DIAGENESIS IN MANGROVE AND SEAGRASS CORES TAKEN FROM NATURAL AND RESTORED BLUE CARBON ECOSYSTEMS IN BISCAYNE BAY, FLORIDA

Mangrove and Seagrass ecosystems are extremely effective at converting atmospheric carbon into organic matter and storing it in lagoonal sediments. These ecosystems, however, are threatened by climate change and coastal development, and many will need to undergo restoration to continue to function as carbon sinks. This study, part of a larger project focusing on these sites, compares sulfur and oxygen microprofiles in surface sediments along with pore water geochemistry from cores collected in a restored and natural mangrove and seagrass community. The data includes DIC concentrations along with d13C isotopic values to help constrain the processes of organic matter cycling in these environments. Taken together, this data may provide insights into the effectiveness of near-shore restoration with respect to the overall blue carbon budget of the ocean.

Sediment cores were taken from an undisturbed site near Matheson Hammock(MH) State park, one from a seagrass meadow, and one beneath the mangrove canopy. Similar cores were extracted from a restored seagrass and mangrove site near Bill Baggs(BB) on key Biscayne. Unisense sulfur and oxygen microsensors were used to record the sharp gradient at the water/sediment interface. Initial results showed similarities between the natural and restored sites, with oxygen depleted within ~1mm of the top of the cores. Dissolved sulfide concentrations at the restored site were significantly higher (~40 mg/L) than those at the natural site (~1 mg/L) and increased deeper into the sediment. This suggests anaerobic sulfate reduction as a dominant metabolic pathway within mm’s of the water/sediment interface . Sodium concentrations in overlying water was 7114 and 10131 mg/kg at MH and BB, respectively. At MH concentrations increase in pore waters downcore, while at BB the concentrations remain near seawater values throughout. While DIC increased in concentration downcore at each site, carbon isotopic values suggested either a seagrass endmember contribution (~-12‰) or a mix of processes including the dissolution of CaCO3.

Rapid onset of anoxia is encouraging for carbon preservation, however secondary processes that may dissolve CaCO3 will need further examination.