Paleodischarge of the Mojave River, Southwestern United States, Investigated with Single-Pebble Measurements of 10Be GEOSPHERE; V

Home , Endorheic basin, Lake Manix, Mojave River, Pluvial lake, Silver Lake (Mojave)

Research Paper

GEOSPHERE Paleodischarge of the Mojave River, southwestern United States, investigated with single-pebble measurements of 10Be GEOSPHERE; v. 11, no. 4 Andrew J. Cyr1, David M. Miller1, and Shannon A. Mahan2 1U.S. Geological Survey, 345 Middlefield Road, MS 973, Menlo Park, California 94025, USA doi:10.1130/GES01134.1 2U.S. Geological Survey, Box 25046, MS 974, Denver Federal Center, Denver, Colorado 80225, USA

6 figures; 2 tables; 5 supplemental files ABSTRACT climatic drivers (e.g., Enzel et al., 2003, and references therein; Miller et al., CORRESPONDENCE: [email protected] 2010; Kirby et al., 2012; Lyle et al., 2012; Antinao and McDonald, 2013). How- The paleohydrology of ephemeral stream systems is an important con- ever, placing constraints on the paleodischarge of fluvial systems remains a CITATION: Cyr, A.J., Miller, D.M., and Mahan, S.A., straint on paleoclimatic conditions in arid environments, but remains difficult difficult problem. 2015, Paleodischarge of the Mojave River, southwestern United States, investigated with single- to measure quantitatively. For example, sedimentary records of the size and Reconstructions of the paleohydrology of arid regions, particularly south- pebble measurements of 10Be: Geosphere, v. 11, extent of pluvial lakes in the Mojave Desert (southwestern USA) have been western North America, are often based on geologic and geomorphic evidence no. 4, p. 1158–1171, doi:10.1130/GES01134.1. used as a proxy for Quaternary climate variability. Although the delivery mech- of the size and extent of alluvial fans (e.g., Wells et al., 1987; Bull, 1991; Harvey anisms of this additional water are still being debated, it is generally agreed and Wells, 1994; Harvey et al., 1999) or eolian deposits (e.g., Lancaster and Received 7 October 2014 that the discharge of the Mojave River, which supplied water for several Pleis- Tchakerian, 2003), the stable isotopic records in speleothems (e.g., Winograd Revision received 30 April 2015 Accepted 15 June 2015 tocene pluvial lakes along its course, must have been significantly greater et al., 2006; Wagner et al., 2010), or lacustrine sedimentary records, particularly Published online 15 July 2015 during lake highstands. We used the 10Be concentrations of 10 individual of pluvial lakes (e.g., Enzel and Wells, 1997; Enzel et al., 1992, 2003; Wells et al., quartzite pebbles sourced from the San Bernardino Mountains and collected 2003). A pluvial lake is a closed (endorheic) basin that fills with water during from a ca. 25 ka strath terrace of the Mojave River near Barstow, California, to wetter climate periods. Observations of pluvial lake sedimentary records in the test whether pebble ages record the timing of large paleodischarge of the Mo- Mojave Desert, southern California, indicate that the highest lake levels, and jave River. Our exposure ages indicate that periods of discharge large enough therefore wettest climate conditions, were generally contemporaneous with to transport pebble-sized sediment occurred at least 4 times over the past glacial stages (e.g., Reheis and Redwine, 2008; Reheis et al., 2007, 2012). ~240 k.y.; individual pebble ages cluster into 4 groups with exposure ages of The timing and surface elevations of pluvial lake stages within the Mojave 24.82 ± 4.36 ka (n = 3), 55.79 ± 3.67 ka (n = 2), 99.14 ± 12.07 ka (n = 4) and 239.9 ± River basin have been used to estimate the water balance in the Mojave River 52.16 ka (n = 1). These inferred large discharge events occurred during both watershed and the discharge required to sustain one or more pluvial lakes glacial and interglacial conditions. We demonstrate that bedload materials along the Mojave River course (Enzel and Wells, 1997). Although the source provide information about the frequency and duration of transport events in of the discharge necessary to produce and maintain pluvial lakes along the river systems. This approach could be further improved with additional mea- Mojave River course remains a topic of considerable debate (e.g., Enzel et al., surements of one or more cosmogenic nuclides coupled with models of river 2003, and references therein; Miller et al., 2010; Kirby et al., 2012; Lyle et al., discharge and pebble transport. 2012; Antinao and McDonald, 2013), estimates of the magnitude and timing of Mojave River discharge are based on spatially and temporally disparate alluvial, lacustrine, and marine sedimentary records. To our knowledge, no at- INTRODUCTION tempt has been made to test these hypotheses using a more direct measure of Mojave River paleodischarge. Arid landscapes shaped by ephemeral stream flow are challenging to char- We present 10 new cosmogenic 10Be exposure ages of individual quartzite acterize and model because of the complex spatial and temporal changes in pebbles collected from alluvium deposited on a strath terrace of the Mojave geomorphic processes and the incomplete records of climate factors that drive River near Barstow, California (Figs. 1 and 2). The age of alluvium on the strath those processes. Innovative approaches such as modeling the residence time is constrained by new optically stimulated (OSL) and infrared stimulated (IRSL) of sand in ephemeral streams based on luminescence ages (e.g., McGuire and luminescence ages. We hypothesize that pebble exposure ages record episodic Rhodes, 2015), using various types of tracers to track sand (e.g., Crickmore, large discharge events in the Mojave River. The geology and geomorphology 1967; Rathburn and Kennedy, 1978; Milan and Large, 2014) or coarser material of the Mojave River watershed, and constraints on these characteristics pro- For permission to copy, contact Copyright (e.g., Church and Hassan, 1992; Lamarre et al., 2005) are improving our ability vided by previous research in the region, provide a nearly ideal experimental Permissions, GSA, or [email protected]. to characterize these types of geomorphic systems, as are recent studies of setting to test this hypothesis.

GEOSPHERE | Volume 11 | Number 4 Cyr et al. | Paleodischarge of the Mojave River Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/4/1158/3334974/1158.pdf 1158 by guest on 23 September 2021 Research Paper

117.5°W 117.0°W 116.5°W 116.0°W 115.5°W 35.5° N

CA NV 35.5° N

AZ Si

Area of Fig. 2A

C So M co 35.0° N H a 35.0° N

Legend Elevation (m) Erosion rate (m/m.y.) Mojave River dam 2595 [from Binnie et al., 2007] at the Forks 34.5° N 52–170 290 171–280 Quartzite clast- km 281–660 bearing units 015 0203040 661–1600

1601–2700 117.5°W 116.5°W

Figure 1. Hillshade of 10-m-resolution National Elevation Dataset (NED, http://ned.usgs.gov/) of the region around the Mojave River basin, including the modern extent of the Mojave River water shed (white line; U.S. Geological Survey Water Resources Maps and geographic information system data; http://water.usgs.gov/maps.html), the modern course of the Mojave River and major tributaries (heavy blue lines), the Mojave River basin drainage network (gray lines), the spatial distribution of quartzite clast-bearing conglomerates (hachured areas; Dibblee, 1973; Morton and Miller, 2006), and the locations of catchment-averaged erosion rates reported by Binnie et al. (2007). Playas supported by Mojave River discharge are denoted by blue polygons (H—Harper Lake; C—Cronese Lakes; So—Soda Lake; Si—Silver Lake; M—Lake Manix; ca—Cady subbasin; co—Coyote subbasin; t—Troy subbasin; a—Afton subbasin). The southwestern edge of the Mojave River watershed, highlighted by the black-edged outline, denotes the approximate extent of the high-elevation, low-relief area of the San Bernardino Mountains referred to in the text. CA—California; NV—Nevada; AZ—Arizona.

116.5625°W 116.5542°W 116.54583°W

m 34.9583°

N A B 0150 300450 600

Lake Manix deposits N 34.9583°

Figure 2. (A) Lidar (light detection and ranging) elevation data set (1 m resolu- N tion) of the area near the sampling location, draped over a hillshade of the same Elevation (m) lidar data, and showing the extent of

34.9542° 631 strath terrace fragments where pebbles C were collected (white lines). Arrows indicate the view directions of the photos shown in shown in B and C, taken from the 116.5625°W B 116.5542°W 486 sampling location. (B) View to the southwest from the sample location showing the extent of the terrace surface. (C) View to the northwest from the sample loca- C D tion. Bluffs in the background and the terrace edge in the middle-ground are Lake Manix lacustrine deposits into which the Lake Manix deposits strath is inset and that Mojave River alluvium overlies. The modern channel of the Mojave River is at the base of the bluffs (dark vegetation line). (D) Close-up view of the sampled pavement.

1. The quartzite pebbles are sourced from metasedimentary and sedimen- nardino Mountains, which are ~5%–10% of the ~9500 km2 Mojave River drain- tary rocks exposed primarily in the upper elevations of the San Bernardino age area, receive >1000 mm/yr, whereas the remainder of the Mojave River Mountains (Fig. 1) (Dibblee, 1973; Sadler and Reeder, 1983; Morton and drainage area receives between 125 and 150 mm/yr (as measured near Baker, Miller, 2006). These rocks are dominantly conglomerates, and so the quartzite California; Enzel and Wells, 1997; Reheis et al., 2012). Hydrological records and pebbles that are eroded out of hillslopes are presorted and rounded, mini- historical accounts indicate that the discharge of the Mojave River is strongly mizing the possible effects on 10Be concentrations of grain comminution or correlated with precipitation in the San Bernardino Mountains; nearly the en- sorting during transport (e.g., Belmont et al., 2007; Carretier and Regard, tire mean annual discharge of ~9.5 × 106 m3 is derived from that 5%–10% of 2011; Carretier et al., 2015, and references therein). The only other source of the Mojave River drainage area in the San Bernardino Mountains (Enzel et al., quartzite pebbles in the Mojave River watershed upstream of our sampling 1992, 2003; Wells et al., 2003). Currently much of this discharge is lost to infil- location is from Mojave River alluvium, either in the channel bed or preserved tration along the river bed, with only extreme floods reaching Afton Canyon, in Mojave River alluvial fill terraces (Dibblee, 1960a, 1960b, 1960c, 1960d, 1965, ~65 km east of Barstow. However, late Pleistocene and Holocene sedimentary 1966, 1970, 1973; Dibblee and Bassett, 1966; B. Cox, 2013, personal commun.). records in Mojave Desert pluvial lake basins indicate that the Mojave River has Although there are other occurrences of quartzite in the region (Bowen and Ver previously carried enough discharge for it to have episodically flowed as far as Planck, 1965; Sadler and Reeder, 1983), their extent inside the Mojave River Death Valley during the Pleistocene (Wells et al., 2003). basin is not significant (Sadler and Reeder, 1983). 3. The age of alluvium deposited on the strath is well known, which allows 2. The discharge of the Mojave River is driven primarily by precipitation in us to precisely and accurately account for postdepositional accumulation of the San Bernardino Mountains. The Mojave River headwaters in the San Ber- 10Be, and isolate that fraction of the measured concentration that records the

pebbles exposure time in the hillslope and channel network. In addition to our in the same sample. The site (Fig. 2A) is ~523 m above sea level, 20 m below a new luminescence ages, the sedimentary record of pluvial Lake Manix, the de- nearby highstand beach and inset more than 9 m into Lake Manix sediments. posits of which the strath at our sampling site are cut into, has been examined The alluvium deposited on the strath is 1.2 m thick, with soils developed in the for nearly a century (e.g., Buwalda, 1914; Blackwelder and Ellsworth, 1936; upper 40 cm. The basal section, 80 cm thick, is composed of coarse arkosic Jefferson, 1985, 2003; Steinmetz, 1988; Meek, 1990, Reheis and Redwine, sand with rounded pebbles of quartzite and angular clasts of local metavol-

SUPPLEMENTAL TABLE 1. PHYSICAL CHARACTERISTICS OF PEBBLES ANALYZED FOR 10BE Sample name Mass Volume*Density Pebble axes† Roundness§ Sphericity# abc 2008; Reheis et al., 2012). It indicates that the Mojave River entered the Cady canic rock. It is concordantly overlain by medium to coarse arkosic sand with (g) (cm3) (g/cm3) (mm) (mm) (mm) MRT-2a168.45732.31 70.1 50.634.50.5 0.378 MRT-2b191.35792.42 79.1 47.443.60.8 0.344 MRT-2d249.80922.72 87.9 44.537.10.9 0.326 MRT-2e140.10572.46 78.9 48.425.90.7 0.309 subbasin of Lake Manix beginning ca. 500 ka (Cox et al., 2003), alternately rounded quartzite pebbles. We sampled for luminescence near the top of the MRT-2f157.15622.53 69.9 44.341.10.9 0.359 MRT-2i122.35562.18 62.2 50.831.90.8 0.390 MRT-2j 321.00135 2.38 84.6 67.740.90.4 0.384 MRT-2k180.55722.51 80.9 46.946.50.3 0.326 terminating in the Cady, Troy, and Coyote subbasins until ca. 190 ka, when the lower unit, below the base of soil development (depth of 45 cm). Samples were MRT-2n269.25111 2.43 88.9 63.223.10.3 0.343 *Determined by displacement. †Measured in CAD software. §Estimated from visual chart in Krumbein and Sloss (1956). #Determined as the cube root of the particle volume divided by the volume of the circumscribing sphere. lake expanded into the Afton subbasin (Reheis et al., 2007). There were three collected and processed following Mahan et al. (2007). Cosmic ray dose rate recognized distinct lacustrine phases between 190 ka and ca. 25 ka, when Afton was estimated according to Prescott and Hutton (1994). Detailed methods for 1Supplemental Table 1. Physical characteristics of Canyon was incised and Lake Manix drained into the Soda and Silver Lake ba- measuring the equivalent dose using single aliquot regeneration were pre- pebbles analyzed for 10BE. Please visit http://dx.doi sins to the east (Reheis and Redwine, 2008; Reheis et al., 2012), extending the sented in Mahan et al. (2007). .org/10.1130/GES01134.S1 or the full-text article on Mojave River across the former floor of Lake Manix. Although Mojave River www.gsapubs .org to view Supplemental Table 1. delta sediments extended far into Lake Manix after ca. 40 ka (Reheis et al., 2012), cut-and-fill fluvial deposits of the Mojave River inset into the delta did Cosmogenic 10Be not reach beyond the upper delta until the lake failed and drained. Those de- Cyr, A.J., Miller, D.M., and Mahan, S. A., 2015, Paleodischarge of the Mojave River, posits occur at a lower elevation than our sample location. Pebbles (mean intermediate axis = 5.15 cm; Supplemental Table 11) were southwestern United States, investigated with single-pebble measurements of 10Be:

Geosphere, v. 11, doi:10.1130/GES01134.1. Because of these geologic constraints, pebble exposure ages should be rep- collected from the surface pavement at site MRT2 (Fig. 2). All had surface desert resentative of the total amount of time that each pebble has spent in the Mojave varnish and were on a vesicular A horizon, indicating long-term postdepo- Supplemental File. Pebble sampling and Cosmogenic nuclide sample preparation River hillslope and channel network, providing new insight into when, and how sitional aggradation of eolian dust and a near-surface origin (Wells et al., (http://dx.doi.org/10.1130/GES01134.S2) frequently, the Mojave River carried enough discharge to transport pebble-sized 1995; McFadden et al., 1987). Samples were prepared following standard Pebble Sampling 2 10 9 We collected 47 pebbles from three different locations on the same geomorphic sediment. Our results show that (1) the Mojave River has undergone at least methods (Supplemental File ). The Be/ Be ratios were measured at the surface. Each of these locations wason a different part of this surface, with each part three periods of discharge large enough to transport pebble-sized sediment over Center for Accelerator Mass Spectrometry, Lawrence Livermore National separated by channels that have begun to dissect it. We identified these surfaces as being the past ~240 k.y.; (2) those periods of high Mojave River discharge do not corre- Laboratory (Livermore, California), and normalized to the 07KNSTD stan- remnants of the same, originally continuous, surface based on their shared elevation,

characteristics of the underlying soil, surface morphology, clast composition, and degree spond to glacial periods as defined by global temperature proxies; and (3) trans- dard series (Nishiizumi et al., 2007). Pebble exposure ages were calculated

of pavement and varnish development. Pebbles were collected from the middle parts of port time from the Mojave River headwaters in the San Bernardino Mountains using the CRONUS online exposure age calculator (Balco et al., 2008) (Sup- each surface remnant, as far away from the dissecting channels as possible. Because we to the sample location, ~160 km downstream, can be very short, likely <~2 k.y. plemental Table 23). were concerned about material loss during sample preparation, we selected the largest

pebbles we could find whileattempting to keep the grain size of pebbles from each part of the surface uniform. STUDY AREA In the lab, the mass of each pebble was recorded and the volumes measured by RESULTS displacement. We selected the 10 pebbles for 10Be determination by ranking each pebble 1 through 47 based on mass, summing the ranks, and then selecting the 10 largest pebbles Quartzite pebbles were collected from alluvium mantling a strath terrace of Luminescence Chronology from the surface with the lowest rank sum. This also turned out to be the most laterally the Mojave River ~35 km downstream of Barstow (34.952°N, 116.558°W; Figs. extensive remnant of the now dissected surface. 1 and 2). The pebbles form a well-developed, moderately varnished pavement The quartz OSL age of the sand deposited on the strath surface is 25.4 ± (Figs. 2B, 2C) supported by an ~6-cm-thick vesicular A horizon overlying a weak 1.42 ka. The feldspar IRSL age is 24.3 ± 2.18 ka. The uncertainties are reported 2Supplemental File. Pebble sampling and Cosmogenic to moderately developed cambic B horizon developed in fluvial pebbly sand. at 2s (Table 1). nuclide sample preparation. Please visit http://dx.doi This soil formed in an ~1-m-thick deposit inset ~9 m into and overlying Lake .org/10.1130/GES01134.S2 or the full-text article on www.gsapubs .org to view the Supplemental File. Manix deposits. The deposit on the strath gently slopes north toward the Mojave River channel, and is cut on the north side by a 6 m lower strath with similar soils. Pebble 10Be Exposure Ages

TABLE DR2. CRONUS EARTH CALCULATOR* INPUT PARAMETERS Sample Latitude† Longitude† Elevation§ Elevation/pressure Sample Sample Shielding Erosion [10Be] [10Be]# 10Be stnd.[26Al][26Al] 26Al name flag thickness density correction rate unc. unc. stnd. (DD N) (DD W) (m) (cm)(g/cm3)(cm/y) (x104 (x104 at/g) at/g) MRT-2a 34.951847116.55882841 std22.65 10229.9316.36 07KNSTDN.M.** N.M. -- 10 MRT-2b 34.951847116.55882841 std22.65 101781.1896.30 07KNSTDN.M.N.M.-- Individual pebbles have measured Be concentrations (N ) between MRT-2d 34.951847116.55882841 std22.65 10785.8444.75 07KNSTDN.M.N.M.-- meas MRT-2e 34.951847116.55882841 std22.65 10858.7045.41 07KNSTDN.M.N.M.-- MRT-2f 34.951847116.55882841 std22.65 10162.338.7007KNSTDN.M.N.M.-- METHODS MRT-2h 34.951847116.55882841 std22.65 10762.0944.40 07KNSTDN.M.N.M.-- MRT-2i 34.951847116.55882841 std22.65 10640.7236.21 07KNSTDN.M.N.M.-- MRT-2j 34.951847116.55882841 std22.65 10413.5722.17 07KNSTDN.M.N.M.-- 5 5 MRT-2k 34.951847116.55882841 std22.65 10190.5710.18 07KNSTDN.M.N.M.-- 1.62 ± 0.16 × 10 and 17.81 ± 1.92 × 10 atoms/g quartz at 2 confidence (Table 2). MRT-2n 34.951847116.55882841 std22.65 10453.3023.77 07KNSTDN.M.N.M.-- s *Wrapper script v2.2; Main calculator v2.1; Constants v2.2.1; Muons v1.1 using the default calibration data set. Calculatedon 12/18/2013 †NAD83 §Catchment-average elevation of the modern Mojave River basin. #1-sigma AMS uncertainty **N.M. = not measured Luminescence These concentrations correspond to exposure ages between 20.71 ± 4.25 and 239.9 ± 52.16 ka (Table 2; Fig. 3A). Uncertainties on pebble ages are the 2s 3 Supplemental Table 2. CRONUS Earth Calculator in- Luminescence from quartz and feldspar separates was used to provide age external uncertainties related to reference 10Be production rate (Balco et al., put parameters. Please visit http://dx.doi.org/10.1130 /GES01134.S3 or the full-text article on www.gsapubs control on the alluvium deposited on the strath terrace from which pebbles 2008). A normal kernel density estimate (NKDE) of the 10 pebble ages (Fig. 3A) .org to view Supplemental Table 2. were sampled by measuring OSL on quartz and IRSL on potassium feldspars indicates three distinct multipebble clusters of ages. These clusters have mean

TABLE 1. LUMINESCENCE AGES FOR MOJAVE RIVER STRATH TERRACE Water content* K† Th† U† Cosmic dose additions§ Total dose Equivalent dose Age Sample information (%) (%) (ppm) (ppm) (Gy/k.y.) (Gy) (Gy) n** (ka) M06NS-860 1 (10) 4.21 ± 0.17 9.78 ± 0.38 2.55 ± 0.21 0.29 ± 0.02 5.88 ± 0.14 150 ± 2.21 21 (25) 25.6 ± 1.38†† 7.87 ± 0.12§§ 191 ± 5.73§§ — 24.3 ± 2.18 *Moisture value used in calculation of age was 5% of total saturation moisture. Full saturation is in parentheses after field moisture. †Analyses obtained using NaI low-resolution gamma spectrometer. §Cosmic doses and attenuation with depth were calculated using the methods of Prescott and Hutton (1994). Refer to Mahan et al. (2007) for details. **Number of replicated equivalent doses (De) estimates used to calculate the mean. Figures in parentheses indicate the total number of measurements made, including failed runs with unusable data. ††Dose rate and age for fine-grained (180–250 µm) quartz sand for blue-light optically stimulated luminescence. Exponential + linear fit used on age, errors reported at 2σ. §§Silt fraction (4–11 µm) of feldspar for infrared stimulated luminescence as multiple aliquot additive dose technique. Errors reported at 2σ.

TABLE 2. ANALYTICAL RESULTS OF COSMOGENIC 10Be PEBBLE GEOCHRONOLOGY Sample Location* Production rate † § Lat Long Spallation Muons Quartz Be carrier# 10Be/9Be**,†† [10Be]**,§§ Exposure age**,## (DD N) (DD W) (atoms/g/yr) (g) (µg) (x10-13) (104 atoms/g quartz) (ka) MRT-2a 34.951399 116.562109 7.64 0.239 79.5380 354.74 ± 18.24 7.715 ± 0.3769 22.993 ± 1.636 29.40 ± 2.11 (3.32) MRT-2b 34.951399 116.562109 7.64 0.239 110.8525 332.65 ± 17.10 88.83 ± 1.479 178.118 ± 9.630 239.91 ± 13.78 (26.08) MRT-2d 34.951399 116.562109 7.64 0.239 146.4272 341.49 ± 17.56 50.43 ± 1.230 78.584 ± 4.475 102.32 ± 5.98 (10.90) MRT-2e 34.951399 116.562109 7.64 0.239 69.4932 345.90 ± 17.78 25.82 ± 0.3108 85.870 ± 4.541 112.08 ± 6.09 (11.72) MRT-2f 34.951399 116.562109 7.64 0.239 81.0043 340.55 ± 17.51 5.778 ± 0.0791 16.233 ± 0.870 20.71 ± 1.12 (2.12) MRT-2h 34.951399 116.562109 7.64 0.239 60.1604 347.80 ± 17.88 19.73 ± 0.5365 76.209 ± 4.440 99.14 ± 5.92 (10.63) MRT-2i 34.951399 116.562109 7.64 0.239 75.5376 346.60 ± 17.82 20.90 ± 0.4875 64.072 ± 3.621 83.02 ± 4.79 (8.78) MRT-2j 34.951399 116.562109 7.64 0.239 177.6354 342.52 ± 17.61 32.10 ± 0.4809 41.357 ± 2.217 53.19 ± 2.89 (5.50) MRT-2k 34.951399 116.562109 7.64 0.239 103.0869 347.84 ± 17.88 8.452 ± 0.1147 19.057 ± 1.018 24.34 ± 1.31 (2.49) MRT-2n 34.951399 116.562109 7.64 0.239 182.8136 346.15 ± 17.80 35.83 ± 0.3575 45.330 ± 2.377 58.38 ± 3.11 (6.01) AC-MB-6 335.24 ± 17.24 0.0428 ± 0.0041 9.583 ± 1.082 — *North American Datum of 1983. DD—decimal degrees. †Constant (time invariant) local production rate based on Lal (1991) and Stone (2000). Refer to CRONUS Earth calculator documentation (Balco et al., 2008). §Constant (time invariant) local production rate based on Heisinger et al. (2002a, 2002b). Refer to CRONUS Earth calculator documentation (Balco et al., 2008). #Carrier mass and uncertainty are based on weighted mean and standard error of ≥5 measurements of carrier [9Be] by inductively coupled plasma–mass spectrometry and 1% uncertainty on mass determination. Carrier solution has a nominal 9Be concentration of 345.18 ± 4.13 µg/g. **Uncertainties are reported at the 2σ external conﬁdence level. ††Ratios are corrected for the background ratio reported for the process blank AC-MB-6. §§Propagated uncertainties include error in the blank, carrier mass, and accelerator mass spectrometer counting statistics. ##Stated uncertainty is the internal uncertainty. Parenthetical uncertainty is the external uncertainty, which includes uncertainties in the 10Be production rate and decay constant.

ages and standard deviations of 24.82 ± 4.36 ka (n = 3), 55.79 ± 3.67 ka (n = 2), from the pebble conglomerate source lithologies in the San Bernardino Moun- and 99.14 ± 12.07 ka (n = 4). There is a single pebble that defines the oldest limit tains (Fig. 1) or have been recycled from Mojave River alluvium adjacent to of our data, 239.9 ± 52.16 ka. the Mojave River channel, either from cut-and-fill terraces or from the Mojave River delta into Lake Manix. The luminescence dates of the alluvium deposited on the strath are indistinguishable from the age of the demise of Lake Manix DISCUSSION (Fig. 3), suggesting that the river cut this channel in the lake floor in <1 k.y. We interpret the 10Be data as exposure ages, i.e., the amount of time that The strath and overlying alluvial deposit that we sampled are inset into the quartzite pebbles have spent near the surface during erosion from peb- lacustrine sediments of Lake Manix, and represents the first known deposit ble conglomerates as prerounded, presorted pebbles from hillslopes in the of the Mojave River to this site. The quartzite pebbles in the alluvium over- San Bernardino Mountains, plus the transport time to where they were sam- lying the strath have been transported to the sample location either directly pled (including any time in storage), plus the exposure time on the strath. It is

Figure 3. (A) Normal kernel density estimate of pebble exposure ages. Individual pebble ages Mojave River pebble and their 2σ external uncertainties are represented as Gaussian distributions (dashed red A ) 2.5 curves). The solid line is the sum of the individual curves. (B) Optically stimulated luminescence -5 exposure ages 0 (solid line) and infrared stimulated luminescence (dashed line) ages of the strath terrace where pebbles were collected. (C) Reconstruction of Lake Manix lake level (redrawn from Reheis et al., 2.0 2012). (D) SPECMAP (SPECtral MApping Project) δ18O record (Imbrie et al., 1989). Gray bars represent the durations of full glacial stages. 1.5

1.0 very likely that each pebble has a complex exposure and burial history (e.g.,

Granger, 2006), including very different initial 10Be concentrations (inheritance; Relative frequency (x1 0.5

Ninh); there are myriad reasonable nuclide buildup and decay scenarios for each of those histories. However, because there is no unique 10Be production and decay path for any given measured 10Be concentration, we use the simple B Luminescence ages )

exposure ages. -4 of strath 0

2 Pebble 10Be Exposure Ages and the Problem of Inheritance

Our interpretation of the pebble 10Be concentrations as a proxy for time spent in transport and storage along the Mojave River assumes that the 1 concentration of 10Be that would have accumulated in the pebbles during exposure and transport on hillslopes in the San Bernardino Mountains prior Relative frequency (x1 to them being shed into the Mojave River network (inheritance) is relatively small. Although this assumption may be valid, it is also possible that the 10Be 10 High C Lake Manix lake level inheritance could be both a significant component of our measured Be con- l (Reheis et al., 2012) centrations and highly variable from pebble to pebble. Whereas recent measurements of 10Be concentrations in modern fluvial sand collected from small San Bernardino Mountains watersheds occupy a narrow range between ~10 × 104 and 20 × 104 atoms/g quartz (Binnie et al., 2007), low-temperature thermochronometric data from the San Bernardino Mountains indicate very low

exhumation rates of a high-elevation, low-relief surface (<20 m/m.y.), and it is Relative lake leve

possible that quartzite pebbles, which are weathering out of Miocene sedimen- Low tary rocks, could persist on low-gradient hillslopes for long and highly variable SPECMAP δ18O amounts of time (Spotila et al., 1998, 2002), resulting in very high and variable 1.5 D 10 Be inheritance in our pebbles. 1.0 We assess the robustness of our interpretation of pebble ages, namely that 0.5 the different aged groups of pebbles represent distinct periods of time that pebbles could be transported by the Mojave River, by using some simplifying 0.0 O 18

assumptions and the results of previous work to explore some possible values δ -0.5 of 10Be inheritance in our pebbles. -1.0 We calculate inheritance, Ninh, in our youngest set of three pebbles using -1.5

Ninh = Nmeas –(Nterr + Ntrans), (1) -2.0 MIS 2 46 8 where N is the measured concentration of 10Be, N is the concentration of meas terr 050100 150200 250300 10Be accumulated during 24.4 ± 2.4 k.y. of exposure on the terrace (the mean Age (ka) and 2s variance of the OSL and IRSL ages), and Ntrans is the concentration

of 10Be accumulated during transport in the hillslope and fluvial network. We profile for that part of the landscape. We can transform the expected 10Be

assume that Ntrans accumulated for a minimum period of time between 1 yr nuclide concentration at any given depth into the apparent age of a pebble and 10.3 k.y., the least and greatest differences between the 2s confidence from that depth to assess the possibility that all of our pebbles were derived intervals of the mean luminescence age of the terrace and the youngest from San Bernardino Mountains hillslopes at the same time but from different 10 pebble Be age. For Nterr we assume constant production rates of 5.97 and depths (e.g., a deep-seated landslide). 0.215 10Be atoms/g quartz/yr by neutron spallation and muogenic processes, respectively. These production rates were calculated using the CRONUS calculator for the sampling location, at an elevation of 521 m and a material Modeled Pebble 10Be Inheritance Compared to Recent 3 10 density of 2.44 g/cm . For Ntrans we assume catchment mean production rates Catchment‑Averaged Be Concentrations of 7.64 and 0.239 10Be atoms/g quartz/yr by neutron spallation and muogenic processes, respectively, also calculated using the CRONUS calculator, consid- We assume that any 10Be not accumulated during 24.4 ± 2.4 k.y. of exposure ering a catchment average elevation of 841 m. We do not account for either on the strath is due to nuclide inheritance and compare those concentrations topographic or snow shielding. Both of these would decrease the 10Be produc- to the renormalized 10Be concentrations of Binnie et al. (2007) (Supplemental

tion rate, resulting in higher modeled concentrations of Ninh. We also do not Table 2 [see footnote 3]; Fig. 4). In general, the pebbles have greater inheri- 10 account for burial during transport, which would increase Ntrans and decrease tance than the measured Be concentrations of fluvial sands. Greater model 10 modeled concentrations of Ninh. Be inheritance of our pebbles can be interpreted in two different ways. The 5 Using these assumptions we calculate Nterr = 1.497 ± 0.146 × 10 atoms/g first is that the pebbles spent more variable, and longer, periods of time on 4 quartz and Ntrans between 8 ± 1 and 8.08 ± 1.04 × 10 atoms/g quartz. The re- hillslopes in the San Bernardino Mountains, where they accumulated larger 4 10 sulting modeled values of Ninh from Equation 1 are between 0.63 ± 0.63 × 10 concentrations of Be (~4–8×) than sand collected by Binnie et al. (2007). atoms/g quartz and 4.01 ± 1.015 × 104 atoms/g quartz. These are the middle of This interpretation implies that pebble transport rates in the high-elevation,

the range between the minimum and maximum modeled Ninh of each pebble, low-relief pebble source areas are low compared to sand, and that some peb- corresponding to the maximum and minimum assumed transport times, re- bles may persist in this lag for much longer periods of time than others (e.g., spectively. We compare these model inherited 10Be concentrations in our peb- Spotila et al., 1998, 2002). In this scenario the pebbles would have all been bles to three different end-member model sources of inherited 10Be. delivered to the Mojave River drainage network and deposited on the strath at 1. We use the renormalized 10Be concentrations of sand-sized sediments about the same time, but had highly variable inheritance.

from high-elevation, low-relief basins in the San Bernardino Mountains However, our modeled pebble Ninh values are also consistent with the (Supplemental Table 2 [see footnote 3]; Binnie et al., 2007), where the peb- interpretation that the four distinct sets of pebbles had similar inherited 10Be ble source areas are found, to account for inheritance. One could argue that concentrations when they entered the Mojave River drainage network and sand is delivered to the channel network by different processes that operate spent different amounts of time in transport to where we sampled them. If

at different rates from those that deliver pebbles, such as landslides and other we consider the youngest set of pebbles, modeled Ninh values are similar processes that deliver material from greater depths beneath the surface (e.g., to half of the 10Be concentrations measured by Binnie et al. (2007) (Fig. 4). Niemi et al., 2005; Yanites et al., 2009; Puchol et al., 2014). Measurements of This indicates that the initial exposure duration of pebbles on high-eleva- 10Be nuclide concentrations in different size fractions indicate that there is no tion, low-relief hillslopes of the San Bernardino Mountains, and therefore consistent trend in concentration as a function of grain size (cf. Carretier et al., hillslope denudation rates at the time that pebbles were delivered to the 2015). Previous work (Olivetti et al., 2012) indicated that coarser size fractions Mojave River network, were similar. This similarity indicates slightly faster have up to two times lower 10Be concentrations, so we halve Binnie et al.’s hillslope denudation rates; recent denudation rates in the headwaters of the (2007) nuclide concentrations to compare possible inheritance related to faster Mojave River, calculated using the Binnie et al. (2007) renormalized 10Be

erosion rates to our model Ninh. concentrations, have a mean of 96 ± 20 m/m.y., whereas denudation rates

2. We use the long-term exhumation rates derived from low-temperature calculated using modeled Ninh values of the youngest set of Mojave River thermochronometric data of Spotila et al. (1998, 2002) to calculate surface pebbles have a mean of 31 ± 14 m/m.y. This threefold difference could be 10Be concentrations in the low-relief watersheds of the upper San Bernardino attributed to greater water availability driving more rapid hillslope transport,

Mountains and compare those to our modeled Ninh values. Even if pebbles or different transport processes, during the times that pebbles were shed form a long-lived lag on the surface, it is doubtful that that lag would persist at into the Mojave River network. In this scenario, pebbles with similar ages time scales longer than this exhumation rate, and so this will place a maximum would have similar inheritance and the difference in exposure ages between

bound on model Ninh values. the sets would be the result of different durations of transport between the 3. We calculate equivalent surface exposure ages of low-relief catchments San Bernardino Mountains source area and where the pebbles were depos- in the upper San Bernardino Mountains and model the expected 10Be depth ited on the strath.

Modeled Pebble 10Be Inheritance Compared to 10Be Concentrations

×10–6 Expected from Long-Term Exhumation Rates 5 A Mojave River Spotila et al. (1998, 2002) used low-temperature thermochronometric data 4 pebble model from the San Bernardino Mountains to infer long-term (~2.5 m.y.) exhumation 10 Be inheritance rates of the mountain range; they inferred an average exhumation rate of 40 m/m.y. for the entire San Bernardino Mountains, but assumed that the exhu- 3 mation rate of the high-elevation, low-relief headwaters of the Mojave River is lower, <20 m/m.y., based on the presence of Miocene-aged sedimentary 2 cover, including the quartzite pebble conglomerates from which our pebbles are derived. Given this estimate of long-term exhumation, and using the larg- 5 1 est modeled Ninh value of 16.155 ± 0.84 × 10 atoms/g produced in the upper 10 m of the surface, assuming a density of 2.44 g/cm3, an elevation of 2050 m, at rates of 16.73 and 0.343 atoms/g by spallation and muons, respectively, the 0 × –5 sample is at secular equilibrium with respect to 10Be. This could be interpreted 10 B 10Be concentrations of as one pebble with a very long exposure time (≥4 m.y.) in the quartzite cobble 2 Binnie et al. (2007)

y lag being mixed with a suite of much younger pebbles that were transported to and deposited on the strath at the same time. We, however, infer from this re-

sult that the modeled Ninh of the oldest individual pebble is not representative 10 of the true Be inheritance. Although we cannot rule out values of Ninh in the

1 older pebbles that are higher than the modeled Ninh of the youngest group, that would imply highly variable processes and process rates continuously deliv-

Relative probabilit ering pebbles to the Mojave River network through time, it is just as likely that pebbles were delivered by similar processes acting at similar rates at distinct times. In this scenario, pebbles enter the Mojave River network with similar 0 Ninh, but in discrete pulses, possibly driven by variability in regional climate ×10–5 that results in greater water availability to transport coarse sediment. C Half the 10Be concentrations 4 of Binnie et al. (2007) (~2× denudation rate) Modeled Pebble 10Be Inheritance Compared to 10Be Concentrations 3 from an Expected Depth Profile

2 The combination of the long-term erosion rate estimated from low-tem-

perature thermochronometry and our largest modeled value of Ninh indicates 1 that the 10Be concentration of the apparent oldest pebble in our data set has reached secular equilibrium; this is something that has been documented only in the most arid landscapes (Klein et al., 1986; Brook et al., 1995; Ivy- 0 0 2 4 6 8 10 12 14 16 18 Ochs et al., 1995; Bierman and Caffee, 2001; Nishiizumi et al., 2005). Given ×105 that recent denudation rates are significantly higher than the longer term 10Be concentration (atoms/g) thermochronologic estimate (Binnie et al., 2007; Spotila et al., 1998, 2002) Figure 4. Probability distribution of Mojave River pebble 10Be in- (Supplemental Table 2 [see footnote 3]), and that part of the San Bernardino heritance modeled by Equation 1 (see text). (A) Compared to the Mountains, though not the pebble source areas, were glaciated at least three distribution of renormalized 10Be concentrations of fluvial sands measured by Binnie et al. (2007). (B) Compared to the 10Be con- times over the Quaternary (Owen et al., 2003), this situation seems unlikely. centrations expected if the Binnie et al. (2007) denudation rates of However, there is another possible way to explain the distribution of our mod- high-elevation, low-relief parts of the San Bernardino Mountains eled N values. were doubled (C), as might be expected during a wetter climate. inh Instead of pebbles with highly variable Ninh being sourced from the surface at the different times, they may have been sourced at the same time, but from

different depths. Mass wasting processes (e.g., landslides, debris flows) can shown in Figure 5C and compared to our modeled Ninh values, shown in Figure supply material from depth that would be relatively coarse, like our pebbles. 5B. The model results demonstrate that it might be possible for some of the This coarse sediment would have a lower 10Be concentration than material at apparently older Mojave River pebbles to have entered the channel network 10 10 the surface. The difference in depth-dependent concentrations of Be could be with Ninh similar to our model values. However, our distribution of Be con- interpreted as differences in exposure age. centrations in our depth profile model is not consistent with the distribution of 10 To explore this possibility, we calculated the Be concentration over a 10 m model Ninh of our pebbles. depth profile beneath a 1 Ma surface lowering at 20 m/m.y. using the same assumptions we used when considering long-term exhumation rates based on low-temperature thermochronometry. We selected 1 Ma as a seemingly Timing of Pebble Erosion and Large Mojave River Paleodischarge reasonable compromise between the ca. 4 Ma age required for secular equilibrium (e.g., Nishiizumi et al., 2005; Granger, 2006) and the Holocene time scale Quartzite pebbles are shed into the Mojave River from a small portion of represented by catchment-averaged erosion rates in the headwaters region of the high-elevation regions of the San Bernardino Mountains (Fig. 1; Dibblee, the Mojave River where the pebbles are sourced (Fig. 1) (Binnie et al., 2007). 1973; Sadler and Reeder, 1983; Morton and Miller, 2006) or recycled from Mo- We also do not consider this scenario outside of the low relief, high-elevation jave River alluvial deposits. The Binnie et al. (2007) renormalized 10Be con- area of the San Bernardino Mountains where the Mojave River headwaters are centrations indicate that quartzite pebble source areas in the San Bernardino located, as it is this part of the landscape where pebbles are originally derived, Mountains have equivalent exposure ages, i.e., the amount of time pebbles and any pebbles from lower elevations within the watershed would overesti- spend in the zone of 10Be production, between ~4.4 k.y. and ~15.4 k.y. (Supple- mate 10Be concentrations due to nuclide production during fluvial transport. mental Table 34). If we were to account for this inheritance in our strath pebble The modeled depth profile is shown in Figure 5A. The 10Be concentrations 10Be concentrations, the ages of the youngest pebbles would be much younger decrease from a surface value of 69.18 ± 0.96 × 105 atoms/g (2s uncertainty of than the strath age from both our luminescence ages and the chronology of 7%) to 1.09 ± 0.08 × 105 atoms/g. We use this depth profile to consider a sce- Lake Manix (Reheis et al., 2012). It is possible that the sampled pebbles en- nario where an equal number of pebbles from 10 cm depth intervals down to a tered the Mojave River drainage network by different, more rapid, geomor- depth of 10 m are shed into the Mojave River at the same time by a landslide. phic processes than the fine sand sampled by Binnie et al. (2007) (e.g., Niemi This would be similar to a large block toppling from a cliff side of a canyon cut et al., 2005; Yanites et al., 2009; Kober et al., 2012). In order to not violate into the high-elevation, low-relief surface of the San Bernardino Mountains. stratigraphic principles, the youngest pebbles must have transport times of The relative probability of these hypothetical pebble 10Be concentrations is ~1 k.y. or less, which is a factor of ~8 shorter than the mean equivalent expo-

0 5 ×10–6

A y 100 4 B Mojave River pebble model 10Be inheritance 200 3

300 2 Figure 5. (A) Model 10Be depth profile beneath a hypothetical high-elevation, low-relief surface in the 400 1 San Bernardino Mountains as described by Spo-

Relative probabilit tila et al. (1998, 2002). (B) Probability distribution

SUPPLEMENTAL TABLE 3. COSMOGENIC 10BE DATA of BINNIE ET AL. (2007) RE-NORMALIZED FOR COMPARISON TO MOJAVE RIVER PEBBLE DATA PRESENTED IN TABLE 2. 10 Sample Location*,† Elev*Elv. flag§ Thick§ Density§ Shield§ Production rate Quartz*Be carrier* [10Be]* Denudation rate§§ Equivalent exposure age§§ 500 0 of Mojave River pebble modeled Be inheritance. 10Be/9Be*,# Lat. Long. Spallation** Muons†† (DD N) (DD W) (m)(cm)(g/cm3)(x10-15)(at/g/y)(at/g/y)(g) (g) (104 at/g)(m/My)(ky) 14 34.934 117.054 1400std 22.651384 ± 22 11.60 0.287 50.52 205.6 ± 2.110.11 ± 0.61 96.67 ± 5.92 (8.98) 7.84 ± 0.47 (0.83) 10 15 34.397 117.076 1540std 22.651248 ± 22 12.82 0.300 29.88 208.7 ± 2.111.06 ± 0.72 95.88 ± 6.33 (9.28) 7.77 ± 0.51 (0.84) (C) Compared to the probability distribution of Be 19 34.404 117.063 1690std 22.651155± 2 14.24 0.315 27.97 155.2 ± 1.68.42 ± 0.65 138.45 ± 10.82 (14.64)5.33 ± 0.41 (0.62) 20 34.375 117.091 1750std 22.651649 ± 27 14.84 0.321 39.70 201.3 ± 2.021.51 ± 0.94 54.95 ± 2.44 (4.71) 13.11 ± 0.58 (1.28) –5 16*** 34.276 117.031 2200std 22.651324 ± 17 20.05 0.368 44.28 208.0 ± 2.19.73 ± 0.55 160.66 ± 9.15 (15.01)4.39 ± 0.24 (0.45) y 1.4 × 17*** 34.279 117.063 1970std 22.651448 ± 19 17.21 0.343 47.08 207.3 ± 2.112.77 ± 0.58 106.66 ± 4.89 (9.22) 6.71 ± 0.31 (0.66) 10 10 18*** 34.280 117.041 2000std 22.651709 ± 39 17.56 0.346 53.28 206.3 ± 2.118.06 ± 1.00 76.24 ± 4.28 (7.08) 9.31 ± 0.52 (0.96) [10Be]Denudation rate§§ Equivalent exposure age§§ Depth profile Be inheritance concentrations expected in coarse fluvial sediment 10 9 ##

Depth (cm) C Be/ Be 600 (x10-15)(104 at/g)(m/My)(ky) 14 ------399 ± 23 ------10.85± 0.63 82.05± 4.80 (7.50) 9.20 ± 0.53 (0.96) 15 ------258 ± 23 ------12.04± 1.07 79.50± 7.23 (9.18) 9.35 ± 0.84 (1.17) 19 ------161± 2------5.97± 0.07 177.77± 2.22 (12.80)4.17± 0.05 (0.37) if an equal number of pebbles were delivered from 20 ------675 ± 28 ------22.87± 0.95 46.62± 1.97 (3.96) 15.39± 0.64 (1.49) 16*** ------337 ± 18 ------10.58± 0.57 133.67± 7.20 (12.25)5.27± 0.28 (0.54) 17*** ------466± 2------13.71 ± 0.59 89.78± 3.89 (7.66) 7.96 ± 0.34 (0.77) 18*** ------737 ± 41 ------19.07± 1.06 65.23± 3.68 (6.08) 10.86± 0.61 (1.12) *As reported in Binnie et al. (2007). †WGS84 1.0 every depth in a 10-m-thick section with the depth §Used in our CRONUS calculations so that Binnie et al.'s (2007) 10Be concentrations would be consistent with our Mojave River pebble data. # 700 Ratiosalready corrected for background (Binnie et al., 2007). **Constant (time invariant) local production rate based on Lal (1991) and Stone (2000). Refer to CRONUS Earth calculator documentation (Balco et al., 2008). ††Constant (time invariant) local production rate based on Heisinger et al. (2002a, 2002b). Refer to CRONUS Earth calculator documentation (Balco et al., 2008). §§Stated uncertainty is the internal uncertainty, which include error in the blanks, carrier mass (1%), and AMS counting statistics. Parenthetical uncertainty is the external uncertainty, which includes uncertainties in the 10Be productionrate and decay constant. profile shown in (A) to the headwaters of the Mo- ##As determined by our renormalization from the NIST standard series to the revised ICN standard series (Nishiizumi et al., 207).0 Equals the reported 10Be/9Be ratio x1.04. ***Samples collected from within the Mojave River Basin. jave River, e.g., in the case of a landslide. 800 0.6 4Supplemetal Table 3. Cosmogenic 10Be data of Binnie et al. (2007) re-normalized for comparison to Mojave 900 Relative probabilit 0.2 River pebble data presented in Table 2. Please visit http://dx.doi.org/10.1130/GES01134.S4 or the full- 1000 1 2 3 4 5 6 5 024681012141618 text article on www.gsapubs.org to view Supplemen- ×10 10 10 ×105 tal Table 3. [ Be] (at/g quartz) [ Be] concentration (atoms/g)

sure age of the catchments in the Mojave River basin sampled by Binnie et al. lakes in the Manix basin during several interglacial periods between ca. 500 (2007). We interpret this to mean that the production and delivery of sediment and 25 ka. In addition, a review of the geomorphic signals of Quaternary to the Mojave River in the San Bernardino Mountains was more rapid during climate change in southwestern North America by Antinao and McDonald the periods of time represented by our pebble ages than it has been over the (2013) showed intermittent shallow Mojave Desert lakes during the latest period of time represented by the Binnie et al. (2007) data. Pleistocene–earliest Holocene. In both cases Antinao and McDonald (2013) We interpret the mean ages of the clusters, identified by the NKDE, as attributed relatively high moisture during interglacial periods to winter at least three periods of discharge in the Mojave River large enough to en- Pacific Ocean sources. However, because of the resolution of our10 Be ages, train new pebble-sized sediment supplied from hillslopes in quartzite source we are unable to infer the water sources, seasonality, or durations of these areas, as well as large enough to remobilize pebbles stored in alluvial deposits wet periods, which could be anything from a single wet season or heavy snow transported during earlier periods of high discharge. The precision of our in- year, to several years or decades of strong Northern Annular Mode resulting dividual pebble ages, and of the age clusters, does not allow us to draw any in increased moisture from atmospheric rivers (e.g., Reheis et al., 2012), or conclusions about the time scale of these transport periods below the time one or more strong El Niño–Southern Oscillation cycles (e.g., Antinao and scale of the stated uncertainty, nor does it allow us to make any inferences McDonald, 2013). Regardless of the main climate drivers responsible for about the specific moisture delivery mechanism, seasonality, or intensity of increased Mojave River discharge, our data indicate that it is likely not as discharge-supplying precipitation. With those caveats in mind, in our inferred simple as glacial = wet, interglacial = dry. This is illustrated by study of ef- scenario the first set of pebbles would have entered the Mojave River drainage fective moisture recorded in soil proxies of U isotopes in opal. Maher et al. network ca. 240 ka, been transported some unknown distance downstream, (2014) argued that more moisture was present in valley-bottom soils during and then deposited. The next event, at least as resolved by our data, occurred MIS 3 and early MIS 2 but distinctly before the Last Glacial Maximum (LGM), ca. 100 ka, during which new pebbles were shed into the Mojave River, trans- and suggested that the LGM was cold and dry but was preceded by a wet ported some distance downstream, and mixed with pebbles from the ca. 240 period, explaining the diachronous behavior of lakes and glaciers. Our data ka events that were remobilized. This process would have been repeated ca. are consistent with this interpretation. 54 ka and ca. 25 ka, at which point the pebbles were deposited on the strath. The periods of time represented by our pebble ages may also represent times of rapid deposition in the delta within Lake Manix, which was later ex- Mojave River Paleodischarge humed. Some of its sediment may have been redeposited on the strath as the Mojave River extended across the floor of the drained lake. Periods of deeper The quartzite pebbles deposited on the strath terrace east of Barstow are water and delta building are known, in a general way, for Lake Manix but are sourced exclusively from the low-relief, high-elevation region of the San Ber- difficult to compare to pebble ages. The delta extended to within 500 m of our nardino Mountains, perhaps cycled multiple times in Mojave River fluvial de- sample site starting ca. 50 ka but pebble-sized sediments are rare in the delta in posits. Although it might be possible for the observed distribution of pebble this area. Coarser clasts are more common >10 km to the west, an area where 10Be exposure ages to be the result of a single event, such as a deep-seated delta sedimentation timing is poorly constrained but in all probability is older landslide, our analyses of different scenarios indicate that it is also likely that than 50 ka. It is possible to argue that the deeper lake episodes coincident with the pebble ages represent periods of time when Mojave River discharge was Marine Isotope Stages (MIS) 6 and 4 may have resulted from enhanced dis- large enough to transport pebble-sized sediment in Mojave River bedload. We charge, causing more gravel deposition in the delta. If so, burial and shielding can use the sizes of our pebbles and some basic measurements of the geom- could explain the age pulses that postdate those two events. etry of the modern Mojave River channel to estimate the magnitude of those Alternatively, more rapid sediment production and delivery from hillslopes past discharge events. to the channel network may be indicative of cooler, wetter conditions during Transport occurs when the driving forces of flow depth, h, and gradient, S, the times represented by our pebble ages, when weathering might have been exceed the resisting forces of the weight and cohesion of sediment, and can be more efficient at generating pebble supply and sustained fluvial discharge expressed as (Heede, 1976): high enough to transport those pebbles (e.g., Miller et al., 2010; Ellwein et al.,

2011). These high discharge proxy ages occur during both glacial and inter ρwghS >θg(ρs −ρw)D, (2) glacial periods (Fig. 3). That we observe age clusters of pebbles during interglacial periods is where g is the gravitational constant, q is Shields’ parameter, estimated as

consistent with the interpretations of other data sets from the region. Based 0.045, D is the grain diameter, and rw and rs are the density of water (0.9997 on sedimentologic and stable isotope data from a core of Lake Manix sedi g/cm3) and sediment, respectively. This relationship can be rearranged to give mentary deposits <1 km to the northeast of our sampling location, Reheis the minimum flow depth necessary to move a specified grain size at a given et al. (2012) inferred sustained shallow (<5 m) to moderate (5–8 m) depth channel gradient,

θD(ρs −ρw) calculation indicates that a discharge at least ~2.5 times the size of the largest h > . (3) Sρw flood on record would be required to move pebbles past Barstow. Average daily flow at Barstow for the largest floods on record (e.g., Enzel and Wells, We determined D by measuring the principal axes of each of the pebbles. 1997) is significantly lower than their corresponding maximum discharge. This The density of each pebble was determined by measuring each pebble’s mass indicates that pebbles moved from their source to the strath in either high-fre- and volume. These characteristics, as well as roundness and sphericity, both quency, very short duration (minutes to hours?) events, or less frequent, determined in a computer-aided design program, are presented in Supple- longer duration (weeks to months?) events. The precision of our 10Be-derived mental Table 1 (see footnote 1). The mean intermediate axis of our 10 pebbles ages precludes us from stating definitively what specific ranges of frequen- is 5.15 cm. Using a mean pebble density of 2.44 g/cm3 and the mean slope of cy-duration combinations were necessary to move pebbles the ~160 km from the Mojave River between the Mojave Dam and the sampling location deter- the headwaters of the Mojave River to the strath where we collected them, mined from the 10-m-resolution National Elevation Dataset (NED; http://ned and therefore prevents us from speculating about the specific type of climatic .usgs.gov/ ) (0.0024; Fig. 6A), the minimum flow depth required to entrain peb- driver responsible for the minimum discharge needed to transport pebbles bles the size that we analyzed is 1.6 m. (e.g., Miller et al., 2010; Ellwein et al., 2011; Antinao and McDonald, 2013; Kirby We also drew 54 thalweg-perpendicular channel cross sections at regular et al., 2012). We can, however, use our pebble ages and the known age of al- intervals, using the same NED, between the Mojave Dam and the sampling site luvium deposited on the strath to estimate the amount of time necessary to (Supplemental Table 45). Using these data we calculate a mean cross-sectional transport pebbles from their source. area of the Mojave River channel of 844 m. Although we recognize that the geometry of the Mojave River channel and the amount and caliber of its bedload may have been different in the past, using this cross-sectional area the mini- Duration of a Given Pebble Transport Event mum discharge required for the modern river to achieve a flow depth of 1.6 m and entrain pebbles of the size we analyzed is 1346 m3/s (flow depth × area). The age of the strath from luminescence (24.35 ± 2.38 ka) and the age of The U.S. Geological Survey (USGS) has maintained a stream gauge at Bar- the youngest quartzite pebble using 10Be (20.71 ± 4.26 ka), are indistinguish- stow (10262500; 34°54′25″N, 117°01′19″W) since 1 October 1930. These flow able within their respective uncertainties. Thus, the minimum amount of time data, to 12 March 2014, are presented in Figure 6B. There are several instances needed to transport quartzite pebbles from their source in the San Bernardino of flow into the current Mojave River terminus at Silver Lake Playa, some of Mountains to our sample location is ~100 yr. Even if the younger 2s bound of which occurred prior to the USGS stream gauge record upstream at Barstow. the OSL age of the alluvium on the strath (21.97 ka) and the older 2s bound These large discharge events have been attributed to large winter floods, tied of the youngest individual pebble (24.97 ka) are considered, the difference is directly to large amounts of precipitation or snow melt in the San Bernardino only 3 k.y. This is consistent with recently reported sand transport rates from Mountains, and occurred in 1862, 1867, 1884, 1891, 1909, 1916, 1922, 1938, two the Mojave River. McGuire and Rhodes (2015) used the observed downstream

SUPPLEMENTAL TABLE 4. MOJAVE RIVER CHANNEL GEOMETRY BETWEEN THE DAM AT THE FORKS AND THE LOCATION WHERE PEBBLES WERE COLLECTED in 1969, 1978, 1980, 1983, and 1993 (Enzel and Wells, 1997, USGS stream gauge increase in IRSL apparent ages, combined with measures of the degree of par- Profile no. Channel width Channel depth Hydraulic radiusCross-sectional Wetted Unit stream area perimeter power (m) (m) (m2) (m) 15010.96505252.88 28510.97858731.10 data). The maximum measured discharge at Barstow over the gauging inter- tial bleaching, to demonstrate cyclical bleaching and burial of grains as they 3 110 1 0.98110 11224.03 4 150 1 0.98150 15217.63 5 150 1 0.98150 15217.63 6 175 1 0.98175 17715.11 3 7 175 1 0.98175 17715.11 val (1 October 1930 to present) was 512.53 m /s on 3 March 1938. Our simple are transported downriver. These observations, combined with a sediment 8 175 1 0.98175 17715.11 9 200 1 0.99200 20213.22 10 100 2 1.92200 10426.44 11 200 1 0.99200 20213.22 12 250 1 0.99250 25210.58 13 250 1 0.99250 25210.58 14 350 1 0.99350 3527.55 15 200 2 1.96400 20413.22 16 400 1 0.99400 4026.61 17 465 1 0.99465 4675.69 18 250 2 1.96500 25410.58 19 500 1 0.99500 50235.29 20 250 2 1.96500 25410.56 1000 21 550 1 0.99550 5524.81 22 375 1.5 1.48562.5 3787.05 23 400 1.5 1.48600 4036.61 24 300 2 1.97600 3048.81 A Mojave River between Mojave Dam 25 225 3 2.92675 23111.75 B Mojave River discharge at Barstow 26 350 2 1.97700 3547.53 27 750 1 0.99750 7523.53 28 400 2 1.98800 4046.61 & sample location east of Barstow 500 29 400 2 1.98800 4046.61 10/1/1930 − 3/12/2014 30 850 1 0.99850 8523.11 31 430 2 1.98860 4346.15 32 430 2 1.98860 4346.15 33 450 2 1.98900 4545.88 34 930 1 0.99930 9322.84 35 500 2 1.981000504 5.29 36 525 2 1.981050529 5.04 /s ) 37 550 2 1.981100554 4.81 3 800 400 3/3/1938 38 375 3 2.951125381 7.05 Figure 6. (A) Mojave River longitudinal profile be- 39 1200 1 0.99120012022.20 40 600 2 1.981200604 4.41 41 600 2 1.981200604 4.41 42 300 4 3.891200308 8.81 tween the Mojave Dam and the sampling location 43 325 2 1.981250629 4.23 44 650 2 1.981300654 4.07 45 740 2 1.981480744 3.57 46 800 2 1.991600804 3.31 300 (arrow). (B) Mojave River discharge at Barstow, Cali 47 800 2 1.991600804 3.31 48 800 2 1.991600804 3.31 1/10-12/2005 49 800 2 1.991600804 3.31 2/25-26/1969 fornia, recorded by U.S. Geological Survey stream 50 850 2 1.991700854 3.11 2/17/198 0 51 925 2 1.991850929 2.86 52 1025 2 1.99205010292.58 53 1175 2 1.99235011792.25 gage 10262500 (34°54′25″N, 117°01′19″W) between 54 1200 2 1.99240012042.20 600 200 2/19-20/1993 Elevation (m) 1 October 1930 and 13 March 2014, highlighting the Discharge (m recorded occurrences of discharge >200 m3/s. 5 100 3/2/1983 Supplemental Table 4. Mojave River channel geom- 1/23/1943 etry between the dam at the forks and the location where pebbles were collected. Please visit http://dx 400 .doi.org/10.1130/GES01134.S5 or the full-text article 120 140 160 180 200 220 240 260 280 300 1/1/1940 1/1/1960 1/1/1980 1/1/2000 on www.gsapubs .org to view Supplemental Table 4. River distance (km) Date

transport model for sand between the Mojave River dam at Forks and Barstow, Lawrence Livermore National Laboratory by Susan Zimmerman and Robert Finkel. This work indicates fine sand transport times between 210 and 800 yr, consistent with our benefitted from discussions with Greg Balco, Brett Cox, David Bedford, Yehouda Enzel, and Alan Hidy. Exploratory 10Be studies on Mojave River pebbles by Lewis Owen motivated our more de- estimated minimum pebble transport time. tailed study. Constructive reviews by Stephen DeLong and three anonymous reviewers, as well as editorial guidance from Jose Hurtado and Shan de Silva, improved the manuscript.

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