Climatic and anthropogenic impacts on δ13C variations in a stalagmite from central China.

In this paper, we present a delta C-13 record that covers the past 750 years at a resolution of 2 - 3 years which was preserved in a precisely dated stalagmite (DY-1) obtained from the Dayu Cave on the south flank of the Qinling Mountains in central China. Between 1249 AD and 1800 AD, climate-induced vegetation changes appear to have been the primary control on delta C-13 values at a centennial scale. Variations in precipitation amounts control the residence time of seepage water and may have affected the dissolution of bedrock, prior carbonate precipitation in the unsaturated zone above the cave, and the degassing of CO2 within the cave. These hydrogeochemical processes are likely to have been the most important controls on delta C-13 levels over annual to decadal scales, and may also have influenced centennial-scale variations. The reduced delta C-13 value of atmospheric CO2 since the Industrial Revolution may have caused the decreasing trend in delta C-13 values seen in stalagmite DY-1 after 1800 AD. Increased visitor numbers in the unventilated Dayu Cave over time produced a large amount of CO2, and maintained a raised level of pCO(2) in the cave air. This artificially enhanced pCO(2) may have decreased the fraction of CO2 degassing, and hence carbonate precipitation, which could partly cause the decreasing trend in the stalagmite delta C-13 seen over the past 200 years.</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

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

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

Characterization and source apportionment of aerosol light extinction in Chengdu, southwest China

To investigate aerosol properties in the Sichuan Basin of China, field aerosol sampling was carried out in Chengdu, China during four one-month periods, each in a different season in 2011. Aerosol scattering coefficient (b(sp)) at dry (RH&lt;40%) and wet (40% &lt; RH&lt;90%) conditions and aerosol absorption coefficient (b(ap)) were measured. Additionally, daily PM2.5 and PM10 samples were also collected. PM2.5 samples were subject to chemical analysis for various chemical components including major water-soluble ions, organic and elemental carbon (OC and EC), trace elements, as well as anhydrosugar Levoglucosan (LG) and Mannosan (MN). A multiple linear regression analysis was applied to the measured dry b(sp) against (NH4)(2)SO4, NH4NO3, organic mass (OM), fine soil (FS), and coarse mass (CM, PM2.5-10), and to the measured b(ap) against EC in all the four seasons to evaluate the impact of individual chemical components of PM2.5 and CM on aerosol light extinction (b(ext) = b(sp) + b(ap)). Mass scattering efficiency (MSE) and mass absorption efficiency (MAE) of the individual chemical components of PM2.5 were estimated based on seasonal regression equations and were then used for estimating b(ext). The annual b(sp), b(ap) and single scattering albedo (SSA) at dry conditions were 456 +/- 237 Mm(-1), 96 +/- 48 Mm(-1) and 0.82 +/- 0.05, respectively. The annual average b(sp) at ambient conditions estimated through hygroscopic curve of aerosol (f(RH)) was 763 +/- 415 Mm(-1), which was 1.7 times of the dry b(sp). The annual average SSA at ambient conditions also increased to 0.88 +/- 0.04. The estimated dry b(ext) was only 2 +/- 9% higher than the measurements and the estimated ambient bext from individual chemical components was only 1 +/- 10% lower, on an annual basis, than that estimated from using f(RH). Secondary inorganic aerosols, coal combustion, biomass burning, iron and steel industry, Mo-related industry, soil dust, and CM to b(ext) were estimated to account for 41 +/- 19%, 18 +/- 12%, 14 +/- 13%, 13 +/- 11%, 5 +/- 4%, 5 +/- 7% and 4 +/- 3%, respectively, of the estimated ambient b(ext).</p

Impact of PM2.5 chemical compositions on aerosol light scattering in Guangzhou - the largest megacity in South China

Daily PM2.5 samples were collected in Guangzhou - the largest megacity in South China, for a period of one month in each season during 2009-2010. Mass concentrations of water-soluble inorganic ions, organic carbon (OC) and elemental carbon (EC) in PM2.5 were determined, and aerosol scattering coefficient (b(sp)) was synchronously measured. The daily PM2.5 mass concentrations ranged from 21.0 to 213.6 mu g m(-3) with an annual average of 76.8 +/- 41.5 mu g m(-3). The highest seasonal average PM2.5 was observed in winter (103.3 +/- 50.1 mu g m(-3)) and the lowest in summer (38.6 +/- 15.7 mu g m(-3)). Annual average PM2.5 mass scattering efficiency (MSE) was 3.5 +/- 0.9 m(2) g(-1), with obvious seasonal variations in sequence of autumn (4.5 +/- 0.2 m(2) g(-1)) &gt; winter (3.9 +/- 0.5 m(2) g(-1)) &gt; spring (3.0 +/- 0.4 m(2) g(-1)) &gt; summer (2.3 +/- 0.3 m(2) g(-1)). To determine the relationship between b and the chemical components of PM2.5, b(sp) was reconstructed in each season using the original IMPROVE formula with a modification of including sea salt aerosols. The estimated bsp using this method was 22 +/- 28% smaller on annual average compared to the measurements. Multiple linear regression of measured b(sp) against (NH4)(2)SO4, NH4NO3, OM (Organic Mass), SS (Sea Salt), FS (Fine Soil), and CM (Coarse Mass) were also performed in all the four seasons. The estimated b(sp) from using the regression equation was 4 +/- 12% larger than the measured values. On average, (NH4)(2)SO4, NH4NO3, OM, SS, FS and CM accounted for 50 +/- 11%, 18 +/- 10%, 19 +/- 5%, 5 +/- 4%, 3 +/- 2% and 5 +/- 6%, respectively, of the estimated b(sp).</p

Characteristics and applications of size-segregated biomass burningtracers in China's Pearl River Delta region

Biomass burning activities in China are ubiquitous and the resulting smoke emissions may pose considerable threats to human health and the environment. In the present study, size-segregated biomass burning tracers, including anhydrosugars (levoglucosan (LG) and mannosan (MN)) and nonsea-salt potassium (nss-K&thorn;), were determined at an urban and a suburban site in the Pearl River Delta (PRD) region. The size distributions of biomass burning tracers were generally characterized by a unimodal pattern peaking in the particle size range of 0.44e1.0 mm, except for MN during the wet season, for which a bimodal pattern (one in fine and one in coarse mode) was observed. These observed biomass burning tracers in the PRD region shifted towards larger particle sizes compared to the typical size distributions of fresh biomass smoke particles. Elevated biomass burning tracers were observed during the dry season when biomass burning activities were intensive and meteorological conditions favored the transport of biomass smoke particles from the rural areas in the PRD and neighboring areas to the sampling sites. The fine mode biomass burning tracers significantly correlated with each other, confirming their common sources. Rather high DLG/DMN ratios were observed at both sites, indicating limited influence from softwood combustion. High Dnss-K&thorn;/DLG ratios further suggested that biomass burning aerosols in the PRD were predominately associated with burning of crop residues. Using a simplified receptor-oriented approach with an emission factor of 0.075 (LG/TC) obtained from several chamber studies, average contributions of biomass burning emissions to total carbon in fine particles were estimated to be 23% and 16% at the urban and suburban site, respectively, during the dry season. In contrast, the relative contributions to total carbon were lower than 8% at both sites during the wet season.</p

Peatland development and climate changes in the Dajiuhu basin,central China, over the last 14,100 years

Peatlands are one of the optimal archives for paleoclimate research and their records contain a detailed history of climatic and environmental variations during their formation. Here we collected a peat profile from the Dajiuhu basin in central China and analyzed several geochemical proxies, such as Total Organic Carbon (TOC), Total Nitrogen (TN), Titanium (Ti) and Aluminum (Al), to investigate the history of peat development. The results show that the peat growth was initiated after around 12.8 cal ka BP at the sampling site and the TOC and TN contents kept stable during the Holocene except during the periods of 10.5&ndash;9.0 cal ka BP and recent 1.5 cal ka BP. However, compared with other TOC records in the Dajiuhu basin, the history of peat development varied greatly in different sampling sites, indicating that the peat development in the Dajiuhu basin probably responds to both climate changes and local geological and hydrological conditions. Investigations of the interactions between climate change and peat development record in this study suggest that peat development presented significant responses to the large and rapid decreases in East Asian Summer Monsoon intensity, such as the 9.2 ka monsoon weakening event. However, the obvious long-term decreasing trend of monsoon precipitation during the mid-late Holocene did not lead to the degradation of the Dajiuhu peat, probably resulting from relatively stable temperature. In addition, the sharp decrease of TOC and TN values in the surface 15 cm of Dajiuhu peat was probably attributed to increased human activity.</p

The Holocene Indian monsoon variability over the southern Tibetan Plateau and its teleconnections

A high-resolution stalagmite oxygen isotope record from Tianmen Cave, located in the south central Tibetan Plateau, characterizes detailed climatic variations between 8.7 and 4.3 ka BP on centennial and decadal time scales, with a temporal resolution of 3 to 7 years. The Tianmen record is in good agreement with speleothem records from Asian monsoon regions (i.e., Dongge Cave, Dykoski et al., 2005; Wang et al., 2005; Qunf Cave, Fleitmann et al., 2003), upwelling records from the Arabian Sea (Gupta et al., 2003, 2005) and peat bog records from southeast Tibetan Plateau (Hong et al., 2003), indicating that the Indian summer monsoon (ISM) gradually weakened as Northern Hemisphere summer insolation declined during the early-mid Holocene and that ISM intensity, rather than temperature, dominates the precipitation delta O-18 on centennial to decadal time scales in the southern TP. On centennial to decadal time scales, the detrended Tianmen record correlates well with the Greenland ice core record, further confirming the mechanistic connection between the ISM and high northern latitude temperature changes. Meanwhile, the Tianmen record bears significant solar activity cycles, suggesting that the solar output changes may affect the variability of the ISM and likely the position of the Intertropical Convergence Zone (ITCZ) as well. Moreover, due to joint effects of changes in monsoon precipitation, moisture source and temperature, the Tianmen delta O-18 record shows much larger amplitude changes than speleothem delta O-18 records from low-elevation Asian Monsoon regions, which is similar to the previously reported Marine Isotope Stage (MIS) 5 record (Cai et al., 20W), suggesting heightened sensitivity of precipitation isotope composition to climate changes over the high-elevation regions and further demonstrating that the stable isotope lapse rate may change under different climatic conditions.</p

A Chinese cave links climatechange, social impacts, and humanadaptation over the last 500 years

The collapse of some pre-historical and historical cultures, including Chinese dynasties were presumably linked to widespread droughts, on the basis of synchronicities of societal crises and proxy-based climate events. Here, we present a comparison of ancient inscriptions in Dayu Cave from Qinling Mountains, central China, which described accurate times and detailed impacts of seven drought events during the period of 1520&ndash;1920 CE, with high-resolution speleothem records from the same cave. The comparable results provide unique and robust tests on relationships among speleothem &delta;18O changes, drought events, and societal unrest. With direct historical evidences, our results suggest that droughts and even modest events interrupting otherwise wet intervals can cause serious social crises. Modeling results of speleothem &delta;18O series suggest that future precipitation in central China may be below the average of the past 500 years. As Qinling Mountain is the main recharge area of two large water transfer projects and habitats of many endangered species, it is imperative to explore an adaptive strategy for the decline in precipitation and/or drought events.</p