This study presents new stable isotope data for precipitation (δ18Op) and drip water (δ18Od) from eight cave sites in the monsoon regions of China (MRC), with monthly to bi-monthly sampling intervals ...from May-2011 to April-2014, to investigate the regional-scale climate forcing on δ18Op and how the isotopic signals are transmitted to various drip sites.
The monthly δ18Op values show negative correlation with surface air temperature at all the cave sites except Shihua Cave, which is opposite to that expected from the temperature effect. In addition, although the monthly δ18Op values are negatively correlated with precipitation at all the cave sites, only three sites are significant at the 95% level. These indicate that, due to the various vapor sources, a large portion of variability in δ18Op in the MRC cannot be explained simply by either temperature or precipitation alone.
All the thirty-four drip sites are classified into three types based on the δ18Od variability. About 82% of them are static drips with little discernable variation in δ18Od through the whole study period, but the drip rates of these drips are not necessary constant. Their discharge modes are site-specific and the oxygen isotopic composition of the stalagmites growing from them may record the average of multi-year climatic signals, which are modulated by the seasonality of recharge and potential effects of evaporation, and in some cases infiltration from large rainfall events. About 12% of the thirty-four drip sites are seasonal drips, although the amplitude of δ18Od is narrower than that of δ18Op, the monthly response of δ18Od to coeval precipitation is not completely damped, and some of them follow the seasonal trend of δ18Op very well. These drips may be mainly recharged by present-day precipitation, mixing with some stored water. Thus, the stalagmites growing under them may record portions of the seasonal climatic signals embedded in δ18Op. About 6% of the thirty-four drip sites are medium-variability drips, with constant and relatively low δ18Od values in the wet season, but with variable and relatively high δ18Od values in the dry season, reflecting flow switching in the karst or evaporation inside the cave.
It is conducive to the sustainable development of human beings in karst regions to research the mechanism of karst rocky desertification (KRD) expansion. Whether the large-scale KRD in southwestern ...China is caused by climate change or human activities is still controversial. In this study, the evolution of the KRD in southwestern China over the past 2000 years was reconstructed through the high-precision
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C record of stalagmites from Shijiangjun (SJJ) Cave, Guizhou Province, China. The
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C of the stalagmites from SJJ Cave exhibited heavy values from the Medieval Warm Period (MWP) to the Little Ice Age (LIA). Furthermore, the
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C records of other stalagmites and tufa from southwestern China also showed the same significant heavy trend. Because the stalagmite
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C could record the change of ecological environment, it indicated that the consistent change of the stalagmites
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13
C may record the process of KRD expansion in the karst regions of southwestern China. During the MWP, the stronger Asian summer monsoon and the northward movement of the rain belt led to a dry period in southwestern China and a wet period in northern China. In contrast, it was wet in southwestern China and dry in northern China during the LIA. In addition, after the Jing-Kang event (JK event, AD1127) occurred at the end of the Northern Song dynasty, the political and economic center of China migrated to southern China for the first time, which changed the population distribution pattern of larger population in the north and smaller population in the south. Therefore, the expansion of KRD in southwestern China was exacerbated in the MWP due to the change of climate in southwestern China, the migration of a large number of people, wars, the large-scale reclamation of arable land, and the cultivation of large areas of crops.
Understanding the hydrological processes of dissolved organic matter (DOM) in the surface karst zone is crucial for the utilization and safety of groundwater resources. However, research on DOM in ...drip water from karst caves is limited. In this study, continuous monitoring was conducted for four years at four drip water monitoring sites (MP1, MP2, MP3, and MP9) in Furong Cave, Southwest China. The three-dimensional fluorescence excitation–emission spectroscopy (3D-EEM) and fluorescence region integration (FRI) methods were employed, along with correlation analysis, to investigate the spectral characteristics, composition, sources, and influencing factors of the DOM in the drip water of Furong Cave. The results indicated that (1) the three-dimensional fluorescence peaks in the drip water were classified into six categories: A, B, C, T, Ti, and M. (2) The dominant source of the drip water DOM is endogenous organic matter. (3) The FRI analysis indicates a relatively high proportion of Type II substances in the drip water, predominantly composed of tryptophan-like substances. (4) The DOM in the drip water of Furong Cave was influenced by various factors, including the mixing effect of “new” and “old” water, water residence time in karst systems, and water–rock interactions (WRI), resulting in the complex responses of drip rates and DOM to surface precipitation and temperature at different drip water sites. This study provides a reference for comparative research on DOM in cave drip water in karst regions, which contributes to a better understanding of the migration mechanism of DOM in karst aquifers under different climate and karst ecological conditions.
This paper explores the influence of the local meteoric conditions, the overlying soil on the concentrations of Ca, Mg, and St, and the ratios of Mg/Ca, Sr/Ca, and Mg/Sr in soil infiltrating water ...（SIW）. An in situ monitoring program was carried out above the Furong Cave throughout 2010 to collect data on SIW, monthly temperature and rainfall, and the geochemical com- position and granularity of soils. The discharge of SIW responded quickly to the local rainfall, and its residence time was the primary factor affecting the Ca, Mg, and Sr content of the SIW. The high concentrations of Ca, Mg, and Sr in the SIW during April should be attributed primarily to the prolonged residence of SIW in the soil during the local dry seasons of winter and early spring. The maximum Mg/Sr ratio also occurred in April. The ratio of Mg/Ca in SIW is positively correlated with pro- longed residence time and with high temperatures, which do not strongly affect the ratio of Sr/Ca. The Mg/Ca ratio was lowest when the Sr/Ca ratio was highest because plant metabolism increased the absorption of Ca and Mg, but not St, and also be- cause higher temperatures enhanced the dissolution of Mg more than that of Sr. These different responses of Mg and Sr to temperature increases resulted in high Mg/Sr ratios during July and August.
•A 12 years’ monitoring work has been conducted in China to explore the change of geochemical characteristics in drip water.•The ratios of trace elements for drip water reflects the variation of ...rainfall.•The δ18O of drip water reflects longer term trends of rainfall, and change of vapor sources dominated by ENSO.
Cave monitoring is crucial for the interpretation of climatic and environmental significances of various geological proxies in speleothem. Therefore, the hydrochemical and stable isotopic compositions (δ18O, δD, Mg/Ca, Sr/Ca, and Ba/Ca) of karst cave waters during 2005–2016 CE were constantly monitored in Furong Cave, Chongqing City, Southwest China. A comparison with local hydrological conditions led to 4 main conclusions as follows: (1) the Mg/Ca ratio is significantly responsive to the changes in drought/wet conditions outside the cave, which increased in drought years and decreased in wet years, respectively. Seasonal variation of Sr/Ca is more significant than those of Mg/Ca and Ba/Ca ratios. (2) Prior calcite precipitation (PCP), incongruent calcite dissolution (ICD), water–rock interaction (WRI), and pCO2 of soil and cave air, may account for the changes in trace element ratios in the epikarst, which resulted in a complex variation of element ratios in the cave drip water. In general, WRI in drought years is stronger than that in wet years, and that in low discharge sites is stronger than in high discharge sites. Seasonal variation of Ca2+ concentration, induced by PCP, exerts significant impact on the evolution of Sr/Ca ratio in drip water. (3) δ18O and δD of drip water are influenced by the “mixing effect”, leading to the result that their seasonal variations are less significant than that of precipitation. (4) At least in the study period, the ratios of trace elements in the drip water in Furong Cave mainly reflected the variations of local hydrological conditions (drought or wet) dominated by precipitation. Because of the “mixing effect” of groundwater, high-resolution δ18O record (e.g., seasonal and annual) of speleothem in Furong Cave may not be recommendable. However, the δ18O is potentially a reliable proxy in speleothems to record the change of rainfall on decadal and longer timescales.
•A six-year cave monitoring work in China, to assess the climatic significance of δ18O and Mg/Ca ratios in drip water.•Changes of δ18O in precipitation and drip water dominated by the change of ...atmospheric circulation patterns, e.g., ENSO.•Mg/Ca in drip water influenced by hydrological condition, with higher/lower values correspond to less/more precipitation.
The scientific explanation of speleothem δ18O in Chinese monsoon region is a greatly debated issue. Modern cave monitoring combined with instrument observation maybe is an essential solution to deal with this issue. During the period from 2011 to 2016, we monitored local precipitation, soil water in three soil profiles, and six drip water sites in Yangkou Cave, which is located in Chongqing City, Southwest China. This article presents measurements about δ18O, δD and Mg/Ca ratios of drip water and compared these geochemical proxies with contemporaneous atmospheric circulations. The main conclusions are: (1) As water migrates from precipitation to soil water to cave drip water, the amplitudes of seasonal variations in δD and δ18O decreased gradually. Due to the existence of complex hydrogeological conditions, the range of variation and the seasonal characteristics of δD and δ18O differ among the drip sites where samples were collected, but the interannual variability is nearly the same. The drip water Mg/Ca ratios are mainly regulated by changes in hydrological conditions in the epikarst zone, with higher values during winter months than that during summer months. (2) When an El Niño event occurs, the Western Pacific Subtropical High (WPSH) is migrated westward, and the production of near-source water vapor from the western Pacific and the South China Sea increases, leading to higher δ18O values in the precipitation and the cave drip water. The drip water Mg/Ca ratios were significantly lower with increased summer precipitation. On the other hand, during La Niña events, the WPSH is migrated eastward, and inputs of water vapor that has traveled greater distances (from the Indian Ocean) become comparatively important, resulting in lower δ18O values in the precipitation and the cave drip water. The drip water Mg/Ca ratios were higher with decreased summer precipitation. In summary, the interannual variability of δ18O in the drip waters of Yangkou Cave reflects changes in water vapor sources caused by atmospheric circulation patterns. Mg/Ca ratios respond to changes of precipitation and CO2 in soil and can be used to reconstruct abnormal drought or flood events.
A number of studies proposed a close link between the climate changes in the Asian summer monsoon (ASM), the Southern Hemisphere (SH), and high latitudes of the Northern Hemisphere (NH). But the ...mechanisms is still an open question. Although speleothems δ18O records have displayed excellent values in the reconstruction of paleoclimate change at centennial - millennial to orbital timescales, the significance of speleothems δ18O and δ13C is still debatable. Here, a high-precise 230Th dated stalagmite from southwestern China was used to reconstruct the changes of ASM and regional hydrological conditions since the last interglacial (3.6–118.1 ka BP) by the coupled δ18O and δ13C. We found: (1) The EASM and ISM are synchronous changing on the orbital timescale, responding to the change of North Hemisphere summer insolation (NHSI) and high latitude climate change. There is a teleconnection between Antarctic temperature change and ASM through the Mascarene High and the Somali Jet. (2) During MIS 5d, the stalagmite δ18O in Yangzi Cave is nearly 1.5‰ higher than that in MIS 2, the maximum of last glacial. This should be attributed to the different isotopic compositions in moisture source of ISM and EASM, and the dynamic changes of monsoon circulations. (3) From MIS 3 to MIS 2, the ASM showed a “stepped” pattern in weakening, consistent with the decreasing NHSI and increasing global ice volume. (4) The changes in stalagmite δ13C indicated the changes of local hydrological conditions and closely correlated with climate changes revealed by stalagmite δ18O. The effective humidity, which determined by temperature, evaporation, and precipitation, dominated the regional hydrological conditions. The change of effective humidity recorded in stalagmite δ13C by influencing surface biomass and soil CO2-production.
•A stalagmite in southwest China reconstructed ASM and regional hydrological environment since the Last Glacial-interglacial.•NHSI and Antarctic temperature coupling impact the change of ASM.•Stalagmite δ13C indicates changes in local surface hydrological conditions.
The climate variability during Dansgaard-Oeschger (D-O) events 19 to 25 (first recognized in the Greenland ice cores) was recorded in the Asian monsoon region, but the climatological dynamic ...mechanism is still poorly understood. Here, we present a new 230Th absolute chronology and high-resolution record of the Asian summer monsoon (ASM) from 117 to 69 kyr B.P. based on 25 230Th ages, 803 oxygen and carbon isotopes, as well as 493 trace element ratios of the YYZ1 stalagmite from Yangzi Cave in Fengdu County, 120 km from Chongqing City, China. The δ18O record supports a strong ASM during marine isotope stages (MIS) 5a and 5c and a weak ASM during MIS 4, 5b, and 5d. Millennial-scale oscillations of D-O 19–25 were also recorded in our speleothem δ18O. Multiple environmental proxies, δ13C and trace element records of YYZ1 stalagmite can also reflect local climate and environmental changes. A drought event was clearly identified between 93.0 and 91.5 kyr B.P., referred to as the 92 kyr event, which exhibited a rapid and abrupt fluctuation (V-shaped) based on our multi-proxy results. We found that D-O 25 event was not obvious in our record and in other Chinese speleothem records. The D-O 25 event occurred within a period of decreasing North Hemisphere summer insolation (NHSI), which led to different climate backgrounds, e.g., the increase of ice volume and the corresponding weakening of the Northern Atlantic thermohaline circulation. Conversely, the D-O events within the high NHSI period or the warm stages (MIS5a, MIS5c) exhibited more obvious fluctuations than those in the low NHSI period or the cool stages (MIS5b, MIS5d). Thus, we suggest that the differences between D-O events are mainly related to the change of the ASM in response to variable NHSI. Furthermore, the ASM during D-O events was also influenced by climate variations in the Southern Hemisphere (SH) and low latitudes, which should not be ignored.
•Variations in Asian summer monsoon (ASM) during MIS 5 were reconstructed in multi-proxy speleothem records.•A drought event was identified in 92 ka.•Most stalagmites records in China showed smaller trends in δ18O changes of DO 25 event.•Weak DO events were related to environmental changes at lower latitudes.
Soil moisture patterns are crucial factors in gully bank erosion and may significantly vary depending on the water conditions before winter freezing. Rainfall events can evoke different moisture ...responses throughout the soil profile of gullies. However, the relative importance of prewinter conditions and rainfall events to the soil moisture responses of gully banks in seasonally frozen regions remains unclear. In this study, a field infiltration experiment of a gully bank with four prewinter soil moisture regimes and three replicates was conducted in October 2017 in the Mollisol region of China. We aimed to elucidate the temporal variations in moisture responses under different prewinter water treatments at various distances to the gully bank after three representative rainfall events in 2018. The response magnitude (∆s) varied widely and was largest in the intermediate layers (30–50 cm) for wetter prewinter regimes (i.e., HI and MED) but was larger in the upper soil layers (0–30 cm) for drier prewinter regimes (i.e., CK and LO). In contrast to the findings of other studies, we found that the relative influences of prewinter conditions and rainfall events on the soil moisture responses varied with soil depth and distance to the gully edge. The soil moisture response increased with rainfall depth, and the effect of prewinter conditions dominated until the end of the rainy season (August). The most important explanatory variables for the response magnitude (∆s) were rainfall events (46%), soil depth (10%), and prewinter conditions (7%). This study increases the understanding of the heterogeneity in soil moisture responses and the relationship to rainfall events under different prewinter conditions on gully banks in the Mollisol region. Understanding subsurface hydrological processes and identifying the relative importance of prewinter conditions and rainfall are beneficial for building gully bank erosion models for seasonally frozen regions.
Gully sites with drier prewinter conditions, that is, sites in the upslope position, south‐facing gully banks, and sites with less snow cover, should receive particular attention to prevent and control gully erosion.
