Carbon isotopes of dissolved inorganic carbon reflect utilization of different carbon sources by microbial communities in two limestone aquifer assemblages
Martin E. Nowak1, Valérie F. Schwab2, Cassandre S. Lazar3, Thomas Behrendt1, Bernd Kohlhepp2, Kai Uwe Totsche2, Kirsten Küsel3,4, and Susan E. Trumbore11Department for Biogeochemical Processes, Max-Planck Institute for Biogeochemistry, Hans-Knöll Straße 10, 07745 Jena, Germany 2Chair of Hydrogeology, Institute of Geosciences, Friedrich Schiller University Jena, Germany 3Aquatic Geomicrobiology, Institute of Ecology, Friedrich Schiller University Jena, Dornburger Str. 159, 07743 Jena, Germany 4German Centre for Integrative Biodiversity Research (iDiv), Halle-Jena-Leipzig, Leipzig, Germany
Received: 10 Oct 2016 – Accepted for review: 07 Nov 2016 – Discussion started: 08 Nov 2016
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 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 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 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.
Our findings demonstrate the large variation in the importance of biotic as well as abiotic controls on 13C and 14C 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.
Nowak, M. E., Schwab, V. F., Lazar, C. S., Behrendt, T., Kohlhepp, B., Totsche, K. U., Küsel, K., and Trumbore, S. E.: Carbon isotopes of dissolved inorganic carbon reflect utilization of different carbon sources by microbial communities in two limestone aquifer assemblages, Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-534, in review, 2016.