Dominant 100,000-year precipitation cyclicity in a late Miocene lake from northeast Tibet

East Asian summer monsoon (EASM) precipitation received by northern China over the past 800 thousand years (ky) is characterized by dominant 100-ky periodicity, mainly attributed to CO2 and Northern Hemisphere insolation–driven ice sheet forcing. We established an EASM record in the Late Miocene from lacustrine sediments in the Qaidam Basin, northern China, which appears to exhibit a dominant 100-ky periodicity similar to the EASM records during the Late Quaternary. Because evidence suggests that partial or ephemeral ice existed in the Northern Hemisphere during the Late Miocene, we attribute the 100-ky cycles to CO2 and Southern Hemisphere insolation–driven Antarctic ice sheet forcing. This indicates a >6–million year earlier onset of the dominant 100-ky Asian monsoon and, likely, glacial and CO2 cycles and may indicate dominant forcing of Northern Hemisphere climate by CO2 and Southern Hemisphere ice sheets in a warm world.This work was funded by the national key research and development program of China (2016YFE0109500), the (973) National Basic Research Program of China (grant no. 2013CB956400), the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDB03020400), the National Natural Science Foundation of China (grant nos. 41422204, 41172329, and 41290253), and the U.S. NSF (grant no. 1545859)

A Conforming Augmented Finite Element Method for Modeling Arbitrary Cracking in Solids

This paper presents a conforming augmented finite element method (C-AFEM) that can account for arbitrary cracking in solids with similar accuracy of other conforming methods, but with a significantly improved numerical efficiency of about ten times. We show that the numerical gains are mainly due to our proposed new solving procedure, which involves solving a local problem for crack propagation and a global problem for structural equilibrium, through a tightly coupled two-step process. Through several numerical benchmarking examples, we further demonstrate that the C-AFEM is more accurate and mesh insensitive when compared with the original A-FEM, and both C-AFEM and A-FEM are much more robust and efficient than other parallel methods including the extended finite element method (XFEM)/generalized finite element (GFEM) and the conforming embedded discontinuity method

Progressive fracture analysis of the open-hole composite laminates: experiment and simulation

A conforming augmented finite element method (C-AFEM) is extended to predict the failure process in inter-ply of fiber-reinforced composite laminates. Combining with cohesive zone model (CZM) element, the delamination between inter-plies can also be captured. Matrix fracture, fiber fracture and delamination in inter-plies will occur when the composite laminate fracture, which is a complicated process, and the current method appears capable of modelling failure mechanism of composite laminates, which is demonstrated mesh-independent, high-fidelity and robust. Different stacking open-hole composite laminates subject to tension load are tested by electrical measurement and Digital Image Correlation (DIC) system. The proposed C-AFEM is applied to model progressive failure of open-hole composite laminates and results are found to agree with experiments very well

A Conforming A-FEM for Modeling Arbitrary Crack Propagation and Branching in Solids

In this paper, an improved conforming AFEM (C-AFEM) for efficient modeling of arbitrary crack propagation and branching is proposed and validated. An explicit formulation for branching cracks has been derived within the C-AFEM framework. The conjugate gradient method is integrated into the C-AFEM formulation to solve the local problem that consists of all elements traversed by single or multiple cracks. Multiple numerical evidences show that this new approach can substantially improve the modeling efficiency. The solution accuracy and numerical robustness are also significantly improved

Detection of Strong Precession Cycles from the Late Pliocene Sedimentary Records of Northeastern Tibetan Plateau

Recent studies demonstrate that aridification is sensitive to eccentricity forcing based on the northeastern Tibetan Plateau Cenozoic fluvio-lacustrine stratigraphic records. However, it is unknown whether this is a bias associated with the fact that higher frequency periods normally have larger age model uncertainties, which tend to decrease spectral power of higher frequency orbital cycles (precession and obliquity) and enlarge lower frequency ones (eccentricity). Here we detect strong and well-resolved (similar to 23 and similar to 19 kyr) precession cycles in the magnetic susceptibility record of late Pliocene fluvio-lacustrine sediments from the Qaidam Basin, northwestern China. We interpret the magnetic susceptibility as a proxy for the degree of chemical weathering in this region, based on the positive correlation between low-frequency magnetic susceptibility and frequency-dependent magnetic susceptibility. The different responses of magnetic susceptibility and a halite content proxy to orbital forcing suggest that aridification is more sensitive to eccentricity forcing than precipitation, but precipitation is more sensitive to precession forcing. This study indicates that different climatic aspects have different sensitivity to orbital forcing. Furthermore, a comparison of the magnetic susceptibility variations in the western Qaidam Basin and global ice volume proxy records suggests ice volume control of Qaidam Basin precipitation and chemical weathering at million-year timescales. Plain Language Summary The cyclicity of Earth's climate is driven by changes in solar insolation associated with variations in Earth's orbital parameters (precession, obliquity, and eccentricity). It remains unclear how different climate processes respond to different orbital forcing. Here, proxy records of aridification and precipitation from the northeastern Tibetan Plateau suggest that variations in aridification were sensitive to eccentricity forcing, while the precipitation changes were sensitive to precession forcing during the late Pliocene. This study indicates that different climatic systems have different sensitivity to orbital forcing

Central Asian Drying at 3.3 Ma Linked to Tropical Forcing?

The processes controlling middle-latitude environmental changes are poorly understood. Both high-latitude ice sheets and tropical oceans can affect climate and environments in middle latitudes, but separating their relative contribution is challenging. Coincidence of intensive Northern Hemisphere glaciations and increased dust accumulation rates at similar to 2.7 Ma on the Chinese Loess Plateau and North Pacific Ocean is often used to indicate high latitudes' dominant role on middle-latitude climate change. Here we present new and compile available evidence from the Qaidam Basin and the Tarim Basin of central Asia demonstrating a phase of drying at similar to 3.3 Ma. The drying is synchronous with proposed timing of closure of the Indonesian Seaway, which has been suggested to cause significant oceanic surface heat adjustments and corresponding decreased heat and moisture transport to middle/high latitudes. These new results suggest a tropical forcing of aridification for the late Pliocene central Asian environmental evolution. Plain Language Summary The timing and forcing mechanisms for past climate changes are instructive to predict future variations. It is widely believed that the aridification of central Asia and changes in monsoonal climate on the Chinese Loess Plateau formed at the Pliocene-Quaternary boundary forced by intensive Northern Hemisphere glaciations at similar to 2.7 Ma. In this study we demonstrate that aridification began earlier within the Qaidam and Tarim Basins, at similar to 3.3 Ma, when no significant change occurred for Northern Hemisphere ice sheets. Instead, this event was aligned with tropical Indian Ocean cooling as a result of closure of the Indonesian Seaway. Thus, we attribute this earlier drying event to tropical forcing rather than cryospheric change. This study reinterprets the forcing of aridification in central Asia and suggests that understanding future environments in this ecologically fragile region needs to consider tropical forcing

Orbital forcing of Plio-Pleistocene climate variation in a Qaidam Basin lake based on paleomagnetic and evaporite mineralogic analysis

Inland Asian aridification is threatening the global ecological system under continued global warming, requiring a full understanding of its forcing mechanisms. Past geological studies in this region focus mainly on initiation timing and million-year timescale variations in aridification. Few studies address the relationship between aridification and insolation forcing. Here we present a paleomagnetic and evaporite mineralogic study of the 476-m-long Huatugou (HTG) section (ca. 3.9 to 2.1 Ma) from the western Qaidam Basin. The results show that the drying of this part of the basin began by at least 3.9 Ma. Furthermore, we show that the relative content variation of evaporite minerals was dominated by 100-kyr cyclicity during the Pliocene. This is consistent with the result from the Xining area during the late Eocene. These results suggest that mid-latitude lacustrine evaporite minerals in semi-arid to arid regions are particularly sensitive to eccentricity forcing, improving understanding of aridification forcing at orbital timescales

Dominant 100,000-year precipitation cyclicity in a lateMiocene lake from northeast Tibet

East Asian summer monsoon (EASM) precipitation received by northern China over the past 800 thousand years (ky) is characterized by dominant 100-ky periodicity, mainly attributed to CO2 and Northern Hemisphere insolation-driven ice sheet forcing. We established an EASM record in the Late Miocene from lacustrine sediments in the Qaidam Basin, northern China, which appears to exhibit a dominant 100-ky periodicity similar to the EASM records during the Late Quaternary. Because evidence suggests that partial or ephemeral ice existed in the Northern Hemisphere during the Late Miocene, we attribute the 100-ky cycles to CO2 and Southern Hemisphere insolation-driven Antarctic ice sheet forcing. This indicates a >6-million year earlier onset of the dominant 100-ky Asian monsoon and, likely, glacial and CO2 cycles and may indicate dominant forcing of Northern Hemisphere climate by CO2 and Southern Hemisphere ice sheets in a warm world