Carbon isotopes of dissolved inorganic carbon reflect utilization of different carbon sources by microbial communities in two limestone aquifer assemblages
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>δ</i><sup>1...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-08-01
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| Series: | Hydrology and Earth System Sciences |
| Online Access: | https://www.hydrol-earth-syst-sci.net/21/4283/2017/hess-21-4283-2017.pdf |
| Summary: | 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>δ</i><sup>13</sup>C and <sup>14</sup>C of DIC. We applied a novel graphical method for tracking changes in the <i>δ</i><sup>13</sup>C and <sup>14</sup>C 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.
<br><br>
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 <sup>14</sup>C
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 <sup>14</sup>C sources with carbonates.
However, oxygen depletion and <i>δ</i><sup>13</sup>C and <sup>14</sup>C 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 <sup>14</sup>C and <sup>13</sup>C in the upper
aquifer complexes result from a number of biotic and abiotic processes,
including oxidation of <sup>14</sup>C-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.
<br><br>
Our findings demonstrate the large variation in the importance of biotic as
well as abiotic controls on <sup>13</sup>C and <sup>14</sup>C 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. |
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| ISSN: | 1027-5606 1607-7938 |