Zhengtang Guo

Onset of Asian desertification by 22 Myr ago inferred from loess deposits in China

The initial desertification in the Asian interior is thought to be one of the most prominent climate changes in the Northern Hemisphere during the Cenozoic era. But the dating of this transition is uncertain, partly because desert sediments are usually scattered, discontinuous and difficult to date. Here we report nearly continuous aeolian deposits covering the interval from 22 to 6.2 million years ago, on the basis of palaeomagnetic measurements and fossil evidence. A total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils. This new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of Asia by the early Miocene epoch, at least 14 million years earlier than previously thought. Regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in Asia.

Climate extremes in Loess of China coupled with the strength of deep-water formation in the North Atlantic

The loess-paleosol sequences of the last 1.2 Ma in China have recorded two kinds of climate extremes: the strongly 18 . developed S4, S5-1 and S5-3 soils corresponding to the marine d O stages 11, 13, and 15, respectively as evidence of 18 three episodes of great warmth and two coarse-grained loess units L9 and L15, corresponding to the marine d O stages 22, . 23, 24 and 38, respectively which indicate severest glacial conditions. The climatic and geographical significance of these events are still unclear, and their cause remains a puzzle. . Paleopedological, geochemical and magnetic susceptibility data from three loess sections Xifeng, Changwu and Weinan suggest that the S4, S5-1 and S5-3 soils were formed under sub-tropical semi-humid climates with a tentatively estimated . . mean annual temperature MAT of at least 4-68C higher and a mean annual precipitation MAP of 200-300 mm higher than for the present-day, indicating a much strengthened summer monsoon. The annual rainfall was particularly accentuated for the southern-most part of the Loess Plateau, suggesting that the monsoon rain belt the contact of the monsoonal . northward warm-humid air mass with the dry-cold southward one might have stood at the southern part of the Plateau for a relatively long period each year. The loess units L9 and L15 were deposited under semi-desertic environments with a tentatively estimated MAT and MAP of only about 1.5-38C and 150-250 mm, indicating a much strengthened winter monsoon, and that the summer monsoon front could rarely penetrate into the Loess Plateau region. Correlation with marine carbon isotope records suggests that these climate extremes have large regional, even global, significance rather than being local phenomena in China. They match the periods with greatestrsmallest Atlantic-Pacific 13 . d C gradients, respectively, indicating their relationships with the strength of Deep Water NADW production in the North Atlantic. These results suggest that the monsoon climate in the Loess Plateau region was significantly linked with the North Atlantic thermohaline circulation on timescales of 10 4 years. q 1998 Elsevier Science B.V. All rights reserved.

Earliest presence of humans in northeast Asia

The timing of the earliest habitation and oldest stone technologies in different regions of the world remains a contentious topic in the study of human evolution. Here we contribute to this debate with detailed magnetostratigraphic results on two exposed parallel sections of lacustrine sediments at Xiaochangliang in the Nihewan Basin, north China; these results place stringent controls on the age of Palaeolithic stone artifacts that were originally reported over two decades ago. Our palaeomagnetic findings indicate that the artifact layer resides in a reverse polarity magnetozone bounded by the Olduvai and Jaramillo subchrons. Coupled with an estimated rate of sedimentation, these findings constrain the layers age to roughly 1.36 million years ago. This result represents the age of the oldest known stone assemblage comprising recognizable types of Palaeolithic tool in east Asia, and the earliest definite occupation in this region as far north as 40° N.

Summer monsoon variations over the last 1.2 Ma from the weathering of loess-soil sequences in China

The loess-soil sequence in northern China contains a near continuous record of Quaternary paleoclimate. Magnetic susceptibility and grain size have so far been the only proxies available to address the long-term changes of the East-Asian paleomonsoon extending back to more than one million years. In this study, the ratio of CBD (citrate-bicarbonate-dithionite)-extractable free Fe2O3 (FeD), a measure of iron liberated by chemical weathering, versus the total Fe2O3 available (FeT) was measured on samples at 10 cm intervals taken from two loess sections deposited over the last 1.2 Ma. Variations of this index are highly consistent with other pedological indicators, but in addition provide a quantitative measurement of the degree of pedogenesis in the Loess Plateau. Since chemical weathering in the region mainly depends upon summer precipitation and temperature, weathering intensity primarily reflects changes in the East-Asian summer monsoon. The new proxy has been used to document a series of summer monsoon changes of global significance, which are not necessarily recorded by magnetic susceptibility.

Distribution and storage of soil organic carbon in China

organic carbon density in soils varies from 0.73 to 70.79 kg C/m 2 with the majority ranging between 4.00 and 11.00 kg C/m 2 . Carbon density decreases from east to west. A general southward increase is obvious for western China, while carbon density decreases from north to south in eastern China. Highest values are observed in forest soils in northeast China and in subalpine soils in the southeastern part of the Tibetan Plateau. The average density of � 8.01 kg C/m 2 in China is lower than the world’s mean organic carbon density in soil (� 10.60 kg C/m 2 ), mainly due to the extended arid and semi-arid regions. Total organic carbon storage in soils in China is estimated to be � 70.31 Pg C, representing � 4.7% of the world storage. Carbon storage in the surface organic horizons which is most sensitive to interactions with the atmosphere and environmental change is � 32.54 Pg C. INDEX TERMS: 0330 Atmospheric Composition and Structure: Geochemical cycles; 1615 Global Change: Biogeochemical processes (4805); 1815 Hydrology: Erosion and sedimentation; 1625 Global Change: Geomorphology and weathering (1824, 1886);

Delayed build-up of Arctic ice sheets during 400,000-year minima in insolation variability

Knowledge of the past variability of climate at high northern latitudes during astronomical analogues of the present interglacial may help to inform our understanding of future climate change. Unfortunately, long-term continuous records of ice-sheet variability in the Northern Hemisphere only are scarce because records of benthic 18O content represent an integrated signal of changes in ice volume in both polar regions. However, variations in Northern Hemisphere ice sheets influence the Siberian High (an atmospheric pressure system), so variations in the East Asian winter monsoon (EAWM)—as recorded in the aeolian dust deposits on the Chinese Loess Plateau—can serve as a useful proxy of Arctic climate variability before the ice-core record begins. Here we present an EAWM proxy record using grain-size variations in two parallel loess sections representative of sequences across the whole of the Chinese Loess Plateau over the past 900,000 years. The results show that during periods of low eccentricity and precessional variability at approximately 400,000-year intervals, the grain-size-inferred intensity of the EAWM remains weak for up to 20,000 years after the end of the interglacial episode of high summer monsoon activity and strong pedogenesis. In contrast, there is a rapid increase in the EAWM after the end of most other interglacials. We conclude that, for both the 400,000-year interglacials, the weak EAWM winds maintain a mild, non-glacial climate at high northern latitudes for much longer than expected from the conventional loess and marine oxygen isotope records. During these times, the less-severe summer insolation minima at 65° N (ref. 4) would have suppressed ice and snow accumulation, leading to a weak Siberian High and, consequently, weak EAWM winds.

Variation of East Asian monsoon precipitation during the past 21 k.y. and potential CO2 forcing

Paleoclimatic research can provide critical insight on causes of change in the East Asian monsoon, which influences the lives of 1.6 billion people today. In this study, we use paleoclimatic indexes from Chinese loess deposits, which have clear climatic implications and are independently dated, to reconstruct the monsoon precipitation since 21 ka. Our results show that monsoon precipitation persistently decreased from 21 ka to ca. 8 ka, and increased after ca. 8 ka, with a precipitation peak at 8–3 ka. These changes in East Asian summer monsoon precipitation are synchronous with changes in high-northern-latitude ice volume/ice cover and atmospheric CO2. These new data suggest that variation of the monsoon precipitation was probably driven by CO2-forced high-northern-latitude temperature changes, shifting the location of the intertropical convergence zone that dominates monsoon precipitation. Our TraCE-21000 modeling experiment supports this interpretation.

Holocene non-orbital climatic events in present-day arid areas of northern Africa and China

A preliminary comparison between the climatic evolution of the arid regions in northern Africa and northern China showed that the variations in continental aridity, on time scales of 104 years, were roughly synchronous over the last 140 ka. Whether this relationship can be established for the Holocene drought events on a century-scale, as reported for tropical and equatorial Africa, is still not known. The comparison of 560 radiocarbon dates on surface fresh water indicators from the Sahara with 158 dates on palaeosols and lake sediments from the arid regions in northern China demonstrates that the Holocene humid phase has been affected in both regions by several drier events, inlaid in the slow trend attributable to orbital forcing. The variations of the southern margins of the deserts, associated with the northern monsoon front, are documented by the latitudinal distribution of these indicators through time. The most startling aspects are a prolonged somewhat drier interval between 7000 and 5600 years BP and the onset of severe aridity at c. 4000 years BP similar to glacial conditions.

Weathering histories of Chinese loess deposits based on uranium and thorium series nuclides and cosmogenic 10Be

The long, continuous deposition of dust in the Chinese loess plateau (Liu et al., 1985; Kukla and An, 1989; Ding et al., 1991a) offers an unique opportunity to study the nature of soil weathering in a wide range of climatic conditions. In this paper we report on measurements of concentrations of U- and Th-series nuclides and of major cations in 150 loess and paleosol samples from five sites, going back 2.5 Ma. Using the results for 10Be concentrations in these soils (Gu et al., 1996), we determined the absolute amounts of water added to several soil units and obtained: (1) first-order leaching constants for U and several cations and (2) the compositions of the soils contributing to the dust-source regions and of the dust at deposition. Further, based on analyses of 230Th in soils deposited in the past ca. 140 ka, we determined when the soils weathered in the source regions. We conclude that most of the weathering in the dust-source regions may have occurred during the interglacials.

Dominant factors controlling glacial and interglacial variations in the treeline elevation in tropical Africa.

The knowledge of tropical palaeoclimates is crucial for understanding global climate change, because it is a test bench for general circulation models that are ultimately used to predict future global warming. A longstanding issue concerning the last glacial maximum in the tropics is the discrepancy between the decrease in sea-surface temperatures reconstructed from marine proxies and the high-elevation decrease in land temperatures estimated from indicators of treeline elevation. In this study, an improved inverse vegetation modeling approach is used to quantitatively reconstruct palaeoclimate and to estimate the effects of different factors (temperature, precipitation, and atmospheric CO2 concentration) on changes in treeline elevation based on a set of pollen data covering an altitudinal range from 100 to 3,140 m above sea level in Africa. We show that lowering of the African treeline during the last glacial maximum was primarily triggered by regional drying, especially at upper elevations, and was amplified by decreases in atmospheric CO2 concentration and perhaps temperature. This contrasts with scenarios for the Holocene and future climates, in which the increase in treeline elevation will be dominated by temperature. Our results suggest that previous temperature changes inferred from tropical treeline shifts may have been overestimated for low-CO2 glacial periods, because the limiting factors that control changes in treeline elevation differ between glacial and interglacial periods.