EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

When “Blue Carbon” turns white: Isotopic evidence of calcification-driven CO2 emissions in a carbonate seagrass meadow

Mary Zeller1, Bryce Van Dam2, Christian Lopes3, Ashley Smyth4, Michael Böttcher1,5,6, Christopher Osburn7, Tristan Zimmerman2, Daniel Pröfrock2, James Fourqurean3, and Helmuth Thomas2
Mary Zeller et al.
  • 1Leibniz Institute for Baltic Sea Research (IOW), Marine Geology, Rostock, Germany (
  • 2Institute of Coastal Research, Helmholtz-Zentrum Geesthacht (HZG), Geesthacht, Germany
  • 3Institute of Environment, Department of Biological Sciences, Florida International University, Miami, Florida, USA
  • 4Soil and Water Sciences Department, Tropical Research and Education Center, University of Florida, Homestead, Florida, USA
  • 5Marine Geochemistry, University of Greifswald, Germany
  • 6Department of Maritime Systems, Interdisciplinary Faculty, University of Rostock, Germany
  • 7Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina, USA

Seagrasses are often considered important players in the global carbon cycle, due to their role in sequestering and protecting sedimentary organic matter as “Blue Carbon”.  However, in shallow calcifying systems the ultimate role of seagrass meadows as a sink or source of atmospheric CO2 is complicated by carbonate precipitation and dissolution processes, which produce and consume CO2, respectively.  In general, microbial sulfate, iron, and nitrate reduction produce total alkalinity (TA), and the reverse reaction, the re-oxidation of the reduced species, consumes TA. Therefore, net production of TA only occurs when these reduced species are protected from re-oxidation, for example through the burial of FeSx or the escape of N2.  Seagrasses also affect benthic biogeochemistry by pumping O2 into the rhizosphere, which for example may allow for direct H2S oxidation.

Our study investigated the role of these factors and processes (seagrass density, sediment biogeochemistry, carbonate precipitation/dissolution, and ultimately air-sea CO2 exchange), on CO2 source-sink behavior in a shallow calcifying (carbonate content ~90%) seagrass meadow (Florida Bay, USA), dominated by Thalassia testudinum. We collected sediment cores from high and low seagrass density areas for flow through core incubations (N2, O2, DI13C, sulfide, DO13C flux), solid phase chemistry (metals, PO13C, Ca13C18O3, AVS: FeS + H2S, CRS: FeS2 + S0), and porewater chemistry (major cations, DI13C, sulfide, 34S18O4). An exciting aspect of this study is that it was conducted inside the footprint of an Eddy Covariance tower (air-sea CO2 exchange), allowing us to directly link benthic processes with CO2 sink-source dynamics.

During the course of our week long study, the seagrass meadow was a consistent source of CO2 to the atmosphere (610 ± 990 µmol·m-2·hr-1).  Elevated porewater DIC near 15 cmbsf suggests rhizosphere O2 induced carbonate dissolution, while consumption of DIC in the top 5-10 cm suggests reprecipitation.  With high seagrass density, enriched δ13CDIC in the DIC maximum zone (10-25 cm) suggests continual reworking of the carbonates through dissolution/precipitation processes towards more stable PIC, indicating that seagrasses can promote long-term stability of PIC.  We constructed a simple elemental budget, which suggests that net alkalinity consumption by ecosystem calcification explains >95% of the observed CO2 emissions.  Net alkalinity production through net denitrification (and loss of N2) and net sulfate reduction (and subsequent burial of FeS2 + S0), as well as observed organic carbon burial, could only minimally offset ecosystem calcification.   

How to cite: Zeller, M., Van Dam, B., Lopes, C., Smyth, A., Böttcher, M., Osburn, C., Zimmerman, T., Pröfrock, D., Fourqurean, J., and Thomas, H.: When “Blue Carbon” turns white: Isotopic evidence of calcification-driven CO2 emissions in a carbonate seagrass meadow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4539,, 2021.


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