Estimation of karst carbon sink and its contribution to CO₂ emissions over a decade using remote sensing imagery

Highlights

• Karst carbon sink is estimated by combining remotely sensed imagery and GEM-CO₂.

• The volume of karst carbon sink displays a “wavy pattern” within a short term.

• But a straight line at an increasing rate of approximately 8.7% for a long term.

• The increase in atmospheric CO₂ content reversely fosters karst carbon sink.

Abstract

Research on the carbon source and sink imbalance in global carbon cycle has demonstrated that the mass of CO₂ consumed by rock weathering is a portion of the “missing carbon sink.” To further understand the karst carbon sink contributions to atmospheric CO₂ emissions, the present study establishes the relationship between the karst carbon sinks and the atmospheric CO₂ emissions. We first estimated the mass of CO₂ consumed by rock weathering using the Global Erosion Model for CO₂ fluxes (GEM-CO₂), based on the distribution of different types of rock in Guangxi, China. The rocks were classified using remote sensing imagery, and we estimated the relationship between the mass of CO₂ consumed by rock weathering and the CO₂ emitted into atmosphere from 2003 to 2012. The analyses reveal that (1) the mass of CO₂ consumed by rock weathering could be rapidly estimated using remote sensing imagery, as it not only considers different types of rocks but also simultaneously considers the geological structure and meteorological data, such as rainfall, temperature, and vegetation coverage; (2) the mass of CO₂ consumed by rock weathering from 2003 to 2012 displays a “wavy pattern”, which might have been primarily caused by the annual variations in rainfall, whose tendency could be fitted by a straight line at an average annual increasing rate of approximately 8.7%; (3) the mass of emitted CO₂ from 2003 to 2012 in Guangxi display a “straight line” at an average annual increasing rate of approximately 12.7%; (4) the mass of CO₂ consumed by rock weathering is positively correlated with the mass of emitted CO₂ with a correlation coefficient of R² = 0.62; this implies that an increasing concentration of CO₂ in the atmosphere reversely fosters the CO₂ consumption by rock weathering; (5) due to different increasing rates, annually 7 × 10⁶ t of CO₂ is accumulated in the atmosphere in Guangxi, and a total of 506.42 × 10⁶ t of CO₂ is accumulated over a decade; (6) the mass of the karst carbon sinks in Guangxi, China, and globally are estimated to be 1.01 × 10⁶ tC·a⁻¹, 1.49 × 10⁷ tC·a⁻¹, and 0.95 × 10⁸ tC·a⁻¹, respectively. Therefore, the results of this study not only help to understand the relationship between karst carbon sink and the global carbon cycle, but also provide an important basis for predicting the variations in CO₂ in the atmosphere and climate change, which could further provide a scientific basis for the rational use of resources and the formulation of environmental protection policies.

Introduction

Discussions about the relationship between global climate change and the increase in atmospheric greenhouse gas concentrations have attracted public attention to Earth's carbon cycle (Zachos et al., 2001; Solomon et al., 2009; Yuan, 2011). Understanding global climate change and developing strategies for the sustainable use of environmental resources are major scientific and political challenges in the present scenario (Hese et al., 2005). Therefore, it is crucial to understand the sources and sinks of carbon dioxide (CO₂) and their spatial and temporal distribution (Scholze et al., 2016). The terrestrial carbon cycle plays a key role in regulating the accumulation of carbon in the atmosphere (Le Quere et al., 2009), and recent studies have shown that the strength of the terrestrial sink is growing (Le Quere et al., 2009; Sitch et al., 2013). Researchers are exploring ways to quantify the terrestrial carbon sink (Guo et al., 2011). It is estimated that the average mass of CO₂ emitted per year into the atmosphere is approximately 7.1 PgC (5.5 ± 0.5 PgC from combustion of fossil fuels and 1.6 ± 0.7 PgC from changes in land use; 1 PgC = 10¹⁵ gC) (Houghton et al., 2012), of which approximately a half (approximately 3.3 ± 0.2 PgC) remains in the atmosphere and a one-third (approximately 2.7 ± 0.6 PgC) is absorbed by the ocean (Siegenthaler and Sarmiento, 1993; Yu et al., 2014). This indicates that approximately 1.8 PgC is an undiscovered residual terrestrial carbon sink, which is called the “Missing Sink” (Houghton et al., 1998; Qu et al., 2004).

In the early 1990s, scientific studies of carbon sinks were primarily focused on how the atmosphere, ocean, and soil contribute to and/or absorb CO₂, and the relationships between those domains. After the 1990s, research became more integrated, with an emphasis on the associated impacts of climate, hydrology, and geology of rock weathering (Zhang, 2011). Most of the studies of the global carbon cycle demonstrated that an unknown carbon sink potentially exist in the continental biosphere. Tans et al. (1990) first proposed that the “missing sink” might occur in a land system, such as the karst systems in the Northern Hemisphere, because karstic dissolution processes consume CO₂ from the atmosphere in a three-phase unbalanced open system of gas, liquid, and solid phases (CO₂-H₂O-CO₃^2-). In addition, this sink would reduce the excess carbon in the biosphere–atmosphere exchanges. Karstic dissolution processes are part of this phenomenon as they consume atmospheric or pedologic carbon (Gombert, 2002). Therefore, the wide global distribution of carbonate rocks and their strong sensitivity to climate change could mean that carbonate rock dissolution by consumption of air/soil CO₂ could play an important role in the global carbon cycle (Cao et al., 2011). The potential role of carbonate weathering might play a role in the evolution of global carbon cycle over the century. At the same time, karst-related carbon sink could quickly respond to future climate change (Zeng et al., 2016, 2019). With decades of study, it is now a consensus that karst areas are one of the major carbon sinks (hereafter called karst carbon sinks) on a global scale.

The karst area in China is approximately 3.44 million km², and in Guangxi they cover approximately 0.234 million km², which accounts for approximately 6.75% of the total karst area in China. Therefore, studying rock weathering in Guangxi could help in understanding the consumption of atmospheric CO₂ in China and on a global scale, particularly, the contributions of the karst carbon sink to atmospheric CO₂ emissions.

Although the quantitative estimation of the karst carbon sink (the CO₂ content consumed by rock weathering is multiplied by $M_C/M_{C{O}_2}=0.27$; M_C = 12 is the relative mass of carbon and $M_{C{O}_2}=44$ is the relative molecular mass of CO₂) has been investigated for more than 20 years, a few questions remain unanswered. This is due to the difficulty in detecting the interaction between CO₂ in the atmosphere and the exposed rock. Therefore, the methods for estimating the atmospheric CO₂ consumed by rock weathering can be categorized as follows.

• Tablet test method. This method was used in the IGCP299 project “Geology, Climate, Hydrology and Karst Formation” to reveal the relationship between the intensity of karst sink and a number of conditions, such as topography, geology, climate, hydrology, and vegetation (Gams, 1985; Yuan, 1997; Jiang et al., 2011; Zhang, 2011). Cao et al. (2011) applied the tablet test method to estimate the carbon sink flux of rock weathering. This is an effective method to obtain scientific data of the study area, establish a model for rate of rock dissolution, and calculate the flux of atmospheric CO₂ consumed by rock weathering.

• Hydrogeochemistry method. Hydrogeochemistry Method calculates the total amount of ions carried by rivers by measuring the concentration of solutes (e.g., $HC{O}_3^{-}$, $K^{+}$, $N{\text{a}}^{+}$, and ${\text{Ca}}^{2+}$ plasma) in the outflow of springs or basins and the runoff of springs or basins. The rate of various types of rock weathering is then calculated based on the distribution of rocks in the basin, and the mass of carbon consumed by weathering is estimated. Hydrogeochemical methods mainly include: the river chemistry method, solute load method, GEM-CO₂, and SiB algorithm.

  • River chemistry method: The River chemistry method follows two main and complementary methods (Mackenzie and Garrels, 1971): It mainly studies the formation, distribution, spatiotemporal variation and influencing factors of river water salinity, pH and main ions, and it Conducts a small-scale study of a rock type of river under a given climatic condition (Amiotte and Probst, 1993; Bluth and Kump, 1994; White and Blum, 1995; Gislason et al., 1996; Louvat and Allegre, 1997), or uses the world's largest river for more comprehensive research (Holland, 1978; Meybeck, 1979, 1987; Amiotte and Probst, 1995).

  • Solute load method: Carbonate rock weathering has a significant correlation with runoff. Larger the runoff, more the rate of carbonate rock weathering. When based on accurate hydrological data and runoff data, the total volume of $HC{O}_3^{-}$ in karst water can be estimated accurately by the solute load method (Corbel, 1959; Liu and Zhao, 2000).

  • GEM-CO₂ model: The GEM-CO₂ model represents the linear relationship between the atmospheric CO₂ consumed by rock weathering and runoff (Amiotte and Probst, 1995). The model was analyzed the relationship between surface runoff and the main dissolved element contents in 232 single lithology basins in France (Meybeck, 1987), and it was summarized a series of empirical relationships and established the formula by assigning the empirical coefficients.

  • SiB: The algorithm for Use of Mole balances for the Assessment of Mineral Weathering Rates (the SiB Model) (Pacheco and Van der Weijden, 2002) estimates the amount of atmospheric CO₂ consumed by rock weathering based on studying the relationship between the rock dissolution rate and runoff volume (Pacheco and Van der Weijden, 1996).

• Dynamical method: The dynamical method obtains the data of reaction rate, activation energy, and pre-exponential factors by analyzing the relationship between the concentration of reactants or products in karst processes and the spending time.

  • DBL (Theoretical Model of Diffusion Boundary Layer): Derybrodt first presented the DBL model to calculate the dissolution rate of calcite in a flowing CO₂-H₂O solution (Dreybrodt and Buhmann, 1991).

  • PWP: Plummer et al. (1978) studied water and rock reactions on calcite surfaces using free-drift and stable pH methods, and found that the water-rock boundary layer will form a water film, the thickness of which is affected by temperature, partial pressure of CO₂, and liquid composition. The authors then presented three basic reaction formulas of the water-rock boundary layer (Qian et al., 2010).

• Other methods: The accuracy of estimations of the flux of atmospheric CO₂ consumed by rock weathering can be improved by adding impact factors or collecting more basic data. Therefore, many scientists have established various models to estimate the flux of the atmospheric CO₂ consumption, such as models that include the factors of rainfall, runoff, dissolution rate, or soil respiration. Sweeting (1980) presented an estimation model related to rainfall. White and Blum (1995), YoshimuraK (1997), Liu and Zhao (2000) presented an estimation model related to runoff. The regression equation of the dissolution rate including the four factors was established by Yao (2003). Zeng et al. (2016, 2019) adopted an improved maximum potential dissolution method (MPD) and discussed the impact of climate change on the karst carbon sinks in southwestern China, and the sensitivity of global carbonate weathering carbon sink flux to climate and land-use changes.

The tablet test method is suitable for estimating regional karst carbon sinks in areas having the same geological conditions. Due to the discontinuity and the special spatial heterogeneity of karst soil, the tablet test method is representative and cannot be applied to other karst regions. The dynamical method is suitable for rock weathering microcosmic studies. However, it is not conducive to large-scale or global-scale research. The hydrogeochemistry method estimates the mass of atmospheric CO₂ consumed by rock weathering in large-scale areas. However, this model does not consider the influence of various factors (e.g., topography, landform, vegetation, and soil) on the rock dissolution rate. These models consider the influence of only one or two factors but ignore the influence of other factors.

The GEM-CO₂ model considers not only the mass of atmospheric CO₂ consumed by carbonate rock weathering, but also the mass of atmospheric CO₂ consumed by silicate rock weathering. More importantly, it estimates the mass of atmospheric CO₂ consumed by rock weathering in large-scale areas. Therefore, in the present study, karst carbon sink in the Guangxi Province was estimated using the GEM-CO₂ model, combined with remote sensing technology. Its contribution to CO₂ emission over a decade was also evaluated. The results show that the CO₂ consumed by rock weathering is involved in the global carbon cycle; it can reflect the climate change, and also help researchers understand the carbon cycle in the global terrestrial ecosystem.