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Vol. 5, No. 5 May 1995 INSIDE • Geosphere Alliance Committee, GSA TODAY p. 100 • Call for Committee Service, p. 102 A Publication of the Geological Society of America • Book Reviews, p. 106 Chasing the Paleomonsoon over China: Its Magnetic Proxy Record Subir K. Banerjee, Institute for Rock Magnetism, Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455 ABSTRACT Magnetic susceptibility enhance- A trench cut into ments in Chinese paleosols compared the loess section to the underlying loess horizons can near Xining, Qinghai be explained by pedogenesis. In re- Province, China. Xin- gions where variations in miner- ing is currently cooler alogic composition and grain size and drier than Xifeng are not large, the magnetic enhance- because Xining lies ment can serve as a proxy for paleo- in the monsoon rain monsoon precipitation. Soil mag- shadow to the west of the Liupan Mountains. netization and current rainfall over However, magnetic many sites, plotted against each parameters show that other, do provide a linear relation, the climate at Xifeng a climofunction. When such climo- and Xining was similar functions are used for old paleosols during both the early and loess, we can obtain relative Holocene and last variations in paleomonsoon precipi- interglacial, about tation that are comparable to general 9 and 120 ka, circulation model (GCM) estimates respectively. Photo for these time periods. by Christopher Hunt. INTRODUCTION Climate change and the effects of climate change on society are inter- esting topics in their own right. Our A vertical cut into the loess section near Xifeng, Gansu Province, China. concerns about them have taken on a Xifeng, which lies to the east of the Liupan Mountains, is currently warmer much more urgent note, however, with and wetter than sites to the west, as revealed by the vegetation and the the realization that increased emissions magnetic properties of the youngest layers. Photo by Christopher Hunt. of greenhouse gases and the resultant induced warming may bring about a more rapid tempo of climate change than has been common in the past. Recent numerical modeling of the for more than a century have used ter northwesterlies and deposited on centration, composition, and grain-size effects of global warming, using atmos- conventional approaches such as the loess plateau (Pye and Zhou, 1989). of the magnetic minerals) has provided pheric general circulation models pollen records and clay/silt ratios to During glacial times, however, the a high-resolution climatic proxy record. (GCMs) have shown that large grain- extract a coarse-resolution paleoclimate input was much higher than during The magnetic minerals responsible are producing areas of China and India record. The successful extraction of a interglacial times. Except for the differ- magnetite, maghemite, and hematite; could become arid and unproductive magnetic reversal stratigraphy from ence in color between buff-brown loess the first two are much more strongly as a result of such global change. Chinese loess by Heller and Liu (1984, (glacial) and darker brown paleosols magnetic and thus contribute greatly Oxygen isotope (δ18O) records 1986) has allowed the dating of the (interglacial), it has been difficult to to the susceptibility signal. However, from the shells of marine foraminifera numerous peaks in magnetic suscepti- study quantitatively the relative without a widely-accepted model for have yielded relative global ice volume bility (χ). These peaks had been known degrees of climatic change (or the the changes in magnitude of all the data and, hence, measures of global to coincide with the warmer inter- precise ages of specific horizons of many magnetic parameters that have temperature. We have learned, for glacials and interstadials when the this uniform body) by the existing been measured up to now, we are example, that the last interglacial paleosol layers had formed. sedimentological and other methods. still waiting to interpret completely (stage 5 in the SPECMAP oxygen iso- Magnetostratigraphic dating by Heller the finer scale details of the magnetic tope stratigraphy; Imbrie et al., 1984) THE ROCK MAGNETIC and Liu (1984, 1986) allowed the corre- signatures. lasted from about 128 to 75 ka, and RECORD lation of loess χ profiles from multiple Kukla et al. (1988) compared the was much warmer than the present sites. High sensitivity of the various detailed χ-record of two sections at The loess deposits of China repre- interglacial (stage 1) that began 10 ka rock magnetic parameters to climate sent eolian dust from the deserts of the ago. If we could obtain reliable, high- change (through a change in the con- north and northwest lifted by the win- China continued on p. 94 resolution (spatial and temporal) records of summer monsoon activities in China for stages 5 and 1, their inter- comparison could help teach us what to expect if the world approached stage 5 conditions of climate in the future. Also, the recently discovered sharp instabilities in climate obtained from Greenland ice cores could be compared with continental paleo- monsoon records. Figure 1. A map The alternating loess and paleosol of the loess plateau layers (total thickness ~100 to 300 m) (~500,000 km2) of China, dissected by the Yellow River (Huang located north of the Qin- He) in central China extend over an ling mountains. Open area of 500,000 km2 (see Fig. 1). They circles mark sampling constitute a 2.6-m.y.-old archive of sites (e.g., Xifeng). high-quality paleoclimatic proxies waiting to be deciphered by modern techniques, even though geologists Note: With apologies to Alexander Frater, author of the eminently readable Chasing the Monsoon (Holt and Co., 1992). China continued from p. 93 genesis. The preferred model of Kukla inferred geologic age. It was observed IN THIS ISSUE et al. called for a relative weakening of that both Xifeng and Luochuan had δ18 χ Chasing the Paleomonsoon Xifeng and Luochuan with the O in the loess layers and a strengthen- the same magnetic accumulation rate over China: Its Magnetic stratigraphy from SPECMAP, which rep- ing in the paleosol layers, as a result of for the past 800 ka. Proxy Record ................... 93 resents the worldwide average changes variable concentration of a global Zhou et al. (1990) and Maher and in global ice volume measured through “rain” of strongly magnetic magnetite Thompson (1991) showed that when About People ....................... 94 the variations in oxygen isotopes 18O in a low-magnetic-susceptibility eolian the paleosol and loess samples are In Memoriam ....................... 94 and 16O in marine foraminifera. The dust (loess) prevalent during glacial studied in detail for their grain-size- Correction .......................... 94 choice of the sites may have been for- times. During interglacial times, when dependent rock magnetic parameters, tuitous, but even without the benefit soils formed slowly and dust arrival it becomes clear that the paleosol layers GSA on the Web .................... 94 of formal statistics it is clear that there diminished, the magnetite rain from are distinguished from loess layers by Forum.............................. 97 is excellent correlation between the χ the troposphere was concentrated, concentration, mineralogy, and a much δ18 Geosphere Alliance Committee and O records (Fig. 2). Many of the thus presenting a relatively large mag- different (and much smaller) grain-size Seeks Input and Action ............. 100 details of the peaks in the magnetic netic susceptibility to the observer. As of the magnetic carriers. The data pre- parameter can be compared with simi- proof, Kukla et al. (1988) presented a sented by Zhou et al. (1990) are shown GSAF Update ...................... 101 lar details in the oxygen isotope stratig- model in which the average magnetic in Figure 3. Both groups of authors Call for GSA Committee Service .... 102 raphy. Because Xifeng and Luochuan susceptibility was multiplied by the thought that the evidence points, at Earth Scientists are about 160 km apart, Kukla et al. thickness of the sediment column least partially, to a pedogenic magnetic and Forest Health Policy ............ 103 (1988) concluded that the magnetic above the sample to produce a plot of variations could not be due to pedo- magnetic mineral accumulation against China continued on p. 95 Assessing Environmental Change . 104 Letter to the Editor ................. 104 Field Cooking Contest ............. 105 Book Reviews ...................... 106 Figure 2. Kukla et al.’s (1988) comparison of the Book Nook ........................ 108 χ depth series from Zifeng 1995 GeoVentures ................. 109 and Luochuan with the SPECMAP time series of GSA Meetings ..................... 110 globally averaged δ18O. Classifieds ......................... 111 Because δ18O values pro- Calendar .......................... 111 vide global ice volume data, they indirectly pro- vide data on global tem- perature fluctuations which May coincide in character with GSA TODAY χ Vol. 5, No. 5 1995 the depth series, provid- ing the first comparable scale comparison between GSA TODAY (ISSN 1052-5173) is published oceanic and continental monthly by The Geological Society of America, Inc., with offices at 3300 Penrose Place, Boulder, Colorado. Mailing climates. address: P.O. Box 9140, Boulder, CO 80301-9140, U.S.A. Second-class postage paid at Boulder, Colorado, and at additional mailing offices. 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