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Biogeosciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 14 May 2019

Research article | 14 May 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

CO2 and CH4 fluxes are decoupled from organic carbon loss in drying reservoir sediments

Tricia Light1,2, Núria Catalán1, Santiago Giralt3, and Rafael Marcé1 Tricia Light et al.
  • 1Catalan Institute for Water Research (ICRA), Emili Grahit 101, 17003, Girona, Spain
  • 2Scripps Institution of Oceanography, University of California San Diego, 8622 Kennel Way, La Jolla, CA 92037, California, USA
  • 3Institute of Earth Sciences Jaume Almera (ICTJA-CSIC), Lluís Solé Sabaris s/n, 08028, Barcelona, Spain

Abstract. Reservoirs are a prominent feature of the current global hydrological landscape, and their sediments are the site of extensive organic carbon burial. Meanwhile, reservoirs frequently go dry due to drought and/or water management decisions. Nonetheless, the fate of organic carbon buried in reservoir sediments upon drying is largely unknown. Here, we conducted a 45-day-long laboratory incubation of sediment cores collected from a western Mediterranean reservoir to investigate carbon dynamics in drying sediment. Drying sediment cores emitted more CO2 over the course of the incubation than sediment cores incubated with overlaying water (206.7 ± 47.9 vs. 69.2 ± 18.1 mmol CO2 m−2 day−1, mean ± SE). Organic carbon content at the end of the incubation was lower in drying cores, which suggests that this higher CO2 efflux was due to organic carbon mineralization. However, the apparent rate of organic C reduction in the drying sediments (568.6 ± 247.2 mmol CO2 m−2 day−1, mean ± SE) was higher than C emission. Meanwhile, sediment cores collected from a reservoir area that had already been exposed for 2+ years displayed net CO2 influx from the atmosphere to the sediment (−136.0 ± 27.5 mmol CO2 m−2 day−1, mean ± SE) during the incubation period. Sediment mineralogy suggests that this CO2 influx was caused by a relative increase in calcium carbonate chemical weathering. Thus, we found that while organic carbon decomposition in newly dry reservoir sediment causes measurable organic carbon loss and carbon gas emissions to the atmosphere, other processes can offset these emissions on short time frames and compromise the use of carbon emissions as a proxy for organic carbon mineralization in drying sediments.

Tricia Light et al.
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Tricia Light et al.
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Short summary
Water reservoir sediments can store large amounts of organic. However, it is unclear what happens to this organic carbon when water reservoirs go dry due to drought, water diversion, etc. Here, we conducted laboratory incubations of reservoir sediment to determine the effect of drying on this stored organic carbon. We found that while some of the organic carbon in water reservoir sediments is released to the atmosphere as reservoirs go dry, other sediment processes can offset these emissions.
Water reservoir sediments can store large amounts of organic. However, it is unclear what...