Coupling \u3csup\u3e210\u3c/sup\u3ePb and \u3csup\u3e14\u3c/sup\u3eC to constrain carbon burial efficiency of blue carbon ecosystems

Blue carbon ecosystems cover a small global area but have the potential to sequester large amounts of organic carbon (OC) from the coastal ocean in the sediment. Organic carbon is continually remineralized and exported in the dissolved form, which is currently only poorly accounted for in blue carbon budgets. Constraining carbon cycling in blue carbon systems is complicated by the range of carbon sources and sinks in the system and high export rates from the system. By coupling 14C and 210Pb chronometers to ascertain the amount of primary production stored within peat, it is possible to study carbon transport through peat systems and examine first order changes in carbon stock through time. Peat cores were collected from three sites in the Charlotte Harbor and Ten Thousand Islands regions of Southwest Florida. Within the top 25 cm of core, the 210Pb chronology extends to different age maxima in each system, from 1937 CE in the salt marsh system to 1892 CE in the riverine mangrove system. Radiocarbon dates for all systems indicate modern deposition. By coupling independent chronometers 210Pb and 14C, I was able to determine an age-depth relationship while also tracing the movement of younger carbon from the surface to depth downcore. To better understand the mechanisms and dynamics of carbon sequestration in these ecosystems, a simplified advection model was constructed to visualize the differences in concentrations of OC at differing depths. Different sensitivity tests were conducted to determine the sensitivity of the model to the method of downward carbon transport. The salt marsh system had the smallest carbon mixing depth (25 years) and proportion (0.2) and highest export value (0.6). Conversely, the basin mangrove system has the deepest mixing depth (120 years) but the lowest export value (0.5). I was unable to fit the data sets from this study to the atmospheric bomb curve, despite the addition of a reactive loss term. I assumed all carbon exported was in the form of DIC and did not control for other speciation, which implies that the DIC is likely an overestimate. My results show that the amount of carbon stored in the basin mangrove system (80.11 Mg C ha-1) was an order of magnitude lower than the riverine mangrove system (691.75 Mg C ha-1), but higher than that in salt marsh systems (59.03 Mg C ha-1). Assuming the cores taken are indicative of the system, riverine mangroves in the Ten Thousand Islands store 5.04 x 106 Mg of carbon in the soil. Making the same assumption, the amount of carbon stored in the Ten Thousand Islands basin mangrove system is an order of magnitude lower at 5.83 x 105 Mg. Salt marsh systems in Charlotte Harbor store 3.5 x 105 Mg of carbon in the sediment. Peats in the Ten Thousand Islands have been dated at 3,500 years. During the time since peats have been forming in the Ten Thousand Islands, there has been 4.89 x 108 Mg C of carbon produced. In Charlotte Harbor, peats have been forming for 2,180 years. During this time, 2.31 x 108 Mg C of carbon has been produced. When I compared the amount of carbon produced to the amount of carbon stored, the percent of carbon stored is exceedingly small