Analysis of development trend of karst carbon cycle and carbon sink effect based on Web of Science
Karstification refers to the chemical dissolution of carbonate rocks by water and the deposition of calcium carbonate due to CO2 degassing. In a macro sense, karstification can be divided into epikarstification and deep-seated (geothermal) karstification. Karst covers about 22 million km2 all over the world, accounting for 15% of the land area, and is of enormous carbon sink potential. Therefore, to alleviate the pressure of carbon neutrality and help China achieve the "Dual Carbon" target, it is of great significance for us to accurately understand the relationship between karst and carbon cycle and the carbon sink effect of karst. As an important tool for us to comprehensively understand the research progress in a specific area, bibliometric analysis can measure the relationship and influence among publications through a series of mathematical and statistical tools, and hence has been applied in many research fields. In order to understand the research hotspots and the development directions in the field of karst carbon cycle and carbon sink effect, this study undertook bibliometric citation analysis on 573 contributions to the literature written from January the 1st, 1991 to June the 6th, 2017, based on the Web of Science core collection. CiteSpace was used to analyze keyword frequency and emergency, country (region) and institution, author impact, research hot spot, co-cited literature and evolution of research topics. In terms of the number of papers published every year, there were fewer papers published before 2007, with a slow increase from 2007 to 2014 and a significant increase after 2015. The analysis of papers published by national (region) found that the research on karst carbon cycle and carbon sink effect presented distinct regional characteristics, which was mainly led by China, followed by the United States, Europe and other regions. Cluster analysis on the top 100 keywords in 573 papers by CiteSpace software generated 11 clusters which could be summarized and integrated into 5 main clusters: 1) the carbon cycle and carbon sink of chemical weathering of carbonate rocks; 2) the carbon cycle and hydrochemical characteristics in karst water; 3) the soil carbon cycle and carbon sink in karst regions; 4) the impact of ecosystem services on karst carbon cycle and carbon sink; 5) the technology of stabilizing carbon isotopes. The analysis of authors and cited references found that the research team led by Liu Zaihua has inherited and developed the viewpoint put forward by Yuan Daoxian that karstification participates in the global carbon cycle and has a carbon sink effect. Besides, Liu Zaihua team proposed a coupled carbonate weathering model, which greatly promoted the research of karst carbon sink. The analysis of references with the strongest citation bursts shows that citations about coupled carbonate weathering model, impact of karst rocky desertification, climate change and change of land use, effects of sulfuric acid as well as ecological engineering on karst carbon sink have attracted widespread attention in a short term and can represent cutting-edge research hot spots. The time line map and the keyword time zone map of keyword clustering can clearly show the research development in the field of karst carbon cycle and carbon sink in three stages: 1) the initial stage from 1997 to 2006, in which relevant studies show that the global amount of atmospheric CO2 uptake from weathering of carbonate rocks was estimated at 0.11–0.608 Gt C·a−1, accounting for 15%–30% of global carbon leakage; 2) the stage of rapid development in research on the chemical weathering of carbonate rocks from 2007 to 2014, in which studies mainly focused on the impact of exogenous acids, runoff conditions, and vegetation cover on the weathering of carbonate rocks in different watersheds; 3) the new stage of research on carbon cycle and carbon sink after 2015, in which studies have been conducted under the guidance of karst critical zone theory based on the atmosphere–biology–soil–water–bedrock system, and they mainly focus on the effect of aquatic photosynthesis, considering the impact of climate change and land-use change. In the future, the research of carbon cycle and carbon sink in karst critical zones should be carried out under the guidance of earth system science theory. At a micro level, studies should focus on the catalysis of carbonic anhydrase in soil, microbial carbon sequestration, photosynthesis of aquatic plants and other biological carbon pumps to improve the potential of karst carbon sink. At a macro level, studies should pay attention to the impact factors such as climate change, change of land use, and change of ecosystem service functions under human activities.
Spatiotemporal patterns of CO2 efflux fluxes from the outflow of karst underground river: A case study of the Panyang river in Bama, Guangxi
Karst basins are key regions for rock weathering and carbon sinks. Because water bodies in karst basins contain high concentrations of dissolved inorganic carbon (DIC), and can affect carbon cycle through both lateral migration of DIC and vertical efflux of CO2, they become research hotspots for CO2 efflux. Although there have been many studies on CO2 efflux fluxes in karst rivers, our understanding is still limited in terms of the spatiotemporal variations in CO2 efflux fluxes of water bodies from the outflow of karst underground rivers. In order to explore the spatiotemporal patterns of partial pressure of carbon dioxide (pCO2) and CO2 efflux flux from the outflow of karst underground rivers, this study focused on the Panyang river in Bama in the karst area of Southwest China. Monthly routine sampling of surface water and groundwater as well as sampling based on rainfall events was conducted from July 2022 to April 2023. The spatiotemporal variations of pH, alkalinity, total dissolved solids (TDS), DIC, dissolved organic carbon (DOC) and pCO2 in both the surface water and underground water were explored. The relationships between DIC, DOC and pCO2, as well as the relationships between TDS, alkalinity, and DIC, were also analyzed. The controlling factors of pCO2 were discussed, and the CO2 efflux flux was estimated. The results showed that alkalinity, TDS, DIC and pCO2 of groundwater in the basin were significantly higher than those of surface water, indicating that karst carbonate weathering released a large amount of DIC into groundwater, and the outflow of groundwater resulted in the CO2 efflux, reducing the DIC content and pCO2 of surface water. There was no significant difference in DIC concentrations and pCO2 of surface water from upstream to downstream, indicating that groundwater could release CO2 into the atmosphere in a short time and quickly reached equilibrium. During the rainy season, DIC and pCO2 of groundwater showed an increasing trend from upstream to downstream, while during the dry season, they showed a decreasing trend, indicating that rock weathering during groundwater recharge is an important source of inorganic carbon in groundwater. Alkalinity, TDS, DIC, pCO2 and CO2 efflux flux of surface water and groundwater during the dry season were significantly higher than those during the rainy season, mainly due to the dilution effect of rainwater during the rainy season. In addition, there were high DOC concentrations during the dry season, and the mineralization of DOC contributed directly to CO2 production, which also led to higher pCO2 in the dry season compared to the rainy season. Overall, there were no significant differences in pH, alkalinity, TDS, DIC, DOC and pCO2 of surface water and groundwater under rainfall events, possibly due to insufficient rainfall or its short duration. However, several samples during rainfall events showed significantly higher pCO2 than in the regular rainy seasons, because the continuous heavy rainfall likely raised the water level of underground rivers and supplied high CO2 concentrations from groundwater to surface rivers. There was no significant correlation between DOC and pCO2, possibly because the carbon input from other sources disrupted the coupling relationship between DOC and pCO2. These sources include soil CO2, organic carbon synthesized by CO2 that was absorbed by photosynthesis of plants, and human activities. A large amount of carbonate and other salt-based ions released from carbonate rocks into groundwater through dissolution increased DIC content, TDS and alkalinity, which contributed to a significantly positive correlation between DIC and indicators such as alkalinity and TDS. The CO2 efflux flux of surface water in the Panyang river basin during the dry season (2.05 ± 1.89 kg C m−2 year−1) was significantly higher than that during the rainy season (0.40 ± 0.30 kg C m−2 year−1), and the CO2 efflux flux of groundwater during the dry season (4.72 ± 4.15 kg C m−2 year−1) was about 4.6 times higher than that during the rainy season (1.03 ± 0.74 kg C m−2 year−1). During the study period, the CO2 efflux fluxes of surface water and groundwater in the Panyang river basin ranged from -0.10 to 9.20 kg C m−2 year−1 and -0.12 to 17.28 kg C m−2 year−1, with average CO2 efflux fluxes of 1.06 ± 1.46 kg C m−2 year−1 and 2.40 ± 3.14 kg C m−2 year−1, respectively, which were much higher than the average CO2 efflux fluxes of major global river basins (0.64 kg C m−2 year−1). Understanding the CO2 efflux flux and its spatiotemporal variations in karst basins is of great significance for us to accurately assess the carbon budget of rivers and to evaluate the role of rock weathering as a carbon sink.
Rock weathering and carbon sink effects under exogenous acid action: A case study of the Yanggong river
Rock weathering in the river basin is not only an essential carbon source and sink mechanism, but also an important link in the global carbon cycle. Rivers are indispensable components of water bodies, and the hydrochemistry of rivers is a representation of the degree to which weathering and denudation occur at the surface of the river basin. Therefore, it is possible for us to collect information on rock weathering in the river basin by conducting an analysis of the chemical compositions of rivers. In turn, the analytical results can be used for the estimation of weathering rates and the amount of carbon dioxide that is consumed by the Earth's atmosphere. The presence of exogenous acids in the process of rock weathering in the river basin has an impact on the geochemical cycling of carbon as well as the carbon source/sink effect. The chemical weathering rate of carbonatite is accelerated by sulfuric acid, but the weathering does not consume atmospheric carbon dioxide. As a result, the effect of sulfuric acid on carbonatite weathering should be taken into consideration when the amount of atmospheric carbon dioxide that is consumed by rock weathering in the basin is calculated. The Yanggong river is a part of the Jinsha river system located in the upper reaches of the Yangtze river. The process of rock weathering and the influence of carbon sinks in the Yanggong river basin are not yet fully understood. As a result of climate warming, the considerable increase in water output from the high-altitude area of this basin has accelerated the water cycle there. This will undoubtedly exert a strong influence on rock weathering rates and geochemical cycling processes that occur within the Yanggong river basin. In addition, a large number of coal layers are distributed in this basin, so the sulfuric acid produced by sulfide oxidation or the dissolution of carbonate rocks by sulfuric acid caused by human activities would also alter rock weathering rates in this basin. For this reason, it is necessary for us to do more research in order to quantify the effect of exogenous acids on rock weathering as well as on the carbon source and sink in the Yanggong river basin. In this study, water samples from main streams and major tributaries of the Yanggong river were collected during the dry and rainy seasons of 2023. The major concentrations of anion and cation, metasilicic acid, and total dissolved solids (TDS) in these water samples were examined. Additionally, different types of rock weathering in the Yanggong river basin were analyzed by the water chemical equilibrium method and the Galy estimation model. Finally, the amount of carbon dioxide that was consumed by rock weathering under the combined effect of carbonic acid and sulfuric acid was estimated. The findings indicated that the ionic compositions of the water system in the Yanggong river basin were mostly derived from the weathering of silicate and carbonate rocks, and the hydrochemical types were either the HCO3-Ca type or the HCO3-Ca·Mg type. Sulfuric acid and carbonic acid worked together to contribute to the process of rock weathering in this basin. The atmospheric CO2 consumption of rock weathering in this basin was 38.35 t CO2·km−2·a−1 when sulfuric acid was not taken into consideration. However, when sulfuric acid participation was taken into consideration, the carbon sink of rock weathering was reduced to 25.54 t CO2·km−2·a−1, with a reduction of approximate 33%, which significantly improved the accuracy of the calculation. The atmospheric CO2 flux consumed by carbonatite weathering contributed 95.5% of the total, which indicated dominance of the atmospheric CO2 flux consumed by carbonatite weathering in the Yanggong river basin. The quantity of atmospheric CO2 consumed by rock weathering in the Yanggong river basin is 4.27×104 t CO2·a−1, suggesting a process of carbon sink. That sulfuric acid participated in the process of rock weathering in the river basin changed the regional carbon cycle is a significant link that cannot be overlooked in the model of the global carbon cycle. Meanwhile, in studies on the carbon source/sink effect of rock weathering in the river basin, it was essential for us to consider the regional geological background, particularly the types of minerals rich in sulfides.
Estimation of the carbon sink of rock weathering by remote sensing and analysis of its spatiotemporal variations
As a type of natural carbon sink, rock weathering plays a critical role in the global carbon cycle by storing atmospheric carbon dioxide (CO2). This process is particularly significant in mitigating climate change, although its contributions are often underestimated or overlooked in broader carbon calculation practices. The present study focuses on Guizhou Province, China, a region that is characterized by extensive karst landforms. These landforms are of particular interest because they are highly effective in capturing atmospheric CO2 through rock weathering. This study aims to explore the spatiotemporal dynamics of the carbon sink of rock weathering from 2001 to 2020. This study integrates various data sources, including remote sensing data, meteorological records, and lithological information, to estimate the carbon sink capacity with the GEM-CO2 model. This study also employs advanced analytical techniques such as Dynamic Time Warping (DTW) and statistical methods to analyze the spatiotemporal evolution of carbon sink. The findings of this study reveal that the rock type is a primary factor influencing the rate of rock weathering and CO2 consumption, followed closely by annual precipitation. The temperature also plays a significant role, although the responses of its effects are observed to be lagged. This indicates that changes in temperature may affect CO2 absorption rates several years after the initial temperature fluctuation occurred. This study identifies that the regions with the highest CO2 consumption through rock weathering are predominantly concentrated in the northeastern, southwestern, southern, and southeastern parts of Guizhou Province. These areas are characterized by widespread formations of carbonate rocks and higher precipitation levels, which can jointly enhance the weathering process and increase carbon sequestration. In contrast, the northwestern regions, which are dominated by silicate rocks and receive lower levels of precipitation, exhibit the lowest levels of CO2 consumption. This discrepancy underscores the importance of both lithological composition and climatic conditions in determining the effectiveness of natural carbon sink. From 2001 to 2020, the annual average karst carbon sink in Guizhou Province ranged from 0 to 1.04×103 t C·km−2·a−1. Although there was a general trend of fluctuation, the overall pattern showed an increase in carbon sequestration capacity. However, the analysis did not reveal any significant single trend over the two decades. This lack of a clear trend suggests a complex interplay between geological and climatic factors that influence carbon sequestration in karst landforms. The variability in carbon sink capacity observed in this study highlights the sensitivity of natural carbon sink to the changes in environmental conditions, particularly in precipitation and temperature. The spatial distribution of carbon sink closely mirrors the distribution of carbonate rocks in Guizhou Province. This correlation emphasizes the critical role that carbonate rocks play in the global carbon cycle due to their high solubility, which can accelerate the process of CO2 absorption. Areas with more annual precipitation were found to have a greater capacity for carbon sequestration, and this result reinforces the importance of hydrological factors in the weathering process. This finding is particularly relevant for the regions that are expected to experience changes in precipitation patterns due to climate change, as it suggests that shifts in hydrological conditions could have a significant impact on the efficacy of natural carbon sink. In addition to these findings, this study also highlights the importance of both geological formations and climatic conditions when the carbon sequestration potential of different regions are estimated. The application of the GEM-CO2 model in this study provides a robust framework for estimating the carbon sink at a regional scale. The effectiveness of this model in this context offers critical data that can be used to guide the development of carbon trading mechanisms and environmental policies aimed at enhancing the natural carbon sink. By integrating geological and climatic data, this model allows a more nuanced understanding of the factors that contribute to the carbon sequestration in karst landforms. The insights gained from this study are invaluable for informing carbon management strategies, particularly in regions with similar geological and climatic conditions. The findings of this study suggest that the carbon sequestration through rock weathering could be a viable component of mitigation efforts for climate change in a wider range. However, the fluctuating nature of carbon sink over the study period indicates that the natural carbon sink is highly sensitive to changes in environmental conditions. This sensitivity underscores the need for adaptive management strategies that can respond to changes in climate and ensure the continued effectiveness of natural carbon sink. Furthermore, this study lays the groundwork for future research to explore the further implications of rock weathering in global carbon cycles. It advocates a more integrated approach that considers both natural and human factors of mitigating climate change. As the climate change continues altering global weather patterns, understanding the role of natural processes like rock weathering in the carbon cycle will be increasingly important for us to develop effective strategies to manage and mitigate the impacts of climate change.
Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes /issues, but are citable by Digital Object Identifier (DOI).
2024, 43(4): 766-779, 809.
doi: 10.11932/karst20240403
2024, 43(4): 810-821, 853.
doi: 10.11932/karst2024y023
Founded in 1982 bimonthly
ISSN: 1001-4810
CN 45-1157/P
Editor-in-chief: Jiang Zhongcheng
Sponsor: Chinese Academy of Geological Sciences
Sponsored by: Institute of Karst Geology, Chinese Academy of Geological Sciences
Cosponsor: International Research Center on Karst under the Auspices of UNESCOKarst Geology Committee of Geological Society of ChinaSpeleology Committee of Geological Society of China
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