Carbon isotopes of dissolved inorganic carbon reflect utilization of different carbon sources by microbial communities in two limestone aquifer assemblages

摘要

pstrongAbstract./strong 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 the i?′/isup13/supC and sup14/supC of DIC. We applied a novel graphical method for tracking changes in the i?′/isup13/supC and sup14/supC of DIC in two distinct aquifer complexes identified in the Hainich Critical Zone Exploratory??(CZE), a platform to study how water 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 NETPATH geochemical modeling program, which accounts for changes in dissolved ions in addition to C isotopes. brbr 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 sup14/supC compared to a deeper, oxic, aquifer complex??(HTL). Carbon isotopes and chemical changes in the deeper HTL wells could be explained by interaction of recharge waters equilibrated with post-bomb sup14/supC sources with carbonates. However, oxygen depletion and i?′/isup13/supC and sup14/supC values of DIC below those expected from the processes of carbonate equilibrium alone indicate considerably different biogeochemical evolution of waters in the upper aquifer assemblage (HTU wells). Changes in sup14/supC and sup13/supC in the upper aquifer complexes result from a number of biotic and abiotic processes, including oxidation of sup14/supC-depleted OM derived from recycled microbial carbon and sedimentary organic matter as well as watera??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. brbr Our findings demonstrate the large variation in the importance of biotic as well as abiotic controls on sup13/supC and sup14/supC of DIC in closely related aquifer assemblages. Further, they support the importance of subsurface-derived carbon sources like DIC for chemolithoautotrophic microorganisms as well as rock-derived organic matter for supporting heterotrophic groundwater microbial communities and indicate that even shallow aquifers have microbial communities that use a variety of subsurface-derived carbon sources./p.