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Radon as a tracer of biogenic gas equilibration and transport from methane-saturated sediments

Authors

  • Christopher S. Martens,

  • Jeffrey P. Chanton


Abstract

Radon 222 activity measurements in sediment gas bubbles can provide key information about in situ bubble equilibration with dissolved gases in pore waters and rates of dissolved gas stripping and transport via ebullition. Activities of the noble gas radon in bubbles will reflect gains or losses resulting only from physical processes and radioactive decay. Gas bubbles in the sediments of Cape Lookout Bight, a small coastal lagoon on the Outer Banks of North Carolina, are at equilibrium with respect to dissolved radon and methane present at concentrations of 1.6 ± 0.3 × 10−15 and 1.5 to over 2.5 mM, respectively. In situ radon activities in sediment gas bubbles at 1.8-atm pressure range seasonally from 257 to 700 disintegrations per minute per liter of gas (dpm Lg−1), whereas methane concentrations range from 86.6 to 91.3%. During summer months, bubble ebullition strips and transports 1.9–4.8% day−1 of the standing crop of radon in surface sediments to the troposphere. The ebullitive mode of gas transport represents an effective mechanism for bypassing oxidizing environments and delivering biogenic gases produced in anoxic sedimentary environments directly to the atmosphere.

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