Modern Yangtze River Research Articles

Rutile and zircon U-Pb dating as a constraint on sedimentary provenance of the Jianchuan Basin: Implications for paleo-drainage evolution

The Yangtze River is the largest river in Asia, and its evolution has been a subject of debate for more than a century. The formation of the First Bend is widely recognized as a key drainage reorganization event that led to the birth of the modern Yangtze River. In this study, we use new detrital rutile UPb ages to constrain the sedimentary provenance of the Cenozoic deposits in the Jianchuan Basin. Our results show that detrital rutile UPb ages of the Eocene sandstones from the Jianchuan Basin are similar to those of the Yalong River and Jinsha River, indicating that the upper Yangtze is likely to have flowed through the Jianchuan Basin in the late Eocene. Moreover, the characteristic zircon age signature of the Yalong and Jinsha Rivers also appear in the Jianchuan Basin, which is consistent with the rutile data. Our new rutile data and previous detrital zircon data suggest that the upper Yangtze River is likely to connect the Red River via the Jianchuan Basin since the late Eocene.

U–Pb detrital zircon ages from late Eocene to early Oligocene Pinghu formation of the East China Sea Shelf Basin: Inferences on synergistic effects between East Asia offshore sedimentation and major rivers supply

Our understanding of the late Eocene to early Oligocene provenance evolution of the East China Sea Shelf Basin has been limited, which hampers our study of the synergistic effects between East Asia offshore sedimentation and major rivers. To address this, we compared newly acquired detrital zircon chronological data from the East China Sea with published detrital U–Pb ages of major rivers in East Asia. Through this analysis, we have been able to reconstruct the provenance changes of sediments in the East China Sea during this crucial period. We have identified two large-scale and three small-scale denudation surfaces that developed in the Pinghu and lower Huagang Formations. These surfaces indicate that sediments were also derived from underlying strata and bedrock, particularly in the lower Pinghu and lower Huagang Formations. The newly acquired detrital zircon U–Pb age range was found to be roughly similar, ranging from 36.2 ± 0.7 to 2954.0 ± 30.3 Ma. The age peaks of the lower Pinghu Formation were more consistent, while the middle, upper Pinghu Formation, and lower Huagang Formation exhibited multiple age peaks. Comparing the compiled zircon U–Pb age peaks of the Chuanjiang segment of the upper Yangtze River with those of the East China Sea Shelf Basin, we found relative similarity. Additionally, the younger zircon grains with ages <500 Ma in the east Cathaysia catchment showed similarities with the East China Sea Shelf Basin. In terms of provenance contribution, the sediments of the lower Pinghu Formation were predominantly provided by the adjacent uplifts in the east Cathaysia catchment, with the Chuanjiang segment also playing a significant role. From the middle Pinghu Formation onwards, the provenance from the Chuanjiang segment became dominant, accounting for over 90 % of the sediments. However, this influence was limited to the northern part of the study area. This suggests a substantial increase in the Yangtze River catchment during this period, potentially related to the complete connection of the modern Yangtze River.

Late Oligocene–Early Miocene incision of the Three Gorges and the initial establishment of an east-flowing Yangtze River

The evolutionary history of the Yangtze River plays a key role for understanding the Cenozoic landscape evolution of East Asia. The Three Gorges currently separate the Sichuan Basin from the Jianghan Basin to the east, and their formation is considered to establish an initial connection between the Upper and Middle reaches of the modern Yangtze River. However, despite their importance in the Yangtze River evolution, the age of the initial incision of the Three Gorges remains an ongoing debate. In this study, we use apatite-fission track and (UTh)/He data to model the thermal history of the crust during the gorge formation. Based on the modeled time–temperature paths of samples on vertical profiles and the 3D thermos-kinematic modeling of the gorge region, we suggest that the gorges were initially carved during the Late Oligocene–Early Miocene. This event connected the drainages to the west and east of the gorge regions, leading to the formation of the modern east-flowing Yangtze River. This is also consistent with the sediment provenance of the Yangtze Gravels in the Lower Yangtze reported in previous studies. The gorge formation could be a response to the base level drop in the Jianghan and Subei Basins, which led to an upstream propagation of high erosion rates in the Three Gorges areas.

Evolution of the Yangtze River and its biodiversity

Documenting the origins of megadiverse (sub)tropical aquatic ecosystems is an important goal for studies of evolution and ecology. Nonetheless, the geological and ecological establishment of the modern Yangtze River remains poorly understood. Here, we reconstruct the geographic and ecological history of an endemic clade of East Asian fishes based on the mitochondrial phylogenomics analysis of Cyprinidae using 15 fossil calibrations. We estimate an ancestral condition of benthic spawning with demersal or adhesive eggs in southern East Asia before ∼23 Ma and a derived condition of riverine spawning with semibuoyant eggs in the Yangtze by ∼18 Ma. These results imply the formation of Yangtze riverine ecosystems around the Oligocene-Miocene boundary in response to plateau uplift and monsoon strengthening. Some of these cyprinids reverted to benthic spawning with adhesive eggs by ∼15 Ma, a time of rising to peak net diversification rates,indicating the formation of potamo-lacustrine ecosystems by the mid-Miocene during a strong East Asian summer monsoon. Our study provides increased spatiotemporal resolution for the co-evolutionary histories of the Yangtze River and its biodiversity and highlights biological evidence concerning the geomorphological dynamics of the Yangtze River.

Luminescence characteristics of muddy sediments in the turbidity maximum zone of the Yangtze River mouth and implications for the depositional mechanisms

Muddy sediments are the most prominent constituents of sedimentary successions in tide-dominated river deltas and have highly complex depositional mechanisms. In this study, we performed fine-grained (4–11 μm) quartz optically stimulated luminescence (OSL) dating on two sediment cores collected at a shipwreck site in the turbidity maximum zone (TMZ) of the modern Yangtze River mouth, China, which were compared with previously published dating results including 45–63 um quartz OSL dating, radionuclide dating, porcelain artifacts recovered from the wreck, macro-plastics, and the morphological history recorded in marine charts. We investigate the luminescence characteristics of muddy sediments trapped in the TMZ and discuss the implications of OSL ages in understanding depositional mechanisms in tide-dominated river mouths. The results indicate that most OSL ages of muddy sediments in the delta front setting are overestimated compared with other dating methods. We suggest that OSL age overestimation reflects the trapping of sediments from offshore in the TMZ imported by saltwater intrusions and storm events. The offshore inputs contain high percentages of residual luminescence and are also subjected to incomplete bleaching due to turbid water conditions and near-bed dispersal in the salt-wedge river mouth. We thus suggest that the reduced bleaching efficiency of muddy sediments in delta front settings needs to be accounted for in understanding sedimentary processes and distinguishing between different sedimentary facies in tide-dominated river mouths. Furthermore, we propose that differences in quartz OSL ages of fine- and medium-grained fractions may arise in response to extreme events.

Late Oligocene-early Miocene Origin of the First Bend of the Yangtze River explained by thrusting-induced river reorganization

The origin of the First Bend of the Yangtze River is key to understanding the birth of the modern Yangtze River. Despite considerable efforts, the timing and mechanism of formation of the First Bend remain highly debated. Inverse river-profile modeling of three tributaries (Chongjiang, Lima, and Gudu) of the Jinsha River, integrated with regional tectonic and geomorphic interpretations, allows the onset of incision at the First Bend to be constrained to 28–20 Ma. The spatio-temporal coincidence of initial river incision and activity of Yulong Thrust Belt in southeastern Tibet highlights thrusting to be fundamental in reshaping the pre-existing stream network at the First Bend. These results enable us to reinterpret a change in sedimentary environment from a braided river to a swamp-like lake in the Jianchuan Basin south of the First Bend, recording the destruction of the hypothesized southwards-flowing paleo-Jinsha and Shuiluo Rivers at ~36–35 Ma by magmatism. During the late Oligocene-early Miocene, the paleo-Shuiluo River was diverted to the north by focused rock uplift due to thrusting along the Yulong Thrust Belt, which also led to exhumation of the Jianchuan Basin. Diversion of the paleo-Shuiluo River can be explained by capture from a downstream river in the footwall of the Yulong Thrust Belt. Subsequent rapid headward erosion, that was caused by thrusting-induced drop of local base level, is recorded by upstream younging ages for the onset of incision and led to the formation of the First Bend. The combination of new ages for the onset of incision at 28–20 Ma at the First Bend and younger ages upstream indicates northwards expansion of the Jinsha River at a rate of 62 ± 18 mm/yr. Our results suggest that the origin of the First Bend was likely triggered by thrusting at 28–20 Ma, after which the Yangtze River formed.

Detrital zircon ages reveal Yangtze provenance since the early Oligocene in the East China Sea Shelf Basin

The origin and evolution of the Yangtze River drainage system play a crucial role in the palaeogeographic and palaeoenvironmental evolution of East Asia. The source-to-sink history of this major Chinese river provides information on the initiation of a topographic gradient in East Asia driven by far-field and near-field effects of plate tectonics and elucidates the subsidence and depositional history of the depocenter in the East China Sea Shelf Basin (ECSSB). Unraveling age constraints on Yangtze provenance, however, remains a big challenge. Here, we review UPb ages of detrital zircon grains from the Oligocene-Miocene successions of the north ECSSB and perform several statistical tests to quantify mixing proportions of different potential source areas that reveal similar age patterns as the modern Yangtze River. We conclude that the initiation of a modern-type Yangtze drainage was established before ~34 Ma, and confirm that in the Oligocene an eastward drainage system was in place from the Yangtze Craton to the ECSSB where huge depositional systems developed on the Chinese continental margin. Monte Carlo models imply that the Oligocene and Miocene sediments of the north ECSSB were mainly supplied by the Yangtze River and the North China Craton - South Korean rivers, and that during the Miocene (< 23 Ma) the Yangtze drainage system extended further southward, reaching the marginal basins of modern Taiwan.

Formation of the Yangtze Three Gorges: Insights from detrital apatite fission‐track dating of sediments from the Jianghan Basin

AbstractThe establishment of the course of the modern Yangtze River is a topic of ongoing debate, with the formation of the Three Gorges, incised in the Huangling Massif, representing the key event under discussion. Downstream of the Three Gorges, the Cenozoic succession of the Jianghan Basin has been investigated to detect the moment of incision into the Three Gorges. Detrital apatite fission‐track data from Eocene to Miocene sediments drilled in the Jianghan Basin suggest that material was supplied by the Dabie, Qinling and Jiangnan Orogens during Eocene to early Oligocene times. At late Oligocene, the source of sediments switched to the Huangling Massif, implying incision of the Three Gorges. We therefore propose that the modern Yangtze River system formed at the late Oligocene.

Formation of the First Bend in the late Eocene gave birth to the modern Yangtze River, China

AbstractThe First Bend on the Yangtze River (China), the point where the river ceases flowing toward the south and heads toward the northeast, has been one of the most strongly debated geomorphic features in Asia because it holds the key to understanding the history of the Yangtze River and is linked to the tectonically driven surface uplift of the southeastern Tibetan Plateau. Mid- to upper Eocene sedimentary rocks preserved in the Jianchuan Basin located immediately southwest of the First Bend demonstrate that a large river system, presumably the paleo–Jinshajiang River (the upper Yangtze), used to flow south through that region. Provenance analysis of sediments over the wider region, based mostly on U-Pb dating of detrital zircon grains, confirms that the once south-flowing paleo–Yangtze River originated in the Tibetan Plateau and flowed into the South China Sea. Inversion of the Jianchuan Basin, starting in or after the latest Eocene and associated with wider plateau surface uplift at that time, caused the river to be diverted toward the northeast where it was confined along tectonic lineaments associated with strike-slip faulting, giving birth to the First Bend, a major step in the formation of the modern Yangtze River.

Chronology and provenance of the Yichang Gravel Layer deposits in the Jianghan Basin, middle Yangtze River Valley, China: Implications for the timing of channelization of the Three Gorges Valley

The Three Gorges Valley, linking the upper and the middle-lower Yangtze River areas is a key area for understanding the geomorphological evolution of the Yangtze River valley, especially the timing of its formation, which remains controversial and occurred following the authors to from the pre-Miocene to Late Pleistocene periods. The 100 m thick Yichang Gravel Layer (YGL), extending over an area >100 km2, is immediately observed downstream of the Three Gorges Valley in the middle Yangtze River Basin also named Jianghan Basin. Studying heavy minerals assemblage and chronology of the YGL deposits can shed lights not only on the geological origin of the sediments and the morphological evolution of the Three Gorges Valley, but also the formation of the Yangtze River Valley. In this study, 18 sediment samples carried out from six representative sections of the unconsolidated YGL and 2 samples from the underlying Neogene and Paleogene bedrocks were analyzed in order to examine their heavy mineral assemblage compositions. These assemblages were compared with modern Yangtze River sediments sampled in Yichang area (n = 5) and present-day sediments from other rivers of the Jianghan Basin (n = 6) to further identify the sediment provenance of the YGL. The statistic results indicate that the heavy minerals identified the YGL dominated by zircon, epidote, titanite, hematite, limonite and ilmenite, are similar to the assemblages observed in the 5 modern Yangtze River sediment samples but differ obviously from both underlying bedrock assemblages, containing kyanite, garnet, rutile and monazite, and other modern river sediments (Jianghan Basin), including hornblende, chlorite, tourmaline, rutile, anatase, spinel and garnet). In addition, 8 sandy samples from the YGL were dated by Electron Spin Resonance (ESR) method using Ti–Li center, showing a deposition age ranging between 1123 ± 139 and 734 ± 183 ka. Based on these observations, we propose that the YGL sediments derived from the upper Yangtze River Basin above the Three Gorges Valley, indicating that the Three Gorges Valley was channelized before 1.12 Ma ago. This result is consistent with previous provenance tracing studies of continuous deposited late Cenozoic sediment from central Jianghan Basin (since 3.0 Ma), which also constrains the incision time of the Three Gorges to ~1.1 Ma.

Provenance changes in fine detrital quartz in the inner shelf sediments of the East China Sea associated with shifts in the East Asian summer monsoon front during the last 6\xa0kyrs

The inner shelf sediments of the East China Sea (ECS) preserve valuable information regarding climatic changes on land through detrital material discharged from large rivers, particularly the Yangtze River. In this paper, we examine the provenance changes of fine-grained detrital quartz preserved in the sediment of the MD06-3040 core retrieved from the mud belt on the inner shelf of the ECS during the last 6 kyrs. The provenance of the fine silt fraction (4–16 μm) of the sediments, which is considered to represent the suspended particulate matter (SPM) from the Yangtze River, is estimated based on the electron spin resonance (ESR) signal intensity and crystallinity index (CI) of quartz. By comparing the ESR signal intensity and CI of quartz in the fine silt fraction of sediments to those from different parts of tributaries in the modern Yangtze River drainage, we can discriminate the SPM sourced from the northwestern and southeastern tributaries of the Yangtze River. Shifts in the East Asian summer monsoon (EASM) front are monitored by changes in the main precipitation areas which are manifested as changes in source rock ages and types implied from the ESR signal intensity and CI of quartz in the fine silt fraction of the studied sediments discharged from the Yangtze River. Temporal changes in provenance of the fine silt fraction through time suggest that shifts in the main location of EASM precipitation (most likely representing the EASM front) occurred at a multi-centennial-to-millennial scale with the deepest northwestward penetration of the EASM precipitation front (an enhanced EASM) occurring during the period 3.5–2.0 kyr BP and a southeastward retreat (a weak EASM) occurring during the periods 6.0–4.1 and 2.0–0 kyr BP.

History of Yellow River and Yangtze River delivering sediment to the Yellow Sea since 3.5 Ma: Tectonic or climate forcing?

Reconstructing the Plio-Pleistocene sedimentary history of the Yellow Sea, is important for understanding the long-term influence of the Yellow River (Huanghe) and/or Yangtze River (Changjiang) on the Asian marginal seas and to further constrain any links between river development, regional paleoenvironmental change, tectonic deformation and/or global climate change. Here we present the first high-resolution clay mineral record combined with Sr-Nd isotopic compositions from a 300-m long sediment core recovered in the western South Yellow Sea. Our study suggests that large-scale transgression in the Yellow Sea occurred after ∼0.8 Ma, possibly related to tectonic subsidence of eastern China coast and the Min-Zhe Uplift. In contrast, the sedimentary environment of Bohai and Yellow Seas was mainly dominated by fluvial/lacustrine deposits at 3.5–0.8 Ma. Provenance analysis suggests a major change in sediment provenance from the Yangtze River to the Yellow River at ∼0.8 Ma, which corresponds to the timing of the final integration of the Yellow River in its present form. At the same time the major channels of the Yangtze River migrated from the South Yellow Sea to the modern Yangtze River delta due to the subsidence of east China coast. The consistence in timing of the provenance transition and large-scale regional marine transgression suggests that tectonic deformation, rather than climate change, is the first-order control on the evolution of the Yellow and Yangtze Rivers and sedimentary environmental change in the Bohai and Yellow Seas.

Sedimentary responses to the cross-shelf transport of terrigenous material on the East China Sea continental shelf

Sedimentary responses to the cross-shelf transport on the East China Sea continental shelf are ascertained based on analyses of the sediment grain size and detrital mineral components of surficial sediments. Sedimentological data show that fine-grained sediments can be transported across the continental shelf and have formed a tongue-shaped zone with an axis located near 29°N that extends from the coast to 124°40′E or beyond. The sediments in this tongue-shaped zone are rich in very fine silt and clay, as are the dominant sediment fractions discharged by the modern Yangtze River indicating a modern terrigenous source. Sediment with grain sizes 7–9 Φ is the main fraction that is transported across the continental shelf. In contrast to the regions in the same longitude, in the tongue-shaped zone the relatively low heavy mineral content, high schistose mineral content and high dolomite content and the mixture of fresh and weathered minerals show that modern Yangtze River sediment has been widely transported to this region. Meanwhile, the simultaneous existence of the fine-grained tongue and a coarse-grained area immediately south of the base of the tongue coincident with the nearshore Kuroshio branch current intrusion area indicate that the Kuroshio intrusion plays an important role in triggering the cross-shelf flows.

The sedimentary evolution of Yangtze River delta since MIS3: A new chronology evidence revealed by OSL dating

Abstract The Yangtze River delta, one of the largest tide-dominated deltas in the world, is a valuable archive for studying the land-sea interaction. Previous investigations have mainly focused on the Holocene Yangtze River delta initiation and the coastal dynamics in response to sea level changes based on analysis of the stratigraphic and sedimentologic evidences. However, the detailed depositional history of the Yangtze River delta prior to Holocene is still poorly understood owing to a deficiency of accurate absolute chronologies. In this study, a 60-m-long core (EGQD14) was drilled in the front edge of modern Yangtze River deltaic plain. The sedimentary features were analyzed based on the data of sediment particle size composition, colors, textures and intra-structure description; a total of 20 sediment samples were collected from this core for optically stimulated luminescence (OSL) dating. The single aliquot regenerative dose (SAR) protocol was applied to fine (4–11 μm in diameter) and coarse (63–100 μm or 100–200 μm) grained quartz extracts to determine their equivalent dose (De). The bleaching levels of different grain-sized fractions were investigated. We found that 63–100 μm grains and fine grains are generally well-bleached, and are appropriate for establishing the chronology of the studying core. However, ages of 100–200 μm grains appear to be problematic as potential feldspar contamination, revealed by remarkable infrared (IR) depletion ratios as low as to 0.3. Thus, ages derived only from 4–11 μm and 63–100 μm fractions were used for the reconstruction of chronological framework. The sedimentary history of the Yangtze River delta since the Marine Isotope Stage 3 (MIS3) was inferred by combination of a 50-ka spanning stratigraphic sequence of the EGQD14 core with previously investigated sedimentary stratigraphy in the study area. This study provided a new chronological evidence of the Yangtze River delta sedimentary evolution over the last ∼50 ka, which generally underwent the terrestrial fluvial, floodplain, tidal flat, estuary-neritic, deltaic and modern Yangtze River delta plain sedimentary environments.

Transparent heavy minerals and magnetite geochemical composition of the Yangtze River sediments: Implication for provenance evolution of the Yangtze Delta

Heavy mineral and detrital magnetite geochemistry were analyzed to extract sediment provenance indexes from different reaches of the modern Yangtze River which were used to trace sediment source of the Yangtze Delta and to speculate its geomorphology change since the Pliocene. Our results show that diagnostic heavy minerals of the upper Yangtze sediment are characterized by clinopyroxene (12% on average) and magnetite (7% on average); the middle reaches by ilmenite, zircon and tourmaline; and the local small rivers by fluorite. Detrital magnetite composition of Ti, Mg, V and Cr is high in the upper Yangtze from the underlying basalt. These diagnostic indexes are then used in the Pliocene sediment core to extract provenance signal of different Yangtze reaches. Analysis of core sediment of the Yangtze Delta reveals that sediment provenance of the Pliocene was from local small rivers. Since the beginning of the Pleistocene, core sediments provenance was similar to that of the middle Yangtze tributaries. After 1.2Ma, high content of pyroxene and magnetite grains that are rich in Ti, Mg, V, Cr imply sediment provenance signals from the upper Yangtze. Sediment provenance shift from short-distance sources to more distant sources indicates that the geomorphology of the Yangtze Delta region has undergone a great transformation since the Pliocene. This dramatic landform change is likely in response to continuous uplift of the Tibetan plateau and accelerated subsidence of the east China coast since the Pliocene.

Dating of the topmost terrace in the Jingxian Basin, Anhui Province: an indication of the establishment of the Qingyijiang River

On the northern fringe of Mt. Huangshan, many river terraces are developed at the junction between mountains and plains. The river terraces are most typical in the Jingxian Basin, Anhui Province, where the Qingyijiang River, one of the longest tributaries of the lower Yangtze River, developed three staircase terraces. The topmost terrace (i.e., the T3 terrace) tread is 38 - 39 m above river level, and the top of the gravels is generally covered by 3 - 10.9 m of red clay deposits. Systematic magnetostratigraphy and electron spin resonance dating are used to research the timing of the establishment of the Qingyijiang River. The main results show that the topmost terrace developed no later than 900 kyr ago and that the appearance of the Qingyijiang River occurred correspondingly at least before 900 kyr ago on the northern fringe of Mt. Huangshan. In view of the almost synchronous age, the establishment of the Qingyijiang River was seemingly a response to the Mid-Pleistocene Revolution when the 100 kyr cycle commenced, and the Kunlun-Huanghe movement during the early-middle Pleistocene transition. In addition, the establishment of the Qingyijiang River possibly reflected the birth of the modern Yangtze River to some degree. Therefore the timing of the formation of the Yangtze River is restricted to no later than 900 kyr ago based on the appearance of the Qingyijiang River

Eastern margin of Tibet supplies most sediment to the Yangtze River

Zircon provenance studies of modern and ancient fluvial systems help reveal the relative contributions and importance of upstream sediment sources. A 2014 study of detrital zircon U-Pb age distributions from the Yangtze River (China) and its tributaries proposed a strong anthropogenic control on sediment flux. Those data, along with other data from the region, were reanalyzed using multiple detrital zircon U-Pb age distribution comparison techniques and a distribution-mixing model to construct an improved and quantitative view of provenance. The variability in the Yangtze River trunk stream U-Pb age distributions is evaluated with respect to trunk-to-trunk stream comparisons, trunk-to-tributary comparisons, and in mixture models that consider tributary and bedrock contributions, the latter using a comprehensive compilation of bedrock source terranes. Uniformity in the zircon age distribution of the Yangtze River trunk stream is established in the upper reaches, downstream of the first bend, and maintained by the left-bank tributaries to its outlet. Whether considering the bedrock source terranes or only the modern Yangtze River sediments, the major source of sediments contributing to Yangtze River is clearly the eastern edge of the Tibetan Plateau (e.g., Songpan Ganze complex, Longmenshan Range), where rock uplift rates are high. The purported increase in anthropogenic impact on sediment yield in the lowlands, at least as viewed through detrital zircon age distributions, is insignificant.

Birth of the Yangtze River: age and tectonic-geomorphic implications

AbstractLarge river systems are an integral and essential component of Earth dynamics. The development of large river systems in Asia is closely linked to the evolving topography driven by both near-field and far-field effects of the interplay among Indian, Eurasian and Pacific plates. Plate tectonics together with climatic changes during the Cenozoic is therefore believed to have determined the evolution of Asian large rivers, yet the age of the Yangtze, the largest in Asia, has been strongly debated over a century, with estimates ranging from 40–45 Ma to a more recent initiation postdating 750 ka. In this paper, I attempt to review the competing views about the age of the Yangtze, and evaluate the estimated pre-Miocene birth of the river based on the provenance of the fluvial sediments from the lower reaches. I further present new geological evidence from the upper stream in southeastern Tibetan Plateau to show the existence of a possible southward paleo-Jinshajiang during the Paleogene, and exploit when and how it might deviated eastward to give birth to the modern Yangtze River. I propose that the present Yangtze River system formed in response to the continental-scale gradient driven by uplifting Tibetan Plateau and regional extension throughout eastern China, synchronous with surface uplift in southeastern Tibet and strengthening of Asian summer monsoon.

Quantitative reconstruction of palaeoclimate from the Middle Miocene Shanwang flora, eastern China

The fossil leaf flora from the Shanwang Formation, Linqu County, Shandong Province, eastern China, is diverse, abundant, well preserved and discretely spaced, all of which make it suitable for the recognition and application of leaf physiognomic characters. This article is the first study of Chinese Tertiary floras using the CLAMP (Climate Leaf Analysis Multivariate Program) approach. Numerous bedding planes yield leaf assemblages through over 20 m thickness of diatomaceous shales within the Middle Miocene Shanwang Formation. The diatomaceous shales, containing abundant fossils, have been divided into 19 units. A working platform (3×3 m) in the Diatomite Quarry of Shanwang was studied for about 3 months in 1998 and as many fossils as possible were recorded layer by layer. This collecting method, used for the first time in the history of Shanwang research, enabled us to record the presence or absence of megafossils in distinct units so that we could investigate leaf physiognomic characteristics and palaeoclimate for each time interval. Leaf fossils from six units (5, 6, 7, 13, 15 and 16) have been used to estimate palaeoclimate parameters of the Shanwang area using the CLAMP 3B dataset. The predicted climate is that of a lakeside setting, at over 1000 m above sea level and within the 17–15-Ma time interval. Meteorological factors have been estimated using the CLAMP approach, including mean annual temperature (MAT; 9.5–11.2°C), warmest month mean temperature (20.2–22.5°C), growing season length (5.9–6.8 months), mean growing season precipitation (83–165 cm) and relative humidity (73–79%). MAT estimates from leaf margin analysis (LMA) range from 12.5 to 15.2°C, which are a maximum of 4° higher than those from our CLAMP analyses and from previous results of LMA studies of the Shanwang flora (10–13°C). Nearest living relatives (NLR) of other biota (pollen, fish, insects, diatoms, microthyriaceous fungi, etc.) indicate warm and moist temperate to subtropical conditions in the Shanwang area during the Miocene. CLAMP predictions are similar to the climate predicted from NLRs although the MAT prediction is a few degrees cooler. The fact that the Shanwang leaf flora is from a lacustrine sequence may have biased the CLAMP analysis towards cooler (and possibly wetter) climates. The modern Yangtze River valley is a reasonable environmental analogue for the Shanwang Middle Miocene, though Shanwang may have had cooler summers, cooler MAT and less seasonality in rainfall. The climate was essentially constant during deposition of the sequence studied, with possible minor variations in precipitation.