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Title:CO2 dynamics and dissolutional processes in the karst vadose zone
Authors:ID Gabrovšek, Franci (Mentor) More about this mentor... New window
ID Kukuljan, Lovel (Author)
Files:.pdf Lovel_Kukuljan.pdf (8,45 MB)
MD5: 5E8975A0AC25B4E0DE392E5DFA0B0809
 
Language:English
Work type:Doctoral dissertation
Typology:2.08 - Doctoral Dissertation
Organization:FPŠ - Graduate School
Abstract:The dynamics and distribution of carbon dioxide (CO2) in karst systems are crucial for understanding fundamental karst processes, namely precipitation and dissolution, which drive karst development both at the surface and underground. The study of CO2 transport provides valuable insights into the role of karst systems in the global carbon cycle and the impact on present climate, but also into the growth of speleothems, which are one of the most reliable terrestrial archives for palaeoclimate reconstruction. Due to the complexity of karst systems, long-term monitoring and high-resolution analyses of cave air and water geochemistry are essential to better understand the controlling factors that affect these processes and their outcomes. In the framework of this dissertation, cave climate and water hydrochemistry monitoring was established in a side-passages of the renowned Postojna Cave in Slovenia during 2017–2021. In the Pisani Passage, high CO2 concentrations, large temporal variations and a heterogeneous distribution of CO2, as well as extreme dissolution features, have already been detected in previous studies. The aim of the present study was to investigate these observations in depth and to find the reasons for their occurrence. This led to creating of a conceptual model for CO2 transport in karst systems that would be valid not only in this case but in karst areas worldwide. The first focus of the study is dedicated to understanding the spatio-temporal dynamics of the partial pressure of CO2 (pCO2) in the Pisani Passage, which is mainly transported by advection (i.e., cave ventilation). Continuous measurements of airflow velocity, air temperature and pCO2 showed (1) that airflow through the karst massif is driven by both the action of the chimney effect and external winds, and (2) that the relationship between the direction of airflow, the configuration of airflow pathways and the connection to the outside explains the observed variations in pCO2. Due to the particular configuration of the airflow pathways, the terminal chamber of Pisani Passage accumulates high levels of CO2 (>10,000 ppm) and forms high vertical gradients of up to 1000 ppm/m. The pCO2 is low and uniform during updraft when outside air flows into the cave chamber through open, unobstructed passages (i.e., high-flow, low-pCO2 pathways). When the airflow reverses direction to downdraft, the chamber is fed by low-flow, high-pCO2 pathways that enter the cave passage through a CO2-rich fracture network embedded in a vadose zone. The spatial distribution of inlets and outlets results in minimal mixing between the low and high pCO2 pathways, leading to high and persistent pCO2 gradients. In addition to the chimney effect driving the seasonal ventilation of the cave, the specific signs of a secondary wind-driven effect were also found; which is the second focus of this study. Wind flow over irregular topography leads to near-surface air pressure variations, and thus, pressure differences between cave entrances at different locations. Pressure differences depend on wind speed and direction and their relationship to surface topography and the location of cave entrances. Winds can act in the same or opposite direction as the chimney effect, either enhancing, diminishing or even reversing the direction of density-driven airflows. In the case of Postojna Cave, north and northeast winds enhance the downdraft and limit updraft, while the opposite is true for south winds, which enhance the updraft and limit downdraft. To investigate the importance of wind-driven flow, a computational fluid dynamics model was used to calculate the wind pressure field over Postojna Cave and the pressure differences between selected points for different configurations of wind speed and direction. These values were compared with those obtained from airflow measurements in the cave and from simple theoretical considerations. Despite the simplicity of the approach and the complexity of the ca
Keywords:cave climate, cave ventilation, carbon dioxide, dripwater geochemistry, speleothem corrosion, Postojna Cave, Slovenia
Place of publishing:Nova Gorica
Year of publishing:2022
PID:20.500.12556/RUNG-7365 New window
COBISS.SI-ID:112456195 New window
NUK URN:URN:SI:UNG:REP:SDPMBNJO
Publication date in RUNG:22.06.2022
Views:2750
Downloads:82
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Secondary language

Language:Slovenian
Title:Dinamika CO2 in procesi raztapljanja v kraški vadozni coni
Abstract:Poznavanje časovne in prostorske porazdelitve ogljikovega dioksida (CO2) v kraških sistemih je ključno za razumevanje raztapljanja in izločanja karbonatov v krasu. Ti procesi poganjajo razvoj krasa na površini in v podzemlju. Prav tako raziskave prenosa CO2 v kraških sistemih nudijo oceno pomena kraških procesov v globalnem kroženju ogljika in posledično njihovega globalnega klimatskega pomena. CO2 ima ključno vlogo pri rasti kapnikov, ki so eden najzanesljivejših kopenskih arhivov za paleoklimatske raziskave. Dolgoročna opazovanja in analize parametrov jamskega zraka in kemičnih parametrov vode v kraških jamah nam omogočajo določitev ključnih dejavnikov in mehanizmov, ki vplivajo na kraške procese. V okviru te disertacije smo v obdobju 2017–2021 vzpostavili monitoring jamskega zraka in geokemije vode v stranskih rovih Postojnske jame v Sloveniji. V Pisanem rovu so bile v prejšnjih študijah že odkrite visoke koncentracije CO2, velika časovna in prostorska spremenljivost CO2 ter sledi intenzivnega raztapljanja matične kamnine in sige. Namen te študije je raziskati in pojasniti ta opažanja z vidika temeljnih procesov v krasu in sestaviti širše veljaven konceptualni model prenosa CO2 v kraških sistemih. V prvem delu se osredotočamo na razumevanje prostorske in časovne spremenljivosti parcialnega tlaka CO2 (pCO2) v Pisanem rovu. Pri tem predpostavimo, da je glavni dejavnik prenosa CO2 naravno prezračevanje kraškega masiva. Analiza samodejnih meritev hitrosti zračnega toka, temperature zraka in pCO2 v visoki časovni ločljivosti je pokazala da: (1) pretok zraka skozi kraški masiv poganjata učinek dimnika (vzgon) in tlačne razlike, ki jih povzročajo zunanji vetrovi, (2) opaženo prostorsko in časovno porazdelitev CO2 lahko pojasnimo z razmerjem med smerjo podzemnega zračnega toka in konfiguracijo zračnih poti, ki povezujejo jamo z zunanjo atmosfero. Zaradi teh dejavnikov v končni dvorani Pisanega rova opazujemo izjemno časovno in prostorsko spremenljivost koncentracije CO2 v zraku. V zimskem režimu, ko zunanji zrak priteka v dvorano skozi glavni jamski vhod in velike dobro prezračene rove z nizkim pCO2, je pCO2 v dvorani stalno nizek. Ko zračni tok obrne smer, v dvorano s površja zrak doteka skozi mrežo razpok, z različnim pretokom in koncentracijo CO2. Prostorska porazdelitev dotokov in iztokov povzroča minimalno mešanje med potmi z nizkim in visokim pCO2, kar vodi do močnih in obstojnih gradientov pCO2. V drugem delu naloge obravnavamo dejavnike, ki vsiljujejo tok zraka v kraškem masivu. Meritve so pokazale, da v primeru Postojnske jame poleg učinka dimnika, pretok zraka vsiljuje tudi zunanji veter. Tok vetra po nepravilnem reliefu povzroča spremembe zračnega tlaka blizu površine in posledično tlačne razlike med vhodi v jamo na različnih mestih. Te razlike so odvisne od zveze med hitrostjo in smerjo vetra ter oblikovanostjo površja in položaj jamskih vhodov. Vetrovi lahko vsiljujejo tok zraka v isti ali nasprotni smeri kot učinek dimnika, tako da povečajo, zmanjšajo, ali celo obrnejo smer zračnega toka, ki bi ga povzročal samo vzgon. V primeru Postojnske jame severni in severovzhodni veter spodbujata poletni režim in omejujeta zimski režim, južni veter pa krepi zimski režim in omejuje poletni režim naravnega prezračevanja zaradi učinka dimnika. V raziskavi smo z uporabo računske dinamike tekočin (CFD) izračunali tlačno polje vetra nad Postojnsko jamo in tlačne razlike med izbranimi točkami za različne konfiguracije hitrosti in smeri vetra. Te vrednosti smo primerjali z ocenami tlačnih razlik, ki smo jih dobili iz meritev pretoka zraka v jami in preprostih teoretičnih izhodišč. Kljub enostavnemu pristopu in kompleksnosti jamskega sistema, smo dobili zadovoljivo ujemanje. Raziskava prinaša prvo kvantitativno oceno relativnega prispevka površinskih vetrov k prezračevanju kraškega podzemlja. V tretjem delu obravnavamo tvorbo korozijskih kotlic, ki jih najdemo pod številnimi curki v končnem delu Pisanega rova. Raziskave temeljijo na sta
Keywords:jamska klima, naravno prezračevanje v jamah, ogljikov dioksid, geokemija curkov, korozija kapnikov, Postojnska jama, Slovenija


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