Leaf wax n-alkane distributions across plant types in the central Chinese Loess Plateau

Terrestrial ecosystems generally contain various plant types (e.g. dicots, monocots, gymnosperms), and an evaluation of the dominant plant type in an ecosystem is the key to understanding geological records in paleoenvironmental research. In this study, we examined n-alkane chain length distributions in terrestrial higher plants in the central Chinese Loess Plateau, and found that average chain length (ACL) could be utilized as an indicator for differentiating gymnosperms from angiosperms. ACL21&ndash;33 was less than 27 for gymnosperms, but more than 27 for angiosperms. Moreover, a derived Pv index from a selected n-alkane ratio, (i.e. (C31 + C33)/RCn(n=27&ndash;33)), provided a relative criterion for distinguishing between dicots and monocots within the angiosperms. When Pv &lt; 0.1, the ecosystem was predominated by dicots, but those with Pv &gt; 0.1, were dominated by monocots. Discrimination of the dominant plant type in an ecosystem is important when leaf wax n-alkanes are used as a proxy for paleoenvironmental reconstructions.</p

Soil-derived sulfate in atmospheric dust particles at Taklimakan desert

Dust-associated sulfate is believed to be a key species which can alter the physical and chemical properties of dust particles in the atmosphere. Its occurrence in the particles has usually been considered to be the consequence of particles&#39; aging in the air although it is present in some crustal minerals. Our observation at the north and south edge of Taklimakan desert, one of the largest dust sources in the Northern Hemisphere, during a dust episode in April 2008 revealed that sulfate in atmospheric dust samples most likely originated directly from surface soil. Its TSP, PM10 and PM2.5 content was proportional to samples&#39; mass and comprised steadily about 4% in the differently sized samples, the ratio of elemental sulfur to iron was approximately constant 0.3, and no demonstrable influence of pollutants from fossil fuel combustion and biomass burning was detected. These results suggest that sulfate could be substantially derived from surface soil at the desert area and the lack of awareness of this origin may impede accurate results in any investigation of atmospheric sulfur chemistry associated with Taklimakan dust and its subsequent local, regional and global effects on the atmosphere.</p

Background-like nitrate in desert air

The atmospheric nitrogen cycle is a key process driving the earth&#39;s environmental evolution. Current model studies require knowledge of NOx soil emissions from various land types, but desert emissions remain unquantified or are not addressed with high confidence. Our measurements at two observatories in Taklimakan desert during a dust episode showed an approximately stable and dust-independent nitrate in the air. Its concentration estimated from PM2.5, PM10 and TSP samples under non-dust, floating dust and dust storm conditions was 3.81&plusmn;1.24&mu;gm-3, 2.95&plusmn;0.69&mu;gm-3, 4.99&plusmn;1.71&mu;gm-3, respectively, despite the more-than-one-order difference of dust loading. This concentration was much larger than that in remote marine and tropical forest air. Comprehensive investigation revealed a similar presence of nitrate in other desert air. The nitrate was hypothesized to be the consequence of the conversion of NOx released from desert soils. These results indicate a background-like nitrate and active reactions of nitrogen compounds in desert air.</p

Amplitudes, rates, periodicities and causes of temperature variations in the past 2485 years and future trends over the central-eastern Tibetan Plateau

Amplitudes, rates, periodicities, causes and future trends of temperature variations based on tree rings for the past 2485 years on the central-eastern Tibetan Plateau were analyzed. The results showed that extreme climatic events on the Plateau, such as the Medieval Warm Period, Little Ice Age and 20th Century Warming appeared synchronously with those in other places worldwide. The largest amplitude and rate of temperature change occurred during the Eastern Jin Event (343-425 AD), and not in the late 20th century. There were significant cycles of 1324 a, 800 a, 199 a, 110 a and 2-3 a in the 2485-year temperature series. The 1324 a, 800 a, 199 a and 110 a cycles are associated with solar activity, which greatly affects the Earth surface temperature. The long-term trends (&gt;1000 a) of temperature were controlled by the millennium-scale cycle, and amplitudes were dominated by multi-century cycles. Moreover, cold intervals corresponded to sunspot minimums. The prediction indicated that the temperature will decrease in the future until to 2068 AD and then increase again.</p

Stable isotope composition alteration produced by the aragonite-to-calcite transformation in speleothems and implications for paleoclimate reconstructions

Aragonite, a mineralogical constituent of speleothems in cave environments, is unstable and susceptible to inversion to calcite, a diagenetic process that involves changes in the mineralogy, texture and geochemistry of speleothems. However, the exact alterations of stable isotope compositions during such diagenesis have not been fully investigated. In this study, two aragonite stalagmites (SN3 and SN15) from the Shennong Cave, southeast China, were found partially inverted to calcite, as determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses, and thin-section inspections under microscope. The fiber relics and textural ghosts of aragonite preserved in coarse and equant mosaic calcite crystals clearly indicate that the calcite in these two stalagmites were inverted from aragonite. The stable isotope compositions (delta C-13 and delta O-18, given in per mil versus VPDB standard) of primary aragonite and secondary calcite were analyzed and compared, along both growth layers and growth axes. The results show that, along growth layers, differences of delta C-13 values between aragonite and calcite are negligible (0.1 parts per thousand-0.2%.), whereas differences of delta O-18 values between aragonite and calcite are significant (0.63 parts per thousand-0.87%0). Comparisons along growth axes show similar results: i.e., differences of delta 13C values are negligible (0.06% +/- 0.22%.) whereas differences of delta O-18 values are significant (0.85 parts per thousand +/- 0.29%.). Most likely, the aragonite in SN3 and SN15 were internally inverted by interactions of trace calcite crystallites and pore water within intercrystalline pore spaces, by a dissolution-reprecipitation process occurring in trapped pore water. In the case of the inversion of aragonite to calcite in speleothems, such as that observed in SN3 and SN15, the delta 13C values could be used in paleoclimate and paleoenvironment reconstructions because they are inherited from those of primary aragonite. Although the delta O-18 values might be cross-calibrated to those of primary aragonite if the aragonite-calcite fractionation offset is known (e.g., 0.85 parts per thousand +/- 0.29%0 in this study), however, the delta O-18 values of secondary calcite should be used with caution in such reconstructions as the delta O-18 offset value is not consistently invariable.</p

Airborne particulate organic markers at the summit (2060 m,a.s.l.) of Mt. Hua in central China during winter: Implications for biofuel and coal combustion

Sugars, n-alkanes and PAHs in PM10 and size-segregated samples collected from the summit (2060 m, altitude) of Mt. Hua in Guanzhong Plain, central China during the winter of 2009 were characterized using a GC/MS technique. Concentrations of sugars, n-alkanes and PAHs in PM10 are 107&plusmn;52, 121&plusmn;63, 7.3&plusmn;3.4 ng m&minus;3, respectively. Levoglucosan and fossil fuel derived n-alkanes are more abundant in the air masses transported from southern China than in those from northern China with no spatial difference found for PAHs, suggesting that emissions from biomass burning and vehicle exhausts are more significant in southern part of the country. Dehydrated sugars, fossil fuel derived n-alkanes and PAHs presented a unimode size distribution, peaking at the size of 0.7&ndash;1.1 &mu;m, whereas non-dehydrated sugars and plant wax derived n-alkanes showed a bimodal pattern, peaking at 0.7&ndash;2.1 and 5.8&ndash;9.0 &mu;m, respectively. Principal component analysis showed that biofuel combustion plus plant emission is the most important source in Mt. Hua, being different from the cases in Chinese urban areas where fossil fuel combustion is the major source. By comparison with previous mountain and lowland observations and aircraft measurements we found that wintertime PAHs in China are still characterized by coal burning emissions especially in the inland regions, although in the country increasing rate of SO2 emission from coal combustion has decreased and emissions of vehicle exhaust has sharply increased.</p

Climate significance of speleothem d18O from central China on decadaltimescale

Speleothem-based oxygen isotopic sequences have been widely used to reveal regional climatic changes worldwide. However, the climatic significance of speleothem d18O variations in monsoonal China on decadal- to annual timescales remains in debate, which limits its application to paleoclimate reconstruction in the world&rsquo;s largest country by population. In this study, we analyzed a seasonally resolved d18O record of an annually layered stalagmite, XL21 whose variability covers a period of 98 years (1912&ndash;2009 AD), from Xianglong Cave, central China. The annual d18O trend shows an inverse relationship with local monsoon precipitation, suggesting speleothem d18O variations in this area can reflect monsoon precipitation at least on decadal timescale. Changes in moisture sources and transport pathways have little effect on the d18O variations in speleothems/monsoon precipitation on decadal timescale, which may be ascribed to similar distances from this region to the two main moisture sources, the Bay of Bengal and West Pacific. There is an anti-phase relationship between speleothem d18O records from central China and India during the last 98 years, which indicates the relationship between Indian monsoon intensity and speleothem d18O from central China on short timescales requires further investigation.</p

Individual and pooled tree-ring stable-carbon isotope series in Chinese pine from the Nan Wutai region, China: Common signal and climate relationships

To investigate the differences in the climatic signals in stable-carbon isotopic composition captured by averaging series from individual trees versus raw wood of trees pooled prior to analysis, we analyzed two groups of Chinese pine (Pinus tabulaeformis Carr.) from the Nan Wutai region of the Qinling Mountains, China. One group included three trees that were analyzed separately, and the other group comprised four other trees that were pooled prior to preparation and analysis. All &delta;13C series were positively correlated (r = 0.50&ndash;0.58, p &lt; 0.0001) for the period AD 1901&ndash;2003. After removing the effects of changing &delta;13C of atmospheric CO2, correlations between the meteorological data and all individual and pooled discrimination (∆13C) series revealed significant negative responses to temperature for several specific months and for mean January to September (TJ&ndash;S) temperature. We used a &ldquo;numerical mix method&rdquo; (NMM, equivalent to unweighted mean), by averaging individual ∆13C series (NPS1+NPS2+NPS3), to generate a new series that more strongly correlated to climate series TJ&ndash;S (r = &minus; 0.67, p &lt; 0.0001). This time interval from January through September (J&ndash;S) includes the growing season and months prior to the growing season, but the temperature prior to the growing season may provide energy necessary for timely initiation of growth. Thus, the mean TJ&ndash;S is significant for plant growth and is consistent with the tree physiology in this region. Our results suggest that the numerical mix method with tree-ring stable isotope data from three trees provides a series quite satisfactory for climatic reconstruction. The relationship of the numerical mix model ∆13C with temperature was stronger than for the pooled series, suggesting numerical mixing of series can be more effective than raw wood sample pooling at least according to the trees in this study.</p

Validation and application of a thermal-optical reflectance (TOR) method for measuring black carbon in loess sediments

In an effort to assess the potential contamination and determine the environmental risks associated with heavy metals, the surface sediments in Liaodong Bay, northeast China, were systematically sampled and analyzed for the concentrations of Cu, Pb, Zn, Cr, Ni, As, and Hg. The metal enrichment factor (EF) and geoaccumulation index (I (geo)) were calculated to assess the anthropogenic contamination in the region. Results showed that heavy metal concentrations in the sediments generally met the criteria of China Marine Sediment Quality (GB18668-2002); however, both EF and I (geo) values suggested the elevation of Pb concentration in the region. Based on the effect-range classification (TEL-PEL SQGs), Cu, Pb, Ni, and As were likely to pose environment risks, and the toxic units decreased in the order: Ni &gt; Pb &gt; Cr &gt; Zn &gt; As &gt; Cu &gt; Hg. The spatial distribution of ecotoxicological index (mean-ERM-quotient) suggested that most of the surface sediments were &quot;low-medium&quot; priority zone. Multivariate analysis indicated that the sources of Cr, Ni, Zn, Cu, and Hg resulted primarily from parent rocks, and Pb or As were mainly attributed to anthropogenic sources. The results of this study would provide a useful aid for sustainable marine management in the region.</p

Miocene climate change on the Chinese Loess Plateau: Possiblelinks to the growth of the northern Tibetan Plateau and globalcooling

The evolution of the Asian monsoon-arid environmental system during the Cenozoic was closely related to the growth of the Himalayan-Tibetan Plateau and global climate change. However, due to inconsistencies in paleoclimatic reconstructions and to various constraints on the timing of the growth of the Tibetan Plateau, the relative impacts of regional uplift and global cooling on Asian climate change remain controversial. Here we investigate the mineralogical composition of a Miocene Red Clay deposit on the western Chinese Loess Plateau in order to infer changes in chemical weathering and monsoon intensity. Variations of four mineralogical ratios (chlorite/quartz, illite/quartz, calcite/quartz, and protodolomite/quartz) reveal that the summer monsoon intensity was relatively strong during the early Miocene (23.5&ndash;18.5 Ma), weakened gradually until 9.5 Ma, and strengthened again in the late Miocene. We synthesized previously published thermochronological data from the northeastern Tibetan Plateau and surrounding mountains, and the results suggest that two phases of the rapid growth of northern Tibet occurred around 24&ndash;17 and 13&ndash;7 Ma. Comparison of paleoclimatic proxies and thermochronological data suggests that the gradual weakening of the summer monsoon intensity from 18.5 to 9.5 Ma paralleled global cooling, whereas two intervals of strengthened monsoon in the early and late Miocene were possibly related to the rapid growth of northern Tibet. Our combination of paleoenvironmental proxies and thermochronological data reveals possible links between Miocene Asian monsoon evolution, phased growth of the Tibetan Plateau, and global climate change, and confirms the interconnection of geodynamic and atmospheric processes in the geological past.</p