INTRODUCTION
The necessity of the research
Paleoclimatology, the study of past climates, has gained prominence in recent decades, enhancing our understanding of current and future climate patterns since systematic weather observations began in the late 1800s This field not only helps identify the causes of past climatic variations but also tests the accuracy of modern circulation models The significance of paleoclimatic research is underscored by its impact on human activities, particularly in agriculture, biological systems, and environmental quality, especially amidst rising global temperatures With atmospheric carbon dioxide levels reaching unprecedented highs—409.9 ppm in 2019 and 417.81 ppm in February 2022—research is increasingly vital The IPCC's 2021 report indicated that the 1.5°C threshold could be crossed in the early 2030s, and COP26 reaffirmed the importance of addressing global climate change through research and action.
Vietnam, situated on the Indochinese peninsula, is significantly impacted by climate change, prompting increased funding for adaptation and mitigation efforts from both the Vietnamese government and international organizations The importance of paleoclimate research has been highlighted in understanding climate change impacts and limitations, as global climate events manifest differently in various regions Although paleoclimate studies in Vietnam are limited compared to global efforts, the use of multiarchives presents a valuable opportunity for further research.
Recent studies have utilized sediments, rainwater, and speleothems to interpret paleoclimate parameters such as temperature, precipitation, monsoon intensity, and humidity In 2016, the EOS Geoscience research group at Vietnam National University initiated a coring expedition in Maar Lake, Highland Vietnam, aimed at reconstructing the paleoclimate of the Holocene and Pleistocene By 2018, Quan et al employed stable isotopes in sediment analysis to identify five significant climatic periods in the Holocene of the Red River Delta, characterized by alternating hot-warm, cold-dry, and warm-wet conditions Briles et al (2019) utilized a multiproxy approach, including pollen and geochemistry, to assess the climatic influences on the tropical island ecosystem of Quan Lan Island over the last 3,000 years of the late Holocene Additionally, Dung et al (2020) examined the Asian Summer Monsoon (ASM) between 30,000 and 22,700 years ago using stalagmite data from Son La Province, Northern Vietnam Despite these efforts in northern Vietnam, there is a notable lack of research in Hoa Binh Province, where the ASM's effects may differ from other regions.
Over the past two decades, studies have highlighted the 4.2 ka BP climate event as one of the two most significant climate occurrences in the Holocene on a global scale (Duan et al., 2021; Harvey).
Research in Southern China has revealed varying climatic conditions surrounding a significant event, with some studies indicating cold and dry weather while others suggest a wetter climate (Zhang et al., 2018; Wang et al., 2005) This thesis analyzes stalagmite NS4, collected from the Nam Son cave in Hoa Binh province on March 10, 2018, to investigate the paleoclimate from 5.0 to 3.5 ka BP, focusing on the period before and after the 4.2 ka BP cooling event in Northern Vietnam The findings aim to serve as a valuable reference for future paleoclimate studies in Vietnam.
The research questions and hypotheses
Table 1.1 contains the research questions and hypotheses
Table 1.1 Research question and hypothesis of the study
Did the changes in stable isotope composition (δ 18 O and δ 13 C) in sample stalagmite NS4 record the global 4.2 ka cooling event?
Stalagmite NS4 can record the paleoclimate in global 4.2ka cooling event
Which is the main factor that affected to δ 18 O value in stalagmite NS4? δ 18 O value in stalagmite NS4 is mainly affected by precipitation
3 How did δ 18 O and δ 13 C value in sample NS4 reflect the changes in regional paleoclimate?
The value of δ 18 O and δ 13 C in sample NS4, in inverse relationship, can reflect the regional changes in paleoprecipitation and paleotemperature at 3.5-5ka BP in Northern Vietnam.
Research objectives and tasks
The research’s objectives and tasks as details in Table 1.2
Table 1.2 Objectives and tasks of the study
1 Comprehend the principles of using paleothems proxies to reconstruct paleoclimate and apply it to interprete the δ 18 O and δ 13 C values in stalagmite
- Achieve the knowledge of principle concepts, include the carbon and oxygen atoms structure and stable isotopes, stalagmite formation process, U-Th dating method, stable analysis methods and monitoring methods
- Literature review about the climate changes studies in the world, Asia and Vietnam which using speleothems proxies
2 Analyze the change of δ 18 O and δ 13 C values through the fractionation processes of precipitation, drip water and deposition
- Collect data of temperature, precipitation, vapor presure, δ 18 O and δ 13 C values, from websites, mete- orological stations and projects
- Use R software for analyzing data (linear modeling, time-series analysis and checking correlation coefficient)
- Monitoring for current value of δ 18 O in precipitation and drip water
3 Reconstruct the changes in paleo- climate at 3.5-5ka in northern Vietnam
- Analyze the recording ability of 4.2ka cooling event in stalagmite NS4
- Analyze the changes and interprete the δ 18 O and δ 13 C values in stalagmite NS4
- Consider the compatibility between the statements on climate change from stable isotope data in stalagmite NS4 with research results in Vietnam and the neighborhood area.
The scope of the research
This research focused on the paleoclimate from 3.5-5.0ka BP in the northern Vietnam
Research conceptual framework
Climate parameters significantly influence biochemical reactions on Earth, and evidence of paleoclimate is recorded in various archives, including ice cores, tree rings, sediments, and speleothems, which contain paleoclimate proxies These proxies preserve past atmospheric compositions in diverse ways, leading to different methodologies in paleoclimatology (Shuman, 2021) This thesis focuses on the correlation between climate factors and the stable isotope values of oxygen and carbon in stalagmites The relationship can be analyzed by examining changes in the oxygen isotope ratio (δ 18 O) across various hydrological cycles, including ocean water, rainwater, drip water, and carbonate precipitates Additionally, comparing isotope values from contemporaneous stalagmites in different caves can provide further insights, although collecting such samples poses challenges Consequently, this study monitors the variations in oxygen isotope values in rainwater, drip water within caves, and precipitated carbonate.
This study investigates climate factors such as precipitation, monsoon intensity, temperature, and humidity as interpreted from δ 18 O To explore the relationship between carbon isotope ratio (δ 13 C) and these climatic factors, changes in δ 13 C values are typically observed in soil, air, and precipitated carbonate or compared across different locations However, due to the limitations of this study, we will reference existing research to interpret the variations in δ 13 C values in the NS4 stalagmite sample The framework of this thesis is illustrated in Fig 1.1.
Matrix of learning outcomes for the master’s thesis
Results of the master’s thesis and and other outcomes include:
- R1: Comprehend principles and methods of using speleothems proxies in paleoclimate studies
- R2: Apply the principles to analyze and interprete data of δ 18 O and δ 13 C value of stalagmite NS4 in Nam Son cave
- R3: Reconstruct the paleoclimate in Northern Vietnam
- O1: Achieve ability in setting up drip water monitoring inside the cave
Details of matrix of learning outcomes as the Table 1.3
Table 1.3 Matrix of learning outcomes for the master’s thesis
Result of the master’s thesis Other outcomes of the master’s thesis
Literature review
1.7.1 Overview on paleoclimate events in the Holocene
The Holocene, which began approximately 11,700 years ago at the end of the Pleistocene, marks a significant period in geological history closely linked to the rise of human civilizations This epoch has seen transformative changes in vegetation, biodiversity, landforms, the water cycle, and non-living resources, all influenced by climate variations and sea level shifts (Padmalal et al., 2022) Advances in research techniques have greatly enhanced our understanding of paleo-climate changes during the Holocene, shedding light on human life, sociocultural developments, and species extinction events (An et al., 2012; Xiao et al., 2004; Yang et al., 2019).
During the Early Holocene, a significant climate shift occurred, leading to warmer and wetter conditions in north-western Russia, with summer temperatures rising from 4°C to 10-12°C (Wohlfarth et al., 2007) This period, known as the first warm phase of the Preboreal, occurred approximately between 11.53ka and 11.50ka BP, as evidenced by the increase in pollen diagrams.
Over the past 8 years, a brief cold period occurred from 11.43 to 11.27 thousand years ago, as evidenced by trapped air in Greenland ice (Kobashi et al., 2008) Following this, Seppọ and Birks (2001) proposed that macrofossil data indicate a rapid and stable warming trend emerged after 10,000 years ago.
BP across the entire northern Scandinavia (Seppọ & Birks, 2001)
The 8.2ka cooling event was first implied from the changes in isotope data in ice cores from the Summit site in Greenland This decreasing in temperature was implied in global scale, from many paleoclimate proxies, however the decreasing amplitude is regional dependance Researchers estimated the air temperature decreased 6 ± 2°C in central Greenland, 0.5-1.5°C in Estonia and Sweden, 0.9-1.8°C from eastern Lativa; and the main reason was attributed to changes in the Surface and deep water circulation in North Atlantic Ocean after the continental ice sheets melted (Borzenkova et al., 2015)
Between 8,000 and 4,500 years ago in north-western Russia, summer temperatures ranged from 2.0 to 2.5°C with annual precipitation between 100-150mm, supporting predominantly spruce and deciduous forests (Arslanov et al., 1999, 2001) In the Baltic Sea basin, pollen and chironomid studies indicate that the warmest period occurred between 7,500 and 5,500 years ago, with temperatures varying from 0.8°C to over 1.5°C Additionally, northern Finland experienced its warmest climate from 7,950 to 6,750 years ago, with July temperatures around 13°C (Borzenkova et al., 2015).
The Late Holocene interval was marked by a cooling and instability in the northern hemisphere's climate, driven by a decline in summer solar radiation This period experienced alternating warm and cold phases of varying intensity across different regions In northern Russia, cooling began approximately 4.5 thousand years ago, followed by a warmer phase from 3.5 to 2.5 thousand years ago, before a colder period emerged post 2.5 thousand years ago (Arslanov et al., 2001) In Poland, around 4.2 thousand years ago, lake sediment analyses revealed a cooler and more humid climate (Borzenkova et al., 2015).
4.2ka cooling event was recorded in global scale and event was considered as the most salient one that influenced on the civilization’s evolution Even a lot of studies on 4.2ka event, the structure and driving mechanism is still unclear (Chen et al., 2021) Carolin et al., (2019) suggested that the 4.2 ka event may be larger in magnitude and longer in duration than previous studies
1.7.2 Overview on paleoclimate studies using speleothems in Asia and Vietnam
Speleothems, formed over thousands of years in karst caves, are influenced by various environmental factors and require the presence of carbonate rocks, adequate water, and CO2 for their formation These formations, including stalagmites, stalactites, and flowstones, serve as valuable records of past local climatic conditions due to their long deposition history Unlike other geological archives, speleothems are pure and ideal for U-Th dating, providing precise calendar ages Their deposition processes are sensitive to the concentration of water and carbon dioxide, leading to the measurement of stable carbon and oxygen isotopes The stable oxygen isotope ratio (δ18O) is a crucial proxy for reconstructing meteoric rainwater properties and cave temperatures, while δ13C offers insights into soil activity and vegetation density.
Paleoclimate studies utilizing speleothems and stable isotope ratio analysis began in Western countries during the 1960s and 1970s, primarily in Europe and the United States In contrast, significant research in Asia, particularly in China and India, emerged in the 1980s and 1990s (Broecker et al., 1960; Cheng et al., 2019; Hendy & Wilson, 1968).
Since the 2000s, advancements in techniques have led to a surge in stalagmite record publications, highlighting significant climate analyses such as the East Asian monsoon spanning 600,000 years, the Indian monsoon covering 280,000 years, the Central Asian westerly climate for 135,000 years, and the Northwestern China westerly climate extending back 500,000 years (Cheng et al., 2019).
The 4.2ka cooling event is variably documented in Asian speleothems, particularly in China Studies by Hu et al (2008) and Liu et al (2013) in Heshang Cave, Hubei, identified a dry period from 4.8ka to 4.1ka BP Similarly, research by Cosford et al (2008) on an aragonite stalagmite in Lianhua Cave, Hunan, revealed a significant decline in monsoonal intensity and extended drought between 4.3ka and 4.0ka Additionally, investigations in Dongge Cave, Guizhou, indicated dry conditions linked to weakened monsoons and solar cycles during the 4.2ka to 4.0ka BP period (Duan et al., 2014; Dykoski et al., 2005; Wang et al.).
In contrast, Zhang et al., (2018), based on the data from a stalagmite SN17 in Shennong cave (Jiangxi, Southeast of China), suggested a wet interval between 4.2ka and 3.9ka
In a study conducted by BP Chen et al in 2021, the analysis of multiple proxies from stalagmite YK1306 in Yangkou Cave, located in Qingdao, Northeast China, revealed significant findings Their research indicated that there was an increase in precipitation between 4.33 and 4.07 thousand years ago, correlating with a strengthening of the Asian Summer Monsoon (ASM).
Figure 1.2 Location map of studied caves and monsoon activities in China
The 4.2ka event is recognized as a critical global phenomenon; however, research in Asia, particularly in monsoon-affected regions, remains limited (Tan et al., 2020; Xiao et al., 2018; Zhang et al., 2018).
Paleoclimate research in Vietnam has been scarce, with only one significant study conducted on paleoclimate reconstruction in northwestern Vietnam, covering the period from 30,000 to 22,700 years ago using speleothem records (Dung et al., 2020) This study reveals evidence of a strong Asian Monsoon System (ASM) in northwestern Vietnam during the Dansgaard-Oeschger events 2-4.
1.7.3 Overview on speleothem sampling and precipitation monitoring for sta- ble isotope interpretation
To achieve reliable paleoclimate data from paleothem samples, it is essential that stalagmites form under isotopic equilibrium conditions, as noted by Tan et al (2006) Consequently, careful sampling is crucial to ensure meaningful results, and the position of the sample must meet specific criteria.
- Slow outgassing of CO2 from the drip water (the speed of drip water slow enough
12 to make sure that the chemical reaction to create stalactites occured slowly and attained the balance point in both directions)
Study area
Nam Son Cave, also referred to as Ton Cave, is a karst cave situated in Nam Son commune, Tan Lac district, Hoa Binh province, northern Vietnam, at an elevation of 971 meters above sea level Discovered in January 2004 by local residents working in the fields, the cave features three expansive chambers, showcasing its geological significance and natural beauty.
In 2008, Nam Son Cave was officially designated as a national scenic relic According to Danviet.vn (2022), the cave is estimated to have formed over the last 250 million years Despite its extensive history, the surrounding area has seen minimal farming activity, which contributed to the cave's late discovery.
Figure 1.3 Location of Nam Son cave (Google Map)
Figure 1.4 A sketch on Nam Son Cave structure (Duong et al., 2021)
A March 2022 survey reveals that vegetation varies with altitude, predominantly featuring C4 plants At the mountain's base, bamboo is the primary vegetation, while shrubs dominate the path leading to the entrance Approximately 60 meters above the entrance, an old forest emerges, characterized by large-diameter trees with rounded tops (Fig 1.5).
Nam Son Cave is situated in a tropical monsoon climate characterized by cold, dry winters and hot, rainy summers Long-term meteorological data from 1961 to 2018 in Mai Chau District, Hoa Binh Province, reveals an average annual temperature of 21.1°C, with peak temperatures of 25.3°C in June and July, and lows of 15.5 to 16.0°C in December and January The region receives an average annual precipitation of 1,342 mm, primarily concentrated between May and October, while rainfall from November to March is relatively low, averaging between 8 to 27 mm Additionally, the relative humidity remains stable throughout the year, fluctuating between 74% and 81%, with an average of 77%.
Figure 1.5 Outside of Nam Son Cave
Figure 1.6 Climate parameters in Mai Chau District, Hoa Binh Province
MATERIALS AND METHODOLOGY
Materials
This research utilizes stable isotope data of carbon (C) and oxygen (O) from 248 subsamples, alongside U/Th dating results from 23 subsamples of the NS4 stalagmite, supported by the Nafosted project coded 105.03 – 2015.35 The NS4 stalagmite measures 269 mm in length and was collected from the last chamber (3rd chamber) of Nam Son cave.
Figure 2.1 Sample for U-Th dating analysis
To interpret paleoclimate conditions through isotopic composition changes in stalagmites, we utilized rainfall data from the Mai Chau meteorological station, the nearest station to Nam Son cave Additionally, we incorporated isotope composition data from rainwater, sourced from the IAEA WISER database, covering the period from 2014 to 2018.
Rainwater samples collected in Hanoi, Vietnam (21.05° N, 105.8° E) from 2004 to 2007 and from 2015 to 2018 were contributed to the IAEA WISER database under projects GNIP/M/XX/01 and GNIP/M/VN/01, respectively, in collaboration with the Partner Institute of Geological Sciences The dataset includes fields such as oxygen isotope, precipitation, temperature, and vapor pressure; however, it is not continuous due to some missing data points, which were not extrapolated Data analysis was conducted using R software to calculate averages and correlations based on the available information.
From March to May 2022, we conducted monitoring of precipitation and drip water, along with an analysis of the isotopic composition in rainwater, drip water, and calcium carbonate formed from dripping water This research is crucial for interpreting paleoclimate conditions, as it reveals significant changes in the isotopic composition of NS4 stalagmites in Nam Son cave.
Background of the methodology
2.2.1 Some principles concept a Stalagmite formation
Caves are the result of CO2-driven dissolution of limestone bedrock in thousand years
The process of cave formation begins with CO2 from decomposed organic matter in the soil reacting with meteoric water to create carbonic acid, which gradually dissolves limestone This reaction leads to the formation of calcium ions and bicarbonate in the water As CO2 is released from dripping water, calcium carbonate precipitates vertically due to gravity, resulting in the formation of stalagmites.
An atom consists of a nucleus containing protons and neutrons, with electrons orbiting around it Protons carry a positive charge, while electrons carry a negative charge, and the number of protons and electrons defines the element Neutrons are neutral and do not affect the charge The combined mass of protons and neutrons determines the atom's mass, which can vary among isotopes due to differing neutron counts Isotopes with more neutrons are termed heavy isotopes Stable isotopes do not emit radiation, contrasting with radioactive isotopes Atoms of the same element possess the same number of electrons, resulting in similar chemical properties; for instance, both carbon-12 and carbon-13 can react with oxygen to form carbon dioxide However, their physical properties differ slightly due to variations in atomic mass, as seen in the stable isotopes of hydrogen, protium and deuterium.
Hydrogen (H) and deuterium (D) can react with various elements to form compounds through similar chemical reactions; however, the significant difference in atomic mass—1.0079 for H and 2.0142 for D—results in distinct physical properties For instance, while water (H2O) has a melting point of 0 °C and a boiling point of 100 °C, heavy water (D2O) has a melting point of 3.81 °C and a boiling point of 101.41 °C (Britannica, 2020) These differing chemical and physical properties of stable isotopes are fundamental to their study and application.
Peterson, (2021) summaried the common 5 stable isotopes used in paleoclimate reconstruction, include Oxygen ( 18 O/ 16 O), Carbon ( 13 C/ 12 C), Nitrogen ( 15 N/ 14 N), Boron
Oxygen is the most abundant element in the Earth's crust, present in various types of rocks, water, and organic materials In nature, oxygen exists in three stable forms.
20 isotopes: 16 O (99.759%), 17 O (0.037%) and 18 O (0.204%) (Sulzman, 2007) Due to the higher mass, the ratio of 18 O/ 16 O is usually used for researching than 17 O/ 16 O
Carbon exists in two stable isotopes: 12 C (98.892%) and 13 C (1.108%) (Sulzman, 2007) It is essential for all living organisms, as it is a fundamental component of organic matter, forming the basis of life on Earth The significance of carbon and oxygen makes them valuable for scientists conducting geological, biological, and chemical research Additionally, isotopic fractionation plays a crucial role in understanding carbon's behavior in various natural processes.
Isotopes of an element have similar chemical properties due to the same number of electrons, but their differing atomic masses influence reaction speeds and isotopic ratios The phenomenon known as "isotopic fractionation" (ε) refers to the enrichment of one isotope over another during chemical or physical processes, with each process exhibiting unique fractionation values This variation in isotopic fractionation is essential, as it impacts the ratio of heavier to lighter isotopes resulting from chemico-biological reactions, and it differs among elements.
“Equilibrium” and “kinetic” are 2 types of isotopic fractionation process As the names,
Equilibrium fractionation occurs during chemical equilibrium reactions, where isotopic ratios remain constant over time once equilibrium is reached In contrast, kinetic fractionation takes place in incomplete and unidirectional processes, significantly influencing the reaction's rate constant Isotopic fractionation values are typically small and are expressed in per mil (‰) Additionally, in chemical reactions, isotopic fractionation is temperature-dependent.
21 pendent (Urey, 1947) while in plants it is influenced by biotic and abiotic factors For example, in the calcite formation process, oxygen isotopic fractionation is 31‰ (αCa-
At 25°C, the carbon isotopic composition of CO3–H2O is 1.031, while at 33.7°C, it reaches 28‰ (Tiwari et al., 2015) Research by Kyle et al (2011) highlights that carbon isotopic fractionation varies between C3 and C4 plants, with C4 vegetation exhibiting higher values attributed to its superior photosynthetic efficiency.
2.2.2 Hendy Test for evaluating the presence of isotopic equilibrium
The Hendy test is a crucial initial step in verifying that deposits formed under oxygen isotopic equilibrium According to Hendy (1971), when isotopic equilibrium is achieved, the calcium carbonate (CaCO3) components are influenced solely by the water components involved in the deposition chemical reaction.
To achieve isotopic equilibrium, the reaction speed must be slow, maintained at a stable temperature with minimal evaporation, making temperature the sole influence on the isotope ratio For speleothems to be deemed ideal for paleoclimate studies, they should exhibit a consistent oxygen isotopic ratio that remains unaffected by variations in the carbon isotopic ratio within a growth layer Samples for analyzing δ18O and δ13C were collected from individual growth layers.
The Hendy Test is commonly utilized to evaluate the suitability of speleothems for paleoclimate studies, particularly to ascertain whether stalagmites have formed in isotopic equilibrium However, Dorale and Liu (2009) identified two significant limitations of the Hendy Test: the first criterion regarding kinetic processes may not accurately reflect equilibrium conditions, and the second criterion concerning the relationship between δ18O and δ13C may not always indicate isotopic equilibrium Consequently, they suggested that the Hendy Test is not essential for all speleothem samples.
2.2.3 Age determination by U-Th method
U-Th dating method was discribed by Edward et al., (1987), based on the analysis the isotope activity ratios of the parent radioactive Uranium and daughter Thorium as Fig.2.3 Differ from Radiocarbon ( 14 C) dating method which measure the end of a decay product with the timespan of 40ka, U-Th dating method measure the activity of ratios
( 230 Th / 238 U) with the result of timespan of 500ka with the high accuracy
The decay process begins with soluble 238 U, which is incorporated into stalagmites during their formation As 238 U undergoes half-life decay, it produces insoluble daughter isotope 230 Th within the speleothems This principle underlies the U-Th dating method, which is applicable exclusively to samples that initially contain no 230 Th, ensuring that all detected 230 Th is a result of the decay process.
Figure 2.3 Principle of U/Th dating method (Chen et al., 2020)
A Decay process B Time since initial fractionation C Define the age of sample Total subsamples (~50mg/each) were collected by hand drill (with small drill bit), drilled in each layer of stalagmite (in the width over 3mm) After collecting, samples
The extraction of uranium (U) and thorium (Th) was conducted in a controlled environment, specifically in dust-free rooms at bench 100, utilizing strong acid at elevated temperatures This process was meticulously designed to minimize errors and involved three main phases for optimal sample treatment.
Methods
2.3.1 Field and monitoring methods δ 18 O is affected by complex processes including changes in the source of water and changes in isotopic fractionation which affected by temperature (Dreybrodt & Scholz, 2011; McDermott, 2004) Therefore, it is neccessary to monitor parameters inside the
Research on cave microclimate parameters, such as saturation and dripwater status, is essential for understanding the processes that influence equilibrium and the kinetic fractionation of oxygen isotopes (Lachniet, 2009) In humid regions, the isotopic compositions of cave dripwater closely resemble local precipitation (Cobb et al., 2007; Fuller et al., 2008; Yonge et al., 1985) Conversely, in arid or semi-arid areas, the δ 18 O values in dripwater tend to be higher than those in local precipitation due to increased evaporation (Ayalon et al., 1998; Bar-Matthews et al., 1997).
Temperature, humidity and CO2 concentration were measured by using 77535 AZ Portable CO2 Analyzer with Temperature & Humidity (AZ Instrument Corp., Taiwan) (Fig 2.5) Details of specifications as Table 2.1
Figure 2.4 A Thermo Finnigan Delta Plus XP mass spectrometer (Czuppon et al., 2018)
Table 2.1 Specifications of Portable CO2 Analyzer - model 77535 AZ (EMIN, n.d.)
Item CO 2 concentration Temperature Humidity
0~9999 ppm (5001~9999 ppm out of scale range)
Item CO 2 concentration Temperature Humidity
Accuracy ±30ppm±5% of reading (0~5000ppm) Other ranges are not specified ±0.6 o C/±0.9 o F ±3%RH (at 25 o C, 10~90%RH); ±5%RH (at 25 o C, others)
Item CO 2 concentration Temperature Humidity
24 hours (Alkaline battery)
Drip water speed was counted by drop counter Stalagmate (Fig 2.6) with detail tech- nical data as Table 2.2
Table 2.2 Technical data of Drop counter Stalagmate (Driptych, n.d.)
Battery type 1/2 AA Lithium thionyl chloride: 1200 mAh @ 3.6 V Battery life (logger off) ≈ 10 years
Battery life (logger on) ≈ 4 years
Sealing IP67 (only when terminal cap is fitted)
The Drip Counter Stalagmate was installed beneath a stalactite in the cave to monitor calcite deposition Initially, visible evidence of calcite was observed on the cave floor, characterized by its white coloration A glass piece was then placed on the ground to collect water droplets directly By measuring the weight of the glass before and after the water dripped, the difference in weight indicated the amount of calcite deposited.
Figure 2.6 Drip counter Stalagmate Figure 2.7 Checking the calcite deposition
Monitoring was established using a drip counter called Stalagmate, positioned on the tube with a collection bottle for drip water placed below (Fig 2.8) Monthly, drip water samples are collected in 50ml-HDPE bottles and sent to Hungary for isotope component analysis, alongside fresh carbonate samples The samples collected in May were personally transported to Hungary by Czuppon following his business trip to Vietnam.
Figure 2.8 Setting up for monitoring in Nam Son cave (2022)
This thesis utilized statistical and graphical techniques such as linear modeling, time-series analysis, display charts, correlation verification, and correlation coefficient determination, implemented through R and MS Excel software.
Scholz et al (2012) evaluated five models for speleothem age modeling, addressing issues such as dataset outliers, age inversions, and significant growth rate changes They found that the OxCal and StalAge models performed well in this context In their study, they created an Age-Depth plot using the StalAge algorithm, which was developed by Scholz & Hoffman in 2011 The model utilized U/Th dating results, incorporating 23 values across three variables: Depths, Ages, and Errors After analyzing the correlation between Depths and Ages, the model produced a comprehensive table of 248 Depth and Age values, corresponding to 248 isotope subsamples of δ 18 O and δ 13 C.
Pearson’s correlation coefficient test was used to identify the dependence between δ 18 O value and three parameters of precipitation, temperature and vapor pressure Correlation coefficient (r) is calculated by the formula:
29 with 𝑥̅ and 𝑦̅ are the average value of variables x and y r value was calculated by using command “cor.test(x,y)”
After determine the correlation of varibales, proposing the variables have linear correlation, continuing use R software for Linear Regression analysis y = α + βx the constanst α and β will be determined by command lm(y~x)
RESULTS AND DISCUSSIONS
Test of stable isotope equilibrium
For stable isotope proxies to be effectively used in paleoclimate studies, stalagmites must form under isotopic equilibrium conditions This is primarily achieved when stalagmites develop in environments with constant temperature and saturated humidity, ideally located far from entrances to minimize kinetic effects These conditions help to maintain the isotopic balance between calcite and water, ensuring accurate paleoclimate reconstructions (Mickler et al., 2004; Tan et al., 2006).
Nam Son Cave, protected by local officials, remains an unspoiled natural wonder Despite having an electric system installed, it has not yet been officially opened for tourism due to the challenging trekking routes, allowing its internal components to remain largely intact Located at an altitude of 971 meters above sea level, the cave houses a year-round freshwater lake covering nearly 1,000 square meters, with depths ranging from over 2 meters to approximately 7 meters (Bui, 2016) The cave's small entrance requires explorers to crawl into the first chamber, where the combination of abundant water and limited air ventilation maintains a relative humidity near 100% and a stable temperature, making it ideal for paleoclimatology studies of the unique speleothems found within.
Table 3.1 presents the monitoring results of temperature and humidity within the cave The data reveals that the temperature in the third chamber, where sample NS4 was located, remains stable between 19.6°C and 19.8°C, despite fluctuations in outside temperatures Additionally, humidity levels inside the cave were consistently recorded at 99.9% relative humidity during three monitoring sessions conducted from March to May.
Figure 3.1 Inside the Nam Son Cave Table 3.1 Checking results of climate parameters
Date Chamber Temperature o C Humidity % CO 2 concentration ppm
CO2 concentration is the factor changing by seasons, dominated by sources of natural and anthropogenic fluxes Natural fluxes include (1) direct diffusion from soils/epikarst,
The primary sources of CO2 in caves include the decay of organic matter, animal respiration, endogenous CO2, and dripwater degassing (Holland et al., 1964) While anthropogenic contributions mainly arise from human exhalation (Faimon et al., 2006), the cave in question has not been exploited for tourism, resulting in negligible levels of anthropogenic CO2 Consequently, it is widely accepted that the cave's CO2 predominantly originates from these natural processes.
32 karst soil (Faimon et al., 2012) and seasonal variations (Ek & Gewelt, 1985; Spửtl et al., 2005; Troester & White, 1984)
The consistent δ 18 O values of -9.31‰ SMOW found in both fast and slow drip water collected from two different stalactites in the 3rd chamber support the hypothesis that the NS4 stalagmite formed under equilibrium conditions.
The Age model
Sample for U-Th dating were collected at 23 points along the central axis of NS4, the result of dating as Table 3.2
The Cross-plot model of Age versus depth was executed automatically as outlined in Chapter 2 Data points at depths of 7, 45, and 50 mm were excluded due to significant errors Inversion age may arise during calcite formation (Lachniet et al., 2012) The calculated age values for depths of 21-23 mm and 35-37 mm exhibit a wide tolerance range of 201-456 years, in contrast to the residual points which show a much narrower range of 23-65 years (Fig 3.2).
Figure 3.2 Polished section (left) and age model (right) of stalagmite NS4
The analysis presented in Fig 3.2 indicates a hiatus in stalagmite growth, transitioning from 36mm to 37mm, which aligns with irregularities observed in age trends confirmed by U/Th dating The growth rate of the stalagmite varied significantly, with a deposition of 225mm over 600 years (from 5,100 BP to 4,500 BP, or stage 1), resulting in a rate of 0.375 mm/year In contrast, during the subsequent 500 years (from 4,500 BP to 4,000 BP, or stage 2), the growth slowed dramatically to just 16mm, equating to a rate of 0.032 mm/year.
Running again model of Cross-plot of Age versus depth in 2 stages, the results show in the Fig 3.3
Table 3.2 230 Th results of stalagmite NS4 from Nam Son cave (2021)
Analytical errors are defined as 2σ of the mean, with the relationship between uranium isotopes given by [238U] = [235U] x 137.818 (±0.65‰) (Hiess et al., 2012) The initial corrected δ234U is calculated using the formula δ234U initial = δ234U measured x e^λ234*T, where T represents the corrected age Additionally, the [230Th/238U] activity can be expressed as 1 - e^(-λ230T) + (δ234U measured /1000)[λ230/(λ230 - λ234)](1 - e^(-(λ230 - λ234)T)), with T indicating the age.
The decay constants for various isotopes are as follows: 230 Th has a decay constant of 9.1705 x 10 -6 yr -1, 234 U has 2.8221 x 10 -6 yr -1 (Cheng et al., 2013), and 238 U has 1.55125 x 10 -10 yr -1 (Jaffey et al., 1971) Age corrections were made relative to the chemistry date of October 9th, 2016, utilizing an estimated atomic 230 Th/232 Th ratio of 4 (± 2) x 10 -6.
Those are the values for a material at secular equilibrium, with the crustal 232 Th/ 238 U value of 3.8 The errors are arbitrarily assumed to be 50%
Figure 3.3 Cross-plot of Age versus depth in 2 stages: Stage 1 (left) and stage 2 (right)
δ 18 O and δ 13 C values in the NS4 stalagmite
The study analyzed 248 subsamples for stable carbon and oxygen isotopes, revealing that δ 13 C values averaged -9.72‰ VPDB, with fluctuations between -12.1‰ and -8.26‰ Notably, δ 13 C values exhibited an overall increasing trend, peaking around 4130 BP before declining sharply to -12.1‰ between 4760 and 4740 BP Similarly, δ 18 O values averaged -9.22‰ VPDB, ranging from -11.54‰ to -7.45‰, and also displayed a general upward trend, with a significant drop to -10.51‰ around 4700 BP and a rise to -7.45‰ by 4800 BP The oscillation amplitude of δ 18 O values was greater than that of δ 13 C values Additionally, both isotopes reached their lowest points around the same time, notably in 4950 BP and 4690 BP.
Retesting using R software revealed a significant correlation between δ 18 O and δ 13 C values (r=0.459, p