In situ determination of porewater gases by underwater flow-through membrane inlet mass spectrometry

Ryan J. Bell, William B. Savidge, Strawn K. Toler, Robert H. Byrne, R. Timothy Short

Limnol. Oceanogr. Methods 10:117-128 (2012) | DOI: 10.4319/lom.2012.10.117

ABSTRACT: An underwater membrane introduction mass spectrometer was deployed in permeable sandy sediment on the Georgia continental shelf (depth = 27 m) to measure in situ dissolved gas concentrations in sediment porewaters. Over a 54-h period, 30 profiles (up to 18 cm deep) were sampled using an automated sediment probe coupled with an underwater positive displacement syringe pump. Porewater was analyzed with a flow-through membrane assembly at constant sample flow rate (0.35 mL/min) and membrane temperature (45°C). Calibration was performed on-site using ambient seawater equilibrated with gas standards. Measurements of methane, nitrogen, argon, oxygen, and carbon dioxide concentrations were used to produce depth-time contours and demonstrate the dynamics of dissolved gases in the porewater. Profiles indicated a well-oxygenated surface layer (1 to 2 cm depth) and anoxia below -3 to -5 cm. Elevated concentrations of methane below the oxycline reveal active methanogenesis in shelf sands despite low (0.05%) organic carbon content. Chemocline depth and sediment ripple height were correlated, suggesting that the porewater environment is controlled by advection-driven interactions between boundary-layer flow and bottom topography. By coupling in situ concentration profiles to independent estimates of sediment-water exchange, it was estimated that maximal oxygen consumption at this site occurs > 2 cm below the interface. Oxygen consumption at this site is estimated as 2.3 mmol m-2 d-1 based on combined dissolved profiles and advection estimates. Raw data and data analysis scripts (Matlab) are available electronically in a Web Appendix.