Biogeochemical cycling in an organic-rich coastal marine basin—I. Methane sediment-water exchange processes
Abstract
Methane produced in anoxic organic-rich sediments of Cape Lookout Bight, North Carolina, enters the water column via two seasonally dependent mechanisms: diffusion and bubble ebullition. Diffusive transport measured in situ with benthic chambers averages 49 and 163 μmol · m -2 · hr -1 during November-May and June-October respectively. High summer sediment methane production causes saturation concentrations and formation of bubbles near the sediment-water interface. Subsequent bubble ebullition is triggered by low-tide hydrostatic pressure release. June-October sediment-water gas fluxes at the surface average 411 ml (377 ml STP: 16.8 mmol) · m -2 per low tide. Bubbling maintains open bubble tubes which apparently enhance diffusive transport. When tubes are present, apparent sediment diffusivities are 1.2-3.1-fold higher than theoretical molecular values reaching a peak value of 5.2 × 10 -5 cm 2 · sec -1. Dissolution of 15% of the rising bubble flux containing 86% methane supplies 170μmol · m -2 · hr -1 of methane to the bight water column during summer months; the remainder is lost to the troposphere. Bottom water methane concentration increases observed during bubbling can be predicted using a 5-15 μm stagnant boundary layer dissolution model. Advective transport to surrounding waters is the major dissolved methane sink: aerobic oxidation and diffusive atmospheric evasion losses are minor within the bight.
- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- March 1980
- DOI:
- 10.1016/0016-7037(80)90045-9
- Bibcode:
- 1980GeCoA..44..471M