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Abstract

We report substantial tidal fluxes of dissolved black carbon (DBC) in a fire‐affected marsh in the northern Gulf of Mexico. DBC was molecularly determined as benzenepolycarboxylic acids in a tidal creek, adjacent rivers, and the coastal ocean. Supported by stable carbon isotope and in situ fluorescence measurements, three sources of dissolved organic carbon (DOC) were identified that mixed conservatively in the coastal system: groundwater from salt marshes, river water, and seawater. Groundwater was the main source of DBC to the creek. The highest DBC concentrations of up to 41 µmol C L−1 (7.2% of DOC) were found in the creek at low tide, compared with < 18 µmol C L−1 in all other samples. Over the studied tidal cycle, we determined a runoff (load per drainage area) of 3700 moles DBC (44 kg C) km−2 of salt marsh. This is high compared with the Apalachicola River, where the annual DBC runoff is on the order of 104 mol (120 kg C) km−2 yr−1. In the marsh, it would require ∼ 20 tidal cycles similar to the one that we studied to remove all black carbon produced during one fire event. Because a spring tide was studied, our estimate is as an upper limit. DBC is ubiquitous in the global ocean, and dissolution and subsequent lateral transport appear to be important removal mechanisms for soil black carbon. Our study, which provides a snapshot in time and space, demonstrates that tidal fluxes may be primary carriers of DBC, and therefore tidal pumping and groundwater discharge cannot be ignored in assessing the continental runoff of DBC.