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Abstract:
Isotopes of dissolved inorganic carbon (DIC) are used to indicate both transit times and biogeochemical evolution
of groundwaters. These signals can be complicated in carbonate aquifers, as both abiotic (i.e. carbonate equilibria) and biotic
factors influence δ13C and 14C of DIC. We applied a novel graphical method for tracking changes in δ13C and 14C of DIC in
two distinct aquifer complexes identified in the Hainich Critical Zone Exploratory (CZE), a platform to study how water
20 transport links surface and shallow groundwaters in limestone and marlstone rocks in central Germany. For more
quantitative estimates of contributions of different biotic and abiotic carbon sources to the DIC pool, we used the
geochemical modelling program NETPATH, which accounts for changes in dissolved ions in addition to C isotopes.
Although water residence times in the Hainich CZE aquifers based on hydrogeology are relatively short (years or less), DIC
isotopes in the shallow, mostly anoxic, aquifer assemblage (HTU) were depleted in 14C compared to a deeper, oxic, aquifer
25 complex (HTL). Carbon isotopes and chemical changes in the deeper HTL wells could be explained by interaction of
recharge waters equilibrated with post-bomb 14C sources with carbonates. However, oxygen depletion and δ13C and 14C
values of DIC below those expected from the processes of carbonate equilibrium alone indicate dramatically different
biogeochemical evolution of waters in the upper aquifer assemblage (HTU wells). Changes of 14C and 13C in the upper
aquifer complexes result from a number of biotic and abiotic processes, including oxidation of 14C depleted OM derived
30 from recycled microbial carbon and sedimentary organic matter as well as water rock interactions. The microbial pathways
inferred from DIC isotope shifts and changes in water chemistry in the HTU wells were supported by comparison with in
situ microbial community structure based on 16S rRNA analyses.
