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

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

Groundwater Diffuse Recharge and its Response to Climate Changes inSemi-Arid Northwestern China

Understanding the processes and rates of groundwater recharge in arid and semi-arid areas is crucial for utilizing and managing groundwater resources sustainably. We obtained three chloride profiles of the unsaturated-zone in the desert/loess transition zone of northwestern China and reconstructed the groundwater recharge variations over the last 11, 21, and 37 years, respectively, using the generalized chloride mass balance (GCMB) method. The average recharge rates were 43.7, 43.5, and 45.1 mm yr-1, respectively, which are similar to those evaluated by the chloride mass balance (CMB) or GCMB methods in other semi-arid regions. The results indicate that the annual recharge rates were not in complete linear proportion to the corresponding annual precipitations, although both exhibited descending tendencies on the whole. Comparisons between the daily precipitation aggregate at different intensity and recharge rates reveal that the occurrence of relatively heavy daily precipitation per year may contribute to such nonlinearity between annual precipitation and recharge. The possible influences of vegetation cover alterations following precipitation change cannot be excluded as well. The approximately negative correlation between the average annual recharge and temperature suggests that changes in temperature have had significant influences on recharge.</p

Quantitative temperature reconstruction based on growth rate of annually-layered stalagmite: a case study from central China

We used the annual growth rate of a stalagmite (XL21) collected from Xianglong Cave, central China, to quantitatively reconstruct regional terrestrial temperature changes over the last 95 years (1912-2006 AD). Based on a significant positive correlation between the growth rate and the observed temperature, a transfer function was designed, and the temperature from the previous September to May (P-9-5) was reconstructed, with an explained variance of 43.5%. Our results show an increasing trend in temperature during the last century, and especially over the last 30 years. The temperature variability from central China recorded here bears a striking similarity to that in the Northern Hemisphere, and also to global trends. However, the cooling between the 1980s and the early 1990s seen in the stalagmite record, which interrupted the warming trend that began in the 1960s, is not observed in the mean conditions found in China, the Northern Hemisphere, neither globally. This methodology for reconstructing historical temperature from stalagmite growth rates overcomes the limitation of the short meteorological observation period and supports the potential of stalagmite lamina climatology.</p

Trace-element variations in an annually layered stalagmite as recorders of climatic changes and anthropogenic pollution in Central China

We analyzed variations in the Sr/Ca, Ba/Ca, REE/Ca (REE: rare earth element), Zn/Ca, and Pb/Ca ratios preserved in an annually layered stalagmite, XL21, from central China. The stalagmite record spans the 95 year period AD 1914&ndash;2008. The Sr/Ca and Ba/Ca ratios have a significant positive correlation with the stalagmite&#39;s growth rate, suggesting that they were primarily controlled by growth-rate variations. Variations in REE/Ca ratios are consistent with local temperature changes, suggesting temperature influenced REE concentrations in the stalagmite over decadal to annual timescales. Higher temperature in this humid area can increase vegetation cover, microbial activity, and organic decomposition in the soil, resulting in enhanced pCO2, organic matter concentration and reduced pH, and consequently increased REE mobilization from the overlying soil layer and host rock. Higher temperatures may also increase the natural Zn mobilization from the overlying soil mediated by organic matter and consequently may have led to increased Zn retention in XL21. An increasing trend is seen in the Pb/Ca ratios from XL21 since 1985, which is consistent with increased lead production in this area, and indicates an increase in mine-derived lead pollution in the local environment over the past 30 years.</p

Centennial-to decadal-scale monsoon precipitation variability in the semi-humid region, northern China during the last 1860 years: Records from stalagmites in Huangye Cave

We developed a composite oxygen isotopic record of cave calcite for the last 1860 years based on three stalagmites from the Huangye Cave in eastern Gansu Province, northern China. The delta(18)O values reflect monsoon precipitation changes, with lower d18O values representing higher precipitation and vice versa. Three intervals of high precipitation were identified at AD 138-450, AD 730-1200, and AD 1860-1960. Two intervals of low precipitation occurred at AD 1320-1410 and AD 1530-1860. The reconstructed monsoon precipitation variations correlate well with other records further east in the eastern Yellow River Basin, suggesting synchronous precipitation changes during the late Holocene in the semi-humid region of northern China on decadal to centennial scales. Peak periods of warfare in dynastic transition times, such as at AD 391-420, AD 601-630, AD 1111-1140, AD 1351-1380, and AD 1621-1650, correspond to sharp declines in precipitation or temperature in semi-humid northern China, indicating a strong connection between climatic and societal changes. Our study suggests that climatic deterioration in semi-humid northern China has played an important role in Chinese societal evolution.</p

Cyclic precipitation variation on the western Loess Plateau of China during the past four centuries

Precipitation variation on the Loess Plateau (LP) of China is not only important for rain-fed agriculture in this environmentally sensitive region, but also critical for the water and life securities over the whole Yellow River basin. Here we reconstruct high resolution precipitation variation on the western LP during the past 370 years by using two replicated, annually-laminated stalagmites. Spatial analysis suggests that the reconstruction can be also representative for the whole LP region. The precipitation variations show a significant quasi-50 year periodicity during the last 370 years, and have an important role in determining the runoff of the middle Yellow River. The main factor controlling the decadal scale variations and long-term trend in precipitation over this region is southerly water vapour transport associated with the Asian summer monsoon. The Pacific Decadal Oscillation is also an important influence on precipitation variation in this region, as it can affect the East Asian summer monsoon and the West Pacific Subtropical High.</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

Global Monsoon Dynamicsand Climate Change

This article provides a comprehensive review of the global monsoon that encompasses findings from studies of both modern monsoons and paleomonsoons. We introduce a definition for the global monsoon that incorporates its three-dimensional distribution and ultimate causes, emphasizing the direct drive of seasonal pressure system changes on monsoon circulation and depicting the intensity in terms of both circulation and precipitation. We explore the global monsoon climate changes across a wide range of timescales from tectonic to intraseasonal. Common features of the global monsoon are global homogeneity, regional diversity, seasonality, quasi-periodicity, irregularity, instability, and asynchroneity. We emphasize the importance of solar insolation, Earth orbital parameters, underlying surface properties, and land-air-sea interactions for global monsoon dynamics. We discuss the primary driving force of monsoon variability on each timescale and the relationships among dynamics on multiple timescales. Natural processes and anthropogenic impacts are of great significance to the understanding of future global monsoon behavior.</p