Transport of gas and solutes in permeable estuarine sediments
In this project, we investigate gas bubbles in sandy estuarine and coastal sediments, and the effect of bubbles on transport of matter through the sand. Results will reveal the role of free gas for physical properties, material transport and biogeochemical reactions in shallow sands. The project will improve interpretation of acoustic survey data from sandy estuarine and coastal sediments (e.g. subbottom seismic profiles) and contribute to the development of high frequency acoustic methods for mapping and profiling of non-cohesive sand sediments.
In the shallow coastal zone, runoff from land results in the highest concentrations of nutrients in the oceans and primary production reaches maximum rates in the water column as well as on the sea bed. High photosynthetic activity and large amounts of buried organic matter promote biogenic gas production. At the sediment surface, m icroalgae and cyanobacteria produce large volumes of oxygen through photosynthesis (Fig. 1). Below this surface layer, where degradation processes depleted oxygen but nitrate is present, denitrification or anaerobic ammonia oxidation (Anammox) release nitrogen gas. Intermediate products of denitrification include nitrogen dioxide (NO2-) and nitrous oxide (N2O), which also can escape as gas. Deeper in the sediment, sulfate reducers produce sulfide gas (HS-), and where sulfate becomes depleted, methanogens can use carbon dioxide and small organic molecules as electron acceptors generating methane.
Where gas production exceeds the solubility and consumption and removal rates of the respective gas in the pore water, gas bubbles form, a common phenomenon in coastal and estuarine sediments (Anderson and Hampton 1980; Richardson and Davis 1998). The first gas species to supersaturate initiates bubbles, then other gases in solution diffuse across the bubble boundaries until diffusion equilibrium is reached. Thus, sedimentary bubbles contain a mixture of biogenic gases reflecting the reactions occurring in the sedimentary production and decomposition processes. Biogenic gas volumes in coastal sediments typically range between 0.1 to 1% of the sediment volume; however, in organic-rich estuarine sediments with low sulfate supply (brackish water) up to 10% of the sediment volume can be gas (Abegg and Anderson 1997; Anderson et al. 1998; Wever et al. 1998) . (Hill et al. 1992) estimated that 30% of Chesapeake Bay is underlain by gas, and that in 60% of that area the gas reaches the sediment–water interface.
Relatively few projects have addressed gas bubbles in sandy coastal sediments. Reasons for this small number of investigations may be that gas in sands occurs mostly as small inconspicuous interstitial bubbles (Ohara et al. 1995) and large gas accumulations and ebullitions may be less frequent. In contrast, free gas in muddy deposits displaces sediment producing disk shaped bubbles and visible cracks (Boudreau et al. 2005) often resulting in large gas accumulations (Richardson and Davis 1998) . Our understanding of formation, size spectrum, movement and spatial and temporal distribution of bubbles in shallow sands, thus, remains very limited (Boudreau et al. 2005) .
The SAX99 experiment (Fleischer et al. 2001; Thorsos et al. 2001) that focused on the acoustic properties of shallow sand deposits in the Northern Gulf of Mexico, revealed that free gas is present in the surface layers of the sand at 18 m water depth. In shallower Gulf sediments, massive accumulation of micro- to millimeter sized bubbles can be observed when favorable light conditions and nutrient availability lead to explosive growth of sedimentary microalgae and cyanobacteria.
Gas bubbles formed in deeper sediment layers can migrate upward and accumulate in sand layers. We found large volumes of free methane gas in subsurface layers of highly permeable sublittoral sand sediments off the coast of Hel Peninsula/Poland in the southern Baltic, which were caused by groundwater upwelling that removed sulfate thereby promoting methanogenesis (Fig. 2).
The presence of gas bubbles in the interstitial space of sands changes the physical properties of the sediment. These changes cause that acoustic properties of saturated and gassy sand are significantly different. Analogous to bubbles in open water, bubble pressure equilibrium in the sediment interstitial water establishes a minimum bubble size in the range of 1 to 10 µm (Anderson and Hampton 1980). Gas bubbles in water are capable of vibratory motion with a sharply peaked resonance at the fundamental pulsation frequency. Likewise bubbles embedded in sand have a resonance frequency; however, this frequency now also depends on the characteristics of the sediment and the pore space. When a critical bubble size is reached, bubbles start moving through the sediment resulting in ebullition. Vertical migration of bubbles of 1-mm diameter or less has been observed in sediments of low cohesion and in cases where channels have been opened through the sediment by the vertical migration of larger bubbles (Hampton and Anderson 1974) .
Our research combines field measurements, laboratory experiments and modeling. Sites of potential gas production are mapped, and sediment samples are collected for gas analyses. The results demonstrate the impact of bubbles on dispersion and interfacial flux of solutes and suspended matter in permeable sediments. Sedimentary bubbles may have significant effects on biogechemical reactions in permeable shallow sea beds by enhancing the exchange of matter between sediment and overlying water. Detailed chemical analyses of the gases in the bubbles reveals the processes responsible for the gas generation, and associated measurements of the spatial and temporal distribution of the main gas generating processes (photosynthesis and methanogenesis) allows prediction of areas of bubble formation.
Coastal eutrophication caused by anthropogenic nutrient release to the oceans result in increased gas production through increased photosynthesis, denitrification, carbon dioxide, sulfide production and methanogenesis. These developments have significant environmental implications because gas seepages affect seawater chemistry and have an as yet poorly assessed effect on atmospheric carbon and methane levels. Sedimentary gases like nitrous oxide and methane are effective greenhouse gases emphasizing the importance of investigations that address formation and release of sedimentary gases.
Abegg, F., and A. L. Anderson. 1997. The acoustic turbid layer in muddy sediments of Eckernfoerde Bay, Western Baltic: Methane concentration, saturation and bubble characteristics. Marine Geology 137: 137-147.
Anderson, A. L., F. Abegg, J. A. Hawkins, M. E. Duncan, and A. P. Lyons. 1998. Bubble populations and acoustic interaction with the gassy floor of Eckernforde Bay. Continental Shelf Research 18: 1807-1838.
Anderson, A. L., and L. D. Hampton. 1980. Acoustics of Gas-Bearing Sediments .1. Background. Journal of the Acoustical Society of America 67: 1865-1889.
Boudreau, B. P. and others 2005. Bubble growth and rise in soft sediments. Geology 33: 517-520.
Fleischer, P., T. H. Orsi, M. D. Richardson, and A. L. Anderson. 2001. Distribution of free gas in marine sediments: a global overview. Geo-Marine Letters 21: 103-122.
Hampton, L. D., and A. L. Anderson. 1974. Acoustics and gas in sediments, p. 249-273. In I. R. Kaplan [ed.], Natural Gases in Marine Sediments. Plenum
Hill, J. M., J. P. Halka, R. Conkwright, K. Koczot, and S. Coleman. 1992. Distribution and Effects of Shallow Gas on Bulk Estuarine Sediment Properties. Continental Shelf Research 12: 1219-1229.
Ohara, S. C. M. and others 1995. Gas Seep Induced Interstitial Water Circulation - Observations and Environmental Implications. Continental Shelf Research 15: 931-948.
Richardson, M. D., and A. M. Davis. 1998. Modeling gassy sediment structure and behavior - Introduction. Continental Shelf Research 18: 1669-1669.
Thorsos, E. I. and others 2001. An overview of SAX99: Acoustic measurements. Ieee Journal of Oceanic Engineering 26: 4-25.
Wever, T. F., F. Abegg, H. M. Fiedler, G. Fechner, and I. H. Stender. 1998. Shallow gas in the muddy sediments of Eckernforde Bay, Germany. Continental Shelf Research 18: 1715-+.