Tide-driven deep pore-water flow in intertidal sand flats

Sulfidic seeps with methane ebullition were observed at the low-water line of intertidal sand flats at a number of locations in the Wadden Sea. Bioturbating fauna was absent in the seep areas but abundant in the more central areas of the tidal flat. At one site, the vertical methane and sulfate distribution in pore water was determined along transects from the low-water line toward the interior of the sand flat. The resulting two-dimensional distributions showed a plume of methane-rich and sulfate-depleted pore water reaching from a depth below 1.2 m beneath the sand surface up to the sediment surface at the low-water line. The d13C of methane released at the seeps was 268.6%, indicating a biological origin. The 14C signature of methane was clearly elevated by anthropogenic radiocarbon, which shows that the methane was formed less than 50 yr ago. The observations indicate an internal circulation, where water enters the sand flats in the central area and exits at the low-water line. Pore-water flow patterns in the sand flat during the tidal cycle were calculated from the surface topography and from the pressure distribution at the flat surface across the tidal cycle. The calculated flow patterns explain the measured methane and sulfate distributions and predict a residence time of the seepage water of about 30 yr. Intertidal sand flats act as one-way valves, passing water from the central surface through the interior of the flat to an outflow zone at and below the low-water line with a velocity of millimeters to centimeters per day. The flow causes permeable tidal flats to emit methane to the surface water and atmosphere in substantial amounts. The Wadden Sea is one of the largest coherent tidal flat systems in the world. It is separated from the North Sea by a chain of barrier islands across 500 km of coastline from Den Helder in the Netherlands, across the German coast to Blåvands Huk in Denmark (Fig. 1). The tidal range is between 3.5 m in the central part and 1.2 m in the northernmost and southernmost areas (van Beusekom 2005). The original ecosystems of the Wadden Sea were salt marshes, mudflats, and sea grass beds that developed after the last ice age. The salt marshes disappeared because of peat harvesting, and the floodplain areas decreased, primarily because of land reclamation. Thus, the sediments of the Wadden Sea are changing via a gradual displacement of silt with sands, especially toward the barrier islands and the large tidal channels where intertidal sand flats now dominate (Lotze 2005). The incentive for this study was methane ebullition and seepage of sulfidic pore water observed at the low-water line of the intertidal sand flat Janssand. A former study (Billerbeck et al. 2006a) concentrated on nutrient fluxes carried by relatively surficial flow. This study aims to identify the source and pathway of the methane-rich water that seeps out near the low-water line, i.e., the deeper part of what was defined as body flow (Billerbeck et al. 2006b). Methane is globally one of the largest reservoirs of organic carbon. It is a powerful greenhouse gas, and therefore the global budgeting of sources and sinks is of great importance. Searches for marine sources of methane are focused on deep-sea seepage through mud volcanoes and gas hydrate bearing sediments. This methane source is fossil organic matter. In these areas most methane transported toward the oxic biosphere is removed by anaerobic methane oxidation coupled with sulfate reduction (Boetius et al. 2000; Elvert et al. 2005; Treude et al. 2005; de Beer et al. 2006). The release rate of methane from the seabed and benthic oxidation rates are still poorly constrained because of lack of understanding of the controlling factors. The kinetics of anaerobic methane oxidation is slow compared with other microbiologically catalyzed sediment processes, and this allows the broad coexistence of sulfate and methane at turnover times of months to years (Jørgensen et al. 2001). Special conditions, for example by pore-water flow or gas ebullition by excessive methane accumulation, enable the methane to escape up through the sulfate barrier (Martens and Klump 1980). Methanogenesis is thought to be of little importance in shallow coastal areas, since the degradation of organic matter is dominated by aerobic respiration and sulfate reduction (Reeburgh 1983). However, tidal estuaries are 1 Present address: Center for Geomicrobiology, Department of Biological Sciences, University of Aarhus, Ny Munkegade 1535, DK-8000 Århus C, Denmark. 2 Present address: National Fisheries Research and Development Agency, Kijang-Kun, Pusan 626-900, Korea. 3 Present address: TNO Built Environment and Geosciences, Princetonlaan 6, 3584 CB Utrecht, the Netherlands.