Two-stage exhumation, uplift, and basinward propagation of the Tian Shan during the late Cenozoic

Quantifying erosion rates over various temporal and spatial scales is the key to understanding the respective roles of tectonic and climatic factors in driving the topographic evolution of an active orogenic belt. This work focuses on the spatiotemporal patterns of denudation rate in the Tian Shan and its implication for topographic evolution by utilizing low-temperature thermochronological analyses. Apatite (U-Th)/He thermochronological analyses and thermal history modelling were conducted on eight samples taken from a ∼ 0.7-km elevation transect at the glaciated headwaters of the Urumqi River in the northern Chinese Tian Shan. The results reveal dominant Mesozoic thermochronological ages and limited Cenozoic exhumation, although the sampled area was subjected to intensive glaciation at least during the late Quaternary. The restrained late Cenozoic exhumation at the glaciated Urumqi catchment is also revealed in the other parts of Tian Shan. The inversion of ∼1780 published thermochronological ages shows that, although the exhumation in the Tian Shan has been enhanced since the late Oligocene due to tectonic rejuvenation and pulsed uplift of the range, the long-term exhumation rates in most of the Tian Shan are generally < 0.2 km/Myr, obviously lower than that in some other active orogenic belts characterized by a more humid climate. Given the fact that the late Cenozoic climate in the Tian Shan was characterized by stepwise aridification due to uplifting topographic barrier of the range for the Westerlies, we propose that climate aridification beginning since the late Oligocene could have acted as the equalizer of range-scale denudation, thus restraining the late Cenozoic denudation of the Tian Shan, a typical reactivated orogen belt in the arid inland of Asia. 

Climatic aridification restrained late Cenozoic denudation of the Tian Shan in the inland of Asia

Climate dictates wildfire activity around the world. But East and Southeast Asia are an apparent exception as fire-activity variation there is unrelated to climatic variables. In subtropical China, fire activity decreased by 80% between 2003 and 2020 amid increased fire risks globally. Here, we assessed the fire regime, vegetation structure, fuel flammability and their interactions across subtropical Hubei, China. We show that tree basal area (TBA) and fuel flammability explained 60% of fire-frequency variance. Fire frequency and fuel flammability, in turn, explained 90% of TBA variance. These results reveal a novel system of scrubland-forest stabilized by vegetation-fire feedbacks. Frequent fires promote the persistence of derelict scrubland through positive vegetation-fire feedbacks; in forest, vegetation-fire feedbacks are negative and suppress fire. Thus, we attribute the decrease in wildfire activity to reforestation programs that concurrently increase forest coverage and foster negative vegetation-fire feedbacks that suppress wildfire. 

Vegetation-fire feedbacks increase subtropical wildfire risk in scrubland and reduce it in forests.

Cenozoic mountain building in Asia has been proposed as an important control over global climate by atmospheric CO2 drawdown through silicate weathering and burial of organic carbon (OC) offshore. Because Asian submarine fans represent the most complete record of Asian orogenic erosion and weathering over the Cenozoic, evaluation of sediment major element chemistry and OC content can be used to estimate CO2 sequestration rates driven by silicate chemical weathering and OC burial. From deep-sea fans in the Indian Ocean and South China Sea, weathering rates are calculated by comparison of weathered sediment to initial bedrock source compositions and then integrated with deposited volumes to derive the first regional weathering budget for India-Asia collision over the last ~25 m.y.. Results indicate the Indus is more important than previously recognized before and during the middle Miocene in sequestering CO2 (~76% of Asian total at 16–14 Ma). This in part reflects the more reactive mafic and ultramafic bedrock sources compared to the largest and major east Himalayan drainage, the Ganga-Brahmaputra, but also greater erosional flux from the onshore Indus drainage at that time. This regional synthesis further concludes that OC burial only represented a minority (20–25%) of the regional carbon budget but became more important after 17 Ma, peaking at ~38% after ~3 Ma. CO2 sequestration rates increased from 17 to 15 Ma, coinciding with the Miocene Climatic Optimum, and remained mostly steady in Asia from ~15–5 Ma as the climate cooled. Only one of three possible sediment flux models for the Bengal Fan predicts increased CO2 consumption rates after 15 Ma and, even then, only predicts steady rates from 11 to 5 Ma. The timing of changes in CO2 consumption rates are not consistent with Asian orogenic silicate weathering acting as the dominant control over late Cenozoic atmospheric CO2. 

The impact of Himalayan-Tibetan erosion on silicate weathering and organic carbon burial

Abstract Background Geological movements and climatic fluctuations stand as pivotal catalysts driving speciation and phylogenetic evolution. The genus Polyspora Sweet (Theaceae), prominently found across the Malay Archipelagos and Indochina Peninsula in tropical Asia, exhibits its northernmost distribution in China. In this study, we investigated the evolutionary and biogeographical history of the genus Polyspora in China, shedding light on the mechanisms by which these species respond to ancient geological and climatic fluctuations. Methods Phylogenetic relationships of 32 representative species of Theaceae were reconstructed based on the chloroplast genome and ribosome 18-26 S rRNA datasets. Species divergence time was estimated using molecular clock and five fossil calibration. The phylogeography and population genetics in 379 individuals from 32 populations of eight species were analyzed using chloroplast gene sequences ( trn H- psb A, rpo B- trn C and pet N- psb M), revealing the glacial refugia of each species, and exploring the causes of the phylogeographic patterns. Results We found that Chinese Polyspora species diverged in the middle Miocene, showing a tropical-subtropical divergence order. A total of 52 haplotypes were identified by the combined chloroplast sequences. Chinese Polyspora exhibited a distinct phylogeographical structure, which could be divided into two clades and eight genealogical subdivisions. The divergence between the two clades occurred approximately 20.67 Ma. Analysis of molecular variance revealed that the genetic variation mainly occurred between species (77.91%). At the species level, Polyspora axillaris consists of three lineages, while P. speciosa had two lineages. The major lineages of Chinese Polyspora diverged between 12 and 15 Ma during the middle to late Miocene. The peak period of haplotype differentiation in each species occurred around the transition from the last interglacial to the last glacial period, approximately 6 Ma ago. Conclusion The primary geographical distribution pattern of Chinese Polyspora was established prior to the last glacial maximum, and the population historical dynamics were relatively stable. The geological and climatic turbulence during the Quaternary glacial period had minimal impact on the distribution pattern of the genus. The genus coped with Quaternary climate turbulence by glacial in situ survival in multiple refuges. The Sino-Vietnam border and Nanling corridor might be the genetic mixing center of Polyspora . 

Phylogeographical and population genetics of Polyspora sweet in China provides insights into its phylogenetic evolution and subtropical dispersal

Evolution of silicate weathering in South China since 30 Ma: Controlling factors and global implications

Expansion of C4 plants in South China and evolution of East Asian monsoon since 35 Ma: Black carbo+0n records in the northern South China Sea

Evolution of Asian drying since 30 Ma revealed by clay minerals record in the West Pacific and its tectonic-climatic forcing

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Evolution of silicate weathering in South China since 30 Ma: Controlling factors and global implications

Expansion of C4 plants in South China and evolution of East Asian monsoon since 35 Ma: Black carbo+0n records in the northern South China Sea

Evolution of Asian drying since 30 Ma revealed by clay minerals record in the West Pacific and its tectonic-climatic forcing

Yi Tang

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Chat about Author

Papers

Evolution of silicate weathering in South China since 30 Ma: Controlling factors and global implications

Expansion of C4 plants in South China and evolution of East Asian monsoon since 35 Ma: Black carbo+0n records in the northern South China Sea

Evolution of Asian drying since 30 Ma revealed by clay minerals record in the West Pacific and its tectonic-climatic forcing

Evolution of silicate weathering in South China since 30 Ma: Controlling factors and global implications

Expansion of C4 plants in South China and evolution of East Asian monsoon since 35 Ma: Black carbo+0n records in the northern South China Sea