseeresponse growth curve fit (see Table S3). Following the results of Thomsen et al. (2005), we added 9% uncertainty to all single-grain De values prior to age analysis. Errors on De and age estimates are reported at 2-sigma standard error and include errors related to instrument calibration, and dose rate and equivalent dose calculations and were calculated in quadrature using the methods of Aitken and Alldred (1972) and Aitken (1985, 1976). 4. Results 4.1. De distribution and partial bleaching As previously mentioned, partial resetting of the luminescence signal was expected to be a major limitation to OSL dating of Kanab Creek sediments, requiring criteria to identify and correct for partial bleaching (e.g. Juyal et al., 2006; Olley et al., 2004; Lepper et al., 2000). For poorly bleached samples, a minimum age model (MAM) is preferred because it statistically selects the younger population of Des that were most likely zeroed during transport, effectively reducing the contribution from grains with residual paleodoses (Galbraith et al., 1999). In addition to applying the MAM where there was evidence of partial bleaching, we used singlegrain dating on seven samples to investigate the De distributions at the individual grain level. Most of the Qa4 samples from Kanab Creek had some population of incompletely bleached grains (see Fig. 2 and Fig. S5 for De distributions). Although we acknowledge that microdosimetry and post-depositional mixing contribute to De scatter, for our samples we assume high overdispersion values (>15% for smallaliquot, >25% for single-grain) are primarily due to partial bleaching as evidenced by large positive skew seen in many samples. It should be noted that Fuchs et al. (2007) found that not

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all partially bleached samples manifested a positively skewed distribution. Therefore, significant positive skew is not viewed as a required parameter to identify partially bleached distributions in this study. Most Kanab Creek OSL samples met the criteria for partial bleaching described above, and the MAM was used to calculate the De for those samples. A simplified 3-parameter MAM (MAM-3) was used for small-aliquot analyses, and the MAM-4 was used for single-grain analyses due to the greater number of De values. For the two single-grain samples that did not exceed the threshold criteria for identifying partial bleaching, USU-363sg and USU446sg, the CAM was used to calculate their De. OSL age information is shown in Table 1. Where small-aliquots and single-grains were analyzed on the same sample, the single-grain results are presented as the most accurate age estimate and used in discussions of the relationship to radiocarbon ages and stratigraphic position (Table 1). 5. Discussion Detailed stratigraphic, geomorphic and geochronologic results suggest that the 30e35 m thick Qa4 fill within Kanab Creek aggraded during the middle to late Holocene. Based on the OSL dataset, the age range for the Qa4 alluvium is 6.7e3.8 ka (Table 1). Radiocarbon ages from the same deposit are similar and range from 6.4 to 3.5 cal kyr BP2010 (Table 2). These age results suggest that there was significant middle Holocene fluvial aggradation along Kanab Creek. In the following, we discuss the relationship between partial bleaching and factors such as: 1) depositional environment, 2) aliquot size (small-aliquot versus single-grain), and 3) method of De calculation (CAM versus MAM). We also compare the resultant OSL chronology to radiocarbon ages and stratigraphic position of the samples. Field sedimentary facies descriptions indicate that the primary mode of sedimentation in the Qa4 fill deposit was overbank deposition of massive sand units (Sm) during flood events (see Table S1, Fig. S2 for sedimentary facies codes and photos). Thick, inchannel plane bed and low-angle cross-bedded depositional facies (Sh, Sl) are also present. These sediment-laden flows produced beds that were >0.5 m thick, which may have contributed to inadequate solar resetting as seen in most samples. Comparison of the sedimentary facies to overdispersion and skew of the De distributions produced mixed results (Fig. 3a). However, deposits with massive (Sm), low-angle crossbeds (Sl), and thick (>0.5 m) plane bed flow (Sh) facies have higher overdispersion values (Table 1, Fig. 3a). Depositional environments associated with these facies appear to reduce the potential for solar resetting of the OSL signal, therefore meter-scale massive, horizontal to subhorizontally laminated units should be avoided as these are likely associated with high sediment load and/or high energy stream flow events. Depositional facies having the lowest overdispersion and lack of significant positive skew in the De distribution are ripple crossbeds (Sr) and thin (<0.2 m) plane beds (Sh) (Table 1, Fig. 3a). We suggest that the transport mechanism and depositional environment associated with these facies may be the most conducive to zeroing of the luminescence signal prior to deposition, and it is recommended that thin (<0.4 m) interbeds of ripplelaminated sand should be targeted for OSL in arroyo settings when possible. Thrasher et al. (2009) found similar results when comparing bleaching potential to glacial lithofacies. The OSL samples with the lowest potential for bleaching were from turbid environments including plane-bedded sandy lithofacies, while rippled sands deposited in waning, shallow flows had the highest potential for solar resetting. We agree with Thrasher et al. (2009) that a lithofacies approach to OSL sampling may increase the

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Fig. 2. De distributions for selected Kanab Creek OSL samples. (a) Small-aliquot De distributions, probability distribution function (left) and radial plot (right). (b) Single-grain De distributions for the samples presented in (a). (c) Small-aliquot and single-grain De distributions for sun-bleached and beta dosed USU-363 (11 h total sunlight exposure).

potential for targeting the well-bleached sediments in potentially problematic settings. Another factor effecting solar resetting is sediment transport distance. As a means of detecting this, we compared small-aliquot to single-grain MAM age ratios in relation to sample location. The upstream-most sample location (location 1, Fig. 3b) had mixed results. However, in the middle reach locations (3 and 4 on Fig. 3b)

small-aliquot to single-grain MAM ratios are close to one, and further downstream (location 7 Fig. 3b) the ratio is slightly <1 indicating single-grain ages are producing older age estimates than small-aliquot. Similar results were seen by Thomas et al. (2005) in well-bleached sediments, possibly due to different measurement stimulation wavelengths and intensities between the two techniques. The results may also suggest that although most samples

USU-423

USU-363

USU-446

USU-360

USU-364

USU-520

USU-359

USU-292

USU-286

USU-290

SG SA

80 60 40 SG threshold

20

Facies

Sh

St

b

CAM:MAM SA CAM:MAM SG USU-290

USU-363 USU-363sg

USU-520 USU-520sg

USU-292 USU-292sg

USU-359

USU-446 USU-446sg

USU-360 USU-360sg

USU-285 USU-286

4

Sr

SA:SG

USU-263

Sl

Sm

USU-364 USU-364sg

0

SA threshold

USU-423 USU-423sg

Overdispersion (%)

100

USU-263

a

USU-285

M.C. Summa-Nelson, T.M. Rittenour / Quaternary Geochronology 10 (2012) 167e174

Ratio

3

2

1

0

2 3 6 1 4 7 (4) (4.5) (11) (13.5) (0.4) (7) Sample location number (mean distance downstream in km)

8 (16)

Fig. 3. (a) Comparison of overdispersion (%) to facies (Table 1). Open squares are USU364sg and USU-446sg, which are below 25% overdispersion (the single-grain threshold identifying well-bleached samples). The massive (Sm) and meter-scale laminated sediments (Sl) have higher overdispersion values than the thin, horizontally bedded (Sh), trough cross-bedded (St) or ripple (Sr) facies. (b) CAM to MAM age ratios versus location (Fig. S1), displaying convergence of CAM to MAM ages with distance downstream. Also shown are small-aliquot to single-grain MAM age ratios and location, which shows a slight decrease in ratio with distance downstream. Both comparisons suggest that sediments are becoming better bleached with further transport distance downstream. Note: Location 5 is a radiocarbon sampling site only.

show some signs of partial bleaching, Kanab Creek sediments become better bleached in the downstream direction, with smallaliquot and single-grain MAM ages converging with greater transport distance. Despite this trend, it should be noted that all single-grain and small-aliquot MAM ages are within error, except for with sample USU-446. CAM to MAM ratios from small-aliquot and single-grain analyses exhibit a stronger decreasing trend with distance downstream, where downstream ratios generally approach one (Fig. 3b). OSL samples that exhibited relatively lower overdispersion values (i.e. USU-364sg, 18.3% and USU-446sg, 17.7%) had smaller differences between the resultant CAM and MAM ages (Table 1). Other samples which required the MAM but had relatively low overdispersion values, USU-423, USU-423sg, and USU-520, also had lower CAM to MAM ratios. We view the convergence of CAM and MAM ages (ratios < 1.3) in a downstream direction as indicative of betterbleached samples due to greater transport distance. The more poorly bleached sediments were found in upstream locations closer to headwater sources (Fig. 3b). As expected, the samples with the greatest CAM to MAM age ratios also have large overdispersion, i.e. USU-360sg (55% OD, CAM:MAM ¼ 2.5). An age overestimation by at least 10 kyr would occur if the CAM derived age estimate was used in this case (Table 1). Application of the MAM to this type of De distribution allowed the portion of grains that were likely to have been zeroed

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at deposition to be identified, allowing more accurate equivalent dose and age estimation. Figure S6 graphically displays OSL and radiocarbon ages with depth below the Qat4 or Qat3 terrace and basin-fill surfaces, from upstream to downstream (locations 1e8). In general, OSL and radiocarbon ages from the Qa4 fill are within error and exhibit similar younging-upward patterns throughout the Qa4 deposit (see Fig. 1 and Fig. S6, Tables 1 and 2). However, three OSL ages (USU-290, -360, -364sg) appear to produce age overestimates considering their stratigraphic position, although it should be noted that only USU-364sg is not within error of samples from similar depths within the Qa4 fill. The two outliers (USU-290, -360) are within error of samples at similar stratigraphic positions and share common problematic characteristics. Both samples are from thick (>1 m), high energy facies (Sm and Sh), were collected near the top of the Qa4 fill, and were only dated using small-aliquots. These two samples make ideal candidates for single-grain dating to help reduce the affects of partial bleaching. OSL results from USU-364sg (location 4) produce an age overestimate when compared to radiocarbon samples from deeper within the stratigraphy at the same site (Fig. 1). The single-grain De distribution for this sample does not display strong positive skew and has a low overdispersion (18.3%, below the 25% threshold) (Fig. S5). We suggest that the De is accurately estimated for this sample and that problems with dose rate estimation are affecting the resultant age. The radiocarbon chronology exhibits good stratigraphic consistency (Table 2, Fig. S6), and ages are in agreement when error is taken into account. At location 4 (Fig. 1 and Fig. S6) there appears to be a slight age inversion between c-5 and c-6, however, their age ranges overlap and are consistently younger than USU-363sg below. The other apparent radiocarbon age reversal is with c-8 and c-9 (location 7 Fig. S6, Table 2). However, Smith (1990) collected c-8 from an inset deposit that crosscuts the deposit from which c-9 was collected, indicating that these ages are in correct stratigraphic order. Despite potential problems with individual sample results, all OSL and radiocarbon ages fall between 6.7 and 3.5 ka, suggesting that the chronology for the Qa4 alluvial fill is generally consistent, reliable and robust. 6. Conclusions Based on radiocarbon and OSL results, the age of the Qa4 alluvial deposit in Kanab Creek is 6.7e3.5 ka and represents a state of fluvial aggradation (30e35 m) during this time. Overall, the sedimentology and stratigraphy suggests an environment with high sediment supply, frequent overbank flood packages and high rates of floodplain deposition (w1.5 cm/yr at location 4; Fig. 1). Most alluvial beds sampled for OSL in this environment exhibit signs of partial bleaching, and we found some correlation between level of inadequate bleaching to sedimentary facies and bed thickness. Our observations suggest meter-scale massive, horizontal to subhorizontal laminated beds should be avoided when sampling for OSL dating in an arroyo setting. Thin, decimeter-scale ripple-laminated facies had better bleaching characteristics and should be targeted for OSL. Additionally we found that based on CAM to MAM and small-aliquot to single-grain age ratios, samples from downstream reaches were better bleached. Although it is not reasonable to collect all OSL samples from the downstream portion of a particular field area, careful sample selection using facies analysis will help target better-bleached sediment from upstream reaches in similar fluvial systems with high sediment supply. Similar to Arnold et al. (2007), we found limitations to smallaliquot OSL dating of arroyo deposits due to high levels of partial

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bleaching in sediments from arroyo systems. This is due to flashy discharge, high sediment load, small catchment size, local sediment supply from hillslopes and bedrock and the contribution of older sediment from arroyo-wall bank collapse in these systems. A minimum age model and single-grain dating were necessary tools to ameliorate problems associated with partial bleaching for many samples. Overdispersion and skew are important parameters to help identify partial bleaching in De distributions, and can be used to help determine which samples should be analyzed using the MAM. In addition, OSL results help to confirm the older radiocarbon chronology of the Qa4 alluvial fill in Kanab Creek from Smith (1990). This chronology will be used in future work reconstructing arroyo cut-fill cycles and potential linkages to past climate change in the region. Acknowledgments Funding for research was provided by USGS EDMAP Grant # 08HQAG0041, Exxon Mobil, Geological Society of America, Society for Sedimentary Geology (SEPM), Utah State University (USU) Luminescence Lab and the USU Department of Geology. We would like to thank an anonymous reviewer for helpful comments in earlier versions of this manuscript. Thanks to Best Friends Animal Sanctuary in Kanab, UT for allowing us to access their property during fieldwork. Editorial handling by: F. Preusser Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.quageo.2012.05.002. References Aitken, M.J., 1976. Thermoluminescent age evaluation and assessment of error limits: revised system. Archaeometry 18, 233e238. Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, New York. Aitken, M.J., Alldred, J.C., 1972. The assessment of error limits in thermoluminescent dating. Archaeometry 14, 257e267. Arnold, L.J., Bailey, R.M., Tucker, G.E., 2007. Statistical treatment of fluvial dose distributions from southern Colorado arroyo deposits. Quaternary Geochronology 2, 162e167. Bailey, R.M., Arnold, L.J., 2006. Statistical modeling of single grain quartz De distributions and an assessment of procedures for estimating burial dose. Quaternary Science Reviews 25, 2475e2502. Baker, V., 1987. Paleoflood hydrology and extraordinary flood events. Journal of Hydrology 96, 79e99. Bøtter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiation Measurements 32, 523e528. Bull, W.B., 1991. Geomorphic Responses to Climate Change. Oxford University Press, New York. Cooke, R.U., Reeves, R.W., 1976. Arroyos and Environmental Change in the American South-West. Clarendon Press, Oxford. Durcan, J.A., Duller, G.A.T., 2011. The fast ratio: a rapid measurement for testing the dominance of the fast component in the initial OSL signal from quartz. Radiation Measurements 46, 1065e1072. Feathers, J.K., 2003. Single-grain OSL dating of sediments from the Southern High Plains. Quaternary Science Reviews 22, 1035e1042. Feathers, J.K., Holliday, V.T., Meltzer, D.J., 2006. Optically stimulated luminescence dating of Southern High Plains archaeological sites. Journal of Archaeological Science 33, 1651e1665.

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