Magnetostratigraphic and paleoenvironmental records for a Late Cenozoic sedimentary sequence drilled from Lop Nor in the eastern Tarim Basinsedimentary sequence drilled from Lop Nor in the eastern Tarim Basin

The Tarim Basin, one of the largest inland basins in the world, is situated in the northwestern China and to the north of the Tibetan Plateau. Continuous Cenozoic deposits have accumulated in this basin, which are crucial for investigating the growth of the Tibetan Plateau and the paleoclimatic evolution in Asian interior. Here we report the magnetostratigraphic and paleoenvironmental records for a Late Cenozoic sedimentary sequence drilled from Lop Nor in the eastern Tarim Basin. Magnetostratigraphic results show that this core has recorded a magnetic polarity sequence from C3Bn to C1n, covering an age range from ca. 7 Ma to the present. Decreased magnetic susceptibility occurred after ca. 5.6&ndash;5.1 Ma, which was interpreted to indicate an enhancement of aridity in the Tarim Basin since this period. We attribute this aridification to the combined effect of global climate cooling and the uplift of the Northern Tibetan Plateau since the late Miocene.</p

Magnetostratigraphy of the Suerkuli Basin indicates Pliocene (3.2 Ma) activityof the middle Altyn Tagh Fault, northern Tibetan Plateau

The left-lateral strike-slip Altyn Tagh Fault (ATF) forming the northern boundary of the Tibetan Plateau accommodates parts of the overall convergence between the colliding Indian and Eurasian plates. Precise dating of the ATF activity is essential for understanding possible mechanisms of Tibetan Plateau deformation and uplift. Here we report a magnetostratigraphic study of the Suerkuli Basin deposits recording depositional changes during the ATF activity. Field investigations reveal a remarkable and widespread change in depositional environment in the Suerkuli Basin, i.e. a transformation from low-energy lacustrine deposits (grayish-green mud-siltstone and brown mud-siltstone) into high-energy alluviul fan deposits (poorly sorted gray pebble and cobble conglomerates). Detailed magnetostratigraphy of the 390-m-thick Daban section, at the southeastern margin of the Suerkuli Basin (38 43.090N, 90 58.840E), shows that this change in depositional facies occurred at 3.2 Ma, accompanied by a remarkable increase in sediment accumulation rate. We attribute this depositional change to the Piocene tectonic activity of the middle ATF although the contribution of the Pliocene global climate deterioration cannot be excluded.</p

Quaternary structural partitioning within the rigid Tarim plate inferred from magnetostratigraphy and sedimentation rate in the eastern Tarim Basin in China

It has been proposed that within the Tarim Basin tectonic activity has been limited since Triassic time. However, on the basis of magnetostratigraphy from the eastern Tarim Basin, which defines the chronology of sedimentation and structural evolution of the basin, we show that the basin interior has been uplifted and partitioned during Quaternary. The magnetostratigraphy was constructed from 2228 samples that yielded acceptable inclination values. Characteristic remnant magnetization (ChRM) with both normal (N1-N11) and reversed (R1-R11) polarity was isolated by thermal demagnetization. The data correlate best with polarity chrons C3r to C1n, which range from 5.39 Ma to recent on the geological time scale 2004 (GTS2004). An abrupt decrease in the sedimentation rate is observed at 1.77 Ma in the Ls1 core. This change does not overlap with known Pleistocene climate-change events. We attribute this sedimentation rate decrease to a structurally controlled local decrease in accommodation space where basin basement uplifts occur. This period of sedimentary environmental change reveals that structural partitioning in the basement of the Tarim Basin occurred since similar to 1.77 Ma, and we speculate that tilting of the Southeast Uplift (a sub-basin unit) within the Tarim Basin began in early Pleistocene time.</p

Late Oligocene–Early Miocenemagnetochronology of themammalian faunas in the LanzhouBasin–environmental changesin the NE margin of the TibetanPlateau

A large number of terrestrial mammalian fossils were reported in the well-exposed Paleogene and Neogene fluvio-lacustrine strata in Western China. Their accurate ages are crucial to understand the mammalian and environmental evolution associated with the step-wise uplift of the Tibetan Plateau. At present their ages are surprisingly poorly constrained. Here, we present a high-resolution magnetostratigraphic dating of the Late Oligocene&ndash;Early Miocene mammal assemblages from a 233-m thick fluvio-lacustrine section in the Lanzhou Basin located at the northeastern margin of the Tibetan Plateau, China. The results suggest that the section spans from the polarity subchron C6Cn.2r to C5En, i.e. ranging from ca 23 to 18 Ma. This magnetochronology provides considerably more robust ages for three associated land mammalian faunal assemblages. Updated ages end the debate on existing ambiguous and distinctly different magnetostratigraphic correlations for those Late Oligocene&ndash;Early Miocene assemblages. The new ages now enable precise correlation of these faunas to the European Land Mammal and North American Land Mammal Ages. The faunal assemblages further suggest a mixed setting of woodlands and grasslands associated with a humid environment in the Lanzhou Basin during the Late Oligocene&ndash;Early Miocene, in contrast to its modern poor vegetation cover and arid environment.</p

New evidence for early presence of hominids in North China

The Nihewan Basin in North China has a rich source of Early Pleistocene Paleolithic sites. Here, we report a high-resolution magnetostratigraphic dating of the Shangshazui Paleolithic site that was found in the northeastern Nihewan Basin in 1972. The artifact layer is suggested to be located in the Matuyama reversed polarity chron just above the upper boundary of the Olduvai polarity subchron, yielding an estimated age of ca 1.7-1.6 Ma. This provides new evidence for hominid occupation in North China in the earliest Pleistocene. The earliest hominids are argued to have lived in a habitat of open grasslands mixed with patches of forests close to the bank of the Nihewan paleolake as indicated from faunal compositions. Hominid migrations to East Asia during the Early Pleistocene are suggested to be a consequence of increasing cooling and aridity in Africa and Eurasia.</p

Magnetostratigraphic evidence of a mid-Pliocene onset of the Nihewan Formation - implications for early fauna and hominid occupation in the Nihewan Basin, North China

The fluvio-lacustrine sediments in the Nihewan Basin of North China, known as the Nihewan Formation, are well-known for an abundance of Early Pleistocene mammalian fossils (known as the Nihewan Fauna sensu lato) and Paleolithic sites. The age at which the sedimentation started is thus crucial for our understanding of early fauna and hominid occupation and infilling history of the basin, but it is poorly constrained to date. Here we report on a detailed paleomagnetic investigation of the Yangshuizhan section that crops out in the northeastern Nihewan Basin, supplemented by rock magnetic analyses into the carriers of the natural remanent magnetization. Magnetite and hematite are shown to be the main carriers of the characteristic remanent magnetization. Magnetostratigraphic correlation to the geomagnetic polarity timescale indicates that the onset of the Nihewan Formation in this section occurs at &sim;3.7 Ma, just below the Gilbert-Gauss boundary and ca 1-Myr earlier than previously established evidence. This pushes the lower limit of the Nihewan Formation back in time from very late Pliocene (&lt;2.8 Ma) to (at least) the mid-Pliocene. Combining the previously established magnetostratigraphic data with the present study, we arrive at a better understanding of the chronological framework and spatio-temporal history of the deposition of the terrestrial Nihewan Formation. Furthermore, it provides new perspectives of early fauna and hominid occupation in the Nihewan Basin.</p

High-resolution record of geomagnetic excursions in the Matuyama chron constrains the ages of the Feiliang and Lanpo Paleolithic sites in the Nihewan Basin, North China

The Nihewan Basin (40 degrees N) in North China is a rich source of Early Pleistocene Paleolithic sites and thus a key area for studying early human evolution in high-latitude (from an early human perspective) East Asia. Here a high-resolution magnetostratigraphic investigation is carried out on a fluvio-lacustrine section in the northeastern Nihewan Basin, which contains the Feiliang and Lanpo Paleolithic sites. Paleomagnetic results suggest that this section records the lower portion of the Brunhes polarity chron and the upper Matuyama polarity chron. Furthermore, the Jaramillo polarity subchron and seven of the nine validated geomagnetic excursions within the Matuyama polarity chron are identified, including the Kamikatsura, Santa Rosa, Intra-Jaramillo, Cobb Mountain, Bjorn, Gardar and Gilsa excursions. The Feiliang artifact layer is located just at the bottom of the Cobb Mountain excursion, thus its age is estimated to be similar to 1.2Ma. The Lanpo artifact layer appears to be coeval with the Gilsa excursion, yielding an estimated age of similar to 1.6 Ma. This study provides new evidence for the presence of early humans in North China before 1.5 Ma and documents the powerful role of geomagnetic excursions: they provide valuable age control points for ongoing efforts to date the early Paleolithic sites.</p

Timing of the Brunhes-Matuyama magnetic polarity reversal inChinese loess using 10Be

In Chinese loess, the Brunhes-Matuyama (B-M) geomagnetic reversal occurs ~25 k.y. prior to the age found in marine sediments. This offset has been attributed by some to post-depositional magnetic overprinting of loess, while others have argued it is due to errors in the loess time scale. Here we solve this long-standing debate by exploiting a new method to extract reproducible records of geomagnetic fi eld intensity from loess with 10Be&mdash;a proxy for global average geomagnetic fi eld intensity&mdash;and using it to show that a pronounced minimum in field intensity (a requirement for dipole fi eld reversal) is recorded in two separate loess records at ca. 780 &plusmn; 3 kyr B.P. This timing is synchronous with the B-M reversal timing seen in marine records, verifying the standard loess time scale as correct, but it is ~25 k.y. younger than the age (depth) of the magnetic polarity reversal recorded in these same Chinese loess sediments, demonstrating that loess magnetic overprinting has occurred.</p

Global Monsoon Dynamicsand Climate Change

This article provides a comprehensive review of the global monsoon that encompasses findings from studies of both modern monsoons and paleomonsoons. We introduce a definition for the global monsoon that incorporates its three-dimensional distribution and ultimate causes, emphasizing the direct drive of seasonal pressure system changes on monsoon circulation and depicting the intensity in terms of both circulation and precipitation. We explore the global monsoon climate changes across a wide range of timescales from tectonic to intraseasonal. Common features of the global monsoon are global homogeneity, regional diversity, seasonality, quasi-periodicity, irregularity, instability, and asynchroneity. We emphasize the importance of solar insolation, Earth orbital parameters, underlying surface properties, and land-air-sea interactions for global monsoon dynamics. We discuss the primary driving force of monsoon variability on each timescale and the relationships among dynamics on multiple timescales. Natural processes and anthropogenic impacts are of great significance to the understanding of future global monsoon behavior.</p

The effects of mid-Holocene fluvio-eolian interplay and coastal dynamics on the formation of dune-dammed lakes in NE Brazil

We analyzed the Late Quaternary coastal evolution of the easternmost tip of South America in Brazil in light of fluvial-eolian interactions controlled by relative sea-level, climate, and coastal physiography changes. The chronology obtained by OSL-SAR of 36 samples coupled with sedimentological analysis from stabilized dunes suggest that eolian activity was primarily controlled by episodes of sediment availability because prevailing SE trade winds have been steadily strong throughout the Holocene. Contrary to the most conventional view linking dune activity to aridity, dune buildup occurred in a period of increased humidity in NE Brazil between 11 ka and 6 ka when a rising relative sea level and higher rainfall enhanced sediment delivery benefiting the construction of transgressive dunefields. The interplay of these advancing dunes with the existing drainage pathways is here investigated using a modern regional analog and through the evolution of Boqueir&atilde;o Lake formed by dune blockage. Analysis of a sediment core from this lake dated between 8.4 and 0.9 ka indicated changes over time in microfossil assemblages, organic geochemistry, and grain size data conforming to fluvial or lacustrine depositional conditions. Between 7.2 and 4.4 ka, during the predominantly regional humid climate, the high abundance of fluvial sponge species correlated with a framework of competent-flow drainage systems diverting from advancing dunes. An abrupt transition from a wetter to a drier climate at 4.4&ndash;4.0 ka stimulated episodes of fluvial damming as indicated by sharp changes from sandy to muddy sediments and anomalous concentration of sponge spicules concurrent with significant mortality rates of fluvial adapted species. From 3.9 ka to the present, the disappearance of sponge spicules and peaking diatom concentration attested to a predominant lacustrine environment. Thus, the formation of Boqueir&atilde;o Lake is mainly a result of the regionally drier climate and not a consequence of increased humidity in coastal NE Brazil.</p