Figure The location of the study areas in Guizhou Province, China (modified after Zhao et al., 2010). (Image by IGCAS)

As global warming becomes more and more serious due to the increase of greenhouse gases, especially CO₂, the global carbon cycle research has attracted increasing attention. Karst processes-related carbon cycle, as a key part of the global carbon cycle and a result of the water-carbonate rock-CO₂ gas-aquatic organism interaction, significantly affects the global carbon budget. Analyzing the relevant karst hydrochemical changes could not only enhance the understanding of the karst processes but also reveal the driving forces and intensity of the karst processes-related carbon cycle.

Prof. LIU Zaihua’s group from the State Key Laboratory of Environmental Geochemistry (SKLEG) at the Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) collected monthly hydrochemical data and stable carbon isotope composition (δ¹³C) of dissolved inorganic carbon (DIC) in karst water samples from September 2007 to October 2012 to study the controlling mechanisms on DIC geochemistry and δ¹³C_DIC under different conditions of climate and land cover in three karst catchments: Banzhai, Dengzhanhe and Chenqi, in Guizhou Province, SW China.

The research finds that the root respiration and organic carbon decomposition with weak carbonate bedrock dissolution result in the lowest δ¹³C_DIC, low pCO₂ and EC, low cation and anion concentrations at Banzhai where there is dense vegetation coverage but thin soil layer with less land use activity. Chenqi has large soil CO₂ input due to a thick soil layer and strong farming activity, and very weak carbonate bedrock dissolution because of a short groundwater flow path and steep surface slope; consequently, the karst water has low pH and δ¹³C_DIC, high pCO₂ and low SIc. At Dengzhanhe, strong carbonate and gypsum bedrock dissolution and lesser soil CO₂ input due to more karst rocky desertification contribute to heavier δ¹³C_DIC, high EC and high cation and anion concentrations.

In addition, there are clear seasonal cycles observed in hydrochemical parameters and δ¹³C_DIC in the karst waters of the study areas, with high pCO₂ and low pH, EC, SIc and δ¹³C_DIC in the warm and rainy seasons, and vice versa during the cold and dry seasons.

Furthermore, a strongly positive shift (>3‰) in δ¹³C_DIC occurred in the drought year, 2011, indicating that δ¹³C_DIC in groundwater systems can be an effective indicator of environmental and/or climate changes. The δ¹³C records of travertine deposited from these karst waters may permit the reconstruction of high-resolution records of paleoclimate change and extreme events.

The study results have been published in Geochimica et Cosmochimica Acta.

Contacts:
LIU Zaihua
State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry
E-mail: liuzaihua@vip.gyig.ac.cn 

LI Hong-Chun
Department of Geosciences, National Taiwan University
E-mail: hcli1960@ntu.edu.tw