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How much of the greenhouse gas methane is transported from the seafloor to the atmosphere is unclear. Here, we present data describing an extensive ebullition event that occurred in Eckernförde Bay, a shallow gas-hosting coastal inlet in the Baltic Sea, in the fall of 2014. A weak storm induced hydrostatic pressure fluctuations that in turn stimulated gas ebullition from the seabed. In a finely tuned sonar survey of the bay, we obtained a hydroacoustic dataset with exceptionally high sensitivity for bubble detection. This allowed us to identify 2849 bubble seeps rising within 28 h from the seafloor across the 90 km² study site. Based on our calculations, the estimated bubble-driven episodic methane flux from the seafloor across the bay is 1,900 μMol m −2 d −1. our study demonstrates that stormassociated fluctuations of hydrostatic pressure induce bulk gas-driven ebullitions. Given the extensive occurrence of shallow gas-hosting sediments in coastal seas, similar ebullition events probably take place in many parts of the Western Baltic Sea. However, these are likely to be missed during field investigations, due to the lack of high-quality data acquisition during storms, such that atmospheric inputs of marine-derived methane will be highly underestimated. Methane is an important greenhouse gas, ranking second in radiative forcing by well-mixed greenhouse gases 1 and with an estimated global net atmospheric emission of about 592 Tg per year 2. A sudden increase of atmospheric methane from possibly biogenic sources in the past decade has been reported, that has the potential to challenge the intergovernmental goals for the reduction of greenhouse gas emissions as set out in the UN Paris Agreement 3. Even though the marine environment hosts massive amounts of methane in the sediment 4 , this system represents a very modest source of atmospheric methane (6-12 Tg CH4 per year) 5,6. Methane can be transferred from the seabed into the water column by porewater-seawater diffusion, fluid flow, or the ebullition of gas bubbles. At the seabed-water interface, anaerobic and aerobic microbial oxidation of methane efficiently reduces the dissolved methane fluxes from the sediment into the overlying water column 7,8. Methane gas bubbles can bypass this microbial sink but their rapid dissolution as they rise in the water column 9 and the subsequent microbial turnover of dissolved methane result in a highly reduced methane flux to the atmosphere 10. However, in shallow depth the efficiency of the water column filter is diminished due to the limited retention time of bubbles in the water column and short diapycnal barriers. Therefore, shallow coastal regions can be considered a significant source of atmospheric methane 9,11,12. The high sedimentation rates of organic matter that typically occur in coastal regions drive methanogenesis in the seabed. As a result, methane inputs into the atmosphere from these regions are much more substantial than those from open waters 5,13-15. Moreover, among these coastal sites, the shallow gas-bear...

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 Eckernförde 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 Eckernförde 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.Plain Language Summary Groundwater that seeps out of the ocean floor is a common global phenomenon. Marine ecosystems are highly dependent on nutrient supply from land, and recent studies have suggested that groundwater seeping out of the seafloor supplies even more nutrients to the world's oceans than rivers do. Also, nearshore freshwater springs have been used as a source of drinking water for decades and large offshore groundwater reserves in continental shelves can potentially prevent future freshwater shortages. We use echo sounding methods to search for indications for submarine groundwater. The methods enable us to carry out detailed investigations of intriguing seafloor depressions (i.e., "pockmarks"). Our accurate measurements give new insights into the morphology and characterization of the Eckernförde Bay pockmarks and reveal a new type of pockmark that is related to seeping groundwater. In the muddy sediment of Eckernförde Bay, methane gas forms in the sediments due to microbial decomposition of biomass. In areas of enhanced groundwater advection, this methane gas is brought closer to the seafloor, where pockmarks form and where we can detect the gas with our sonar system. Given the abundant global distribution of muddy gaseous sediments, our findings have important implications for the future detect...