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

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 FeS_x 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 δ¹³C_DIC 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.