The uplift of the Tibet Plateau (TP) during the Miocene is crucial to understanding the evolution of Asian monsoon regimes and alpine biodiversity. However, the northern Tibet Plateau (NTP) remains poorly investigated. We use pollen records of montane conifers (Tsuga, Podocarpus, Abies, and Picea) as a new paleoaltimetry to construct two parallel midrange paleoelevation sequences in the NTP at 1332 ± 189 m and 433 ± 189 m, respectively, during the Middle Miocene [~15 million years ago (Ma)]. Both midranges increased rapidly to 3685 ± 87 m in the Late Miocene (~11 Ma) in the east, and to 3589 ± 62 m at ~7 Ma in the west. Our estimated rises in the east and west parts of the NTP during 15 to 7 Ma, together with data from other TP regions, indicate that during the Late Miocene the entire plateau may have reached a high elevation close to that of today, with consequent impacts on atmospheric precipitation and alpine biodiversity.
Phenological cold/warm events recorded in Chinese historical documents are used to reconstruct, at 10–30 years' resolution, winter half-year (October to April) temperatures for the past 2000 years in the central region of eastern China. Because of the uneven spatial and temporal distribution of the phenological records, the reconstruction of the regional mean temperature involves two steps: reconstruction for individual sites within the region and calculation of the regional mean. For a single site, the reconstruction involves: identifying the difference in dates in phenological events for both historical and modern records; establishing the conversion function between the date difference and temperature change from the modern records; and converting the historical records into temperature variation. The spatial representativeness of the individual sites is studied by examining the correlation between individual sites and regional mean temperature from modern instrumental data. The correlation is then used as the basis for constructing the regional mean winter half-year temperature for the past 2000 years. From the beginning of the Christian era, climate became cooler at a rate of 0.17°C per century, and around the ad 490s temperature reached about 1°C lower than that of the present (the 1951– 80 mean). Then, abruptly, temperature entered a warm epoch from the ad 570s to 1310s with a warming trend of 0.04°C per century; the peak warming was about 0.3–0.6°C higher than present for 30-year periods, but over 0.9°C warmer on a 10-year basis. After the ad 1310s, temperature decreased rapidly at a rate of 0.10°C per century; the mean temperatures of the four cold troughs were 0.6–0.9°C lower than the present, with the coldest value 1.1°C lower. Temperature has been rising rapidly during the twentieth century, especially for the period 1981–99, and the mean temperature is now 0.5°C higher than for 1951–80. The most interesting aspect over the past 2000 years has been the rapid transitions between cold and warm periods.
A series of important geological events occurred in the Tibetan Plateau area during the Jurassic, such as the collision of the Lhasa and Qiangtang Terranes, the closure of the Meso-Tethyan Ocean, the opening of the Neo-Tethyan Ocean and the cessation of the mega-monsoon. The similar to 3000 m thick Jurassic sedimentary sequence in the Qiangtang Basin on the central Tibetan Plateau, which is called the Yanshiping (YSP) Group, recorded these geological events. However, the chronology of the sequence is surprisingly poorly constrained. Here, we perform a detailed palaeomagnetic analysis on the similar to 1060 m thick middle and upper portions of the YSP Group (the Xiali and Suowa Formations) in the YSP section of the eastern Qiangtang Basin. Three bivalve zones at stratigraphic intervals of similar to 40-140, 640-800 and 940-1040 m are identified, which yield a Bathonian-Callovian age for the Lower Xiali Fm., a Callovian-Oxfordian age for the Lower Suowa Fm. and an Oxfordian-Kimmeridgian age for the Upper Suowa Fm. A total of 544 oriented palaeomagnetic samples were collected from the section. By combining thermal and alternating field demagnetizations, clear characteristic remanent magnetization (ChRM) directions are isolated for most of the samples. The robust ChRM directions pass fold and reversals tests, which support the primary nature of the ChRMs and yield a palaeopole at 76.8A degrees N/297.2A degrees E (dp = 2.2A degrees, dm = 3.7A degrees). A total of 27 normal and 26 reversed polarity zones were successfully recorded in the section. Combined with fossil age constraints, results suggest that the section is plausibly composed of a Callovian-Early Kimmeridgian age sedimentary sequence.
<p>Cenozoic changes in climate, erosion, and atmospheric circulation in Asian interior continents can be reconstructed from records of eolian dust deposition from sediments of the North Pacific Ocean (NPO). Through a careful investigation of Nd isotope as eolian dust source tracer, the well-known core GPC3 in the central NPO has provided so far the most complete Asian dust records since ~40 Ma. Nd isotope in the GPC3 eolian dust thus documented an integrated history of Nd isotopic change of very fine eolian dust contributed from various geological terranes in Asian dust source areas. Unraveling this ~40 Myr-long Nd isotopic change in the NPO provides a first order constraint on the provenance change of the Asian dust source areas as a whole. However, this work cannot be done without an explicit Nd isotopic history for each geological terrane within the broad Asian dust source areas, since the Asian dust source area can be at least divided isotopically into two regions with distinct Nd isotopic values, e.g., the northern Tibetan Plateau (NTP) and the Central Asian Orogen (CAO). In this work, we present new data of river sediment Nd isotopic data around the entire Qaidam and Xining Basins to yield a more comprehensive Nd isotopic regimes at the NTP with compiling previously reported data. We have established an integrated Cenozoic Nd isotopic records of finer dust in the NTP based on previous records and our new Nd isotopic records in the Xining Basin from 52 to 17 Ma and Linxia basin from 23 to 5 Ma using both bulk sediments and clay fractions (<2 &#956;m). After comparison of the reconstructed Nd isotopic variation in fine dust at the NTP with that in the NPO, we have further assessed the relative contributions of NTP and CAO to the Asian dust preserved in the NPO during the last 40 Myr, which indicates a dominant late Oligocene-Neogene uplift and growth of the mountains at the NTP and the CAO regions.</p>
Abstract Cretaceous‐Miocene sedimentary rocks in the Nepalese Lesser Himalaya provide an opportunity to decipher the timing of India‐Asia collision and unroofing history of the Himalayan orogen, which are significant for understanding the growth processes of the Himalayan‐Tibetan orogen. Our new data indicate that detrital zircon ages and whole‐rock Sr‐Nd isotopes in Cretaceous‐Miocene Lesser Himalayan sedimentary rocks underwent two significant changes. First, from the Upper Cretaceous‐Palaeocene Amile Formation to the Eocene Bhainskati Formation, the proportion of late Proterozoic‐early Palaeozoic zircons (quantified by an index of 500–1200 Ma/1600–2800 Ma) increased from nearly 0 to 0.7–1.4, and the percentage of Mesozoic zircons decreased from ca. 14% to 5–12%. The whole‐rock 87 Sr/ 86 Sr and εNd( t = 0) values changed markedly from 0.732139 and −17.2 for the Amile Formation to 0.718106 and −11.4 for the Bhainskati Formation. Second, from the Bhainskati Formation to the lower‐middle Miocene Dumri Formation, the index of 500–1200 Ma/1600–2800 Ma increased to 2.2–3.7 and the percentage of Mesozoic zircons abruptly decreased to nearly 0. The whole‐rock 87 Sr/ 86 Sr and εNd( t = 0) values changed significantly to 0.750124 and −15.8 for the Dumri Formation. The εHf( t ) values of Early Cretaceous zircons in the Taltung Formation and Amile Formation plot in the U‐Pb‐εHf( t ) field of Indian derivation, whereas εHf( t ) values of Triassic‐Palaeocene zircons in the Bhainskati Formation demonstrate the arrival of Asian‐derived detritus in the Himalayan foreland basin in the Eocene based on available datasets. Our data indicate that (1) the timing of terminal India‐Asia collision was no later than the early‐middle Eocene in the central Himalaya, and (2) the Greater Himalaya served as a source for the Himalayan foreland basin by the early Miocene. When coupled with previous Palaeocene‐early Eocene provenance records of the Tethyan Himalaya, our new data challenge dual‐stage India‐Asia collision models, such as the Greater India Basin hypothesis and its variants and the arc–continent collision model.
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