Project: A New Method for Assessing the Magnitude and Impact of Shallow Seawater/Pore water Exchange in Salt Marsh Systems

NSF Award Abstract:
Tidal marshes are critical junctions between marine and terrestrial chemical processes. The chemical and physical processes that tidal marsh sediments undergo determine the nature of the transformation and accumulation of particulate organic carbon (POC) at the marine to terrestrial interface. Physical transport of pore water through marsh sediments greatly influences the availability of carbon and nutrients that can undergo transformative reactions at the marine to terrestrial interface. Past studies have shown that there is a discrepancy between predicted rates of pore water flow and calculated estimates from tracer studies. Because this flow is crucial to the chemistry of sediment at the terrestrial to marine interface, which is the first junction of the chemical species exported to the ocean, it is important to explain the discrepancy between expected rates of flow and observed rates. This study will use a known tracer method based on natural isotopic ratios (224Radon/228Thorium) and will develop a new method (223Radon/ 227Actinium) to evaluate the export of POC due to pore water flow in tidal marsh sediments, which will have much broader impacts to the overall marine carbon budget. Results from this work will also be integrated into workshops that will educate students and teachers from outside of oceanography about oceanographic carbon and climate research. Presentations will also be developed in collaboration with the South Carolina State Museum as part of the Science Café series. Additionally, one graduate student and one undergraduate student will be supported by this funding.

Predicted advection rates of pore water in very fine-grained tidal marsh sediments have been shown to significantly differ from rates calculated using tracer studies. Because tidal marshes are a crucial chemical link at the marine terrestrial interface, and advection has a significant impact on the availability of chemical species to undergo diagenetic reactions at this interface, it is of significant importance to evaluate this discrepancy. Export of particulate organic carbon (POC) and dissolved organic carbon (DIC), in particular, are of interest, due to the impact on microbial communities within sediments. This research aims to validate the method of 224Ra/228Th disequilibrium to calculate pore water exchange rates. Previous studies using this method have provided evidence for more rapid pore water exchange than would generally be expected in such fine-grained sediments, which are less permeable than coarser grained sediment. Additionally, this research will attempt to develop the disequilibrium method for 223Ra/227Ac. The researchers will evaluate the impact of advection rates on the export of chemical species in salt marshes.