230Th/234U and 36C1 dating of evaporite deposits from the western Qaidam Basin, China: 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 Salt Lakesces Institute, Académica Sínica, Xining, Qinghai, 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- years (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 Greenland (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 Pleistocene ice has dust concentrations of the Gobi Desert, Tarim Basin, 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 deserts 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 yardangs, 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. United States, 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 salts, 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 Pliocene 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.