Carbon and oxygen isotopic constraints on paleoclimate and paleoelevation of the southwestern Qaidam basin, northern Tibetan Plateau

We investigate the growth of the northern Tibetan Plateau and associated climate change by applying oxygen and carbon isotopic compositions in Cenozoic strata in the southwestern Qaidam basin. The X-ray diffraction and isotopic studies reveal that the carbonate minerals are mainly authigenic and they do not preserve any evidence for detrital carbonate and diagenesis. The isotope data show large fluctuations in the δ^(18)O and δ^(13)C values in the middle-late Eocene, indicating relatively warm and seasonal dry climate.The positive correlation of the δ^(18)O and δ^(13)C values in the Oligocene and the positive shift of the δ^(13)C values from the Eocene to Oligocene suggest that the climate changed to arid in the Oligocene. However,the δ^(18) values show negative shift, which is closely related to the global cooling event. During the Miocene, the δ^(13)C values vary between-2‰ and-4‰, whereas the δ^(18)O values show continuous negative shift. The mean δ^(18) values decrease from-8.5‰, in the early Miocene to-10.0‰, in the late Miocene. The stable isotope-based paleoaltimetry results suggest that the elevation of the southwestern Qaidam basin was approximately 1500 m in the middle-late Eocene and Oligocene. Subsequently, during Miocene the crustal uplift process started and the elevation reached approximately 2000 m in the early Miocene and 2500 m in the late Miocene, which suggests large-scale growth of the northern Tibet Plateau during the Miocene.

Application of the Material Balance Method in Paleoelevation Recovery: A Case Study of the Longmen Mountains Foreland Basin on the Eastern Margin of the Tibetan Plateau

We applied the material balance principle of the denudation volume and sedimentary flux to study the denudation-accumulation system between the Longmen Mountains （Mts.） and the foreland basin. The amount of sediment in each sedimentation stage of the basin was estimated to obtain the denudation volume, erosion thickness and deposit thickness since the Late Triassic Epoch, to enable us to recover the paleoelevation of the provenance and the sedimentary area. The results show the following： （1） Since the Late Triassic Epoch, the elevation of the surface of the Longmen Mts. has uplifted from 0 m to 2751 m, and the crust of the Longmen Mts. has uplifted by 9.8 km. Approximately 72% of the materials introduced have been denuded from the mountains. （2） It is difficult to recover the paleoelevation of each stage of the Longmen Mts. foreland basin quantitatively by the present-day techniques and data. （3） The formation of the Longmen Mts. foreland basin consisted of three stages of thrust belt tectonic load and three stages of thrust belt erosional unload. During tectonic loading stages （Late Triassic Epoch, Late Jurassic-Early Cretaceous, Late Cretaceous-Miocene）, the average elevation of Longmen Mts. was lower （approximately 700-1700 m）. During erosional unloading stages （Early and Middle Jurassic, Middle Cretaceous and Jiaguan, Late Cenozoic）, the average elevation of Longmen Mts. was high at approximately 2000-2800m.

Altitudinal variation in Ginkgo leaf characters: Clues to paleoelevation reconstruction

After detailed studies of leaf area, leaf shape index, petiole length, stomatal density, stomatal index, and carbon isotope discrimination of Ginkgo biloba L., growing in Northwest China, the change trends of these parameters with an altitude gradient and their differences between sun and shade leaves were assessed. The results show that leaf area, petiole length, and stomatal parameters have no obvious linear relationship with altitude, but the carbon isotope discrimination shows a negative correlation with altitude, which has a potential to be an applicable paleo-altimeter. The results also suggest that the differences in stomatal density and stomatal index between sun and shade leaves had more influence on paleoelevation reconstruction than that in other parameters. Based on the linear and nonlinear correlations between carbon isotope discrimination and altitude, the paleoelevation evolution during the Middle Jurassic of the Yaojie Basin, Lanzhou in Gansu Province was estimated. The results show that the paleoelevation of the Yaojie Basin increased at first, and then decreased from the Aalenian to the early Bajocian and then to the late Bajocian in the Middle Jurassic.

Differential surface uplift: Cenozoic paleoelevation history of the Tibetan Plateau

The surface uplift history of the Tibetan Plateau has provided a key boundary criterion for various geological, climatic,and environmental events since the Cenozoic. The paleoelevation history of the plateau is organically associated with interactions amongst deep geodynamics, earth surface processes, and climate change. Understanding of plateau uplift history has been advanced by the development of a number of paleoaltimetries and their application to studies of the Tibetan Plateau: the paleogeomorphic scenario for the Early Eocene Tibetan Plateau is thought to include two high mountains, the ca. 4500 m Gangdese Mountains to the south, and the ca. 5000 m Qiangtang Center Watershed Mountains to the north. Between these ranges, a low-elevation basin(ca. 2500 m) is thought to have been present. The Himalayas in the southern Tibetan Plateau was close to sea level at this time,while the Hoh Xil Basin in the north reached an elevation of no more than 1500 m. Thus, the so-called "Roof of the World" Tibetan Plateau formed subsequent to the Miocene. Nevertheless,, the uplift histories of the different terranes that comprise this plateau currently remain unclear, which constrains the uplift history reconstruction of the entire Tibetan Plateau. Additional paleoelevation data from different areas, obtained using multi-paleoaltimeters, is required to resolve the forms and processes of Tibetan Plateau uplift and extension.

How does the elevation changing response to crustal thickening process in the central Tibetan Plateau since 120 Ma?

When and how the Tibetan Plateau formed and maintained its thick crust and high elevation on Earth is continuing debated.Specifically,the coupling relationship between crustal thickening and corresponding paleoelevation changing has not been well studied.The dominant factors in crustal thickness changing are crustal shortening,magmatic input and surface erosion rates.Crustal thickness change and corresponding paleoelevation variation with time were further linked by an isostatic equation in this study.Since 120 Ma crustal shortening,magmatic input and surface erosion rates data from the central Tibetan Plateau are took as input parameters.By using a one-dimensional isostasy model,the authors captured the first-order relationship between crustal thickening and historical elevation responses over the central Tibetan Plateau,including the Qiangtang and Lhasa terranes.Based on the modeling results,the authors primarily concluded that the Qiangtang terrane crust gradually thickened to ca.63 km at ca.40 Ma,mainly due to tectonic shortening and minor magmatic input combined with a slow erosion rate.However,the Lhasa terrane crust thickened by a combination of tectonic shortening,extensive magmatic input and probably Indian plate underthrusting,which thickened the Lhasa crust over 75 km since 25 Ma.Moreover,a longstanding elevation>4000 m was strongly coupled with a thickened crust since about 35 Ma in the central Tibetan Plateau.

Global-scale altitude effect on leaf wax n-alkane δD values in terrestrial higher plants

Numerous field studies have shown that leaf wax n-alkane δD values(δD_(n-alkane))can be used as a proxy for paleoelevation reconstruction.However,the lack of systematic global analysis of δD_(n-alkane) values with respect to altitude limits the reliability of δD_(n-alkane )values for paleoaltimetry at the global scale.In this study,we analyzed the variation of δD_(n-alkane) values with altitude in terrestrial higher plants based upon our compiled globalδDn-alkane database.The general decrease in δD_(n-alkane) values with altitude corresponds to the variation in precipitationδD(δD_(prec))with altitude,suggesting that the δD_(n-alkanevalues) effectively reflect δD_(prec) values.When eliminating the effect of latitude at the global scale,the global δD_(n-alkane)-altitude lapse rate for all species was-1.01±0.10‰(100 m)-1,while these lapse rates were-1.03±0.13‰ and-0.47±0.13‰(100 m)^(-1) for dicots and monocots,respectively.Our study reveals that it is reliable to use δD_(n-alkanevalues) to gain information about paleoelevation at the global scale,and the global δD_(n-alkane-altitude) lapse rates provide an alternative to reconstruct paleoelevation at some δD_(n-alkane)-uninvestigated areas.