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Long chain n-alkanes of leaf waxes are widely used in paleoclimatic reconstructions. The application of these proxies depends on profound understandings gained from studies of the relationship between climatic factors and long chain n-alkanes in living plants. However, a lot of uncertainties still exist in the relationship due to the interactions among climatic factors, geographical and topographical factors, and phylogeny. In addition, unrealistic or inaccurate climatic data also introduce errors. To evaluate the effect of climate with minimized interfering factors, we conducted long-term measurements of leaf wax n-alkanes of the plants growing on Dongling Mountain, north China from 2004 to 2019, where a meterological observatory is located. On the whole, total n-alkane concentration (Σalk) was positively correlated mean annual precipitation (MAP); average chain length (ACL) was negatively related to MAP; carbon preference index (CPI) had poor correlations with all climate factors. Compared with Σalk, ACL is a better indicator that reflects annual precipitation. The coefficient of ACL vs. MAP is 0.0015/mm. However, MAP accounted for only 40% of the variability in the year-averaged ACL of all plants together due to the significant interspecific differences in the response of leaf wax n-alkane distribution to climatic parameters; thus, ACL should be employed in parallel with other proxies to quantitatively reconstruct paleoprecipitation. We conducted a sample reconstruction of paleoprecipitation using ACL records preserved in the annually laminated sediments of Maar Lake Twintaung, Myanmar. The results of reconstruction using ACL are completely consistent with the results of reconstruction using carbon isotope ratios of n-alkanes, and are highly consistent with historical documents.

Many studies have documented that the delta(13)C values of plants increase with altitude both on a global scale and locally in humid climates, while in semi-arid areas the opposite trend has been found. The study reported herein was conducted in a mountainous area of China characterized by a temperate semi-humid climate. The delta(13)C values of C(3) species do not exhibit a consistent variation along an altitudinal gradient and the observations suggest that the pattern of increasing delta(13)C with altitude cannot be generalized. In the study area, in addition to environmental factors such as changing air pressure and light, the interaction between temperature and plant water balance determines the delta(13)C-altitude variations in C(3) plants. The delta(13)C of the leaves of C(4) plants is found to increase with altitude with a mean gradient of 0.9 per thousand/km. The altitudinal trend of C(4) plants is attributed to the combined influences of water availability and other factors rather than temperature.

Variations in carbon isotopic ratios (δ13C) of C3 plants and distribution of C4 plants were investigated along an altitudinal transect on the eastern slope of Mount Gongga, and the environmental effects on them were discussed. It is shown that plants with C4 photosynthetic pathway mainly occur at altitudes below 2100 m a.s.l., suggesting that the low summer temperature is responsible for the distributional pattern. In addition, δ13C of C3 plants increases with elevation at the region above 2000 m a.s.l. with the characteristics of humid climate, and the increase rate in δ13C for C3 plants is about 1.3% per kilometer. Temperature determines the altitudinal trend of δ13C.

Carbon isotope ratios (δ13C) of plants, litter and soil organic matter (0–5 cm, 5–10 cm and 10–20 cm) on the eastern slope of Mount Gongga were measured. The results show that δ13C values of plants, litter and soil organic matter all decrease first and then increase with altitude, i.e. δ13C values gradually decrease from 1200 to 2100 m a.s.l., and increase from 2100 to 4500 m a.s.l. The δ13C altitudinal variations are related to the distribution of C3 and C4 plants on the eastern slope of Mount Gongga, because C4 plants are observed to grow only below 2100 m, while C3 plants occur at all altitudes. There are significantly positive correlations among δ13C of vegetation, δ13C of litter and δ13C of soil organic matter, and litter, 0–5 cm, 5–10 cm and 10–20 cm soil organic matter are 0.56‰, 2.87‰, 3.04‰ and 3.49‰ greater in δ13C than vegetation, respectively. Considering the influences of rising concentration of atmospheric CO2 and decreasing δ13C of atmospheric CO2 since the industry revolution on δ13C of plants, 1.57‰ is proposed to be the smallest correction value for reconstruction of paleovegetation using δ13C of soil organic matter.

Leaf δ 13C of different plant functional groups (trees, shrubs and forbs; evergreen and deciduous; annual, biennial and perennial) were examined on the Changbai Mountains, China. Life form has a significant influence on plant δ 13C, suggesting that leaf δ 13C is also ideal for distinguishing functional groups species in temperate and frigid zones with high humidity. Additionally, the difference of water-use efficiency (WUE) is significant among different plant functional groups. δ 13C and WUE are in the following order of forbs < shrubs < trees. Within shrubs, δ 13C and WUE are higher for evergreen shrubs than deciduous shrubs. Differences in δ 13C and WUE are significant among different lifespan herbs, and δ 13C and WUE follow the order of annual herbs > biennial herbs > perennial herbs, not in accordance with the pattern obtained by previous studies in deserts, suggesting that the ranking of δ 13C and WUE among annual, biennial and perennial herbs may be dependent on local water availability.

The relative contributions of C3 and C4 plants to vegetation at a given locality may be estimated by means of δ13C of soil organic matter. This approach holds a great potential for paleoecological reconstruction using paleosols. However, two main uncertainties exist, which limits the accuracy of this application. One is δ13C-enrichment as the plant carbon becomes incorporated into soil organic matter. The other is due to environmental influences on δ13C of plants. Two types of data were collected and analyzed with an objective of narrowing the error of paleovegetation reconstruction. First, we investigated δ13C variations of 557 C3 and 136 C4 plants along a precipitation gradient in North China. A strong negative correlation is found between the δ13C value of C3 plants averaged for each site and the annual precipitation with a coefficient of −0.40‰/100mm, while no significant coefficients were found for C4 plants. Second, we measured δ13C of soil organic matters for 14 soil profiles at three sites. The isotopic difference between vegetation and soil organic matter are evaluated to be 1.8‰ for the surface soil and 2.8‰ for the soil at the bottom of soil profiles. We conducted a sample reconstruction of paleovegetation at the central Chinese Loess Plateau during the Holocene and the Last Glacial (LG), and conclude that, without corrections for δ13C-enrichment by decomposition, the C4 abundance would be overestimated. The importance and uncertainties of other corrections are also discussed.

Significant correlations between leaf carbon isotope discrimination (Delta) and altitude and between gas exchange and altitude have been reported in previous studies, raising the question of whether the altitudinal variations in discrimination and gas exchange can be attributed to genetic differences among populations from different altitudes. Studies that focus on in situ analysis cannot distinguish the effects of genetic variation from environmental variation. This article describes an experiment in which seeds of Plantago depressa (C3 species) and Setaria viridis (C4 species) collected from a wide altitudinal range were grown in the same environment. Carbon isotopic ratios (delta(13)C) and gas exchange of the seedlings were measured. The progenies of P. depressa and S. viridis no longer display any significant Delta decreases with the altitude of origin as seen in situ. Furthermore, photosynthetic rate, stomatal conductance, the ratio of intercellular to ambient CO(2) and intrinsic water use efficiency for P. depressa and S. viridis grown in the greenhouse are also not significantly related to the altitude of origin. The observations suggest that altitudinal variations in Delta and gas exchange are not because of genotypic differences, independent of photosynthetic type.

Carbon isotope ratio of leaf dry matter, δ 13 C, was measured on species occurring within Baiyin desert community, consisting of valley, slope and ridge microhabitats, and within Shandan desert community, consisting of Gobi desert and seasonal flooded creek microhabitats, in Northwest China. δ 13 C of C 3 species increased with a decrease in soil water availability, suggesting that water-use efficiency (WUE) increased with decreasing soil moisture, whereas for all C 4 species, δ 13 C tended to decrease with decreasing soil water availability, suggesting that WUE also increased with decreasing soil moisture. Above results indicated that water-use pattern was conservative under drought for C 4 and C 3 plants. In this present study, C 4 species' occurrences within different microhabitats were investigated and C 4 plants were observed to be absent and/or scarce within relatively lower soil moisture microhabitats, whereas they occurred and/or even had a high abundance within relatively higher soil moisture microhabitats, suggesting limited moisture available was a key factor of limiting C 4 distribution in arid region of Northwest China.

This paper presents a large data set on carbon isotope composition (δ13C) of modern soils which were collected under the main vegetation communities along an altitude of 1250–5500 m above sea level in the Qinghai-Tibetan Plateau. The δ13C values of 198 samples range from −28.6 to −15.1‰ versus PDB and exhibit a clean relation to different vegetation communities from forest (−25.9 ± 1.2‰) to shrub (−24.7 ± 1.4‰), steppe (−23.1 ± 1.3‰), alpine meadow (−23.6 ± 0.7‰), alpine desert steppe (−21.3 ± 1.6‰), and alpine desert (−18.9 ± 2.5‰). We attributed the observed variability in δ13C values to that the mean annual precipitation (MAP) and the mean annual temperature (MAT) are the main factors controlling the distribution of vegetation types in the Tibetan Plateau, which causes the change in carbon isotope composition of modern soils at any given altitude. The result of both linear and nonlinear regression analyses also confirms that MAP and MAT are the major factors affecting the δ13C values of surface soils. In the absence of favorable moisture and temperature conditions, low pCO2 alone is not sufficient to cause the distinct changes in carbon isotope composition of modern soils in the Tibetan Plateau. This study provides some fundamental information on the carbon isotope composition of terrestrial carbon pools and bears some practical significance for the use of carbon isotope data to document vegetation changes and environmental conditions of the high plateau in the past.

Plant specimens are collected from the areas between latitude 27 ° 42′N and 40 ° 57′N, and longitude 88 ° 93′E and 103 ° 24′E, with an altitudinal range from 2210 to 5050 m above the sea level in Qinghai-Tibetan Plateau. The stable carbon isotope analysis indicates that two of Chenopodiaceae and six of Poaceae in the samples are C4 plants. Four of the C4 plants are found in 11 spots with altitudes above 3800 m, and Pennisetum centrasiaticum, Arundinella yunnanensis and Orinus thoroldii are present in six spots above 4000 m, even up to 4520 m. At low CO2 partial pressure, that sufficient energy of high light improving C4 plant’s tolerance of low temperature and precipitations concentrating in growing season probably are favorable for C4 plants growing at high altitude in Qinghai-Tibetan Plateau.

13C of 367 C3 herbaceous plants was measured in loess area in northern China. Their δ13C values vary between −21.7%. and −30.0%., with a mean of −26.7%.. In the center of Loess Plateau (semimoist area) with annual precipitation of 400–600 mm, the δ13C values of C3 herbaceous plants range from −24.4%. to −28.5%., with a mean of −27.5%.. In the west of Loess Plateau (semiarid and arid area) with annual precipitation less than 400 mm, they range between −21.7%. and −30.0%., with a mean of −26.2%.. Annual precipitation is the main factor that makes δ13C values of C3 herbaceous plants in the west greater than those in the central Loess Plateau. The composition of δ13C in C3 plants increases with deceasing annual precipitation, and the mean change is −49%./100 mm.

Abundance of C3 and C4 photosynthesis plants can be inferred relatively from stable carbon isotopic composition of organic matter in soils. The samples from five sequences of the last glacialinterglacial loesssoil in the Chinese Loess Plateau have been measured for organic carbon isotopic ratios (δ13 Corg). The organic carbon isotope data show that relative abundance (or biomass) of C4 plants was increased ca. 40% for each sampling site from the last glacial maximum (LGM) to Holocene optimum, and increased southeastward on the Loess Plateau during both periods of LGM and Holocene. Statistic analyses on the steady maximum δ13 Corg values of Holocene soils and modern climatic data from the Loess Plateau and Inner Mongolia indicate that the C4 plant abundance increases with increasing temperature and decreasing precipitation. The C4 plant abundance is related much closer with mean April temperature and precipitation than annual. These results lead us to deduce following conclusions. First, temperature is the major factor for control on variations in C4 plant abundance in the Loess Plateau from the last glacial to interglacial. In the absence of favorable temperature condition, both of low moisture and low atmospheric CO2 concentration are insufficient to drive an expansion of the C4 plants in the plateau. Second, δ13 Corg in the loess-paleosol sequences, as a proxy of the relative abundance of C4 plants in the Loess Plateau, could not be used as an indicator of changes in the summer monsoon intensity unless the temperature had changed without great amplitude. Since all C4 plants are grasses, finally, the increase of the C4 plants supports that forest has not been dominant in the ecosystem on the Loess Plateau during Holocene although precipitation and atmospheric CO2 were largely increased relative to those during LGM.

The sedimentology and geomorphology of landforms along Keriya River, that traverse the Kunlun Mountains and Taklamakan Desert, were studied to examine the relationship between environmental change and landscape evolution in mountains and desert margins of the Taklamakan Desert. The region is divided into four geomorphic provinces: desert with active dunes and sand seas; desert pediment with fans; and foothills with deep gorges and glaciated mountains. A succession of three fans with dunes is differentiated on the basis of morphostratigraphy and relative weathering. The oldest fan and dunes are radiocarbon dated to ∼29 ka. This fan probably formed as a response to increased meltwater and sediment loads as glaciers retreated at the end of the local last glacial maximum in the Kunlun Mountains. Periglacial and glacial features throughout the region show that, during the last glacial, the snowline was lowered to ∼4000 m a.s.l., permafrost existed down to an altitude of ∼1800 m a.s.l., and that the likely temperature depression was ∼10°C. Radiocarbon dates on sand dunes and tamarisk hills help define the age of the lower two terraces to >1.5 ka and a few hundred years BP. This suggests that these terraces may have formed in response to increased precipitation during more humid times at ∼2 ka and during the Little Ice Age, respectively. The landforms and sediments in this region are polygenetic in origin, being transported, eroded and deposited by glacial, fluvial, aeolian and lacustrine processes. Particle size characteristics show that the fluvial sediments were substantially modified down basin. In contrast, the aeolian sediments were modified as they were transported toward the mountains from the desert by the dominant northerly winds. This complex interaction between environmental change and earth surface processes provides an important framework for examining the environmental consequences of the recently growing population and intensified agricultural activities along the mountain and desert margins of the Taklamakan Desert.

Loess and paleosol samples from three distant sections (Xining, Xifeng, Jixian) of the Chinese Loess Plateau were analyzed for chemical and Nd–Sr isotopic compositions in order to obtain information about the paleoclimatic variation during the last 140 ka. These three sections represent three different climatic conditions, from arid in the west (Xining) to more humid in the east (Jixian) across the Loess Plateau. Isotopic analyses show that all the three sections display a restricted range of 143 Nd/ 144 Nd ratios (or e Nd ≈−10±0.5), indicating the dominance of relatively young and uniform upper crustal sources for the eolian dust. The loess and paleosol samples have typical upper crustal 87 Sr/ 86 Sr ratios (0.714–0.718), and their variation can partly be attributed to different carbonate/silicate ratios in the samples. REE data from the three sections are indistinguishable from each other; they show typical upper crust patterns characterized by (La/Yb) N ≈10 and Eu/Eu*≈0.65. The high degree of chemical and isotopic homogeneity suggests that the dust source region must have undergone multiple recycling and thorough sedimentary mixing processes. Various chemical indicators have been used to monitor the pedogenetic intensity in these sections. Although results for the Jixian section agree reasonably well with the known climatic conditions on the Loess Plateau, the other two sections yield results not in agreement with some well-accepted proxies like magnetic susceptibility or grain-size distribution. Some chemical proxy indicators established in our previous work on the Luochuan section, such as the systematic U-depletion and negative Ce anomalies in REE patterns in soils, are not clearly recorded in the present case. The discrepancy between different climatic proxies could be related to the effective soil water budget, temperature, as well as the rate of development of each proxy.

To understand the origin of the ultrafine pedogenic components responsible for the magnetic susceptibility (MS) enhancement remains a major challenging problem, in linking the magnetic signal with paleoclimatic conditions. Here we examine the influence of natural fires on the MS signal of both plants and modern soils and in particular the MS difference between C3 and C4 plant ashes and their influence possibly on soil magnetic susceptibility. We demonstrate that burning of C3 and C4 plants can enhance MS signal of modern soils. We show that C4 plants have greater potential to enhance the MS signal. The average MS value of C4 plant ashes is (532±61)×10−8m³ kg−1, much higher than the average MS value of (120±65)×10−8m³ kg−1 of C3 plant ashes. The Fe2O3 concentration in C4 plant is two to four times higher than that in C3 plant. One burning of the grassland, mainly consisting of C4 plants, can enhance MS value of the surface soil up to about 30–40%.

The magnetic enhancement of paleosol is observed in the Chinese loess. The origin of this magnetic enhancement is still very uncertain. It is, however, a key problem in correctly understanding the paleoclimatic significance of changes of magnetic susceptibility and in transferring the magnetic signals to paleoclimatic parameters. Two main models have been proposed to explain the mechanism of magnetic enhancement in paleosols: a depositional model and a pedogenic model. Together with composition and concentration, grain size distribution of magnetic minerals also plays an important role to the magnetic enhancement of paleosols. Systematic susceptibility measurements were carried out on the samples of the upper part (S0 to top of L2) of three loess sections from Jixian, Xifeng, and Xining, along an east–west transect in the loess plateau, China. The samples with the highest value of magnetic susceptibility in S1 and the lowest value in L2 of each section were selected as the representatives. These representatives were separated into different grain size fractions based on Stokes’ law for coarse grains and by centrifuge for fine grains. Measurements of magnetic susceptibility and mass have been carried out on these fractions. Results show that for the loesses magnetic susceptibility changes little in the fractions with different grain size and for the paleosols it increases with decreasing grain size. The magnitude of changes is bigger in the east (Jixian and Xifeng) than that in the west (Xining). The fraction with the finest particle size in paleosols does not show very high magnetic susceptibility values. A new approach is used to estimate the contribution of each fraction to total magnetic susceptibility. The contribution comes mainly from coarse grains (>10 μm) for loess samples. It reaches about 90%. The main contribution comes, however, from the particles with medium size (10–0.2 μm). The very fine grained particle (

The anisotropy of magnetic susceptibility (AMS) is a rapid and precise tool for determining paleowind directions. During the past several decades, AMS has been used for determining paleowind directions of Chinese loess. The AMS for a loess-paleosol profile in early Quaternary at Longdan in the Linxia Basin, Gansu Province, northeastern fringe of Qinghai-Xizang (Tibetan) Plateau has been obtained. The primary results show that the AMS of the Longdan profile has its typical normal eolian dust deposit. The AMS is mainly controlled by magnetic foliation (F) and the shape parameter of the AMS ellipsoid is oblate. The declination of maximum susceptibility (K1) was used to determine the paleowind direction of that area in early Quaternary and it was found that the paleowind direction is biased towards NW-SE.

Carbonate concretions are commonly found in the loess in China. Oxygen and carbon isotopic compositions of carbonate concretions collected from 14 different paleosols (S1–S14) in the Lishi Formation of Luochuan loess section, China were analysed. The isotopic compositions of concretion in loess are useful tools in paleoclimatic study. Usually the more developed the paleosol is, the less negative the δ18O values and the more negative the δ13C values are. The oxygen isotopic composition of these paleosols varies within a range of about 1.3‰. The carbon isotopic composition, however, shows larger variations from one paleosol to the other, within a range of 5.3‰. An attempt was made to semi-quantitatively estimate the paleosol formation temperature and precipitation based on the carbon and oxygen isotopic data obtained.

SUMMARY Results of the first detailed study of the climate proxy record in the loess-palaeosol sequence at Xining-one of the few palaeoclimate sites in the currently arid western Loess Plateau of China-illustrate the importance of making many types of rockmagnetic measurements other than susceptibility. A multiparameter approach yielded confirmation that here, as elsewhere in the Loess Plateau, the susceptibility enhancement in palaeosols was caused primarily by ultrafine magnetite and maghaemite. Nevertheless, magnetic enhancement was caused not exclusively by changes in relative grain size, but also by variations in concentration and mineralogy of the magnetic fraction. The effects of concentration variations were removed through normalization of susceptibility and anhysteretic remanence with saturation magnetization and saturation remanence, respectively. The resulting signal was ascribed more confidently to variation in magnetic grain size, which in turn was interpreted as a better proxy of pedogenesis than simple susceptibility. Variations in magnetic mineralogy were also determined to constrain interpretations further. The data were then used to discuss climate history at Xining. Finally, results from Xining were compared with other western sites and contrasted with eastern sites. In summary: (1) data is presented from a new Loess Plateau site which also appears to yield a global climate signal; (2) a demonstration is made of a more rock-magnetically robust way to separate concentration, composition and grain-size controls on susceptibility and other magnetic parameters; and (3) models are provided for inter-regional comparisons of palaeoclimate proxy records.

Many studies have documented that the delta(13)C values of plants increase with altitude both on a global scale and locally in humid climates, while in semi-arid areas the opposite trend has been found. The study reported herein was conducted in a mountainous area of China characterized by a temperate semi-humid climate. The delta(13)C values of C(3) species do not exhibit a consistent variation along an altitudinal gradient and the observations suggest that the pattern of increasing delta(13)C with altitude cannot be generalized. In the study area, in addition to environmental factors such as changing air pressure and light, the interaction between temperature and plant water balance determines the delta(13)C-altitude variations in C(3) plants. The delta(13)C of the leaves of C(4) plants is found to increase with altitude with a mean gradient of 0.9 per thousand/km. The altitudinal trend of C(4) plants is attributed to the combined influences of water availability and other factors rather than temperature.

The Nuanquangou loess section in the Longxi Basin has been examined in terms of stratigraphic investigation, including14C dating, paleomagnetic polarity, spore-pollen diagram, and mineral and chemical analyses. The results indicate that the section is one of the ancient loess sections with a continuous sedimentation and composed of homogeneous materials with abundant carbonate. The Brunhes / Matuyama boundary and the Jaramillo event were recorded in the section. Stratigraphically, this section can be divided into the Holocene and the Pleistocene loess. The latter includes Malan, Lishi and Wucheng loess in descending order. They were developed at about 1.2 Ma ago, whereas the Nuanquangou loess section possesses some local characteristics different from those observed in other areas.