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The MITAS (Methane in the Arctic Shelf/Slope) expedition was conducted during September, 2009 onboard the U.S. Coast Guard Cutter (USCGC) Polar Sea (WAGB-11), on the Alaskan Shelf/Slope of the Beaufort Sea. Expedition goals were to investigate spatial variations in methane source(s), vertical methane flux in shallow sediments (<10 mbsf), and methane contributions to shallow sediment carbon cycling. Three nearshore to offshore transects were conducted across the slope at locations approximately 200 km apart in water column depths from 20 to 2100 m. Shallow sediments were collected by piston cores and vibracores and samples were analyzed for sediment headspace methane (CH4), porewater sulfate (SO42−), chloride (Cl−), and dissolved inorganic carbon (DIC) concentrations, and CH4 and DIC stable carbon isotope ratios (δ13C). Downward SO42− diffusion rates estimated from sediment porewater SO42− profiles were between −15.4 and −154.8 mmol m−2 a−1 and imply a large spatial variation in vertical CH4 flux between transects in the study region. Lowest inferred CH4 fluxes were estimated along the easternmost transect. Higher inferred CH4 flux rates were observed in the western transects. Sediment headspace values ranged from −138 to −48‰, suggesting strong differences in shallow sediment CH4 cycling within and among sample locations. Measured porewater DIC concentrations ranged from 2.53 mM to 79.39 mM with δ13CDIC values ranging from −36.4‰ to 5.1‰. Higher down-core DIC concentrations were observed to occur with lower δ13C where an increase in was measured, indicating locations with active anaerobic oxidation of methane. Shallow core CH4 production was inferred at the two western most transects (i.e. Thetis Island and Halkett) through observations of low coupled with elevated DIC concentrations. At the easternmost Hammerhead transect and offshore locations, and DIC concentrations were not coupled suggesting less rapid methane cycling. Results from the MITAS expedition represent one of the most comprehensive studies of methane source(s) and vertical methane flux in shallow sediments of the U.S. Alaskan Beaufort Shelf to date and show geospatially variable sediment methane flux that is highly influenced by the local geophysical environment. 相似文献
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A realistic-geometry global baroclinic tidal model forced with a single tidal constituent is used to investigate the generation of the internal tide and the associated radiated baroclinic energy flux. The model internal wave spectrum is populated at discrete frequency multiples of . The subharmonic is particularly energetic at its turning latitude of . Poleward only integer superharmonics of are significantly excited. The subharmonic turning latitude (SHTL) disturbance has high vertical wavenumber and shear, provided that internal tide energy level exceeds a threshold value. Under these circumstances, Richardson numbers smaller than 1/4 occur in the upper few hundred meters in both the realistic-geometry model and in a complimentary idealized geometry two-dimensional (2D) model. In the 2D model, the disturbance enables Richardson number dependent diapycnal entrainment to effect a modification of the stratification of the upper 400 m of the ocean, and poleward cross-SHTL energy flux falls to 10% of its pre-instability value due to energy transfer to the non-propagating (i.e., inertial) subharmonic. Realistic-geometry simulations suggest a more modest 40% decrease in net flux, although the strongest beams are almost entirely shut down. The predicted energy flux-convergence implies a thermocline dissipation rate in the 28.5–30.0°N latitude band of , with an associated diapycnal diffusivity of . North of Hawaii the implied regional dissipation rate reaches with an associated thermocline diffusivity of . Investigations of subgridscale parameterization and resolution sensitivity suggest that the basic character and magnitude of the predictions are robust to details of the numerical solutions. The present results are taken as further evidence that an increase in shear-driven turbulent mixing in the upper ocean is predicted at special latitudes. It is suggested that the search should be directed to regions where intense low-mode internal tide beams cross their subharmonic turning latitude. 相似文献
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