Abstract
Submarine groundwater discharge into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine ecology, the coastal sedimentary environment, and its potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical pore water and water column investigations at a well-known groundwater seep site in Eckernforde Bay (Germany). We aim to better constrain the effects of shallow gas and submarine groundwater discharge on high-frequency multibeam backscatter data and to present acoustic indications for submarine groundwater discharge. Our high-quality hydroacoustic data reveal hitherto unknown internal structures within the pockmarks in Eckernforde Bay. Using precisely positioned sediment core samples, our hydroacoustic-geochemical approach can differentiate intrapockmark regimes that were formerly assigned to pockmarks of a different nature. We demonstrate that high-frequency multibeam data, in particular the backscatter signals, can be used to detect shallow free gas in areas of enhanced groundwater advection in muddy sediments. Intriguingly, our data reveal relatively small (typically <15 m across) pockmarks within the much larger, previously mapped pockmarks. The small pockmarks, which we refer to as "intrapockmarks," have formed due to the localized ascent of gas and groundwater; they manifest themselves as a new type of "eyed" pockmarks, revealed by their acoustic backscatter pattern. Our data suggest that, in organic-rich muddy sediments, morphological lows combined with a strong multibeam backscatter signal can be indicative of free shallow gas and subsequent advective groundwater flow.