Continental margins represent a potentially large, but largely unconstrained, flux of CO2 between the coastal ocean surface and atmosphere, with efforts to predict this flux generating estimates of either a sink or a source of approximately 1 Pg C yr-1, which is significant globally relative to, e.g., pelagic air-sea exchange. Large variability inherent in these settings makes extrapolation based on sparse sampling difficult. Further confounding these efforts are uncertainties as to the boundaries of the coastal margins, both in seaward and landward extent. There is sensitivity of global margin air-sea CO2-flux calculations to the inclusion or exclusion of estuaries. Other marginal environments, such as salt-marshes, mangrove forests, and tide flats may exchange large quantities of CO2 directly with the atmosphere, and it is unclear whether these fluxes are accounted for in regional to global scale CO2 budgets. In addition to air-sea gas exchange, the modes and magnitudes of carbon transport across other key boundaries in the coastal oceans are poorly understood. Net exchange between pelagic and coastal oceans; between estuaries and coastal oceans; between the coastal water column and seafloor; and between subaerial coastal environments and the atmosphere are all areas that have received limited study.