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

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.