Weijun Luo

Significance and dynamics of drip water responding to rainfall in four caves of Guizhou, China

In rainy season, NaCl is adopted to trace sources of cave drip water, time scales of drip water responding to precipitation, and processes of water dynamics in four caves of Pearl watershed in Guizhou, China (Liangfeng cave in Libo, Qixing cave in Duyun, Jiangjun cave in Anshun and Xiniu cave in Zhenning). Because of the variety of karst cave surroundings, interconnections of water transporting ways, water dynamics processes etc., time scales of drip-water in four caves responding to rainfall is 0–40 d. According to the characteristics of water transport in cave roof, pathways of water movement, types of water head etc., drip water of four caves can be divided into five hydrodynamics types. The differences of time scales, and ways of water-soil and water-rock interaction during water transporting in cave roof make it difficult to correctly measure speleothem record and trace material sources. In addition, there exist great differences in water dynamic conditions among the four caves. So the interpretation of the paleoenvironment records of speleothem must be supported by the understanding of hydrodynamics conditions of different drip sites. Based on the data got from drip sites in four caves, drip conductivity accords with precipitation, which indicates that element contents in speleothem formed by drip water record the change of karst paleoenvironment. But results of multi-points study are needed to guarantee the correctness of interpretation.

Temporal and spatial variations in hydro-geochemistry of cave percolation water and their implications for four caves in Guizhou, China

Soil water and cave drip water from four cave systems in Guizhou, Southwest China, were monitored and sampled monthly from April 2003 to May 2004 to understand spatio-temporal variations in hydro-geochemistry of cave percolation water. The results indicated that among the 5 drip water sites from the Liangfeng Cave (LFC), there were no significant differences among the hydro-geochemical parameters. In the Xiniu Cave (XNC), the drip rates were variable and responded quickly to precipitation events in the 3# (No. 3) drip site with variable water head; both bulk and variation in drip rates were smaller in the 2# with constant water head. However, in the Qixing Cave (QXC) and Jiangjun Cave (JJC), drip rates, concentrations of major ions and saturation index for calcite (SIC) were smaller, and Mg/Ca ratios in Group I (1#, 2#, 6#, 7# and 8# in the QXC; 2# and 3# in the JJC) were larger than those in Group II (3#, 4#, 5# and 9# in the QXC; 1# and 4# in the JJC). These differences might be the result of different hydrogeological processes above the caves, which are divided into five categories based on hydro-geochemistry data. The formation of some proxies in speleothem, such as Mg/Ca, is likely to be affected by those processes.

Characteristics of soil water movement in a grass slope in a karst peak-cluster region, China

Soil water is very important in hilly areas with thin soil layers and deep groundwater tables, such as the karst peak-cluster region of Southwest China. An investigation into soil water movement can provide insights into management of shallow water resources and soil nutrients, as well as prevention of groundwater pollution. In this study, O-18 and H-2 tracers were used to trace soil water movement in planar soil mass type microhabitats in the middle part of a steep hillslope covered by grasses in a karst peak-cluster region of China. From May 2008 to July 2009, samples of precipitation and two types of soil water, which had different integrated degrees of mobility and were of different depth intervals or depths, were collected. The hydrogeochemical characteristics were compared between precipitation and soil water, and these data were applied in convolution-based lumped parameter models. Our results indicated that vertical piston flow, rather than lateral flow along the soil-bedrock interface, played an important role in soil water percolation at least in the upper soil layer approximately 7cm over the permeable bedrock. The mixing effect and preferential flow might also play a role in soil water percolation. In general, the evaporation effect on soil water was weak except for the uppermost 10cm soil matrix water during winter. The lower limits of mean transit time of soil matrix flow passing through 5, 15, 25, 35, and 41.5cm depths were 4.81, 7.70, 16.19, 21.85, and 27.44days, respectively. Our study demonstrated the crucial functions of the soil reservoir in regulating the water cycle and could provide guidance on conservation of soil water and hydrological studies. The applied method was proved to be a suitable approach for investigating soil water movement on a monthly scale.

Micro area transportation of residues: A style forming the red weathering crusts of carbonate rocks

Owing to differential erosion, the red weathering crusts of carbonate rocks usually show special profile structures that are characteristic of thick-bedded, intensely weathered profiles and strongly undulate basement rock surface (i.e. alternate distribution of solution grooves and stone teeth). In this work a typical red weathering crust of dolomite rock (i.e. Pingba profile) is selected in a karst terrace in Guizhou Province, China. Via field geology, geochemistry, mineralogy, grain size analyses, and in comparison with two neighboring limestone soils (i.e. Luoli profile and Longdongbao profile) as well, we discussed the formation process of the red weathering crusts of carbonate rocks, and demonstrated that micro-area transportation of acid-insoluble residues of carbonate rocks is a style of forming the red weathering crusts, especially the thick-bedded those of carbonate rocks. The weathering crusts at solution grooves, at the scope of the depth upwards from weathering front, are in-situ weathering residues (i.e. residual layers); over this depth upwards, are accumulated deposits of different weathering intensity (i.e. accumulation layers) from upper adjacent stone teeth situation by micro-area transportation. This process is a direct cause leading to intense undulation of weathering indices with the depth. The latter generally compose the principal part of the profiles. At solution grooves, the chronostratigraphy of the red weathering crusts of carbonate rocks shows that, at residual layers, upwards from weathering front, the weathering age is from new to old; at accumulation layers, from bottom to top, the weathering age is from old to new. The weathering front is a main place of geochemical reaction, and at this limited-thick interface, with quick dissolution of carbonates, acid-insoluble residues begin to obviously decompose. However, ensuing evolvement of weathering crusts is very slow. When they is up to gravity balance (i.e. flat surface and no micro-area transportation), under the action of weathering from exterior to interior, at the scope of the depth from the surface downwards, the weathering profiles are gradually reconstructed and take on the developing feature of normal weathering crusts.