Analysis of carbondioxide (CO2) sequestration capacity in Berambai Cave, Samarinda, East Kalimantan, Indonesia

Climate change is a result of global warming triggered by human activities, especially those related to the use of fossil fuels and land use change. These activities increasingly produce gases in the atmosphere, especially carbon dioxide (CO2), which causes the Earth’s temperature to rise. As CO2 continues to increase in the atmosphere, CCS (Carbon Capture and Storage) can be one of the solutions to store carbon dioxide in the long term on the earth’s surface. Karst ecosystems consisting of limestone and dolomite are one of the potential carbon stores. The purpose of this study is to estimate the carbon dioxide sequestration capacity of Berambai Cave as an effort to reduce carbon dioxide emissions in Samarinda. Administratively, this research location is in Berambai Village, North Samarinda District, East Kalimantan Province. The existence of the Pulau Balang Formation which has limestone successions in the study area has undergone a karstification process. Karstification is a natural process that can store carbon dioxide through chemical reactions. To determine the carbon dioxide sequestration capacity of Berambai Cave, the alkalinity titration method was carried out using underground rivers and secondary data analysis from BMKG Samarinda. Berambai Village has an annual rainfall of 2,705 mm/year and an average temperature of 27.48°C. Evapotranspiration is 1,383.14 mm/year and runoff are 13.21 dm. The CaCO3 concentration in Berambai spring is 140 mg/L. The carbon dioxide sequestration capacity of Berambai Cave is 73.97 m3/year/km2 contributing to reducing atmospheric carbon emissions, especially in the Samarinda area.


Introduction
Historically, carbon dioxide (CO2) has not been a pollutant monitored by the Environmental Protection Agency for public health reasons.Some of it is naturally present in the atmosphere.Carbon dioxide is essential to the life cycle of plants, and is not harmful to humans, when in moderate concentrations.However, the level of carbon dioxide in the atmosphere has increased by more than 40% in the last half century, raising concerns about the resulting climate change.Because carbon dioxide (CO2) is one of the greenhouse gases that contribute to global warming.Global warming is a global phenomenon triggered by human activities, especially those related to the use of fossil fuels and activities that change land use.In recent years, there has been increasing interest in carbon capture and storage (CCS) methods, with natural carbon sequestration being one widely used approach [1].Efforts to reduce carbon emissions have been researched and proposed by experts through the estimation of the global carbon cycle.The calculation of global carbon estimation is intended to identify the potential for carbon dioxide absorption to balance the value of existing carbon dioxide emissions [2].Carbon sequestration potential on land and oceans is needed to offset increasing carbon dioxide (CO2) emissions, karst ecosystems are one of the potential carbon sinks.The karst landscape system, which is mostly composed of limestone and dolomite, functions to absorb carbon dioxide (CO2) [3].Carbon sequestered in karst landscapes is known as inorganic carbon.Inorganic carbon is sequestered through the process of karst landscape formation called karstification [2].The source of greenhouse carbon dioxide (CO2) sequestration in the atmosphere has a close relationship with the karstification process [4].The process of carbon dioxide (CO2) absorption in karst areas occurs during dissolution of limestone (karstification).The karstification process can occur in areas with a wet tropical climate with high rainfall and conducive temperatures and dense vegetation cover.The interaction between carbon dioxide (CO2), water (H2O), and carbonate rock (CaCO3) is known as the karst dynamic system (KDS).This process begins with the dissolution of carbon dioxide (CO2) in water to form H2CO3. H2CO3 solution has unstable properties, so it breaks down into HCO3 2-and H + .This H + ion will then decompose limestone (CaCO3) into Ca 2+ and HCO3 - [5].
Equations ( 1) for limestone and (2) for dolomite show that 1 or 2 moles of CO2 will be required, when 1 mole of CaCO3, or CaMg (CO3)2 is dissolved.Although the carbon dioxide (CO2) required for rock weathering does not directly come from the atmosphere (some of it comes from the soil), the carbon uptake process during rock weathering can still be considered as a sink for atmospheric carbon dioxide (CO 2 ) because the carbon dioxide (CO 2 ) released from the soil to the atmosphere will decrease along with the carbon uptake process during rock weathering [6].Indonesia has a large karst landscape.Indonesia's karst landscape covers 140,000 square kilometers.Indonesia's karst landscapes are spread throughout the Indonesian archipelago.The characteristic of karst landscapes in Indonesia is high rainfall and covered by forest ecosystems.Rainfall in Indonesia ranges from 2,000 to 3,000 mm/year which can accelerate the dissolution of karst landscapes.In addition, forest cover in karst landscapes can increase soil carbon dioxide concentrations, making water more effective in dissolving carbonate rocks [2].There has not been much research on carbon dioxide sequestration in karst areas in Indonesia.So, it requires further research to determine the level of carbon absorption in each karst landform to offset the increase in carbon dioxide (CO2) emissions in Indonesia.Therefore, this research focuses on estimating the carbon dioxide sequestration capacity of Berambai Cave even though it is not included in karst due to its small area.However, Berambai Cave undergoes a karstification process which indicates dissolution that absorbs carbon dioxide (CO2).

Methods
The location of this study Figure 1 is at coordinates (0°19'55.0"S -117°12'07.8"E).The research area is included in the Kutai basin which is in the east of Kalimantan Island and is in the Pulau Balang formation which is aged from Early Miocene to Middle Miocene.More precisely, Berambai Cave is in North Samarinda District, Samarinda City, East Kalimantan Province.The journey to the research location has a distance of 20 km and can be reached in approximately 45 minutes using a vehicle from the city of Samarinda and needs to walk with an estimated time of 45 minutes to get to Berambai Cave.Berambai Cave has a high topography because it is located along the Samarinda anticlinorium, with a tropical climate with an average temperature of 27.5˚ C and high average rainfall.The method used in this research is to measure the concentration of CaCO3 dissolved in the Berambai Cave underground river with alkalinity testing.
This research was conducted at Berambai Cave using primary and secondary data.All secondary data were obtained from the Samarinda weather observation station, namely the meteorological, climatological, and geophysical agency (BMKG).Secondary data in the form of rainfall data, temperature, evapotranspiration, and runoff.Primary data was obtained from field observations by sampling and measuring the discharge at the Berambai Cave underground river spring.Annual rainfall data was obtained from three rain observation stations owned by meteorological, climatological, and geophysical agency (BMKG) Samarinda Sungai Pinang Station, Lempake Station, and Sungai Siring Station.After that, the annual rainfall data was processed using Arcgis with the Isohyet method to determine regional rainfall.Temperature data is also obtained from meteorological, climatological, and geophysical agency (BMKG) stations and processed using the Mock method, due to the absence of temperature data at the closest station to the research location, namely Sungai Siring station, data from other stations is and processed using the formula from Mock by considering the elevation of each station.After that the two data (rainfall and temperature) are used to determine the amount of evapotranspiration that occurs in the study area, the calculation of evapotranspiration using the method of Turc Lungbein.Runoff is obtained from the reduction of rainfall and evapotranspiration from the previous calculation.Primary data is the amount of CaCO3 concentration dissolved in water to determine the level of weathering of carbonate rocks.The amount of CaCO3 concentration was obtained by taking water samples from springs in the Berambai Cave underground river using a purposive sampling method and then tested using the alkalinity titration method to determine the solubility of CaCO3.After obtaining primary and secondary data, the carbon dioxide sequestration capacity was calculated using the formula from equation (3).
Where V is the volume of carbon dioxide absorbed in the karstification process (m 3 /year/km 2 ), E is the runoff (dm), and T is the concentration of CaCO3 in the water coming out of the spring (mg/l).The flow chart of this study can be seen in Figure 2.

Results and Discussions
Indonesia is a country that is passed by the equator line which causes Indonesia to have a tropical climate, this is because the sun will shine throughout the year.This tropical climate is very good for the karstification process because it has sufficient rainfall and sunlight for the development of the karstification process and the development of organisms around Berambai Cave.Annual rainfall in tropical karst areas which has a higher value than rainfall in non-tropical karst areas makes the dissolution process run more intensively.The greater the intensity of rainfall in tropical karst areas can accelerate and increase the intensity of the dissolution process in the karst area so that carbon dioxide sequestration from the atmosphere will also increase.Rainfall in Indonesia has a value of 2,000 -3,000 mm/year so that it can accelerate the dissolution process in the Karst landscape.In addition, forest cover in karst landscapes can increase soil carbon dioxide levels to produce water that is more aggressive in dissolving carbonate rocks.The presence of these two factors causes karstification to run more intensively so that the potential for carbon dioxide sequestration from the atmosphere in the Indonesian Karst region also increases [2].Meanwhile, the calculation of regional rainfall resulted in a value of 2,705 mm/year.The high rainfall in the Samarinda region is sufficient to dissolve limestone in the karstification process because the more water that carries carbon dioxide into the epicarst layer makes the dissolution process run more intensively.Temperature drives the karstification process, especially in relation to organism activity.Areas with warm temperatures such as in the tropics are ideal places for the development of organisms that further produce abundant CO2 in the soil.The study area has a temperature of 27.48˚C which is good for organism development.The rainfall and temperature data were used to determine the amount of evapotranspiration in the study area using the Turc Lungbein formula of 1,383.14mm/year and to determine the amount of runoff as a store of water that is not absorbed into the soil, by subtracting rainfall and evapotranspiration [5].Runoff in the study area is 13.21 dm, determining runoff is very important because the weathering of carbonate rocks is very sensitive to runoff, namely the greater the runoff, the more intensive the weathering of carbonate rocks [8].The concentration of CaCO3 was obtained from the results of sampling in the spring water of the Berambai Cave underground river and analyzed by the alkalinity test method to determine the concentration of dissolved CaCO3 in water.From the results of the spring water sample test at Berambai Cave, the CaCO3 concentration was 140 mg/L.From the calculation of rainfall, temperature, evapotranspiration and analysis of Berambai Cave spring water, the results can be seen in Table 1.From the results in Table 1, the carbon dioxide sequestration capacity of Berambai Cave is 73.97 m 3 /year/km 2 calculated using equation (3).Although Berambai Cave is not included in the karst landscape due to several factors such as its small area, it still contributes to the carbon dioxide capture and storage (CCS) process.Because the source of greenhouse CO2 sequestration in the atmosphere has a close relationship with the karstification process.When corrosion occurs in carbonate rocks, it results in the loss of CO2 in the atmosphere [4].This karstification process can be seen from the development of the underground river in Berambai Cave in Figure 3 as a medium for dissolving carbonate rocks.The dissolution of carbonate rocks in the underground river of Berambai Cave is part of a dynamic system that is an important part of the Earth's surface system.The carbon cycle processes that occur at Berambai Cave on a watershed scale include: (a).dissolution of carbonate rocks that release atmospheric or soil CO2 into water and produce inorganic carbon; (b).movement and conversion of inorganic carbon along with water flow (c).inorganic carbon turns into organic carbon through photosynthesis of aquatic plants, part of the organic carbon deposits at the bottom of the river mixed with sediments [9].Therefore, the absorption of carbon dioxide during the karstification process is important as part of the global carbon cycle which plays a role in reducing excess carbon dioxide in the atmosphere.As well as preserving existing ecosystems to maintain the flux of carbon dioxide in the atmosphere.

Conclusion
Berambai Cave, located in Berambai Village, North Samarinda District, East Kalimantan, has the potential as a carbon capture and storage (CCS) because it plays a role in carbon dioxide sequestration in the Samarinda area.From the research results, it is known that the carbon dioxide sequestration capacity in Berambai Cave is 73.97 m 3 /year/km 2 .The dissolution process of carbonate rocks in Berambai Cave can contribute to reducing atmospheric carbon emissions.The contribution of inorganic carbon uptake value can help realize carbon emission reduction programs at the regional level.This is influenced by several factors such as rainfall, temperature and evapotranspiration that affect the process and level of karstification in Berambai Cave.

Figure 2 .
Figure 2. Flowchart of the research.