230Th/234U and 36C1 dating of evaporite deposits from the western Qaidam Basin, : Implications for glacial-period dust export from Central Asia

FRED M. PHILLIPS J- Geoscience Department, New Mexico Tech, Socorro, New Mexico 87801 MAREK G. ZREDA TEH LUNG KU SHANGDE LUO j-^ DepartmentDepartment ofof GeologicalGeological Sciences,Sciences, UniversityUniversity ofof SouthernSouthern California,California, LosLos Angeles,Angeles, CaliforniaCalifornia !90089 QI HUANG Lakesces Institute, Académica Sínica, , , People's Republic of China DAVID ELMORE* PETER W. KUBIK* J> Nuclear Structure Research Laboratory, University of Rochester, Rochester, New York 14627 PANKAJ SHARMA* 1

ABSTRACT this change of regime was enhanced transport and Nees, 1986). Arid climates reduce vege- of dust. The association of thick loess depos- tational cover, thereby allowing increased The western Qaidam Basin contains numer- its in the mid-latitudes with maxima in global wind erosion and dust input to the atmo- ous hydrologically closed lakes and playas. We ice volume has been recognized for many sphere. Therefore, as long as an adequate have measured uranium, thorium, and chlo- (von Richthofen, 1882). The scale of supply of dust size particles remains in the rine radioisotopes in sediments from drill cores the dust transport that accompanied the gla- source region, increasingly arid climates will at two of these, Gasikule Lake and Dalangtan cial maxima, however, has only recently result in increased dust flux to the oceans." dry playa, in order to date cycles of high and been documented. Polar ice, from both the Although this mechanism for enhanced low lake level. The sediment chronologies from (Hammer and others, 1985) and dust mobilization is plausible, two consider- the two sub-basins are generally concordant, the Antarctic (De Angelis and others, 1987; ations warrant further consideration. The and the U/Th ages indicate transitions from Petit and others, 1990) ice sheets, shows first is that the postulated dust source areas high to low lake levels at 302 ± 56 ka, at 138 ± markedly higher dust concentrations during are very arid under the present climate re- 6 ka, and at 16.3 ± 2.2 ka. These ages indicate glacial intervals. In mid-latitude ice caps, the gime. In fact, at the present time, vast areas that high lake stands in the Qaidam Basin were late ice has dust concentrations of the Gobi , , and Qaidam terminated at the end of continental glacial that are several times those of the Holocene Basin are virtually devoid of vegetation. maxima. This result is unexpected because ice (Thompson and others, 1989). Heavy ac- These areas are capable of generating huge previously the large eolian dust fluxes out of cumulations of dust from central Asia are dust storms under the present climatic regime Central Asia during glacial maxima have been found in sediments from the northern Pacific, when winds are strong and persistent (Kukla considered to be associated with increased and intervals of high dust deposition can be and An, 1989). Increased aridity in these un- aridity in the region, not increased humidity. correlated with both the marine oxygen-iso- vegetated is unlikely to enhance the The lacustrine evidence, combined with previ- tope record of glaciations and with the epi- availability of dust over current conditions. A ous data from ice cores, suggests instead that sodes of loess deposition on the Chinese loess second consideration is that the necessary the persistence of strong winds may have plateau (Hovan and others, 1989, 1991). climatic connection, increased aridity in Cen- played a more important role in enhanced dust Analysis of sediment cores from this area tral Asia during glacial periods, has not been fluxes than did variations in dust availability shows that during glacials the dust flux in- convincingly demonstrated. Kukla (1987) due to changes in aridity. creased by two to four times the average in- and Kukla and An (1989) have reviewed the terglacial flux (Hovan and others, 1989; evidence (largely paleoecological) published INTRODUCTION 1991). in the Chinese literature for the influence of What explains this enhanced glacial dust aridity on loess deposition in central China. The Quaternary glacial/interglacial cycles flux in the North Pacific? Hovan and others Based on a combination of pollen, gastropod, involved a wide range of climatic phenom- (1991), relying on previous research (Rea and and vertebrate faunal data, the ecological ena. The global climate regime was perva- Leinen, 1988), link loess deposition and en- community in the Loess Plateau region can sively altered; one of the chief indications of hanced marine dust fluxes to aridity: "The best be described as switching from a savan- mass flux of dust to the ocean reflects the nah-like environment with abundant herbs supply of dust-sized particles in the source and broadleaf trees during interglacials to a * Present addresses (Elmore and Sharma): Pur- region (Prospero, 1985; Rea and others, steppe-like environment characterized by due Rare Isotope Measurement Laboratory, Pur- 1991), which for time scales of years to per- short grass and low shrubs (artemisia) during due University, West Lafayette, Indiana 47907; glacials. We make two observations concern- (Kubik) Paul Scherrer Institut, ETH - Höngger- haps several 100,000 [sic] years is controlled berg, CH-8093 Zürich, Switzerland. by climate (Rea and others, 1985; Prospero ing the significance of these reconstructions.

Geological Society of America Bulletin, v. 105, p. 106-1616, 5 figs., 5 tables, December 1993.

1606

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/12/1606/3381693/i0016-7606-105-12-1606.pdf by guest on 28 September 2021 DATING EVAPORIATE DEPOSITS, CHINA

(1) The paleoecological reconstructions , dunes, and other topography asso- bles, as in Luo and Ku (1991), are one-stan- clearly pertain to climatic conditions in the ciated with deflational basins. dard deviations derived from counting statis- loess deposition area, not the loess source ar- This study focused on radiometric dating tics and the least-squares fitting of the eas, which lie to the north and west of the of materials from two drill cores in the far isochrons. All of the ages are in correct strat- Loess Plateau. (2) It is not clear to what de- western Qaidam Basin, but 36C1/C1 ratios in igraphical order except for samples U-4 and gree changes in temperature affected the veg- inflow waters and salt lakes and flats around U-3 from ZK 2605, but these ages are indis- etational replacements, as compared to the basin were also measured. The most in- tinguishable within the analytical uncertain- changes in the balance of precipitation to tensively studied core was ZK 2605, drilled ties. We prefer the age on U-3 because of its evaporation. Thus, in the southwestern on the eastern margin of Gasikule Hu (Turk- smaller uncertainty. , the artemisia steppe extended ish, Gas Kol), at the western tip of the far southward during the last glacial maxi- Qaidam Basin. Gasikule is a shallow saline 36C1 Measurements mum (Van Devender and others, 1987), but lake with a surface area of about 100 km2. It this was unquestionably a time of more fa- forms the terminal sink of two small rivers, Chlorine-36 has a half-life of 301 ka and is vorable water balance in the region (Benson the Alar He and the Timuleek He. The produced, both in the atmosphere and at the and Thompson, 1987). Similarly, Prentice Gasikule basin was created by the uplift of an Earth's surface, by the action of cosmic ra- and others (1992) have shown that full-glacial anticlinal ridge to the northeast of the lake, diation (see Bentley and others, 1986a, for a artemisia steppe in the northeast Mediterra- blocking drainage into the lower Mangya sub- review of 36C1 geochemistry). The 36C1 meas- nean area (presently occupied by broadleaf basin. The lake is bounded on the southeast urements were performed on purified AgCl trees) is consistent with a more humid glacial by extensive salt flats. The core was drilled obtained from halite crystals from cores or climate. near the center of these salt flats, close to the surface , or from chloride in surface-wa- A direct method of assessing changes in present lake shore. ter samples. The samples were converted to aridity is to reconstruct fluctuations in the A second core (ZK 402) is from near the AgCl as described in Jannik and others (1991) surface area of closed-basin lakes. In cases center of the Dalangtan Playa. Dalangtan is a and Bentley and others (1986b). The purified 36 where precipitation directly on the surface now-dry salt flat about 100 km east of and dried AgCl was analyzed for the C1/C1 lake is negligible, the ratio of lake surface area Gasikule, on the northern margin of the ratio at the University of Rochester by accel- to drainage basin area is equal to the ratio of Qaidam. The playa is bounded in the south- erator mass spectrometry (Elmore and oth- 36 runoff to lake evaporation: a quantitative west and northeast by anticlinal ridges ers, 1979). The measured C1/C1 ratios are measure of the water balance (Street-Perrott formed from interbedded lacustrine silts and given in Tables 1, 4, and 5. and Harrison, 1985). This type of analysis is silly evaporites of age. The Aljun useful in Central Asia because most of the Mountains (Altan Shan), bounding Dalang- DISCUSSION area consists of hydrologically closed basins. tan Playa to the north, are a relatively low and 23( !34 36 In this paper, we report W U and C1 arid range in this region and at present do not 36C1 in Surface Waters and Saline Playas data on lacustrine cores from the western yield any perennial inflow to the playa. Qaidam Basin of Qinghai Province, China. Both of the cores studied were drilled by Bentley and others (1986a) have calculated 23(> 234 We use the rh/ U ages to determine the the Qinghai Bureau of Geology and Mineral the 36C1/C1 ratio in meteoric deposition over timing of lake-level fluctuations, and we com- Resources, in 1984 (ZK 402) and 1986 (ZK North America, based on estimates of mete- pare the inferred water-balance histories with 2605). Although the cores were logged in de- oric 36C1 production and stable chloride dep- the timing of glacial-stage dust enhancement tail as they were drilled, the cores themselves osition. Such calculations have not been in order to evaluate the role of continental were not preserved. The material we ana- performed for central Asia, but similar geo- aridity on dust production. lyzed was obtained from sectioned samples graphical patterns should exist. Based on ex- collected by scientists of the Institute of Salt perience in analogous situations in North STUDY AREA Lakes, Academia Sinica (ZK 2605), and of America, 36C1/C1 ratios in the range 500 x the Qinghai Bureau (ZK 402). These sec- 10"15 to 1,000 x 10"15 could be expected in The Qaidam Basin is a large (120,000 km2) tioned samples are stored at the Institute in surface waters and young salt deposits. compressional basin to the northeast of the Xining, Qinghai. The data in Table 4 indicate generally Kunlun Shan, the range bounding the north- lower ratios of 36C1/C1 in river waters, ranging ern margin of the (Fig. 1). METHODS AND RESULTS from 34 x 10"15 to 507 x 10"ls. These low The climate within the basin is very arid. An- ratios are probably caused by the addition of nual precipitation of about 25 mm is greatly Measurement of U-Series Isotopes ancient sedimentary chloride (with low 36C1/ exceeded by the annual potential evaporation CI) leached out of rocks at higher elevations of 3,000 mm. Due to its relatively high eleva- Analyses were done on halite-rich layers in the drainage basins. Sedimentary salt tion (2,700 m above sea level), the basin is a from the cores. The samples were prepared sources are apparently widespread. Rockhill cool desert, with January and July means of for U and Th isotope analysis, counted, and (1894, p. 184) described the bottom and sides -10 °C and 25 °C, respectively. Streams 230Th/234U ages were calculated using the iso- of the Naichi gol (Nalin gol) valley at mid- draining the much more humid Qilian Shan to chron method, as described in Luo and Ku elevations as so uniformly covered with salt the north and Kunlun Shan to the south ter- (1991). The analytical and age results for core efflorescences that "the salt makes them look minate in either small saline lakes or large salt ZK 2605 have been presented earlier (Luo as if covered with deep snow." flats. Other than salt-tolerant grasses around and Ku, 1991) and are summarized in Table The 36C1/C1 ratios of the saline playas and some of the influent streams, the basin floor 1, and those for ZK 402 are listed in Tables 2 lakes (5 x 10"15 to 98 x 10~15) are even is devoid of vegetation. It is characterized by and 3. The uncertainties indicated in the ta- lower than the surface-water inflows. This

Geological Society of America Bulletin, December 1993 1607

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/12/1606/3381693/i0016-7606-105-12-1606.pdf by guest on 28 September 2021 PHILLIPS AND OTHERS

* ' ' ' .1 FI^N OR BRACKISH LAKE 39° .¿'vi¿Ï^fe

.. ! \ • ^ 'i |:::::• | PLAYAS 11 :":'--. KUNTEYI P. " 0 50 100 km 1 i I i i I^G AS I KU LÈ V'\" 38e- 12,13fzK-2605 DALANGTAN R """'^V '"••..-''

37

36°.

I 91

Figure 1. Sample locations in the Qaidam Basin. Numbers without prefixes refer to QBW (water) samples. Prefix "S" indicates "QBS" (surface salt) samples. Core locations are indicated by "ZK" prefix. Figure modified from Bowler and others (1986).

further reduction can be explained by addi- yas probably originates from recycling of in Figure 2. Ages could also be calculated tional dilution of the surface-water 36C1 by these older lacustrine evaporites, rather than from the 36C1/C1 values, assuming that all hal- sedimentary salt. Many of the salt flats bor- surface-water inflow from the surrounding ite was deposited with an initial ratio equal to dering the saline lakes contain solution mountains. The Pliocene evaporites are ex- the value from Gasikule lake waters: 88 x "pipes" that discharge brine from depth into pected to have 36C1/C1 ratios close to 1 x 10 15. Most of the ages calculated in this the lakes. These brines could result from the 10"15 (Bentley and others, 1986a). Pliocene fashion, however, are systematically older discharge of deep-basin fluids along fault evaporites, however, do not crop out in the than the 23t>Th/234U ages by about 80 ka. zones (Lowenstein and others, 1989) and Gasikule basin. Three of them (QBC-5, QBC-6, and QBC-17) would be expected to have very low 36C1/C1 yielded anomalously young or negative ages. 23<) 234 ratios. Furthermore, extensive areas of Measurements on Core ZK 2605 We prefer the Th/ U ages because in the Pliocene evaporite halite and have Th/U system (1) solution of old evaporites will been exposed by Pleistocene and Holocene Both 36C1 and U/Th disequilibrium in the bring in only the uranium parents to the dep- anticlinal uplift. Most of the halite in the sed- evaporites can potentially yield chronological ositional site, (2) both the parent and the iments that fill the intervening synclinal de- information. The distribution of 230Th/234U daughter nuclides are measured, and (3) pressions such as Kunteyi and Dalangtan pla- ages with depth in the ZK 2605 core is shown the assumptions can be tested by the degree

1608 Geological Society of America Bulletin, December 1993

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/12/1606/3381693/i0016-7606-105-12-1606.pdf by guest on 28 September 2021 DATING EVAPORIATE DEPOSITS, CHINA

m 23<> TABLE 1. V/ ni AGES (FROM LUO AND KU, 1991), 36C1/C1 DATA, CALCULATED INITIAL ^Cl/Cl RATIOS (RJ AT sum-dominated layers are higher than those TIME OF EVAPORITE DEPOSITION, AND CALCULATED CHLORIDE RESIDENCE TIMES IN LAKE WATER (At) FOR SAMPLES FROM CORE ZK 2605 AT GASIKULE in adjoining halites, whereas those in the hal- ite-dominated layers are too low to produce 23(> 234 Sample no. Depth ^/^^Th Measured Calculated Rd At ages matching those from the Th/ U (m) Age» (ka) '"CI/IO15C1 (36C1/1015C1) (ka) method. Cl-l 0 4.9 ± 1.5 We interpret the pattern of stratigraphy QBC-ZK2605-1 2.14 (16) 81 ± 5 84 150 QBC-ZK2605-3 8.88 (16) 84 ± 14 87 120 and isotopic measurements described above a-3 14.5 16.3 ± 2.2 using the model of lake sedimentation cycles QBC-ZK2605-5 20.9 (25 to 130) 133 ± 14 >141 QBC-ZK2605-« 24.75 (30 to 130) 97 ± 10 104 0 shown in Figure 3. Figure 3a shows the a-9 33.0 138 ± 6 QBC-ZK2605-8 33.46 (140) 51 ± 4 70 320 Gasikule basin under modern conditions. U-3 43.7 167 ± 8 QBC-ZK2605-11 45.02 (165) 53 ± 5 77 230 Lake levels are relatively low, and the lake is QBC-ZK2605-13 60.06 (165) 47 ± 6 69 330 surrounded by extensive salt flats. Due to the U4 61.5 158 ± 17 QBC-ZK2605-16 67.27 (170) 44 ± 6 65 400 very low precipitation (less than 25 mm/yr), U-5 69.9 173 ± 30 QBC-ZK2605-17 90.05 (240) 78 ± 5 135 there is insufficient runoff to carry clastic sed- U-6 109.7 302 ± 56 iments onto the salt flats from the surround- QBC-ZK2605-19 111.65 (300) 29 ± 5 58 500 QBC-ZK2605-20 113.65 (300) 28 ± 2 56 540 ing topographic highs. Small amounts of hal- ite are probably being deposited on the lake 'Ages in parentheses are interpolated. bed, but most of the influent chloride is prob- ably stored in the lake brines. Figure 3b il- lustrates a transgressive lake cycle. Early in the cycle, increased runoff carries silt over of fit to the isochron as well as by strati- Inspection of the lithologie log, the 23(>Th/ the salt flats. Shallow ground water seeping graphic consistency. In contrast, the daughter 234U ages, and the 36Q/C1 ratios reveals con- through this silt layer may evaporate and de- in the 36Q system (36Ar) cannot be distin- nections among the three. The core lithology posit gypsum. As the lake rises in response to guished from atmospheric-origin argon, and can generally be divided into two classes: in- the increased inflow, it forces the clastic dep- the initial 36C1/C1 ratio must be assumed. In tervals dominated by silt and gypsum, and osition away from the basin center as the cy- this case, an initial 36C1/C1 ratio of 70 x 10 15 ones dominated by halite. The 230Th/Z34U cle progresses. The diluted lake brines may would be required to bring the majority of ages show that the silt- and gypsum-domi- dissolve some halite from the former basin 36C1 ages into agreement with the 23(W!34U nated intervals were deposited slowly (6 to 30 floor until the brine becomes resaturated with ages. Such a low initial ratio cannot be justi- cm/ka), whereas the halite intervals were rap- respect to NaCl. Figure 3c shows the regres- fied on the basis of present surface inflow, idly deposited (140 cm/ka and greater). The sive phase of the lake cycle. As the inflow lake water, or salt flat brine measurements. 36C1/C1 ratios measured in the silt- and gyp- diminishes and the lake volume shrinks, the

TABLE 2. RADIOCHEMICAL DATA ON EVAPORITES OF CORE ZK402, QAIDAM BASIN, NORTHWEST CHINA

Sample no. Residue aU 234D a2Th 230Th 1MU/WV Uncorrected (%) (dpm/g) (dpm/g) (dpm/g) (dpm/g) age (ka)

Yl, 0.0-2.70 m 1 0.03 0.0032 ± 0.0002 0.0034 ± 0.0003 0.0021 ± 0.0001 0.0022 ± 0.0001 1.075 ± 0.113 0.636 ± 0.062 108 + 28 2 0.14 0.0099 ± Ö.0005 0.0099 ± 0.0005 0.0064 ± 0.0005 0.0067 ± 0.0005 0.995 ± 0.072 0.676 ± 0.058 123 ± 27 3 0.26 0.0248 ± 0.0013 0.0265 ± 0.0013 0.0244 ± 0.0009 0.0228 ± 0.0008 1.072 ± 0.070 0.858 ± 0.053 202 ± 44 Y10, 18.17-20.77 m 1 0.51 0.0182 ± 0.0006 0.0194 ± 0.0006 0.0132 ± 0.0005 0.0192 ± 0.0006 1.068 ± 0.045 0.987 ± 0.046 >300 2 6.86 0.155 =t 0.006 0.168 ± 0.007 0.136 ± 0.004 0.180 ± 0.005 1.085 ± 0.048 1.071 ± 0.052 >300 3 7.91 0.361 ± 0.010 0.383 ± 0.010 0.361 ± 0.010 0.441 ± 0.012 1.063 ± 0.028 1.152 ± 0.043 >300 Y15, 37.12-39.14 m 1 3.13 0.112 ± 0.003 0.118 ± 0.004 0.106 ± 0.003 0.0% ± 0.003 1.045 ± 0.038 0.815 ± 0.035 179 ± 23 2 9.84 0.216 ± 0.006 0.232 ± 0.006 0.258 ± 0.008 0.202 ± 0.007 1.074 ± 0.033 0.872 ± 0.037 212 ± 30 3 10.3 0.376 ± 0.011 0.404 ± 0.012 0.372 ± 0.011 0.326 ± 0.010 1.073 ± 0.035 0.809 ± 0.035 173 ± 21 Y20, 49.4-51.2 m 1 1.75 0.132 ± 0.004 0.149 ± 0.004 0.045 ± 0.002 0.141 ± 0.004 1.125 ± 0.036 0.945 ± 0.038 268 ± 47 2 5.61 0.383 ± 0.009 0.433 ± 0.010 0.147 ± 0.006 0.423 ± 0.012 1.130 ± 0.030 0.977 ± 0.036 311 ± 63 3 13.1 1.170 ± 0.031 1.320 ± 0.034 0.464 ± 0.017 1.260 ± 0.036 1.300 ± 0.030 0.948 ± 0.037 271 ± 46 Y24, 57.95-59.35 ni 1 0.43 0.088 ± 0.002 0.104 ± 0.003 0.0190 ± 0.0008 0.106 ± 0.003 1.182 ± 0.033 1.022 ± 0.036 >300 2 2.18 0.310 ± 0.008 0.348 ± 0.008 0.0681 ± 0.0023 0.362 ± 0.009 1.123 ± 0.023 1.038 ± 0.035 >300 3 4.89 0.719 ± 0.017 0.826 ± 0.019 0.184 ± 0.006 0.865 ± 0.021 1.149 ± 0.023 1.046 ± 0.035 >300 Y36, 86.99-89.73 m 1 10.2 0.423 ± 0.013 0.457 ± 0.013 0.286 ± 0.010 0.459 ± 0.014 1.080 ± 0.033 1.003 ± 0.042 >300 2 39.7 1.60 ± 0.04 1.66 ± 0.04 1.21 ± 0.03 1.63 ± 0.04 1.035 ± 0.020 0.985 ± 0.034 >300 3 54.8 2.78 + 0.08 3.03 ± 0.08 2.10 ± 0.05 2.96 ± 0.07 1.088 ± 0.026 0.978 ± 0.035 332 ± 76 Y47, 110.77-113.77 m 1 0.17 0.0212 ± 0.0005 0.0B1 ± 0.0006 0.0081 ± 0.0003 0.0234 ± 0.0006 1.088 ± 0.036 1.014 ± 0.037 >300 2 0.81 0.0820 ± 0.0030 0.0874 ± 0.0031 0.0304 ± 0.0010 0.0833 ± 0.0019 1.066 ± 0.047 0.953 ± 0.040 298 ± 73 3 3.5 0.296 ± 0.007 0.326 ± 0.007 0.141 ± 0.005 0.321 ± 0.009 1.102 ± 0.027 0.983 ± 0.035 335 + 76 Y56, 134.56-136.33 m 1 0.04 0.0052 ± 0.0002 0.0055 ± 0.0003 0.0034 ± 0.0001 0.0060 ± 0.0002 1.063 ± 0.065 1.090 ± 0.066 >300 2 0.39 0.0206 ± 0.0007 0.0221 ± 0.0007 0.0106 ± 0.0005 0.0249 ± 0.0007 1.071 ± 0.045 1.129 ± 0.048 >300 3 1.16 0.0739 ± 0.0024 0.0890 ± 0.0025 0.0413 ± 0.0021 0.0745 ± 0.0029 1.094 ± 0.046 0.921 ± 0.046 250 ± 51 Y81, 223.52-226.73 m 1 0.05 0.0028 + 0.0002 0.0032 ± 0.0002 0.0020 ± 0.0001 0.0031 ± 0.0002 1.160 ± 0.108 0.965 ± 0.081 284 ± 119 2 0.10 0.0064 ± 0.0003 0.0069 ± 0.0003 0.0040 ± 0.0003 0.0075 ± 0.0004 1.078 ± 0.065 1.088 ± 0.071 >300 3 0.41 0.0222 ± 0.0014 0.0239 ± 0.0014 0.0203 ± 0.0012 0.0251 ± 0.0013 1.079 ± 0.088 1.049 ± 0.083 >300

Geological Society of America Bulletin, December 1993 1607

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/12/1606/3381693/i0016-7606-105-12-1606.pdf by guest on 28 September 2021 PHILLIPS AND OTHERS

TABLE 3. ISOCHRON DERIVED AUTHIGENIC PWu), AND f23*^234^ AND AGES, USING DATA FROM TABLE 2 bined and solved for the initial conditions given above to yield Sample no. Depth f^rh/^U), Age (m) (ka) Ri Evaporites from core ZK402 Rd = (3) Y1 0.0-2.7 0.960 ± 0.050 0.266 ± 0.045 27.9 ± 9.1 A-36At Y10 18.2-20.8 1.073 ± 0.024 0.566 ± 0.030 89.5 ± 8.3 Y15 37.1-39.1 0.971 ± 0.021 0.571 ± 0.047 92.4 ± 12.6 36 Y20 49.4-51.2 1.090 ± 0.019 0.857 ± 0.119 200 ± 76 where Rd is the C1/C1 ratio at the time of Y24 58.0-59.4 1.200 ± 0.015 0.917 ± 0.036 229 ± 29 36 Y36 87.0-89.7 1.249 ± 0.016 1.12 ± 0.06 >300 halite deposition, R( is the C1/C1 ratio of 0.976 ± 0.022 1.03 ± 0.17 >300 Y47 110.8-113.8 chloride in water flowing into the lake (= i36/ Y56 134.6-136.3 1.124 ± 0.030 1.02 ± 0.58 >300 Y81 223.5-226.7 1.100 ± 0.051 1.05 ± 0.22 >300 ia), and At = t0 - td. Equation 3 may then be solved implicitly for At if Rd and R: can be measured or estimated. Such an approach to NaCl in the lake water is precipitated as hal- which the meteoric 36C1 is diluted by ancient estimating the length of lake cycles is obvi- ite. If the lake regression is sudden, a thick sedimentary chloride and diminished by de- ously a simplification and will yield only ap- bed of halite can be deposited rapidly. cay due to long residence time. This mecha- proximate cycle lengths, because all lake cy- The rate of halite deposition provides evi- nism can explain the high 36Q/C1 ratios meas- cles may not go to desiccation, and thus dence for the depositional environment. Sub- ured in the minor halite contained in silty or chloride may be retained from one cycle to aerially deposited halite from saline playas is sandy intervals of ZK 2605, and which was the next, and because it does not account for normally deposited relatively slowly, and probably precipitated subareally from evap- any chloride dissolved from the lake bed dur- thus thick halite units require long accumu- oration of shallow ground water in these clas- ing transgressions. These factors would lation times. The halite is usually dirty or tic deposits. cause the cycle length to be overestimated. thinly interbedded with clastic sediments. Nevertheless, the 36C1/C1 ratio at the time of Rapid deposition of thick units composed of Chloride and 36C1 Budgets deposition does contain useful information clean halite requires evaporation of a large about the longevity of lake cycles.

reservoir of brine. Radiometric ages on such Suppose that at time t = tQ the lake is dry The thickness of the halite beds also con- units thus date the desiccation of a large lake and begins a refilling cycle. At that time, the tains information on the minimum water which persisted prior to the deposition of the inventory of chloride in the lake water (on a depth during the lake cycles. The WTh ages 36 halite. Clear examples of these changes are molar basis), Ma, is zero, as is the C1 in- of the uppermost halite bed (0-20 m) and the found in the halite units deposited near 165 ka ventory, M36. During the part of the cycle beds between 33 and 70 m indicate that these and 16 ka at Gasikule. prior to precipitation of halite, at t = td (where halite beds were deposited very rapidly, The 36C1/C1 ratio of the halite deposited "d" indicates deposition), the accumulation probably in too short an interval to be re- during regressions need not necessarily equal of chloride in the lake waters is given by solved by the U/Th method. Under these cir- that of the water flowing into the lake. If the cumstances, the minimum water depth nec-

lake were stable at a low level for a long pe- dMc¡ essary to precipitate the halite can be riod prior to the transgressive-regressive cy- lo (1) calculated using halite solubility in water of dt 3 cle, the 36C1 stored in the lake water would 360 g/L and halite density of 2.16 g/cm .

decay before it was ultimately precipitated, where ic, is the average influx of chloride to These values indicate that each meter of hal- yielding halite with lower ratios than the in- the lake and t is time. Similarly, but account- ite deposited requires the evaporation of a flow. Alternatively, during transgressive ing for radiodecay, the accumulation rate of minimum of 6 m of halite-saturated brine; the phases, higher local precipitation would bring 36C1 is given by depth would be greater if the brine were 36 undersaturated. in meteoric chloride with higher C1/C1 ratios dM-ib than that in the river inflow and lake water, in ¿36 - \36M36 = —— (2) A simple reconstruction of the lake-level dt history was performed using the following 36 where i36 is the influx of C1 to the lake basin procedure. (1) The core chronology was in- 36 J6 and \36 is the decay constant of C1 (2.30 x terpolated between the measured U/Th ages, TABLE 4. C1/C1 RATIOS OF SURFACE WATER AND 6 SALINE PLAYA SAMPLES FROM THE 0AIDA M BASIN 10~ yr~'). Jannik and others (1991) have using as a guideline the sedimentation cycle shown that these two equations can be com- outlined in Figure 3. The interpolated chro- 36 Sample no. Location C1/10 "a

Surface water samples QBW-1 Oarhan River 133 ± 13 TABLE 5. CHL0RINE-36 AND PALEOMAGNETIC DATA FROM CORE ZK 402, DRILLED AT DALANGTAN DRY PLAYA QBW-2 Nuomuhong River 378 ± 27 QBW-3 Takole River 34 ± 7 QBW-4 Gotaud River 409 ± 46 36 15 Sample no. Depth C1/10 C1 Apparent QBW-6 Dabuxum Lake 36 ± 6 age (ka) QBW-7 Tataleng River 391 ± 29 QBW-8 Yuka River 507 ± 29 QBW-9 Dezengmahai Lake 9 ± 6 QBS-4 Surface 41 ± 4 0 QBW-10 Alar River 99 I 5 QBC-ZK-402-Y1 0-2.7 40 ± 10 11 ± 125 QBW-11 Timuleek River 138 ± 16 QBC-ZK-402-Y15 37.1-39.1 26 ± 5 198 ± 75 QBW-12 Gasikule Salt Fiat brine 98 ± 15 QBC-ZK-402-Y24 57.9-59.3 30 ± 4 136 ± 62 QBW-13 Gasikule Lake 88 ± 10 QBC-ZK-402-Y45 101.8-103.8 18 ± 4 357 ± 90 Playa halite samples ZK-402-Y115 320 750 (B/M reversal) QBC-ZK-402-Y144 420.1-422.9 1.7 ± 3 >940 QBC-6UI-1 Oarhan Playa drill core, 39.3 m 28 t 5 QBS-2 Oarhan Playa 36 ± 5 36 36 15 QBS-3 Kunteyi Playa 39 ± 5 Note : apparent C1 age s calculated using an assumed initial C1/Ci ratio of 41 x 10" .

1610 Geological Society of America Bulletin, December 1993

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/12/1606/3381693/i0016-7606-105-12-1606.pdf by guest on 28 September 2021 DATING EVAPORITE DEPOSITS, CHINA

0 paleomagnetic datum and the oldest U/Th age (302 ka) at 109.7 m is noteworthy for two characteristics: the very low sedimentation rate compared to the sediments above, and 36 15 20 the very low C1/C1 ratio of about 57 x 10" at the time of deposition of the halite at 111 and 113 m. This interval of 30 m of sediment represents 440 ka of depositional time. In contrast, during the following 300 ka, 110 m 40 of sediment was deposited. This difference in sedimentation rate can be attributed to the lack of halite beds in the lower interval, ex- •4 ••••: ••••.»: cept for the very top. We interpret this slow 60 sedimentation of "drab gypsum and silt" as ja subaerial deposition under a climate more hu- « mid than that responsible for the present salt- &a. flat conditions (because under present condi- : * tions, runoff is not sufficient to carry clastic 80 sediment onto the salt flats), but not humid * v * - enough to raise the lake above the sample (T * elevation. The lake cycle was initiated about # 800 ka ago, as inferred from the calculated 36 100 # C1 residence time of 500 ka plus the U/Th i • *» »• age of 302 ka for the first halite formation (Ta- ble 1). The very long chloride residence time implies relatively constant climatic condi- tions, with no fluctuations sufficient to cause 120 desiccation. t The interval between about 302 and 173 ka was characterized by a higher sedimentation A • rate of silt and gypsum, indicating near-subaer- 140 ial deposition, perhaps under a more humid 200 400 0 200 climate. This is supported by the high Rd in 15 Lithology 230Th/234u age [ka] the middle of the interval (135 x 10" at 90-m depth), indicative of local inflow rather than lake water. The chloride residence time of 400 ka inferred from the 36C1/C1 ratio at 67 E 1 EB m is much longer than the time since the pre- Halite Sand Silt Clay Gypsum vious halite deposition episode at 300 ka. This Figure 2. Stratigraphic column, U/Th ages, and 36C1 data for core ZK 2605 from Gasikule. indicates either incomplete desiccation at 300 Measured 36CI/C1 ratios are indicated by diamonds and calculated 36C1/C1 ratios (based on U/Th ka or else substantial re-solution of halite de-

ages) at the time of deposition (Rd) by squares. posited at that time, rendering the residence time an upper-limit estimate. Incomplete des- nology is indicated in Figure 2. (2) The inter- cycle. Values used in the calculations and the iccation is much more likely than re-solution, polated U/Th ages were used to calculate the calculated results are given in Table 1. The given the almost 40 m of silt deposited on top 36 C1/CI ratios (Rd's in Table 1) at the time of lacustrine history reconstructed using this of the halite from the previous lake cycle. 36 deposition (td) of the halite sampled for C1. procedure is illustrated in Figure 4. The re- This pattern of low calculated depositional 36 (3) Equation 3 was used to calculate the ac- construction is referenced to the elevation of C1/C1 ratio continues through the lake cy- cumulation time (At) of the chloride in the the sediment surface at the core site at the cles dating to 138 ka, again probably indicat- lake water. The initial 36C1/C1 ratio (R,) was time of deposition, to show changes in the ing that the lake regressions did not proceed assigned a value of 100 x 10~15, based on the water depth; it does not include lake-bed to desiccation. The next cycle after 138 ka (no 36 ratios measured in the Alar River (the major aggradation. U/Th age available) shows a C1/C1 ratio (104 1S 36 inflow), the Timuleek River, and the ground- x 10" ) nearly equal to the measured C1/C1 water inflows on the salt flats (samples QBW- Lacustrine History Based on the ZK 2605 Data of the lake inflow, indicating that the lake 11, 12, and 13 in Table 4). This calculated completely desiccated at the end of the pre- residence time was then used as an estimate Chronological control is provided at the vious cycle. There appears to be a hiatus be- of the duration of the lake cycle. (4) Finally, base of the studied interval by the Brunhes/ tween —120 and 20 ka. Either the lake was at the individual halite bed thicknesses (Fig. 2) Matuyama magnetic reversal (750 ka; Izett a low level and the core site on a stable salt were used to estimate the minimum water and others, 1988) at 137.85-m depth (Huang flat (as at present) or else at a higher level, but depth that evaporated at the end of the lake and Phillips, 1990). The interval between this any evaporites deposited during the interval

Geological Society of America Bulletin, December 1993 1611

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/12/1606/3381693/i0016-7606-105-12-1606.pdf by guest on 28 September 2021 PHILLIPS AND OTHERS

(a) Unfortunately, with no present inflow from which to obtain samples, it is not pos- sible to measure the 36C1/C1 entering the lake. This, combined with the relatively small number of U/Th and 36C1 measurements, does not permit the reconstruction of a de- tailed lacustrine history. The general se- quence of events, however, is consistent with that reconstructed at Gasikule. The oldest fi- nite U/Th ages (229 and 200 ka) lie at the top of a 50-m-thick sequence dominated by clas- tics. This appears to correlate with the clas- (b) tic-dominated section from 108 to 70 m (300 to 200 ka) at Gasikule. The ages of 158 and 173 ka for the halite terminating the thick clastic section at Gasikule are within the an- alytical uncertainty of the 200 ± 76 ka age on the corresponding halite at Dalangtan. The next two lacustrine cycles (between 50- and 20-m depth) at Dalangtan both have ages close to 90 ka. They may correlate with the poorly constrained lacustrine cycle indi- cated by clastic sediments at 30-m depth in ZK 2605, shown with question marks in Fig- (c) ure 4. The final lacustrine episode terminated with halite deposited during the interval 19 to 37 ka. Independent regional evidence (Fang, 1991) indicates that the earlier part of this pe- riod was the wettest part of the last 40 ka. Dalangtan evidently desiccated earlier than the final transgression at Gasikule due to the Figure 3. Conceptual model for transgressive-regressive lacustrine cycles at Gasikule. (a) smaller inflow to Dalangtan. Steady-state arid conditions (present time). Low lake level and extensive salt flatswit h insufficient The depositional 36C1/C1 ratios calculated runoff to transport clastics over salt flats.Diamond s indicate salt precipitation, (b) Transgression. at Dalangtan, based on the U/Th ages, vary Increased runoff deposits silt over salt flats, lake rises to cover salt/silt flats, and some halite may from 36 X 10~1S to 51 x 10"15, much lower be redissolved from lake bed. (c) Regression. Reduction in runoff/evaporation ratio forces lake- than at Gasikule. This lower ratio is ex- level reduction; halite is precipitated from lake waters. pected, given the expanses of Pliocene evaporites exposed within the Dalangtan Ba- were redissolved during a late transgression. langtan basin, implies a somewhat different sin. Although the variations in initial ratio do Both alternatives are shown in Figure 4. The interpretation of the depositional history than not allow accurate 36C1 dating of the salts, the high 36C1/C1 ratio measured at 20.9 m occurs at Gasikule. The Dalangtan sediments (Fig. 5) fairly regular decrease of the 36C1/C1 ratio at the very beginning of the last lake cycle and are characterized by repeated sequences of with depth does indicate that 36C1 dating

presumably resulted from local inflow ac- silts and clays overlain by mirabilite (Na2S04 • might be used at Dalangtan or similar playas companying the deposition of clastic silts 10H20) or bloedite [Na2Mg(S04)2 • 5HzO] for approximate age estimates beyond the during the beginning of the transgression. Fi- that grades upward into halite. These record range of U/Th. nally, the U/Th ages indicate that the last lake cycles of infilling, accompanied by clastic cycle ended ca. 16 ka with the deposition of deposition, followed by desiccation and COMPARISON WITH OTHER LOCAL more than 14 m of halite. evaporite sedimentation, with the least solu- PALEO C LI MATE RECORDS ble evaporites deposited first. The playa is Results from Core ZK 402 now dry and contains no clastic sediments on The lake-level reconstruction in Figure 4 top of the uppermost halite bed, which was shows lake highs in the intervals 350-300 ka, Nine U/Th and five 36C1/C1 measurements deposited at about 28 ka, the end of the most 175-140 ka, and 30-15 ka. These highs ap- were performed on samples from the ZK 402 humid interval during the late Quaternary parently correspond to marine oxygen-iso- core (Tables 2, 3, and 5). The distribution of (Fang, 1991). The Dalangtan basin is evi- tope stages 8, 6, and 2 (Imbrie and others, U/Th ages and of 36C1/C1 ratios with depth is dently flooded only during the wettest peri- 1984). Although the uncertainty for the age of illustrated in Figure 5. Unlike core ZK 2605, ods, and during drier ones receives virtually the oldest interval (302 ± 56 ka) allows only which was drilled on the margin of Gasikule no sediment at all. The sediment record is weak correlation with stage 8, the U/Th ages lake, ZK402 was drilled near the lowest point thus probably quite discontinuous. In con- of 138 ± 6 ka and 16.3 ± 2.2 ka for the ter- of the now-dry Dalangtan playa. This, cou- trast to Gasikule, clastic sediments probably minations of the two most recent major lake pled with the much smaller inflow to the Da- record only maximum lake levels. cycles fix the endings of the wet episodes in

1610 Geological Society of America Bulletin, December 1993

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/12/1606/3381693/i0016-7606-105-12-1606.pdf by guest on 28 September 2021 DATING EVAPORIATE DEPOSITS, CHINA

0 that aid in a more accurate resolution of the chronology and magnitude of lake fluctua- tions in the region. Chen and Bowler (1986) obtained ages ranging from 37 to 29 ka from shells sampled on a beach ridge about 25 m 50 above the present elevation of the Qarhan salt pan. At Xiao Qaidam, a sub-basin on the northern margin of the Qaidam depression, Bowler and others (1986) reported that an os- 100 tracode shell concentrate yielded a 14C age of 14.7 ka. At Kunteyi playa, in the northwest corner of the Qaidam Basin, they determined two ages of 14 and 15.5 ka on oolitic grains Figure 4. Reconstructed lake- 150 from a beach about 40 m above the present level history at Gasikule for the playa elevation. past 350 ka. Lake-surface eleva- Age These dates are consistent with the U/Th tions are reconstructed relative to [ka] ages for the same period from Gasikule and the sediment surface at the core also with the 14C stratigraphic data from Qar- location. Arrows indicate U/Th 200 han. All of these data indicate that the lakes ages from Table 1. Dashed lines of the Qaidam Basin rose to relatively high indicate relatively poorly con- levels before 37 to 29 ka and maintained those strained lake levels. highstands, with some late oscillations, until 250 about 14 ka, when a regression that was prob- ably fairly rapid led to desiccation no later than 9 ka. This inferred history can be checked using data from a nearby ice core.

300 Ice-Core Data

Thompson and others (1989) drilled an ice core from the top to the base of the Dunde ice 350 cap at the summit of the Qilian Shan, on the ) 0 50 100 eastern border of the Qaidam Basin. Chem- Water level [m above core location] ical and particulate analysis of the ice core has provided an excellent record of atmo- spheric deposition in the region. They found the deglaciation phases of isotope stages 6 others, 1983; Bowler and others, 1986), both that latest Pleistocene ice contained two to and 2. High lake levels (and thus relatively to the east of Gasikule in the Qaidam Basin four times the dust concentration of Holo- favorable water balances) during glacial max- (Fig. 1). The best data (from organic carbon cene ice, consistent with greater deposition ima, especially the last one, are in clear con- disseminated in the core) indicate clastic dep- of dust on the Loess Plateau and the Pacific flict with the hypothesis that increased dust osition under relatively dilute lake-water con- Ocean (Hovan and others, 1989; 1991) during deposition during glacial maxima on the centrations for some indeterminate time prior glacial maxima. The dust concentration was Loess Plateau (Kukla and An, 1989) and in to ca. 24 ka. Subsequently, three episodes of well correlated with light oxygen-isotope the North Pacific (Hovan and others, 1989) is halite precipitation occurred in the lake be- values in the ice itself—strong evidence of caused by increased aridity in the Central fore desiccation took place between 16 and 9 decreased summer temperature. The dust Asian source areas. To help to evaluate the ka. This history is in good agreement for that concentration, however, was negatively cor- reliability of our reconstruction, we compare inferred for Gasikule during the same period. related with the concentrations of major an- it with three independent sources of paleohy- Gasikule may not have responded with halite ions in the ice. At 129.2 m in the core, there drological information: (1) other lacustrine deposition to the arid episodes between 24 is an abrupt transition. The dust concentra- stratigraphic evidence from the Qaidam Ba- and 16 ka because of its greater relative in- tion above this depth is lower by a factor of sin, (2) shoreline evidence from the same flow, whereas Dalangtan, with much more three than below it, but the concentrations area, and (3) ice-core evidence from the restricted sources of inflow, apparently des- of sulfate and chloride are higher by a fac- nearby Qilian Shan. iccated earlier, during the first episode of hal- tor of three to four above 129.2 m than below ite precipitation at Qarhan. it. This transition is explained by Thompson Other Lacustrine Stratigraphic Records and others (1989) as a result of an abrupt cli- Shoreline Evidence mate transition in the Qaidam Basin, going Radiocarbon chronologies have been pub- from a relatively humid but windy climate lished for Lake Qarhan (Chen and Bowler, Chen and Bowler (1986) and Bowler and during which the lake and playa bottoms 1985, 1986; Bowler and others, 1986) and others (1986) have published 14C ages on were protected from deflation by being under Dabuxun (a sub-basin of Qarhan) (Huang and shells from shorelines in the Qaidam Basin water, to a less windy but more arid climate

Geological Society of America Bulletin, December 1993 1607

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/12/1606/3381693/i0016-7606-105-12-1606.pdf by guest on 28 September 2021 PHILLIPS AND OTHERS

0 (that is, from about 40 to 24 ka). Lake levels in western China were generally somewhat Km lower during the last glacial maximum. With regard to the termination period of the last glaciation, Fang (1991) summarized: "In re- gion R^ [which includes the Qaidam Basin], lake levels rose dramatically and reached 50 their late-glacial and postglacial maxima be- tween 15,000 and 12,000 yr B.P How- ever, widespread evidence . . . indicates that western China, like other areas of the coun- try, experienced prolonged and intense arid- IS® ity between 16,000 and 10,000 yr B.P." Our m lacustrine chronology reconciles these some- 100 what contradictory summaries by indicating a late-glacial episode of very favorable water g balance culminating between 16 and 14 ka fi o. and immediately succeeded by an episode of o Q sudden desiccation. Gasikule is near the center of the southern 150 dust-source areas in eastern Central Asia (the Tarim and Qaidam Basins). The northern dust-source areas (the Dzungar Basin and ) are more distant. Unfortu- m nately, paleohydrologic data from these areas are both scanty and contradictory. Discus- sions of previous research by Flint (1971, 200 p. 672) and Fang (1991, p. 49) indicate that the pattern of water-balance fluctuations there iiii 1—r may have been similar to that of the Qaidam LLLLLI Basin. Tffff Not included in Fang's (1991) review are more recent detailed lacustrine studies in the region by Kelts and others (1989), Lister and -ou others (1991), Kashiwaya and others (1991), 0 250 500 0 25 and Gasse and others (1991). None of these 23 36 15 36 studies includes data from the period prior to Lithology »»Th/ ^ age [ka] C1/10 CI Apparent C1 age [ka] 14 ka, but all of them do indicate that lake levels were very low in the early part of the period between 14 and 10 ka, in agreement Œ with our reconstruction. The general agree- ment between our reconstruction and the re- Halite Mirabilite/bloedite Mud or clay Mud and evaporites sults of the geographically widely distributed Figure 5. Stratigraphie column, U/Th ages, 36CI/C1 data and apparent 36C1 ages for core ZK studies cited by Fang (1991), as well as the 402 at Dalangtan playa. Dashed line in "U/Th age" column connects oldest finite U/Th age with more recent ones discussed above, suggests Brunhes/Matuyama paleomagnetic boundary at 340-m depth. Circles with arrows indicate in- that the Gasikule reconstruction has regional definite (limiting minimum) U/Th ages. Symbols for the 36CI/C1 data points are the same as in significance for the water-balance history of Figure 2. the dust-producing area in Central Asia.

IMPLICATIONS FOR GLACIAL-PERIOD during which the desiccation of the lakes REGIONAL COMPARISONS AND ARIDITY made available abundant sulfate and chloride CAUSES OF LAKE FLUCTUATIONS salts for transport to the ice cap (Thompson Both independent stratigraphic and shore- and others, 1989). Thompson and others Fang (1991) has recently reviewed the re- line data, as well as ice-core data, strongly (1989) used age-depth extrapolation to esti- sults of studies on late Quaternary lake fluc- support the reconstruction of the last glacial mate the age of this transition as about 12 ka; tuations in China. His summary of results maximum in the Qaidam Basin as relatively given the uncertainties in this extrapolation, from western China is in general agreement more humid than the Holocene. Although in- this is in reasonable agreement with the 14C with our synthesis from the Qaidam Basin. dependent data are not available for earlier and U/Th ages of 14 to 16 ka for the corre- His compilation indicates very high lake lev- periods, this correspondence for the last sponding lacustrine sediment transition. els during the middle of the last glaciation maximum indirectly supports the high lake

1610 Geological Society of America Bulletin, December 1993

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/12/1606/3381693/i0016-7606-105-12-1606.pdf by guest on 28 September 2021 DATING EVAPORIATE DEPOSITS, CHINA

levels reconstructed for previous glacial max- creased wetness. Simple global heat-balance itation on the high peaks of the Kunlun Shan, ima. This observation raises two questions. models have suggested that the Earth may be but the increases probably were not sufficient (1) Can the lake-level pattern inferred for the characterized by a bimodal (or multimodal) to induce the growth of vegetation on the ba- Qaidam Basin be explained in terms of global climate regime, with the present mode being sin floor. climatic patterns? (2) How can the observed characterized by vigorous poleward oceanic Increased precipitation may even favor dust fluxes associated with glacial maxima be heat transport. An alternative (glacial) mode dust mobilization in hyperarid deserts (Pye, explained if they are not caused by increased has markedly reduced oceanic heat trans- 1989). At the present time, much of the to- aridity? port, but correspondingly increased atmo- pography of the Qaidam Basin floor is stabi- We believe that the first question can be at spheric advection (Birchfield and others, lized due to eolian deflation down to a limit least partially addressed by comparing the in- 1990). In addition to transporting additional imposed by resistant layers of halite and gyp- ferred Qaidam lacustrine history with other heat, the increased atmospheric circulation sum. Increased precipitation would leach lacustrine records from similar latitudes. Spe- would transport more water vapor, helping to more of the evaporite minerals out of the soil cifically, the Qaidam reconstruction in Figure explain the elevated lake levels. Presumably, surface, releasing the silt interbedded with 4 bears a striking resemblance to long-term the increased circulation would also have the the evaporites and making it available for de- lake-level reconstructions in the southwest- effect of enhancing both dust mobilization flation. Increased precipitation could also re- ern United States (compare with Oviatt and and transport. sult in increased runoff which would trans- others, 1987; McCoy, 1987; Lao and Benson, Previously, the high rate of dust deposition port deflatable silt to alluvial fans and playas. 1988; Jannik and others, 1991). Aside from in the North Pacific during glacial maxima In general, it seems likely that in the hyper- the general similarity in the timing of high and has been ascribed to increased aridity rather arid climate of the Qaidam Basin, the positive low lake levels, the best-dated event in both than to enhanced atmospheric circulation be- effects of readily available fine sediment due areas is a relatively sudden decline from high cause the eolian mass-accumulation rate of to increased runoff would outweigh any neg- lake levels at close to 13 ka (Benson and oth- dust was observed to be uncorrelated to the ative effects on dust availability due to veg- ers, 1990). This correspondence both in long- median grain size of the dust (Prospero, 1985; etation growth (Pye, 1989). term trends and in very rapid transitions at Rea and others, 1985). If increased wind ve- these two distant (but identical in latitude) ar- locity were responsible for increasing dust CONCLUSIONS eas suggests that their climatic histories are mobilization, then larger grains should be as- linked by global transitions in zonal atmo- sociated with higher dust deposition. More We have used lithologic logs, WTh iso- spheric circulation. We believe that a strong recent evidence may have weakened this in- chrons, and 36C1/C1 ratios to reconstruct the case has been made for an association of ference (Olivarez and others, 1991), but we lake-level history in the Gasikule and Dalang- tropical aridity (and low lake levels) with gla- suggest that the frequency and persistence of tan sub-basins of the western Qaidam Basin. cial maxima (Street and Grove, 1979; Kutz- strong winds may have played a more impor- Our reconstruction indicates that transitions bach and Street-Perrott, 1985; Pokras and tant role than did intensification of wind ve- from high to low lake level were associated Mix, 1987; Lezine and Casanova, 1991). locity. Our own field observations, along with with the terminations of marine isotope Such an association could help to explain en- those of many others (for example, Zhang, stages 2, 6, and probably 8; thus high lake hanced glacial-dust fluxes in tropical ocean 1984), demonstrate that present-day winds in levels were associated with global glacial cores. Such a strong case has not been made, the Central Asian deserts are capable of rais- maxima. The ratio of lake-surface area to ba- however, for the mid-latitudes. Instead, the ing basin-scale dust clouds. Such dust storms sin area is equivalent to the ratio of precipi- preponderance of the evidence seems to in- occasionally penetrate as far east as Beijing tation to evaporation (Street-Perrott and Har- dicate that (in the Northern Hemisphere, at (Liu and others, 1981), but these events are rison, 1985); thus the lacustrine history least), most or all of the 30°-45° latitude belt not common. Winds no stronger than those indicates that glacial maxima were relatively was characterized by more positive water of the present, but blowing several times as more humid and interglacials more arid. This balances during glacial maxima than during often per and for longer periods per pattern conflicts with the widely accepted interglacials (Street and Grove, 1979; Smith event, would result in greatly enhanced dust idea that increased aridity in the Central and Street-Perott, 1983; Szabo, 1990; Fang, transport to the Loess Plateau and Pacific Asian source area was largely responsible for 1991; Prentice and others, 1992). This in- Ocean. enhanced dust fluxes in China and the North crease in relative wetness can probably be Enhanced dust mobilization is not neces- Pacific during glacial maxima. attributed mainly to three factors: (1) de- sarily incompatible with a more humid cli- We believe that the hypothesis of mid-lat- creases in temperature which resulted in de- matic regime in the study area. At the present itude aridity during glacial maxima, and con- creased evaporation, (2) southward shifts in time, the central Qaidam Basin is unvege- sequent increased dust availability, should be the belt of maximum mid-latitude precipita- tated. Pollen studies on ZK 2605 (Huang Qi, re-examined. As an alternative, we suggest tion (now about 45°-60°) due to a general unpub. data) show that throughout the entire that greater persistence of strong winds was equatorward contraction of zonal circulation, dated interval at Gasikule, the pollen spectra the major factor in producing the enhance- and (3) enhanced rates of atmospheric cir- are dominated by herbaceous species, prob- ment of glacial-period dust transport. More culation which advected increased amounts ably originating from the small riparian area persistent strong winds on a global scale may of marine moisture into the continental at the point where the Alar and Timuleek Riv- prove to be a hallmark of full glacial climate. interiors. ers flow into Gasikule. The pollen counts Enhanced atmospheric circulation is of present no evidence for increased vegetation ACKNOWLEDGMENTS particular importance because it may provide on the now-barren slopes of the drainage ba- a link between the apparently paradoxical sin. The discharge responsible for the rise in This research was supported by the U.S. connection of greater dust transport with in- lake levels originated from increased precip- National Science Foundation through Grant

Geological Society of America Bulletin, December 1993 1607

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/12/1606/3381693/i0016-7606-105-12-1606.pdf by guest on 28 September 2021 PHILLIPS AND OTHERS

EAR-8816278 and by the Re- Hammer, C. U., Clausen, H. B., Dansgaard, W., Neftel, A., a shallow early or middle Wisconsin-age lake in the Bonne- Kristinsdottir, P., and Johnson, E., 1985, Continuous impu- ville Basin, Utah: Quaternary Research, v. 27, p. 248-262. search Fund administered by the American rity analysis along the Dye 3 deep core, in Langway, C. C., McCoy, W. D., 1987, Quaternary aminostratigraphy of the Bon- and Dansgaard, W., eds., Greenland ice core: Geophysics, neville Basin, western United States: Geological Society of Chemical Society through grant 20875-AC2. geochemistry, and the environment: Washington, D.C., America Bulletin, v. 98, p. 99-112. Field work in the Qaidam Basin was sup- American Geophysical Union Monograph 33, p. 90-94. Petit, J. R., Mounier, L., Jouzel, J., Korotkevich, Y. S., Kotlyakov, Hovan, S. A., Rae, D. K., Pisias, N. G., and Shackleton, N. J., V. I., and Lorius, C., 1990, Paleoclimatological and chrono- ported by the Salt Lakes Institute, Académ- 1989, A direct link between the China loess and marine SlsO logical implications of the Vostok dust core record: Nature, records: Aeolian flux to the North Pacific: Nature, v. 340, v. 343, p. 56-58. ica Sínica, Xining. We thank Xia Ming for p. 296-298. Pokras, E. M., and Mix, A. C., 1987, Earth's precession cycle and help in the sample collection and Stewart S. Hovan, S. A., Rea, D. K., and Pisias, N. G., 1991, Late Pleistocene Quaternary climatic change in tropical Africa: Nature, v. 326, 36 continental climate and oceanic variability recorded in north- p. 486-487. Smith for laboratoiy work on the CI sam- west Pacific sediments: Paleoceanography, v. 6, p. 349-370. Prentice, 1. C., Guiot, J., and Harrison, S. P., 1992, Mediterranean Huang, Q., and Phillips, F. M., 1990, Preliminary study on chlo- vegetation, lake levels, and paleoclimate at the last glacial ples. David K. Rea provided helpful com- rine-36 dating of halite in salt lakes from Qaidam Basin: Chi- maximum: Nature, v. 360, p. 658-660. ments, and Daniel Muhs and Paul Bierman nese Science Bulletin, v. 35, p. 32-36. Prospero, J. M., 1985, Records of past continental climates in deep- Huang, Q., Bowler, J. M., Polach, H. A., and Head, J., 1983, Car- sea sediments: Nature, v. 315, p. 279-280. offered constructive reviews of the bon isotope study of lakes Qarhan and Frome sediments: Chi- Prospero, J. M., and Nees, R. T., 1986, Impact of the North African nese Journal of Oceanology and Limnology, v. 1, p. 314-325. drought and El Nino on mineral dust in the Barbados trade manuscript. Imbrie, J., Hays, J. D., Martinson, D. G., Mclntyre, A., Mix, A. C., winds: Nature, v. 320, p. 735-738. Morley, J. J., Pisias, N. G., Prell, W. L., and Shackleton, Pye, K., 1989, Processes of fine particle formation, dust source re- N. J., 1984, The orbital theory of Pleistocene climate: Support gions, and climatic changes, in Leinen, M., and Samthein, REFERENCES CITED from a revised chronology of the marine 8,sO record, in M., eds., Paleoclimatology and paleometeorology: Modern Berger, A., Imbrie, J., Hays, J., Kukla, G., and Saltzman, B., and past patterns of global atmospheric transport: Dordrecht, Benson, L., and Thompson, R. S., 1987, The physical record of eds., Milankovitch and climate, understanding the response The Netherlands, Kluwer Academic, p. 3-30. lakes in the Great Basin, in Ruddiman, W. F., and Wright, to orbital forcing, Part 1: Norwell, Massachusetts, D. Reidel, Rea, D. K., and Leinen, M., 1988, Asian aridity and the zonal west- H. E., Jr., eds., North America and adjacent oceans during p. 269-305. erlies: Late Pleistocene and Holocene record of eolian dep- the last deglaciation, The geology of North America, Volume Izett, G. A., Obradovich, J. D., and Mehnert, H. H., 1988, The osition in the northwest Pacific Ocean: Palaeogeography, K-3: Boulder, Colorado, Geological Society of America, Bishop ash bed (middle Pleistocene) and some older (late Palaeoclimatology, Palaeoecology, v. 66, p. 1-8. p. 241-260. Pliocene and Pleistocene) mineralogically and chemically Rea, D. K., Leinen, M., and Janecek, T. R., 1985, Geologic ap- Benson, L. V., Currey, D. R., Dorn, R. I., Lajoie, K. R., Oviatt, similar ash beds in California, Nevada, and Utah: U.S. Ge- proach to the long-term history of atmospheric circulation: C. G., Robinson, S. W., Smith, G. I., and Stine, S., 1990, ological Survey Professional Paper 1675, 37 p. Science, v. 227, p. 721-725. Chronology of expansion and contraction of four Great Basin Jannik, N. O., Phillips, F. M., Smith, G. I., and Elmore, D., 1991, Rea, D. K., Hovan, S. A., and Janecek, T. R., 1991, Late Quater- lake systems during the past 35,000 years: Palaeogeography, A ^Q chronology for lacustrine sedimentation in the Pleis- nary flux of eolian dust to the pelagic ocean, in Hay, W. W., Palaeoclimatology, Palaeoecology, v. 78, p. 241-286. tocene Owens River System: Geological Society of America ed., Geomaterial fluxes, glacial to Recent: Washington, D.C., Bentley, H. W., Phillips, F. M., and Davis, S. N., 1986a, Chio- Bulletin, v. 103, p. 1146-1159. National Academy of Science. rine-36 in the terrestrial environment, in Fritz, P., and Fontes, Kashiwaya, K., Yaskawa, K., Yuan, B., Lin, J., Gu, Z., Cong, S., Rockhill, W. W., 1894, Diary of a journey through Mongolia and J.-C, eds., Handbook of environmental isotope geochemis- and Masuzawa, T., 1991, Paleohydrological processes in Sil- Tibet in 1891 and 1892: Washington, D.C., Smithsonian try, Volume 2B: Amsterdam, The Netherlands, Elsevier, ing-Co (Lake) in the Qing-Zang (Tibetan) Plateau based on Institution. p. 427-480. the physical properties of its bottom sediments: Geophysical Smith, G. I., and Street-Perrott, F. A., 1983, Pluvial lakes of the Bentley, H. W., Phillips, F. M., Davis, S. N., Airey, P. L., Calf, Research Letters, v. 18, p. 1779-1781. western United States, in Wright, H. E., Jr., ed., Late Qua- G. E., Elmore, D., Habermehl, M. A., and Torgerson, T., Kelts, K., Zao, C.-K., Lister, G., Qing, Y.-J., Hong, G.-Z., ternary Environments of the United States; Porter, S. C., ed., 1986b, Chlorine-36 dating of very old ground water: I. The Niesser, F., and Bonani, G., 1989, Geological fingerprints of Volume 1, The late Pleistocene: Minneapolis, Minnesota, Great Artesian Basin, Australia: Water Resources Research, climate history: A cooperative study of , China: University of Minnesota Press, p. 190-212. v. 22, p. 1991-2002. Ecologae Geologae Helvetiae, v. 82/1, p. 167-182. Street, F. A., and Grove, A. T., 1979, Global maps of lake-level Birchfield, G. E., Wang, H., and Wyant, M., 1990, A bimodal cli- Kukla, G., 1987, Loess stratigraphy in central China: Quaternary fluctuations since 30,000yr B.P.: Quaternary Research, v. 12, mate response controlled by water vapor transport in a cou- Science Reviews, v. 6, p. 191-219. p. 83-118. pled ocean-atmosphere box model: Paleoceanography, v. 5, Kukla, G., and An, Z.-S., 1989, Loess stratigraphy in central China: Street-Perrott, F. A., and Harrison, S. P., 1985, Lake levels and p. 383-396. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 72, climate reconstruction, in Hecht, A. D., ed., Paleoclimate Bowler, J. M., Huang, Q., Chen, K., Head, M. J., and Yuan, B., p. 203-226. analysis and modeling: New York, John Wiley & Sons, 1986, Radiocarbon dating of playa-lake hydrologic changes: Kutzbach, J. E., and Street-Perrott, F. A., 1985, Milankovitch forc- p. 291-340. Examples from northwestern China and central Australia: ing of fluctuations in the level of tropical lakes from 18 to 0 kry Szabo, B. J., 1990, Ages of travertine deposits in eastern Grand Palaeogeography, Palaeoclimatology, Palaeoecology, v. 54, BP: Nature, v. 317, p. 130-134. Canyon National Park, Arizona: Quaternary Research, v. 34, p. 241-260. Lai, D., 1991, Cosmic-ray tagging of erosion surfaces: In situ pro- p. 24-32. Chen, K., and Bowler, J. M., 1985, Preliminary study on sediment duction rates and erosion models: Earth and Planetary Sci- Thompson, L. G., Mosley-Thompson, E., Davis, M. E., Bolzan, characteristics and evolution of palaeoclimate of Qarhan Salt ence Letters, v. 104, p. 424-439. J. F., Dai, J., Yao, T., Gundestrup, N., Wu, X., Klein, L., Lake in Qaidam Basin: Scientia Sinica (ser. B), v. 28, Lao, Y., and Benson, L., 1988, Uranium-series age estimates and and Xie, Z., 1989, Holocene-late Pleistocene climatic ice core p. 1218-1231. paleoclimatic significance of Pleistocene tufas from the La- records from Qinghai-Tibetan Plateau: Science, v. 246, Chen, K., and Bowler, J. M., 1986, Late Pleistocene evolution of hontan Basin, California and Nevada: Quaternary Research, p. 474-477. salt lakes in the Qaidam Basin, Qinghai Province, China: v. 30, p. 165-176. Van Devender, T. R., Thompson, R. S., and Betancourt, J. L., Palaeogeography, Palaeoclimatology, Palaeoecology, v. 54, Lezine, A. M., and Casanova, J., 1991, Correlated oceanic and con- 1987, Vegetation history of the deserts of southwest North p. 87-104. tinental records demonstrate past climate and hydrology of America; the nature and timing of the late Wisconsin-Holo- Clemens, S. C., and Prell, W. L., 1990, Late Pleistocene variability North Africa (0-140 ka): Geology, v. 19, p. 307-310. cene transition, in Ruddiman, W. F., and Wright, H. E., Jr., of Arabian sea summer monsoon winds and continental arid- Lister, G. S., Kelts, K., Chen, K. Z., Yu, J.-Q., and Niessen, F., eds., North America and adjacent oceans during the last de- ity: Eolian records from the lithogenic component of deep-sea 1991, Lake Qinghai, China: Closed-basin lake levels and the glaciation: The geology of North America, Volume K-3: sediments: Paleoceanography, v. 5, p. 109-146. oxygen isotope record for ostracoda since the latest Pleis- Boulder, Colorado, Geological Society of America, p. 323-352. De Angelis, M., Barkov, N. I., and Petrov, V. N., 1987, Aerosol tocene: Palaeogeography, Palaeoclimatology, Palaeoecol- concentrations over the last climatic cycle (160 ka) from an ogy, v. 84, p. 141-162. von Richthofen, F., 1882, On the mode of origin of the loess: Ge- Antarctic ice core: Nature, v. 325, p. 318-321. Liu, T. S., Gu, X. E., An, Z.-S., and Fan, Y. X., 1981, The dust fall ological Magazine (new ser.), v. 9, p. 293-305. Elmore, D., Fulton, B. R., Clover, M. R., Marsden, J. R., Gove, in Beijing, China, on April 18, 1980: Geological Society of Zhang, D. E., 1984, Synoptic-climatic studies of dust fall in China H. E., Naylor, H., Purser, K. H., Kilius, L. R., Beukins, America Special Paper 186, p. 149-157. since historic times: Scientia Sinica, v. 27, p. 835-836. Lowenstein, T. K., Spencer, R. J., and Pengxi, Z., 1989, Origin of Zreda, M. G., Phillips, F. M., Elmore, D., Kubik, P. W., Sharma, R. P., and Litherland, A. E., 1979, Analysis of ^Cl in envi- 36 ronmental water samples using an electrostatic accelerator: ancient potash evaporites: Clues from the modem nonmarine P., and Dorn, R. I., 1991, Cosmogenic C1 production rates Nature, v. 277, p. 22-25. Qaidam Basin of Western China: Science, v. 245, in terrestrial rocks: Earth and Planetary Science Letters, Fang, J.-Q., 1991, Lake evolution during the past 30,000 years in p. 1090-1092. v. 105, p. 94-109. China, and its implications for environmental changes: Qua- Luo, S., and Ku, T.-L., 1991, U-series isochron dating: A general- ternary Research, v. 36, p. 37-60. ized method employing total-sample dissolution: Geochimica Flint, R. F., 1971, Glacial and Quaternary geology: New York, John et Cosmochimica Acta, v. 55, p. 555-564. Wiley. Olivarez, A. M., Owen, R. M., and Rea, D. K., 1991, Geochemistry Gasse, F., Arnold, M., Fontes, J.-C., Fort, M., Gibert, E., Hua, A., of eolian dust in Pacific pelagic sediments: Implications for Li, B., Li, Y., Lin, Q., Melieres, F., Van Campo, E., Wang, paleoclimatic interpretations: Geochimica et Cosmochimica MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 10,1992 F., and Zhang, Q., 1991, A 13,000-year climate record from Acta, v. 55, p. 2147-2158. REVISED MANUSCRIPT RECEIVED APRIL 22, 1993 western Tibet: Nature, v. 353, p. 742-745. Oviatt, C. G., McCoy, W. D., and Reider, R. G., 1987, Evidence for MANUSCRIPT ACCEPTED APRIL 29,1993

Printed in U.S.A.

1610 Geological Society of America Bulletin, December 1993

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/12/1606/3381693/i0016-7606-105-12-1606.pdf by guest on 28 September 2021