Fang Xiaomin

The Late Cenozoic uplift of the Liupan Shan, China

Based on paleomagnetic measurements and morphostratigraphy of red bed/clay sequences from pediments of the Liupan Shan and the Longdong Basin, the following results are revealed. The red bed/clay sediments became to accumulate at around 8.1 MaBP, which implied that the plantation surface developed since Late Cretaceous was broken by active fault, and its development was terminated. The Liupan Shan began to slightly uplift. The Liupan Shan experienced a small-scale uplift around 5.2 MaBP, inferred from the appearance of fine gravel sediments at that time. Consequently, a pediment was developed. The Liupan Shan accelerated uplift since about 3.8 MaBP at a large scale, which caused the deep incision of the rivers and the termination of fluvial and lacustrine deposition. Meanwhile, typical eolian red clay appeared since then. This uplift process is well correlated and in response to that of the Tibetan Plateau and the mountains around it.Based on paleomagnetic measurements and morphostratigraphy of red bed/clay sequences from pediments of the Liupan Shan and the Longdong Basin, the following results are revealed. The red bed/clay sediments became to accumulate at around 8.1 MaBP, which implied that the plantation surface developed since Late Cretaceous was broken by active fault, and its development was terminated. The Liupan Shan began to slightly uplift. The Liupan Shan experienced a small-scale uplift around 5.2 MaBP, inferred from the appearance of fine gravel sediments at that time. Consequently, a pediment was developed. The Liupan Shan accelerated uplift since about 3.8 MaBP at a large scale, which caused the deep incision of the rivers and the termination of fluvial and lacustrine deposition. Meanwhile, typical eolian red clay appeared since then. This uplift process is well correlated and in response to that of the Tibetan Plateau and the mountains around it.

An 8.1Ma calcite record of Asian summer monsoon evolution on the Chinese central Loess Plateau

Carbonates in loess-red clay sequences consist mainly of calcite and dolomite. The EDTA analysis of carbonates in different size fractions and magnetic susceptibility reveal that calcite is a sensitive index of summer monsoon. The chemical analysis of carbonates and calcite from an 8.1 Ma loess-red clay sequence at Chaona on the Chinese central Loess Plateau shows that the evolution of the Asian summer monsoon experienced four stages, namely 8.1–5.5 Ma, 5.5–2.8 Ma, 2.8–1.5 Ma and 1.5–0 Ma, with increasing intensification and fluctuation, suggesting a possible combining impacts of uplift of the Tibetan Plateau and global changes on the Asian summer monsoon.

Late Eocene pollen records and palaeoenvironmental changes in northern Tibetan Plateau

The Eocene palaeovegetation landscape and palaeoclimate reconstructed from the pollen records in the Jiuquan Basin, northwest China provide some important information on the early uplift of the Tibetan Plateau and the origin and evolution of the aridification in northwest China. The records show the arid-semiarid scrubs with open forest palynofloras controlled by the subtropical high existed in northwest China during the 40.2–33.4 Ma. Four pollen assemblages are found: Nitrariadites-Cheno-podipollis-Pinaceae assemblage (40.2–37.9 Ma) is followed by Chenopodipollis-Nitrariadites assemblage (37.9–34.6 Ma), Pinuspollenites & Abietineaepollenites-Chenopodipollis assemblage (34.6–33.9 Ma), and Chenopodipollis-Nitrariadites assemblage (33.9–33.4 Ma). The percentage of thermophilic types is in anti-correlation with that of the dry types, which means the palaeoclimate is relatively warm-wet or cold-dry during most of that time. Such aridity may be related to the water vapor reduction and the planetary wind system movement northward in response to the cooling caused by small-ephemeral ice-sheets.

Distribution of lipids in modern soils from various regions with continuous climate (moisture-heat) change in China and their climate significance

Total 26 modern soil samples were collected from various regions under different climate conditions from tropical to arid temperate in China and systematically analyzed for their organic matters by GC/MS in order to evaluate climatic impacts on soil organic components. Abundant lipids molecules were recognized, including n-alkanes, n-alkenones, and long-chain branched alkanes. We find the predominance of main peaks of long-chain n-alkanes (nC29, nC31, nC33) and short-chain ones (nC16, nC17, nC18) records information of soil generation in warm-humid and cold-dry regions. The proportion of n-alkanes (nC16+nC17+nC18) to (nC29+nC31+nC33) varies in good agreement with moisture-heat conditions and thus probably can serve as a new index for climate change. The ratios of C21−/nC22+, nC17/nC31 and (nC15+nC17+nC19) / (nC27 + nC29 + nC31) of n-alkanes, indicating respectively input ratios of lower bacterial alga, aquatic organisms, and higher plants and terraneous organisms, co-vary well in different climate regions from forest to grassland and desert, suggesting that they have also reflected the difference of climates between monsoon region and inland one. The C21−/C22+ ratio of n-alkan-2-one records largely the discrepancy of temperature from north to south of China bordered by the Qinling Mountains, but less humidity. The C21−/C22+ ratio of n-alkan-3-ones changes well with climatic factors, such as temperature and humidity. The biogenic source of series A-D long-chain branched alkanes may be derived from some kinds of special epiphyte that most likely live in weak oxic-anoxic and moisture-heat environments, suggesting their distribution record as well some information on climatic change. All these researches demonstrate that the distributions of lipids molecules in modern soils in China record well signals of climates from quite different climatic regions, and can serve as important climatic proxies to reveal climatic change over China.

Sedimentary n -alkanes record of precipitation D/H ratios in arid regions of the Tibetan Plateau

The hydrogen isotope ratios of sedimentary leaf waxes have been investigated in various environments and are considered as a promising proxy for paleohydrological and paleoclimatic reconstruction. However, there is a growing concern regarding whether or not sedimentary leaf waxes can be used as an environmental proxy in extremely arid regions, where intensified evaporation strongly enriches the deuterium both in water and in leaf waxes. Here we present a record of the n -alkane D/H ratios of surface sediments from 21 lakes. The data were collected from 2012–2014. The n -alkanes extracted from lake surface sediments range from n C16 to n C35, with the most abundant compounds being n C23, n C25, n C29 and n C31. In addition, there is an evident odd-over-even preference for long-chain n -alkanes, with carbon preference index (CPI) values varying between 3.41 and 10.75, suggesting that the long-chain sedimentary n -alkanes mainly originated from terrestrial plants in arid regions on the Tibetan Plateau. The D/H ratios of the precipitation ( δ Dp) atall the lakes in this studywere calculated using the Online Isotopes in Precipitation Calculator. The OIPC calculates precipitation isotope ratios at a given site by combining an empirical model for isotopic trends related to latitude and altitude with detrended interpolation based on the isotope data from stations in the Global Network of Isotopes in Precipitation. The mean annual hydrogen isotopes ratios of precipitation ( δ Dann) at the lakes in this study vary from - 78‰ to - 115‰, with lower precipitation δ D values occurring in the southern TP and higher values in the northern TP and the Qaidam Basin. The δ D values for summer precipitation (July–September, δ Dsum) range from - 68‰ to - 126‰, with a similar range of variation for δ Dann. The sedimentary n -alkane D/H ratios are strongly correlated with the D/H ratios of annual precipitation ( r 2=0.84) and summer precipitation ( r 2=0.91), suggesting that the hydrogen isotope signal in sedimentary n –alkanes is mainly derived from precipitation, and that the n -alkane D/H ratios may reflect the D/H ratios of precipitation in arid regions on the Tibetan Plateau. Water is the ultimate source of hydrogen in plant tissues, and organic matter derived from terrestrial plants should record the isotopic ratio of soil water, which is ultimately recharged by precipitation. The δ D values of n -alkanes in lake surface sediments could thus be expected to be correlated with the δ D values of precipitation, on a large spatial scale. Furthermore, precipitation on the Tibetan Plateau mainly occurs from July to September, which accounts for ~70% during the year, and the n -alkanes of leaf wax are synthesized during the plant growing season. Therefore, it is unsurprising that the best correlation between D/H ratios of sedimentary leaf wax and precipitation occurs in the summer months. The apparent hydrogen isotopic fractionation between sedimentary n -alkanes and precipitation averages - 113‰, evidently smaller than that in relatively humid regions. A possible reason for the smaller apparent fractionation might be spatial differences in soil evaporation and plant transpiration. Mean annual precipitation amount and mean annual evaporation at the studied lakes is 207 and 2343 mm, respectively. However, in the humid areas, such as the southeastern TP, the mean annual precipitation amount and mean annual evaporation are 1298 and 795 mm, respectively. Due to the strong evaporation in arid regions, the δ D values of soil water and plant water are more isotopically-enriched relative to precipitation than those in humid regions. Overall, our results demonstrate that sedimentary n -alkanes can be used as a proxy indicator of variations in the isotopic composition of summer precipitation, in order to document past changes in summer monsoon intensity and hydrology.

Grain-size records of Cenozoic Lacustrine sediments from Linxia Basin and the aridification of Asian Inland

High-resolution grain size records from Maogou section in Linxia Basin were characterized with relative stable fluctuation from 29Ma and 7.4 Ma, and changes in several sedimentary facies responded to Tibetan uplift in some short intervals. At about 7.4Ma, aeolian dusts gradually entered into Linxia Basin, and input fluxes of aeolian dusts had two rapid increased phases respectively at about 6.4Ma and 5.3Ma, suggesting that NW China drying climate began at 7.4 Ma and strengthened rapidly at 6.4Ma and S.3Ma. Comparing of our records with the Tibetan uplift and global climatic records, we found that the gradual uplift of the Tibetan Plateau and global cooling, especially the onset of Arctic ice sheet, since about 9-8 Ma, may be the two important forcing mechanisms for the fast aridification of Asian inland.

The evolution of river topography of Northern Tibet since 8Ma B P

River sediments derived from mountains are sensitive to record the information of uplift and erosion of mountains. Fluvial and alluvial sediments, which mainly consist of sand and gravel, of 21 sites from 14 inland rivers in the north side of Qilianshan Mts. were sampled. The volume of sediments sampled is 1 m. The result indicates that the variations of grain-sizes are closely related to the altitude difference between the highest peak of mountains in drainage basins and the sampling sites,and this relationship can be modeled based on the statistical analysis. This model is then used to the Laojunmiao Section which is located at the north of Tibetan Plateau to reconstruct the ancient topography of the Shiyouhe Drainage Basin since 8Ma. The reconstruction curve suggests that topography of Shiyouhe drainage uplifted fluctuantly since 8Ma B.P. and abruptly uplifted at the ages of 6.36Ma B.P. and 2.70Ma B.P.. There are three evident stages during the evolution of topographic difference. Mean topographic difference is about 500 m during 8-6.2Ma B.P., about 1,500 m during 6.2-2.7Ma B.P., more than 2,000 m during 2.7Ma B.P. to present days. The result basically agrees well with the claiming that northern Tibet uplift by phases since 8Ma B.P., after that come through an evident uplift at the age about 6Ma B.P., and accelerate to uplift since 3.66Ma B.P..