Profile distribution of soil inorganic carbon along a chronosequence of grassland restoration on a 22-year scale in the Chinese Loess Plateau

Understanding the distribution of soil inorganic carbon (SIC) is meaningful and necessary for assessing soil carbon sequestration. However, changes in SIC following vegetation restoration are still not well studied in the Chinese Loess Plateau (CLP), which has undergone extensive changes from farmland to grassland. Here, we chose three study sites, farmland (FL), grassland restored for 12 years (RG-1) and grassland restored for 22 years (RG-2). The changes of SIC storage were determined under the conversion from a farmland ecosystem to extensively used grassland during the past twenty years. Results showed that the SIC content decreased with vegetation restoration, which averaged 18.5, 13.2 and 11.8 g kg(-1) in FL RG-1 and RG-2, respectively. The SIC content was significantly lower in the top 80 cm in RG-1 and RG-2 than in FL, but showed little variance at depths of 80-200 cm among the study sites. It illustrated that the portion of carbonate was most likely released to the atmosphere as CO2 temporarily due to soil acidification and the increase of soil moisture in restored grassland. The lower delta C-13 value of SIC in RG-1 and RG-2 than in FL was most likely due to the soil carbonate exchanging with more soil respired CO2, with increasing vegetation coverage in restored grassland. Our results indicate that the SIC storage deceases in the restored grassland in situ probably due to the decrease of soil pH and the increase in soil water content The SIC plays an important role when assessing the capacity of carbon sequestration with vegetation restoration in the semi-arid CLP.</p

Organic stable carbon isotopic composition reveals late Quaternary vegetation changes in the dune fields of northern China

Vegetation changes during the late Quaternary in the dune fields of northern China are not well understood. We investigated organic carbon stable isotopic composition of surface soils, related mainly to the ratio of C-3 and C-4 plants, across a range of arid to subhumid climates in this region. Isotopic composition is weakly related to both temperature and moisture (multiple R-2=0.53), with the highest delta C-13 (greatest C4 abundance) in the warm, subhumid Horqin dune field. In late Quaternary, eolian stratigraphic sections of the Mu Us and Horqin dune fields, but not in the much colder Otindag dune field, delta C-13 is higher in organic carbon from paleosols than in eolian sands. This contrast, most evident for paleosols recording a major early to middle Holocene phase of dune stabilization, is interpreted as evidence for expansion of C4 plants due to increased effective moisture, high temperature because of high insolation, and decreased disturbance related to eolian erosion and deposition.</p

Cenozoic Environmental Changes in the Northern Qaidam Basin Inferred from n-alkane Records

Cenozoic climatic and environmental changes in the arid Asian interior, and their possible relations with global climatic changes and the Tibetan Plateau uplift, have been intensively investigated and debated over past decades. Here we present 40-Myr (million years)-long n-alkane records from a continuous Cenozoic sediment sequence in the Dahonggou Section, Qaidam Basin, northern Tibetan Plateau, to infer environmental changes in the northern basin. A set of n-alkane indexes, including ACL, CPI and Paq, vary substantially and consistently throughout the records, which are interpreted to reflect relative contributions from terrestrial vascular plants vs. aquatic macrophytes, and thus indicate depositional environments. ACL values vary between 21 and 30; CPI values range from 1.0 to 8.0; and Paq values change from &lt;0.1 to 0.8 over the past 40-Myr. We have roughly identified two periods, at 25.8-21.0 Ma (million years ago) and 13.0-17.5 Ma, with higher ACL and CPI and lower Paq values indicating predominant lacustrine environments. Lower ACL and CPI values, together with higher Paq values, occurred at &gt;25.8 Ma, 17.5-21.0 Ma, and &lt;13.0 Ma, corresponding to alluvial fan/river deltaic deposits and shallow lacustrine settings, consistent with the observed features in sedimentological facies. The inferred Cenozoic environmental changes in the northern Qaidam Basin appear to correspond to global climatic changes.</p

A climate threshold at the eastern edgeof the Tibetan plateau

Proxy records of summer monsoon moisture at Lake Qinghai on the northeastern Tibetan Plateau reveal a late Quaternary climate history that is subtly different from that of speleothems from southern and eastern China. Total organic carbon and authigenic carbonate in two independently analyzed and dated cores indicate (1) relative stability and aridity during the glacial interval, (2) small variations during the B&oslash;lling-Aller&oslash;d and the Younger Dryas intervals, (3) comparatively abrupt change at the late Pleistocene/Holocene transition, and (4) relatively high variability during a wet early Holocene. Taken together, the data suggest that a climate threshold exists for penetration of Asian monsoon rainfall onto the Tibetan Plateau, a threshold that was crossed at the beginning of the Holocene. Conceptually, the threshold simply may be related to the topographic barrier that the eastern margin of the Tibetan Plateau presents to the landward penetration of the monsoon, or it may be related to nonlinearities in the climate system itself, such as sudden shifts in the configuration of the Westerly jet stream. Different mechanisms for producing a threshold are not mutually exclusive and may have combined to affect the dynamics of the Asian monsoon. In any case, the threshold is related to the presence of the Tibetan Plateau, which has a profound influence on the Asia monsoon system.</p

Variation of the winter monsoon in South China Sea over the past 183 years: Evidence from oxygen isotopes in coral

Oxygen isotope (&delta;18O) data in winter months in living coral Porites lutea collected from the Xisha Islands, South China Sea is significantly correlated to the measured winter monsoon velocity (WMV) with a correlation coefficient of 0.63. Based on 40-year (1961&ndash;2000) instrumental data, a transfer function between the WMV and the winter &delta;18O is established: WMV = 2.819 &delta;18O + 19.615 (N = 40, p &lt; 0.0001). Based on the calculated WMV from &delta;18O over the past 183 years, the sequence of winter monsoon velocity (WMV) from year 1818 to 2000 in the South China Sea is re-constructed. The sequence can be divided into three stages: the first stage shows a decreasing trend of 0.009 m/s&middot;yr from 1818 to 1954, the second stage indicates an increasing trend of 0.011 m/s&middot;yr from 1955 to 1976, and the last stage shows a decreasing trend of 0.026 m/s&middot;yr from 1977 to 2000. The maximum reduction in winter monsoon velocity over the last 24 years (1977&ndash;2000) is approximately 20% of the average over the last 183 years (1918&ndash;2000). The variation of wind velocity shows two complete cycles over the past 183 years. In particular, the strongest and weakest winter monsoon velocities in the last two centuries occurred in the 1830s and 1940s, respectively. The variation in winter monsoon velocity in the 20th century is closely linked to the surface temperature of the South China Sea, as well as air temperature over continental China. In addition, the winter monsoon was weak during two warm periods, i.e. the 1940s and 1980s. Statistical analysis of the winter monsoon velocity anomaly as well as the El Ni&ntilde;o (warm event) and La Ni&ntilde;a (cold event) phenomena indicates that 70% of El Ni&ntilde;o events correspond to the weakness of the winter monsoon.</p

Salinity control on long-chain alkenone distributions in lake surface waters and sediments of the northern Qinghai-Tibetan Plateau, China

Long-chain alkenones in lacustrine settings are potentially excellent biomarkers for the reconstruction of past terrestrial environmental conditions, and have been found in many different types of lakes around the globe. A wider range of factors influence the occurrence and distribution of alkenones in lake sediments and waters when compared to marine systems. Lake environmental conditions, such as temperature (in particular) and salinity, are among the key factors controlling alkenone distributions in lacustrine settings. Here we investigated alkenone distribution patterns in lakes of the northern Qinghai-Tibetan Plateau, China, and their possible relationship with environmental conditions, by analyzing paired samples of suspended particulate matter in surface waters and surface sediments. Salinity of investigated lake waters ranges from almost 0 to similar to 100 g/L, while temperature variation among the lakes is minimal, effectively eliminating temperature effects on the alkenone distribution patterns observed here. We show that (1) alkenone concentrations vary substantially between the lakes, yet controlling mechanisms remain elusive; (2) C(37)/C(38) ratios are substantially lower in the lakes of the Qaidam Basin than in the Lake Qinghai region, probably indicating different alkenone producers in the two regions; and (3) large variations in %C(37:4) (the percentage of the C(37:4) alkenone), determined from both surface waters and sediments, are negatively correlated with salinity. We suggest that the %C(37:4) index could be used as a salinity indicator at least on a regional scale, with careful considerations of other potentially complicating factors. However, potential reasons for why salinity could significantly affect %C(37:4) values need further investigation.</p

Total organic carbon isotopes: A novel proxy of lake level from Lake Qinghai in the Qinghai-Tibet Plateau, China

The isotopic compositions of total organic carbon (TOC) in lakes have been widely used to interpret paleoclimatic changes and the depositional environments of lake sediments. However, the main factors that affect the carbon isotopes of TOG (source of organic material, water condition and others) may vary in different lake sediment records, which have limited the applicability of organic carbon isotopes in explaining biogeochemical and environmental changes in lakes. In this study, the organic carbon isotopic compositions of aquatic plants and surface sediments from Lake Qinghai and the living terrestrial plants and surface soils around the lake were systematically investigated to identify the sources of TOC in the sediments and the significance of the organic carbon isotopes of sedimentary TOG. We found that the aquatic plants in the deep water areas (&gt;10 m) were primarily dominated by Cladophora, but submerged plants (Potamogeton and Ruppia L) are the dominant species in shallow water (&lt;10 m). The Cladophora have negative delta C-13(org) values (-33.6 parts per thousand to -28.6 parts per thousand) that are caused by C(3-)ike photosynthesis, but the submerged plants have enriched delta C-13(org) values (-17.8 parts per thousand to -15.4 parts per thousand) that are caused by C-4-like photosynthesis. In addition, the delta C-13(org) values of Cladophora become more negative with increasing water depth because of the slow photosynthetic rate caused by the weak light intensity at depth. The isotopic data indicate that the carbon isotopes of organic material in the surface sediments are primarily controlled by the types of aquatic plant and that the delta C-13(org) values of TOC can be used to indicate the variation of the water depth (lake level). The organic carbon isotopic data from the IF core showed that the water was shallow (&lt;10 m) because of intense evaporation related to high temperatures even though precipitation sharply increased in the warm period during the early-mid Holocene. The lake level reached its maximum level at 3 ka.</p

Precipitation changes recorded in the sedimentary total organiccarbon isotopes from Lake Poyang in the Middle and Lower YangtzeRiver, southern China over the last 1600 years

Lake Poyang, the largest freshwater lake of China, is well known for its ecological and hydrological importance as a dynamic wetland and lake system. The climatic change and its effect on ecological and hydrological system of the lake has aroused considerable interest in recent years. So, a sediment core from Lake Poyang was analyzed for d13C total organic matter (d13Corg) to study the hydrological changes in the Lake Poyang catchment. By comparing with modern meteorological data (precipitation and temperature) and hydrological data (volume of runoff), the result showed that the d13Corg values in the sediment are negatively correlated with the total runoff of the 5 main rivers supplying water to Lake Poyang and precipitation in this area over the past 50 years. Therefore, the d13Corg values of the Lake Poyang sediment can serve as reliable proxies for reconstruction of river discharge and regional precipitation, with negative d13Corg values representing increased precipitation in the Lake Poyang area. On this basis, the river discharge and regional precipitation variation in the Lake Poyang catchment over the past 1600 years were discussed. The d13Corg values suggest that the precipitation was highest in the period 340e880 AD, and then decreased with fluctuations to the minimum at approximately 1110 AD. During the period of Medieval Warm Period (1110e1350 AD, MWP), the precipitation was generally in the high level except low amount during period of 1140 AD to 1220AD. The precipitation was in low amount during the Little Ice Age (1350e1620 AD, LIA), except for a sudden increase at approximately 1550e1560 AD. The precipitation continuously increased with little fluctuation from 1610 AD until now. In general, changes in regional precipitation and river discharge were dominated by variations in the Asian Summer Monsoon (ASM) precipitation in the Lake Poyang area, consistent with other records from the ASM-controlled areas of China.</p

An evaluation of alumina reaction tube conditioning for high-precision H-2/H-1 isotope measurements via gas chromatography/thermal conversion/isotope ratio mass spectrometry

RATIONALE The condition of the pyrolysis reactor is very important for obtaining stable, precise hydrogen isotopic ratios using gas chromatography/thermal conversion/isotope ratio mass spectrometry (GC/TC/IRMS). However, few studies of the conditioning process have been conducted, and little is known about the best methods for high-precision hydrogen isotope analysis. METHODS We investigated delta H-2 variations and observed the changes in carbon coating using six different conditioning methods for the pyrolysis alumina tube: (i) no treatment; (ii) conditioning with 4 mu L hexane; (iii) conditioning with 2 mu L hexane; (iv) conditioning with 2 mu L hexane followed by backflushing overnight; (v) conditioning with 10 s of backflushing with methane; (vi) conditioning with 3 s of backflushing with methane. RESULTS Conditioning the alumina tube can improve the pyrolysis efficiency of organic compounds because a moderate amount of carbon acts as a catalyst in high-temperature regions of the alumina tube. Carbon actually flows in the tube and is difficult to confine to the high-temperature region. Insufficient amounts of carbon in the high-temperature regions lead to incomplete pyrolysis of organic compounds and lower delta H-2 values due to kinetic fractionation of hydrogen isotopes. In contrast, excess hexane or methane can lead to higher delta H-2 values, probably due to enrichment of deuterium in the hydrocarbon residue. CONCLUSIONS The delta H-2 values obtained by Method 6 are closest to the TC/EA delta H-2 values and are more precise than those obtained by other methods, perhaps because this method introduces a moderate, consistent amount of carbon with each sample injection.</p

Onset of frequent dust storms innorthern China at ~AD 1100

Dust storms in northern China strongly affect the living and health of people there and the dusts could travel a full circle of the globe in a short time. Historically, more frequent dust storms occurred during cool periods, particularly the Little Ice Age (LIA), generally attributed to the strengthened Siberian High. However, limited by chronological uncertainties in proxy records, this mechanism may not fully reveal the causes of dust storm frequency changes. Here we present a late Holocene dust record from the Qaidam Basin, where hydrological changes were previously reconstructed, and examine dust records from northern China, including the ones from historical documents. The records, being broadly consistent, indicate the onset of frequent dust storms at ~AD 1100. Further, peaked dust storm events occurred at episodes of high total solar irradiance or warm-dry conditions in source regions, superimposed on the high background of frequent dust storms within the cool LIA period. We thus suggest that besides strong wind activities, the centennial-scale dust storm events over the last 1000 years appear to be linked to the increased availability of dust source. With the anticipated global warming and deteriorating vegetation coverage, frequent occurrence of dust storms in northern China would be expected to persist