Spatial distribution and budget for submarine groundwater discharge in Eckernforde Bay (Western Baltic Sea)

Submarine groundwater discharge (SGD) from subseafloor aquifers, through muddy sediments, was studied in Eckernforde Bay (western Baltic Sea). The fluid discharge was clearly traced by Rn-222 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 >22% of the seafloor of the bay area was affected by freshwater admixture and active fluid venting. A maximal discharge rate of >9 L m(-2) d(-1) was computed by modeling pore water profiles. Based on pore water data, the freshwater flow from subseafloor aquifers to Eckernforde Bay was estimated to range from 4 x 10(6) to 57 x 10(6) m(3) yr(-1). 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 11degrees, 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.