An Zhisheng

The long-term paleomonsoon variation recorded by the loess-paleosol sequence in Central China

Abstract The loess-paleosol sequence in Central China during the last 2.5 Ma is a good record of the Asian monsoon variation, which could be considered as the concrete realization of global paleoclimatic cycles. The geological and biological evidence of the loess-paleosol sequence indicates that the loess was mainly the dust deposits transported by the northerly wind of winter monsoons and also suffered to some extent from pedogenesis caused by weakened summer monsoons, and that the paleosol accreted by the rather slow dust accumulation was closely related to strengthened summer monsoons. An alternating occurrence of loess and paleosol in the Luochuan section implies the variation history of warm-humid climate with a summer monsoon dominance and cold-dry climate with a winter monsoon dominance. The magnetic susceptibility of loess and paleosol can be regarded as a proxy index of Asian monsoon variation. An analysis of the susceptibility curve and structure characters of Luochuan section shows that the Asian monsoon circulation was strengthened from Early Pleistocene to Late Pleistocene, and the 500 ka BP point and 1200 ka BP point seem to be a threshold and a turning point respectively in terms of the long-term paleomonsoon variation in Asia.

New absolute time scale for the Quaternary climate in the Chinese Loess region by grain-size analysis

Continuous loess sequences in China provide not only information for paleoclimatic reconstructions, but also permit the establishment of a reliable absolute time scale for the Quaternary. An empirical relationship is developed between the grain size of the loess and the sedimentation rate in a short, well-dated reference section (step 1). This relationship is extrapolated to the complete Quaternary section (step 2). After derivation of the sedimentation rate (by measuring the grain size) in all the layers of the sequence, the duration of deposition of each layer is obtained (by dividing thickness by sedimentation rate) and subsequently the age of each point in the sequence is calculated (by adding together all the duration values of the overlying layers). The chronology of the extrapolated part of the section obtained in this way is independent of dates provided by other methods and may also be checked by them.

MILLENNIAL-SCALE CLIMATIC OSCILLATIONS DURING THE LAST INTERGLACIATION IN CENTRAL CHINA

Repeated southward excursions of North Atlantic polar water during the last interglaciation (δ 18 O stage 5, 130‐74 ka) are recorded by planktonic foraminifera and ice-rafted detritus in North Atlantic sediment cores, and Greenland ice-sheet cores display quasi-synchronous fluctuations. Comparable high-frequency variations in the East Asian winter monsoon climate are discernible in three loess-paleosol profiles in central China that span the last interglaciation. Peak values of the >40 μm quartz fraction and bulk sediment samples from the S1 (last-interglacial) accretionary paleosol complex reflect major dust-flux events when winter monsoon winds strengthened. Frequent oscillations of the dust flux and nine significant dust events are recorded. Six events, falling between ca. 110 and 70 ka, are correlated with cold peaks (C19‐24) identified in North Atlantic cores. Two comparable dust peaks occur within paleosol S1SS3 (= substage 5e); the older of these, dating to ca. 121 ka, may correlate with a brief cold event recently recognized in high-resolution marine and terrestrial climate-proxy records.

New magnetostratigraphic dates of Lantian Homo erectus

Skeletal remains of Homo erectus found in Pleistocene loess at two sites near Lantian in central China are of greatly different geologic age. The cranium found in the fossil-bearing strata at Gongwangling is about 1.15 myr old whereas the remains found at the Chenjiawo locality in middle Pleistocene loess are about 0.65 myr old. The dating is based on new paleomagnetic polarity determinations and on the lithostratigraphic position of the fossils in the loess-paleosol sequence. Our results confirm that both localities are older than the first occupation of Zhoukoudian. New dates, palaeoenvironmental settings, and morphological features of the hominids from Lantian localities have significant bearing on the understanding of adaptive radiations of the middle and late hominids in Asia.

Cyclic Quaternary alluviation and terracing in a nonglaciated drainage basin on the north flank of the Qinling Shan, central China

Abstract Pleistocene alluvial terraces of the nonglaciated Ba River drainage basin on the north flank of the Qinling Shan are capped by a succession of loess units and paleosols that correlate with the standard marine isotope chronology and are used to date the subjacent alluvial gravels. Alluvial fills were deposited during isotope stages 2, 6, 8, 12, and 16, whereas terracing occurred during interglacial stages 1, 5, 7, 11, and 15. The apparent absence of terraces dating to stage 14 and stage 4 may be due to the lesser intensity of these glaciations compared to that of stage 2, although disruption of the alluvial regime by local tectonism is a likely alternative for the lack of a stage 4 terrace. A stage 10 terrace was not positively identified from available exposures; its possible absence could be related to post-stage 12 uplift. Aggradational episodes correlate with glaciations and loess deposition, whereas degradational episodes correlate with interglaciations or interstades and soil formation, implying that climate is the primary control on Quaternary paleohydrology. This in turn points to variations in the Earths orbital geometry as the major factor that modulates both climate and, ultimately, the fluvial system in the Qinling Shan. In this region, glaciations were dominated by a cold, dry winter monsoon climate, whereas during interstades and interglaciations a warmer and wetter climate prevailed, implying strengthening of the summer monsoon. Both the loess/paleosol and the alluvial records are consistent with climate-model simulations spanning the last 18,000 yr that show a change from cold, dry conditions during the last glacial maximum to a climate warmer and wetter than present during the first half of the Holocene.

Palaeomagnetic stratigraphy of Lake Bungunnia: Plio-pleistocene precursor of aridity in the murray basin, Southeastern Australia

Abstract The Murray-Darling river system in southeastern Australia formerly flowed into a large lake, Lake Bungunnia, before reaching the sea. Fine-grained clastics of this system, the Blanchetown Clay Formation, extend over an area of about 68,000 km2 underlying the Mallee region with its surface array of dunes and salt lakes. Palaeomagnetic analyses are used to date the major hydrologic transition from the expansive freshwater system to the saline deflation playas and dunes that characterize this landscape today. Two sites were selected for sampling; one a cliff section on the left bank of the Murray River at Chowilla, the other from within sediments of Lake Tyrrell, a saline playa cut into sediments of the older freshwater system. Two sequences were sampled at Tyrrell; one by coring to 30 m in the basin centre, the other a composite cliff and 5-m core on the western margin of the playa. Post-Bungunnia sediments representing modern landforms were sampled in both surficial dune-palaeosol units of the Woorinen Formation and in saline sediments, the Tyrrell Beds, within the modern saline playa. The sequence of multiple aeolian units of the Woorinen Formation and gypseous sediments of the Tyrrell Beds are normally magnetized indicating deposition within the past 0.7 million years. By contrast, the uppermost horizons of bedded Blanchetown Clay below a surface zone of pedogenic alteration are reversed. The Brunhes-Matuyama (B-M) boundary, key to the age of draining of Lake Bungunnia by deepening the overflow channel to the sea, is obscured within the zone of pedogenesis separating unaltered Blanchetown Clays from overlying aeolian deposits. The onset of aeolian and saline gypseous deposits characteristic of the arid-zone facies is placed soon after the B-M boundary, about 500,000 years ago. Sequences through the Blanchetown Clay pass down through the Matuyama reversed chron into the Gauss normal chron with the lowermost section on the Tyrrell west bank recording a basal reversed zone of possible Gilbert age. Thus the origins of Lake Bungunnia extend back beyond basal Pleistocene into the late Pliocene. The lakes duration covers more than 2 million years spanning the Plio-Pleistocene boundary in this important region of southeastern Australia.

The global surface energy system and the geological role of wind stress

Abstract Deserts and the Loess Plateau in North China constitute an environmental coupling system which has undergone periodic changes in the Quaternary period, attributable mainly to the variations of the global surface energy system. Thirty-seven loess-paleosol formations of the last 2.5 Ma have been identified in the Loess Plateau, representing thirty-seven cycles of climate from cold to warm and of wind intensity from strong to weak. In historical literature, more ‘loess falling’ events in relatively cold-dry periods have been recorded, than in comparatively warm-humid periods. Loess is being currently deposited. Analyses on three contemporary ‘dust storm’ events show that the scope, mass fluxes, and manners of loess deposition are controlled by different atmospheric circulation systems.

The paleosol complex S5 in the China Loess Plateau — A record of climatic optimum during the last 1.2 Ma

Having investigated the loess-paleosol sequence in various loess deposition basins in the China Loess Plateau, authors have found that the paleosol complex S5 is of special significance in terms of Pleistocene paleoclimate. Paleopedological, micromorphological, palynological and paleomagnetic evidences indicate that this paleosol complex formed in the interval of 460,000 to at least 560,000 years BP, which was a climatic optimum.

Preliminary palaeomagnetic study of loess from the Wucheng section, North China

One hundred and seventeen oriented specimens of loess (in the form of cubes with an edge of 4 cm) were collected from the Wucheng loess section in Xixian County, Shanxi Province. After magnetic cleaning in an alternating field the remanent magnetization of specimens was measured with an astatic magnetometer (sensitivity 4×10−3 emu/cm3).The normally magnetized zone of the Brunhes normal epoch is recorded in upper Pleistocene Malan loess and middle Pleistocene Lishi loess which contain typical Zhoukoudian fauna. The reversely magnetized zone of the Matuyama reversed epoch is observed in lower Pleistocene Wucheng loess, whose fossils are found corresponding to Villafranchian Nihowan fauna.One hundred and seventeen oriented specimens of loess (in the form of cubes with an edge of 4 cm) were collected from the Wucheng loess section in Xixian County, Shanxi Province. After magnetic cleaning in an alternating field the remanent magnetization of specimens was measured with an astatic magnetometer (sensitivity 4×10−3 emu/cm3).