Submarine groundwater discharge (SGD) from subseafloor aquifers, through muddy sediments, was studied in Eckernförde Bay (western Baltic Sea). The fluid discharge was clearly traced by 222 Rn enrichment in the water column and by the chloride profiles in pore water. At several sites, a considerable decrease in chloride, to levels less than 10% of bottom-water concentrations, was observed within the upper few centimeters of sediment. Studies at 196 sites revealed that Ͼ22% of the seafloor of the bay area was affected by freshwater admixture and active fluid venting. A maximal discharge rate of Ͼ9 L m Ϫ2 d Ϫ1 was computed by modeling pore water profiles. Based on pore water data, the freshwater flow from subseafloor aquifers to Eckernförde Bay was estimated to range from 4 ϫ 10 6 to 57 ϫ 10 6 m 3 yr Ϫ1 . Therefore, 0.3-4.1% of the water volume of the bay is replaced each year. Owing to negligible surface runoff by rivers, SGD is a significant pathway within the hydrological cycle of this coastal zone. High-resolution bathymetric data and side-scan sonar surveys of pockmarks, depressions up to 300 m long, were obtained by using an autonomous underwater vehicle. Steep edges, with depths increasing by more than 2 m within 8-10 m in lateral directions, equivalent to slopes with an angle of as much as 11Њ, were observed. The formation of pockmarks within muddy sediments is suggested to be caused by the interaction between sediment fluidization and bottom currents. Fluid discharge from glacial coastal sediments covered by mud deposits is probably a widespread, but easily overlooked, pathway affecting the cycle of methane and dissolved constituents to coastal waters of the Baltic Sea.For coastal areas, Sonrel (1868) reported the discharge of freshwater from submarine springs and speculated on their use and risks for sailors. Since then a few studies have considered the importance of fluid discharge from sediments for nutrient budgets of coastal environments, formation of offshore plankton blooms, hydrological cycles, or the release of trace elements and gases such as radon from the seafloor (Johannes 1980;Valiela et al. 1990;Moore 1996;Cable et al. 1997;Laroche et al. 1997). For the South Atlantic Bight, on the eastern coast of the United States, Moore (1996) deduced that the quantity of submarine groundwater discharge (SGD; here taken as fresh plus salt water) represented 40% of the river input into the study area. Although these figures probably overestimate the contribution of submarine seepage 1 Corresponding author (mschlueter@awi-bremerhaven.de). AcknowledgmentsWe thank the captains and crews of RVs Littorina, A. v. Humbold, and Alkor for their support during several cruises. We are grateful to W. Lemke and J. Harff from the Institute of Baltic Research (IOW, Warnemünde) for providing the vibro corer system. Jayne Wolf-Welling is gratefully acknowledged for improving the English text. We thank DeBeers Marines/Maridan for conducting the AUV dives. Application of GIS was supported by Dr. A. Schäf-er. The pap...

No abstract

No abstract

No abstract

No abstract