Inorganic carbon cycle in soil-rock-groundwater system in karst and fissured aquifers
Anorganski ogljikov cikel v sistemu tla-kamnina-podzemna voda v kraško-razpoklinskih vodonosnikih

Koceli Ajda, Kanduč Tjaša, Verbovšek Timotej
2013 Geologija  
Izvle~ek V prispevku so predstavljene sistemati~ne analize izotopske sestave ogljika (δ 13 C CaCO3 ) v karbonatnih kamninah osrednje Slovenije, ki predstavljajo kra�ko-razpoklinske vodonosnike, in deleži doprinosa ogljika iz procesov raztapljanja karbonatov ter razgradnje organske snovi v vodonosnike, izra~unane iz ena~b masne bilance. Analiziranih je bilo 59 vzorcev kamnin (predvsem dolomitov) od zgornjepermske do zgornjetriasne starosti. Vzorci karbonatnih kamnin so bili upra�eni in zmleti na
more » ... ra�eni in zmleti na frakcijo 45 µm ter za dolo~itev δ 13 C CaCO3 analizirani z masnim spektrometrom za analizo stabilnih izotopov lahkih elementov -IRMS. Enaka metoda je bila uporabljena tudi za dolo~itev izotopske sestave raztopljenega anorganskega ogljika (δ 13 C DIC ) v podzemni vodi za 54 od 59 vzorcev. Vrednosti δ 13 C CaCO3 se spreminjajo v razponu od -2,0 ‰ do +4,1 ‰, s povpre~no vrednostjo +2,2 ‰. Vrednosti so tipi~ne za morske karbonate z δ 13 C CaCO3 okoli 0 ‰, se pa vrednosti δ 13 C CaCO3 v na�i raziskavi razlikujejo med posameznimi skupinami glede na nastanek in starosti. Zgodnjediagenetski dolomiti imajo relativno vi�je vrednosti δ 13 C CaCO3 v primerjavi z ostalimi analiziranimi vzorci. Najnižje vrednosti imajo cordevolski in ba�ki dolomiti, najverjetneje zaradi pozne diageneze, pri kateri je skozi že sedimentirane dolomite krožila meteorna voda z nižjo izotopsko sestavo ogljika ter posledi~no znižala vrednosti δ 13 C CaCO3 . Vrednosti δ 13 C DIC se spreminjajo v razponu od -14.6 ‰ do -8.2 ‰. Vi�je vrednosti izotopske sestave anorganskega ogljika (δ 13 C DIC ) (-8.2 ‰) v podzemni vodi kra�ko-razpoklinskih vodonosnikov kažejo na nizki delež talnega CO 2 v vodonosniku in hitro infiltracijo, medtem ko nižje vrednosti δ 13 C DIC (-14.6 ‰) kažejo na ve~ji delež talnega CO 2 v vodonosniku in s tem na po~asnej�o infiltracijo. Izra~un deleža doprinosa ogljika iz organske snovi/ raztapljanja karbonatov v kra�ko-razpoklinske vodonosnike kaže na približno enak delež (50 % : 50 %). Abstract The paper presents a systematic analysis of the isotopic composition of carbon (δ 13 C CaCO3 ) in carbonate rocks in central Slovenia, representing karst and fissured aquifers, and share of carbon contributions from carbonate dissolution and degradation of organic matter in aquifers, calculated from the mass balance equation. 59 samples of rocks (mainly dolomites) from Upper Permian to Upper Triassic age were analyzed. Samples of carbonate rocks were pulverized and ground to fraction of 45 µm and for determination of δ 13 C CaCO3 analyzed with mass spectrometer for analyses of stable isotopes of light elements-IRMS. The same method was used for determination of isotopic composition of dissolved inorganic carbon (δ 13 C DIC ) in groundwater for 54 of 59 locations. Values of δ 13 C CaCO3 are in the range from -2.0 ‰ to +4.1 ‰, with an average δ 13 C CaCO3 value of +2.2 ‰. These values are typical for marine carbonates with δ 13 C CaCO3 around 0 ‰, although δ 13 C CaCO3 values differ between groups depending on the origin and age. Early diagenetic dolomites have relatively higher values of δ 13 C CaCO3 compared to other analyzed samples. The lowest values of δ 13 C CaCO3 were observed in Cordevolian and Ba~a dolomite, probably due to late diagenesis, during which meteoric water with lower isotopic carbon composition circulated in the process of sedimentation. Values of δ 13 C DIC range from -14.6 ‰ to -8.2 ‰. Higher δ 13 C DIC values (-8.2 ‰) indicate a low proportion of soil CO 2 in the aquifer and rapid infiltration, while lower values (-14.6 ‰) indicate higher proportion of soil CO 2 in the aquifer and slower infiltration. Calculated contributions of carbon from organic matter / dissolution of carbonates in the karstic and fissured aquifers show a similar proportion (50 % : 50 %).
doi:10.5474/geologija.2013.014 fatcat:mm3xzxczhnc7lfckubqinlcw24