Hongbo Zheng

Pre-Miocene birth of the Yangtze River

The development of fluvial systems in East Asia is closely linked to the evolving topography following India–Eurasia collision. Despite this, the age of the Yangtze River system has been strongly debated, with estimates ranging from 40 to 45 Ma, to a more recent initiation around 2 Ma. Here, we present 40Ar/39Ar ages from basalts interbedded with fluvial sediments from the lower reaches of the Yangtze together with detrital zircon U–Pb ages from sand grains within these sediments. We show that a river containing sediments indistinguishable from the modern river was established before ∼23 Ma. We argue that the connection through the Three Gorges must postdate 36.5 Ma because of evaporite and lacustrine sedimentation in the Jianghan Basin before that time. We propose that the present Yangtze River system formed in response to regional extension throughout eastern China, synchronous with the start of strike–slip tectonism and surface uplift in eastern Tibet and fed by strengthened rains caused by the newly intensified summer monsoon.

Late Oligocene–early Miocene birth of the Taklimakan Desert

Significance The formation of the Taklimakan Desert marked a major geological event in central Asia during the Cenozoic, with far-reaching impacts. Deposition of both eolian sand dunes in the basin center and the genetically equivalent loessite along the basin margins provide evidence for the birth of the Taklimakan Desert. This paper resolves a long-standing debate concerning the age of the Taklimakan Desert, specifically whether it dates to ∼3.4–7 Ma, currently the dominant view. Our result shows that the desert came into existence during late Oligocene–early Miocene, between ∼26.7 Ma and 22.6 Ma, as a result of widespread regional aridification and increased erosion in the surrounding mountain fronts, both of which are closely linked to the tectonic uplift of the Tibetan–Pamir Plateau and Tian Shan. As the world’s second largest sand sea and one of the most important dust sources to the global aerosol system, the formation of the Taklimakan Desert marks a major environmental event in central Asia during the Cenozoic. Determining when and how the desert formed holds the key to better understanding the tectonic–climatic linkage in this critical region. However, the age of the Taklimakan remains controversial, with the dominant view being from ∼3.4 Ma to ∼7 Ma based on magnetostratigraphy of sedimentary sequences within and along the margins of the desert. In this study, we applied radioisotopic methods to precisely date a volcanic tuff preserved in the stratigraphy. We constrained the initial desertification to be late Oligocene to early Miocene, between ∼26.7 Ma and 22.6 Ma. We suggest that the Taklimakan Desert was formed as a response to a combination of widespread regional aridification and increased erosion in the surrounding mountain fronts, both of which are closely linked to the tectonic uplift of the Tibetan–Pamir Plateau and Tian Shan, which had reached a climatically sensitive threshold at this time.

Environmental and cultural changes during the terminal Neolithic : Qingpu, Yangtze delta, eastern China

The lower Yangtze, eastern China, was colonized by several Neolithic and Bronze Age cultures giving rise to possibly the highest concentration of prehistoric sites in the world. Early Neolithic cultures in the delta region cultivated rice (Oryza sativa) and agricultural developments appear to have occurred throughout the Neolithic with abrupt socio-economic changes possibly associated with the terminal Neolithic. Given the extensive history of environmental exploitation and well-preserved archaeological sites, the Yangtze delta is an ideal setting to explore the complex interactions between humans and their environment. Multiproxies of environmental changes, namely pollen, charcoal and phytoliths, in a 14C AMS-dated sequence of sediments from an exposed profile at Qingpu, Yangtze delta, were investigated. 14C AMS dating indicates that the age range of the sedimentary sequence analysed is from c. 1800 to 6000 BP, and therefore encompasses the terminal Neolithic and subsequent Bronze Age in the region. This paper reviews this sediment-based evidence in the light of current understanding of human—environment interactions during a critical phase of the development of the Yangtze delta and associated human cultures.

Cenozoic sediments in the southern Tarim Basin: implications for the uplift of northern Tibet and evolution of the Taklimakan Desert

Abstract Cenozoic sedimentary successions along the southern margin of the Tarim Basin, western China, reach up to 10 km in thickness. The two studied sections, the Yecheng and Aertashi, comprise c. 4.5 km and c. 7.0 km of clastic sedimentary rocks respectively. The base of the Yecheng section has been dated palaeomagnetically to be about 8 Ma. Age control of the Aertashi section is based on 87Sr/86Sr measurements (for the basal marine bed), together with magnetostratigraphy and regional stratigraphic correlation. The lower part of each section is mainly composed of fine-grained mudstone and fine sandstone, which makes up the Wuqian Group (Miocene). The palaeoenvironment is low-energy, meandering and braided streams. The middle part is composed of red mudstone, sandstone with thin conglomerate beds, which make up the Artux Formation (Pliocene). The palaeoenvironment is a distal- to mid-fan environment. The uppermost part of the section, known as the Xiyu Formation (Plio-Pleistocene), consists of cobble and boulder conglomerate intercalated with massive siltstone lenses, which formed as proximal alluvial fan and aeolian deposits. Neogene red beds passing upward into upward-coarsening conglomerate and debris-flow deposits record the change in palaeoslope related to uplift of the northern margin of Tibetan Plateau. The formation of aeolian dunes at c. 8 Ma, and underlying playa lake deposits (as at Aertashi), may indicate an arid, enclosed basin in the southern Tarim after this time. Sedimentological characteristics, together with grain size distribution and geochemistry of siltstone bands in the Xiyu and Artux Formations, point to an aeolian origin. This indicates that the Taklimakan Desert and the regional climate regime may have been fully developed by the Early Pliocene. The onset of aeolian sedimentation in the southern Tarim Basin coincided with uplift of the northern Tibetan Plateau, as inferred from the lithofacies change. Tibetan Plateau uplift resulted in the shift of sedimentary environments northwards into the southern Tarim Basin, and could well have triggered the onset of full aridity in the Taklimakan region as a whole.

A sediment-based record of Lateglacial and Holocene environmental changes from Guangfulin, Yangtze delta, eastern China:

Multiproxies of past environmental conditions, comprising 53 sediment samples analysed for their lithostratigraphic properties (mainly their charcoal, phytoliths and pollen contents) from an AMS 14C-dated sequence of sediments accumulating at Guangfulin, Yangtze delta, are presented. The oldest sediments recovered date to the Lateglacial when a mosaic of mixed (conifer-deciduous) temperate forest and wetland vegetation characterized the study area. The Lateglacial—Holocene transition and much of the early Holocene record to c. 7400 yr BP appears to be missing from the sequence. The earliest evidence possibly representing human activities in the study area (the remains of cereals and indicators of forest) date to c. 7000 yr BP. A large increase in macrocharcoal remains c. 4700 yr BP is a more certain indication of human activities close to the study site, and may indicate the first occupation of what is now the location of a major archaeological excavation at Guangfulin. Technological changes during the Eastern Zhou Dynasty (770—221 BC) may be responsible for an increased abundance of rice (Oryza sp.), and possibly also foxtail or Chinese millet (Setaria italica), detected in the Guangfulin record after c. 2400 yr BP. An abrupt sedimentary change at c. 4000 yr BP may represent a short-lived episode of catchment instability. Aside from this, the sediment record from Guangfulin contains no evidence of dramatic environmental changes that could have led to a major decline in agricultural productivity c. 4000 yr BP, as has been suggested for the lower Yangtze by some researchers, who associate this with the cultural transition from Liangzhu to Maqiao. The findings do, however, add weight to the argument that developments in rice-based agriculture on the Yangtze delta varied both spatially and temporally.

Monsoon evolution and tectonics-climate linkage in Asia: an introduction

PETER D. CLIFT1*, RYUJI TADA2 & HONGBO ZHENG3 School of Geosciences, Meston Building, University of Aberdeen, Aberdeen, AB24 3UE, UK Department of Earth and Planetary Science, University of Tokyo, Science Building #1, 7-3-1 Hongo, Tokyo 113-0033, Japan Institute of Surficial Geochemistry, School of Earth Science and Engineering, Nanjing University, 22 Hankou Road, Nanjing, People’s Republic of China

Late Pliocene–Pleistocene expansion of C4 vegetation in semiarid East Asia linked to increased burning

Plants using the C 4 photosynthetic pathway, commonly tropical and subtropical grasses, increased in abundance in East Asia dur- ing the late Cenozoic. Determining the exact timing and likely fac- tors leading to this major vegetation change requires region-specific studies. Here variations in pyrogenic carbon mass accumulation rate (PyC-MAR) and isotope composition (δ 13 C PyC ) from an ~7-m.y.- long depositional sequence from the central Loess Plateau, China, suggest increased biomass burning and an increased contribution to combusted material from C 4 taxa from 2.6 Ma. Changes in the composition of PyC after 0.6 Ma likely reflect the effects of lower temperatures, particularly during glacial periods, and changes in seasonality of precipitation. Increased PyC-MAR without concomi- tant changes in δ 13 C PyC at ca. 0.15 Ma appears to indicate a decou- pling of feedbacks between changes in climate, fire regime, and veg- etation, and may mark the onset of anthropogenic burning in the region. These new data suggest that C 4 taxa were present on the Loess Plateau from at least the late Miocene, rising to prominence at ca. 2.6 Ma following changes in climate and, critically, an increase in biomass fires.

Desertification and dust emission history of the Tarim Basin and its relation to the uplift of northern Tibet

Abstract The potential links between uplift of the Himalaya and Tibetan Plateau and desertification of inland Asia have been a long-considered problem in geology. Although a close link between the two has been suggested by theoretical climatic simulations, not enough geological data has existed to test the theory. Here, we conducted semi-quantitative field observations of a Neogene fluvial sequence at the Yecheng section on the southwestern margin of the Tarim Basin in order to confirm the origin and mode of deposition of the aeolian siltstone, determine the onset timing, evaluate quantitatively the temporal evolution of its deposition and its relationship to the tectonically driven surface uplift of NW Tibet. The results suggest a close link between the uplift of northwestern Tibet, alluvial fan formation, dust emission from Taklimakan Desert and the deposition of loess on the alluvial fans.

Monsoon Evolution and Tectonic–Climate Linkage in Asia

The Earth’s climate varies through geological time as a result of external, orbital processes, as well as the positions of continents, growth of mountains and the opening and closure of oceanic gateways. Climate modelling suggests that the intensity of the Asian monsoon should correlate, at least in part, with the uplift history of the Tibetan Plateau and the Himalaya, as well as the evolution of gateways and the retreat of shallow seas in Central Asia. Long-term reconstructions of both mountain building and monsoon activity are key to testing the proposed links. This collection of papers presents a series of new studies documenting the variations of the Asian monsoon on orbital and tectonic timescales, together with the impact this has had on environmental conditions. The issue of which proxies are best suited to measuring monsoons is addressed, as is the effect that the monsoon has had on erosion and the formation of the stratigraphic record both on and offshore.

Reply to Sun et al.: Confirming the evidence for Late Oligocene−Early Miocene birth of the Taklimakan Desert

In Zheng et al. (1), we applied radioisotopic methods to precisely date a volcanic tuff preserved in the Xiyu Formation, revised the magnetostratigraphy of the Cenozoic successions (2), and determined the initial desertification of the Taklimakan to be Late Oligocene to Early Miocene.